Thursday, April 6, 2017

Hot Water BITX 40 Hack

Fred's idea really resonated with me.   My first SSB rig was an HW-32A, the 20 meter version of the rig shown above.  If -- as I suspect -- these rigs are anything like the HW-101, they are not aging well. Heath's drive for economy resulted in rigs that don't hold up to well over time. I remember the sound of the  plastic HW-101 dial clutch cracking when I pushed the button.

BITX40 Modules to the rescue! Put a mono-band board inside an old mono-band rig.   There are a lot of possibility here.  Some ideas:

-- Put that Heath VFO to use.  Maybe convert it to solid state.  Or just put the LCD from an Si5351 in the window (Pete did this with an HW-101).

-- Get the S-Meter wiggling.  

-- Keep the final amplifier circuitry in there and let the BITX drive it.  This will give you a QRO option.  Bill N2CQR 

Hello Fellows,
Attached is a picture of my BITX-40 V3 adapted to a Heath kit Single Bander HW22. This is a work in progress but what a neat way to bring an old boat anchor into the present.
The only parts of the HW 22 used were the front panel and case and knobs. Modifications yet to be  incorporated include: AGC , a USB port on the front panel to access the Arduino, and a PTT/CW mode switch.
I enjoy your pod cast and web site…Best of 73 KC5RT.

Wednesday, March 29, 2017

ND6T's Forward and Reverse Power Meter for the BITX

RF Monitor
by Don Cantrell, ND6T

In response to several requests for a VSWR and output power monitor I have developed a simple circuit to be easily added to the BITX (or any transceiver) and some software to get it working. I used a 20 dB directional coupler design, sometimes called a “Stockton bridge”, that is simple, broadband, and requires no adjustment.

The sensor circuitry is mounted on a small piece of printed circuit board that is held in place under the mounting nut of the BNC antenna connector, replacing the ground lug that was originally supplied. The (brown) wire from the 2-conductor “ANT1” plug feeds through the center of one toroid core (T1) and solders to the BNC jack, the other (black) solders to the new board's ground plane. Two new wires connect to the Raduino plug for reading and display of the results.

I began with a square of un-etched printed circuit board stock, one inch on each side, and drilled a 3/8” hole near one corner to fit over the antenna jack. Leaving enough room for the mounting nut, I glued 4 small pieces of PC board as solder pads. The two transformers are T37-43 ferrite cores wrapped with 10 turns of AWG#24 wire in a single layer. The 50 ohm terminations are ½ watt 51 ohm resistors. I hand-selected two of the ones closest to 50 ohms but this is not critical (what's an ohm between friends?) ¼ watt will work fine at these levels. I used two 1N34A diodes for detectors.

I mounted all of the components upright to save space. A short jumper wire feeds from T1 secondary through the center of T2 to the forward detector as that primary winding. Note that you do not wrap this around the core, just pass it through like the antenna lead on T1.

Don't have any #24 wire? Use whatever you have that will fit 10 turns. Don't have a T37 core? A T50 or even T60 will work. It is best, however, to stick to 43 mix ferrite unless you adjust the turns accordingly. Capacitor voltage or temperature coefficient is not important. Got lots of room around the antenna connection on your build? Then go large with the PC board and give yourself some space for component mounting and ease of connection.

Testing is simple. If windings have the proper polarity then, when you transmit into a dummy load, there should be a couple of volts DC on the “Forward” solder point and a very low voltage on the “Reflected” pad. The coupler works the same in both directions, meaning that if it were built outside of the BITX, then you could reverse it and get the same readings by switching voltage pads. A nice check of operations. 7 watts on my build resulted in a bit over 2 volts volts on the output, well under the maximum 5 volts allowed on an Arduino® A/D input.

I included some formulas in the software to average the peaks during SSB operation and displaying it every 3 seconds. The results are also displayed after a half-second key down in straight-key mode. The power reads out as watts in both forward and reverse directions. As usual, the Arduino sketches are available free by emailing

Not using a digital readout? Just attach meters and adjustment pots to the output connections. If you want just one single-movement meter then use a switch and a trim pot for each switch position.

de ND6T

Tuesday, March 28, 2017


BITX CW Update
By Don Cantrell, ND6T

Since the last post (February 25, 2017 “Putting the BITX Raduino on CW”) I have made a few changes in the sketches, added a variable keying speed control, and added a Straight Key mode. In the ensuing weeks it has performed nicely on the air, superb reports from everyone.

The primary reason for the initial sluggishness in calling up the original keying routine was that the display updating necessitated a sizable delay to keep it from flickering. By moving the display update out of the loop() and putting the CW into a “while()” structure made the delay essentially disappear. It also cleaned up the code, modularizing it to make it easier to re-configure and to steal whole sections to use in other sketches. I decided to use iteration counts (how many times the subroutine ran) to create the time-out to return from CW transmit to receive. This is called “semi-break in” operation and is used on many rigs. This is still primarily a 'phone rig, after all. I was reminded about how fast the Arduino works by the number of iterations it performed in 1.5 seconds...100,000! Zoom! Yes, it read the paddle inputs 100,000 times in one and a half seconds.

The next big change was a speed control. I am not a fan of menus. I want to turn a knob and go. So I drilled another hole in the front panel of the BITX and mounted another 10K linear taper pot. I wired it like the tuning control: +5 volts to one side, chassis ground to the other, and the wiper to an unused A/D pin on the Raduino. Since the new speed control was right near the tuning control, I just wired from that for the end connectors.

The CW function reads the value and converts directly to Words Per Minute. The display shows the two-digit speed at the end of line 1 right after the “Idle” indicator on my program. I have it set for 10 to 35 WPM, pretty much my normal range. It can be changed to cover whatever range the operator would like by a couple of simple changes in the code. That code is still extremely short, small, and in simple Arduino.

I am not a person that shuns Straight Key mode, though. In fact, I rather prefer it. It is simple, understandable, and has character. Much like a persons' hand-written messages. An operator's fist is as distinctive as his call. And who can send CW slower than 10 WPM on a paddle?

With the current sketch, turning the keyer speed control to less than 10 WPM replaces the speed indication with “SK” and directs the program flow to the straight key function. Again, after 1.5 seconds of no key closure, the BITX returns to receive.

The only glitch occurs if the straight key uses a 2-conductor plug or has the sleeve and ring conductors shorted and is plugged into the 3-conductor key jack. Do that and the Raduino sees it as a request for a “dash” from paddle. This is handled by reading the speed control. If the control is set for “SK” then all is well. Leave it on a keyer speed position with such a straight key plugged in then you can expect the BITX to start transmitting “ DAH DAH DAH...” until you realize your error.

Although I was initially worried about key clicks due to the fast rise time of the keying envelope I don't see much difference from my other CW rigs. My Icom 746pro is not bothered just 100 Hz away from the 50dB over S9 signal when using a 50Hz receive filter. No one I have worked notices a problem. One operator using my conversion thought it had some chirp but it turned out to be receiver overload instead. Not the BITX, another receiver, relax.

Dmitry, EI3JQ, reported that he had to use a lower value series resistor at the signal injection point at C132. He says that the 470 ohm resistor resulted in low output. Further reports are still pending but I expect that different builds will show variation. It is my opinion that the CW output should be no more than the SSB PEP level to maintain signal purity and lessen component strain. The average power difference is nearly 10 dB and should be taken into account. Dmitry had much lower power, though. He reports nice CW contacts both in Europe and Russia now.

I also re-wrote the sketches to reduce the number of synthesizer and display refreshes. The newer versions of the etherkit Si5351 libraries ( v 2.0.2  and v 2.0.3) shut off the synthesizer outputs for a brief time when programmed for frequency. This creates a clicking in the audio. My earlier sketches were constantly updating so it was particularly irritating. Jerry, N5KYD, found this problem. I suggest using v 2.0.1 until the library is further corrected but my newer sketches have profitted from the experience.

If anyone wants a copy of the current sketch with CW, for either the 40m Raduino BITX or the 60 meter version, just write me at: ND6T@ARRL.NET  for a copy. There are changes made almost daily, lots to improve. The fun is in the innovation, after all!

de ND6T

Thursday, March 9, 2017

A Band Scope for the BITX

The use of an RTL-SDR dongle with a discarded Android phone as the display seems to me to be a very BITX-appropriate use of road-kill technology.  Bill N2CQR

Thoughts and experiments on BITX Band Scope
by Ken Marshall G4IIB

I have explored several very simple ways of implementing a band scope on my BITX. To test the concept I am using HDSDR on a PC and an HF converted RTL SDR dongle (a la Sprat 162). We can turn the sound off on our SDR software as we are only interested in the display. I have also tested it in demo mode on an android phone using SDRTouch as seen above.

Method 1 implementing a band scope on the first IF stage (output of Q2). This gives a classic tunable Pan-adapter. You tune your SDR software (HDSDR or SDRTouch) to the IF frequency 12Mzh and you will see a band scope and waterfall display.  You tune stations in the normal way on the BITX and you will see that once you resole the LSB signal on the BITX it resides within the 12Mhz pass band near 12Mhz on the waterfall.  See below you will also see that the lower portion of the 40M band is to the right note the CW section 1260 – 1280 and the high end of 40M is to the left  this is inversion due to my use of high side vfo. This will not be he case if you are using the conventional low side VFO ( the waterfall will change if you use high side VFO to change to USB as implemented on the Raduino using the fixed BFO crystal, if that’s what you use). What you see on the scope is not what you get in that every thing is referenced to 12Mhz you have to do a conversion in your head to know where you are within the 40M band, but it does show you the activity on the band which you can tune to via the BITX.

The next image shows the mess the waterfall gets in to when you tune the band. The trick when tuning is to ignore the waterfall and concentrate on the peak in the band scope of the signal you want to listen to as it moves into the 12Mhz pass band when tuning the BITX. 

As previously indicated the hardware mod itself is very simple, tack a 10 or 20pF cap (whatever low value you have) onto the crystal side of C23 and connect this to your dongles antenna connection via screened lead.

Method 2  implementing a band scope on the RF amp stage (output of Q1). This gives an untunable Pan-adapter/band scope. You tune your SDR software (HDSDR or SDRTouch) to 7.1Mzh and you will see a band scope and waterfall display of the whole of 40M.  You tune stations in the normal way on the BITX the band scope does not move as you tune all of the signals you see are in the correct place on the band. If you see a signal on the band scope at say 7140 tune the BITX to 7140 to listen to it, the band scope does not change. As the main purpose of the band scope is just to give a visual indication of what activity is on the band you may find this approach simpler to use.

Again the hardware mod itself is very simple, this time  tack a higher value cap 10nF onto C13 and connect this to your dongles antenna connection via screened lead.

The main downside to methods 1 & 2 are the amount of spurs that can be seen in either of the displays as above. This is a known issue with these cheap RTL SDR dongles and not easy to overcome. However, most of the spurs seem to be generated within the BITX. This is hardly surprising given that I'm running an arduino, which has its own internal clock, which is driving an SI5351 producing VFO and BFO frequencies, harmonics from these and those produced by the RTL devise itself will be generating and mixing all sorts of other harmonics. Contrast the above with the SDR on its own antenna not connected to the BITX. Hardly any spurs see below. This would be another option (method 3) to use the dongle with either a PC or android device as a band scope using a separate RX antenna.

 I have loaded SDRTouch on my android phone and it works but only for 60 seconds in demo mode. Not really long enough to test and fully evaluate the functionality. I have to pay£7 for a key to use the full spectrum functionality. I'm not sure at this stage which android devise I want to run it on or the use I'll make of it. But I may consider it in the future. This is why most of my testing has been carried out via HDSDR on a PC. I have tried other android SDR apps but SDRTouch seems to be the only one to offer I or Q channel inputs necessary to operate HF modded dongles.

Whilst methods 1 & 2 work they would benefit from further experimentation. Reducing or eliminating the spurs would be nice but difficult. I have since found that screening the Arduino and using coax instead of just screened lead to connect to the RTL helps. As it stands even with the spurs it still gives useful band information. A band scope found on $2000 plus rigs it is not! But for the outlay of $13 for an SDR dongle and an android device it could be a useful addition to the BITX.

In summary, both methods are very easy to implement but I feel that method 2, although not a classic tunable band scope, is probable the easiest option from a user perspective. Try it on your PC or android and see what you think.

Post Script check out these V3 dongles from the people. They have improved screening, spur suppression, lower noise floor and are already modded for HF. It's only 19 of your American bucks.

Saturday, February 25, 2017

Putting the BITX Raduino on CW

Clean CW on the Raduino
 by Don Cantrell, ND6T

Although the original Raduino v 1.01 sketch included a CW mode I wasn't pleased with the signal produced from the side tone insertion into audio. Even when filtered to where it sounded acceptable there were multiple artifacts accompanying the tone. Since I like a 700 Hz tone there were three additional harmonics spread across the spectrum within the 2.8 KHz spectrum. These are attenuated by the degree of filtering but unless I used active filtering they were quite evident. Additionally, there was the suppressed carrier and the opposite side band both showing at several milliwatts. All this would not be detectable if my signal was quite weak, but there are many times when nearby stations will be annoyed by these “companions”.

I decided to use the CLK1 output of the Raduino module (P3 pin 11) inserted, at operating frequency, into the RF Power Amplifier predriver (Q13). I used a 470 ohm resistor soldered to C132 (either side, CLK1 has a blocking capacitor) fed with a shielded RG-174/u cable. I grounded the shield at the synthesizer at P3 pin 12 but left the other end open, dressed back ½” , and covered with tubing. The 470 ohm resistor throttled the CW output to 5 watts. If you want to chance it, a 220 ohm resistor at that point will output 10 watts. I prefer 5.

 The T/R relays are keyed by placing a 2N7000 NMOS transistor from ground to the PTT pin on the microphone jack. Source to ground, Drain to PTT, and Gate to P3 pin 1 of the Raduino.

Another 2N7000 is placed across R44, the base of Q4 to ground. Source to ground, Drain to the base side of R44, and the Gate tied directly to the gate on the 2N7000 T/R driver that was just installed. This transistor disables the post-mixer transmit buffer in order to eliminate the remaining carrier and anything coming in from the microphone.

The result is a clean and clear CW tone.

In my software I use a side tone that matches my transmit offset. A 700 Hz offset will produce a 700 Hz side tone, making it easy to spot my transmit signal in the receiver. This side tone appears on P3 pin 2 on my build and is injected into the speaker jack through a 1 uF capacitor. Those users who have chosen to remove C113 from the audio output amplifier can use that capacitor: Just lay it on top of the jack and solder one end to the “tip” connector pin (the brown wire). Want a different level? Then either change the capacitor to a different value or put a resistor in series with the 1 uF. Another variable resistor on the front panel, perhaps?

I wrote a primitive little iambic keyer routine and placed it in a “while” loop. The structure is timed by a simple iterative count and jumps back out to normal operation after about one and a half seconds of inactivity. That is the “semi-break in” time out and is changeable in software to meet your needs. The key inputs are fed into P3; pin 3 for the dits, pin 4 for the dahs.

Two transistors, one resistor, and a capacitor. Less than $1 USD in parts!

The CW routine is called by holding the paddle to either side. The relays are operated and the fun begins. Speed is set in the start of the program as a variable, “wpm”, and is fixed on my simple sketch. You can set up a simple pot (just like the tuning control) to generate a number to be directly scaled to the word-per-minute rate and place that control right on the front panel for speed control. Place the WPM rate on the second line of your display. Class!

You could do the same thing with the offset/side tone frequency. Go crazy and amaze your friends!

I can provide a current copy of the 60m sketch that includes the CW program. It is written in simple Arduino, no fancy C++ calls. I like simple. Also included is the added tuning of 5 to 6 MHz. Nice to calibrate with WWV, listen to trans-oceanic aircraft and short-wave broadcasts. Write me at  An adaptation for 40 meters will also be available when I get to it.

It certainly serves as a successful proof of concept. It is not, however, a great CW transceiver. Yet.

*       Activation of the CW function is awkward and slow. It is delayed by the iteration of  the main loop, including the display. Solvable by an interrupt routine, perhaps?

*          It needs a straight key mode.

*          Keying waveform is too abrupt. Perhaps an keyed integrator in the PA power plug circuit?

*          Receive bandpass it pretty wide for CW. Audio filter?

Plenty of room for improvement and more modifications. Isn't that the whole point of the project? Incremental improvements and improvisations, learning (both hardware and software), collaboration and open-source hardware and software. These are grand times.

de ND6T

Tuesday, February 21, 2017

Cap Stack Hack: Putting the BITX40 on 60 Meters

Putting the BITX40 Module on 60 Meters
by  Don Cantrell, ND6T

I have been enjoying the BITX40 with  the AD9850 DDS VFO but when the Raduino was announced I was overwhelmed with curiosity. I ordered a new BITX complete with Raduino. What would I do with another 40 meter BITX? How about 60 meters?

60 meters in the U.S. is pretty limited, currently only 5 fixed channels and limited to 100 watts effective radiated power, and so it is seldom addressed in commercially built gear. It has good propagation for nearby communication and there is a possibility that a new segment (not channels!) will soon be authorized for QRP operation. The BITX would be perfect since it is easy to program for new allocations.

While waiting for the new BITX I considered conversion strategies by using the AADE filter modeling program. First, I modeled the existing 40 meter RF bandpass filter and then modified it for 60 meters. My first idea was to change just the series capacitors in the original. It worked!

The nice curve to the right (in black) is the original RF band pass response. Changing the three series-tuned capacitances produced the response to the left (in red). Yes, it is twice as broad, but that is what I wanted since it would include WWV and a number of frequencies that I enjoy.

The conversion does not require removing any hardware. Just add four common value capacitors! Parallel the three 100 pF capacitors (C2, C4, and C6) in the RF bandpass filter with 100 pF capacitors. I  just stacked them. The fourth additional capacitor is a 220 pF capacitor across L7 (in the transmit low-pass network) to attenuate to second harmonic (now > 50 dBc). Voltage ratings are not a problem.

Stacking is done simply if you have some 1206 sized capacitors. Just sweeten the solder fillets on each end of the target capacitors with just a touch of extra solder. Place the additional part atop the target part. Hold it down while re-heating the solder at each end and you will see the capillary action pull the solder up onto the ends to complete the joint. Ta-DA!

No SMD capacitors in your junk box? Just use ceramic disks, then. The 220 pF needs to have leads  in order for it to reach across the L7 terminals and it goes underneath the board. You can use a ceramic disk like I did, but a silver mica or any other RF type would work well.

That was too easy. The only thing left to do is to load new operating software and put it on frequency.

I wrote an entirely new sketch for the Raduino. Instead of tuning a band, we now have (currently) just five channels. The new program has you tuning the knob to the far clockwise end to start the channels  changing from #1 to #5 and then starting at #1 again. I will add other channels for WWV and the broadcasters to suit me and will institute tuning when the allocation is approved in this country. This way I don't need to modify more hardware. As it now stands, I can just turn the knob to where it slowly  scans all of the channels and walk away until I hear activity. To stop channel selection, turn the knob counter-clockwise anywhere out of the channel select zone.

I will provide the 60 meter sketch to anyone that is interested free (of course) if they will write me at unless Bill or Pete can think of another way for me to post it. You can use it as a basis for any kind of experimental platform that you would like since it is very small and simple.

To put it right on frequency it would be best to use a frequency counter. This is due to the variations in BFO frequencies and the reference oscillator on the Raduino module. You could do it without  equipment by “change and re-try” but a counter (even a borrowed one) would be easier.

This calibration is the subject of a previous article.

de ND6T

Monday, February 20, 2017

VU2XE's CAD files for a BITX Box

Kiran VU2XE built a nice BITX box using newly developed CAD skills. 
He suggests that you could take these files to a local laser/CAD shop and have them reproduce  box in aluminum. 

Here (I hope!) are the CAD files.

Sunday, February 19, 2017

Calibrating the Raduino

Don's discussion of the calibration problem is especially relevant for the "channelized" 60 meter band in the USA -- he has been working on a Module modified for that band.   But I think accurate calibration is also important on 40 SSB.  The vast majority of phone QSOs on 40 take place on whole integer kHz frequencies:  7164 kHz, NOT 7164.3 kHz!  If you call CQ on a non-integer frequency, people get confused and irritated. Often they will tell you that your signal is "distorted."  I have the Si5351 on my DigiTia well calibrated (thanks to Tom AK2B) and I keep the increment at 1 kHz.  This keeps people happy.   Bill N2CQR
Raduino Calibration
by  Don Cantrell, ND6T

The BITX and Raduino combination can be used as shipped without calibration as long as you do not operate right at the boundaries of the assigned frequency allocations. The old saw “Good 'nuff fer gum'mit work” applies for most use. Certainly it is more accurate (and stable) than the stuff we old geezers used “back in the day”. But what if you want all of the accuracy that the equipment is capable of producing? That digital readout is tempting.

Ashar Farhan composed a brilliant solution in his Raduino sketch. He included a software calibration that uses a push button press to enable the operator to tune a known frequency signal “by ear” to establish an offset to be applied to the VFO frequency. This puts it very close, much better than the uncalibrated result, since it accounts for both  BFO and Si5351 reference variations. There remains a small intrinsic error (insignificant in a single band application) in that the correction is frequency proportional. If you were to use the scheme for wide frequency ranges it could be a small problem.

When I converted my new BITX40 to 60m I realized that my channels were several kilohertz off frequency. In this country these channels are only 2.8 Khz wide. I couldn't even hear them. I measured the Beat Frequency Oscillator frequency at C106, the blocking capacitor to the balanced modulator/product detector, and found that it was 11.999045 MHz. Other BITXs will be significantly different since the crystals are carefully selected to match those in the crystal IF filter.

I wrote this value into my operating program file as the “BFO frequency” but found that I was still not on frequency. The only other reason for this discrepancy, aside from a math error, would be the reference oscillator for the Si5351 Phase Locked Loop.

I wrote my 60m operating system from scratch. It is simple since I am a novice at such things. No internal calibration routine. Instead I rely upon the accuracy of the Raduino reference oscillator value.

To find what that is, without disturbing the oscillator loading with a probe, I wrote a short and simple sketch for the Raduino.


 Calibration program for Raduino

 Don Cantrell,ND6T  v 1.0           7 Feb 2017

 Compiles under etherkit Si5351 library v 2.0.1

 This source file is under General Public License version 3.

 Generates the reference clock frequency so that it can be

 measured and substituted as the corrected frequency of the

 particular oscillator.


#include <si5351.h>

Si5351 si5351;

void setup() {



    si5351.set_pll(SI5351_PLL_FIXED, SI5351_PLLA);

    si5351.output_enable(SI5351_CLK2, 1);

    si5351.set_freq(25e8 , SI5351_CLK2);


void loop() {


The crystal frequency is assumed to be exactly 25 MHz, at least that is what the Si5351 thinks. So we ask it to generate a 25 MHz signal and then read what it actually is. Simple.

If you are comfortable with re-loading the original Raduino sketch (or whatever sketch that you have been running) and are familiar with the different versions of the libraries and procedures that are needed to do this then, and only then, you are ready to continue. If you aren't comfortable with that then stop right here! You can easily “upgrade yourself out of service” as most of us have realized, much to our chagrin. If you find yourself in that corner then you may not have anyone to rescue you.

That said, the next step is to replace the operating system with the little calibration sketch and let it run for a few minutes so that the oscillator will stabilize (it can drift 10 Hz or so while warming up). There is no display routine for the Raduino so ignore the display (I told you this was a simple program). Measure the Raduino output frequency where it feeds the “DDS1”jack at pin number 1 there on the BITX board. Record your result from the frequency counter and use that value in the setup line for the Si5351 in place of the 25000000 value. Don't forget to add the “L” (for Long integer) if the old value had it.

In the original Raduino sketch v 1.0.1 this appears at line number 589.

“ si5351.init(SI5351_CRYSTAL_LOAD_8PF,25000000l); “

Reload your revised operating sketch and enjoy. It certainly solved my problem. My 60m BITX now seems to stay within 1 Hz, or so, of where it should be (after a few minutes “warm up”, of course).

Keep this value to use whenever you use this particular reference oscillator. Your frequencies will now be as accurate as your frequency counter and component drift will allow.

de ND6T

Saturday, February 18, 2017

Bitx40 Spectral Output

Wayne, NB6M has written in. he says :

I find that its second harmonic suppression doesn’t meet our current (since 2003) standard that all spurious output must be at least 43 dB below the level of the fundamental.  At the bottom of the band the second harmonic was only 38 dB below the fundamental, and the rig just barely met the standard at the high end of the band.

There is a very easy fix for this, simply parallel L7 with a 100 pF C0G (NP0) cap of suitable voltage rating.  After doing so in my unit, the worst case scenario, at the bottom of the band, had the second harmonic some 56 dB below the fundamental and it was now some 60 dB below the fundamental at the top of the band.

Thanks for the hack Wayne, we'll see how we can incorporate this in the next batch of the PCBs. In th meantime the amateurs in the USA can make this fix.

Another Approach to RF Gain Control (on ND6T's BITX60 Module)

Another Approach to RF Gain Control
by  Don Cantrell, ND6T

I am lazy. So on my first BITX I devised the simplest way that I could think of to install a usable RF gain control.  With so little effort, I was pleasantly surprised that it worked so well. Still, the range was not spectacular, just 17 dB. Could it be improved upon with more (please excuse the following four-letter word!) work?

Most of the best QRP designs have used a simple potentiometer at the receiver input. Simple is good, and the arrangement has proven itself to be both effective and cheap. The problem is that the only place available for it in the BITX design that is not shared with the transmit function is the connection between  pins number 14 on the two relays. Without any components in the path or any solder pads (other than the connections to the relays beneath the board) I had to do surgery on that beautiful board. Still, I had to try the idea.

Locating the conductor run on the top surface of the board, I used a hobby knife to carefully scrape the solder mask from about 5 mm of that trace  toward the front panel from R143. Not the trace that connects to R135 and R144, but the adjacent long one, the one that disappears beneath the relays. Be careful not to scrape too deeply, just to where the copper is clean and bright enough to accept solder. In the center of that cleared zone carefully remove a 1mm section of the trace, separating the path. Tin both sides of the cut and use an ohm meter to ensure a clean separation.

As homage to Ashhar Farhan's legacy of plug-in connections on the BITX40,  I removed a two pin portion of right-angle pin connector stock to fabricate a make-shift header. With small pliers I bent the short pins down to make contact with the foil when the plastic part of the connector was flush to the flat surface of the board. A drop of gel cyanoacrylate glue helped hold it in place. I soldered the previously formed short pins to the trace and was ready for the control installation.

Unlike my original RF control project that used just DC supply current, this design should use shielded cables to and from the control. I used RG174/u miniature coax, grounded at the low side lug of the 1K ohm potentiometer (both runs), the lug on the other side will go to the center conductor of the cable going toward K1 (that's toward R142). The other coax will connect to the wiper on the pot. The connector ends of those coax runs will solder to a two conductor female plug like the BITX uses for power, antenna, and speaker. That plug connects to your new header.

This arrangement gives nice, smooth control of 26 dB on my build. That is a lot of effort for just the additional 9 dB of control. However, now for the good news: You can still connect a switch between R15 and R16 (my previous RF gain control project) and use that as a 17 dB attenuator. That would give a 43 dB total attenuation!

If you choose to remove the control you can use a shorting plug on your new header to restore the radio to normal receive. If you want to install a proper Automatic Gain Control in the future, this jack might prove quite useful.

It could be worth all the trouble after all.
de ND6T

Don's BITX60

Friday, February 17, 2017

FREE "Roadkill" Mounting Hardware for Digital Displays

Cases and Display Mounting
by  Don Cantrell, ND6T

I was recently informed that there are still a few builders out there that are unaware of the bounty of cabinets, cases, and hardware available for next to nothing.

I am referring to data switch units, those gadgets used to share printers and serial port devices before the age of the Universal Serial Bus (USB) and “wireless”. Most of them used the old DB-9 and DB-25 connectors that are no longer in favor so they show up in thrift stores and swap meets for free or close to it. The cabinets are perfect for most projects, easy to open and very sturdy. The front panel usually has just one hole, for the switch, and is often the right size for a control.

The switches aren't something that you would normally use but the wire from them to the rear panel mounted jacks is frequently stranded, brightly color-coded, and a good length. A touch of the soldering iron releases them intact. A good source of small gauge hook-up wire.

The connectors on the rear panel are usually attached by 3/16  inch hex 4-40 threaded short stand-offs at each end. They were there to provide a means of holding the connecting plugs in place. So each jack has two of these stand-offs.

The mounting hardware supplied with the BITX40 board can be used to mount the Raduino module to the front panel but those 11mm stand-offs are a bit long and place the front of the display behind the panel. That's fine if you want to place a clear plastic cover over it but I prefer to extend the display through the panel as much as is allowed by the backlight and the connector clearance. About an 1/8” (3.5mm).

Those short 3/16 inch stand-offs from the data switchers work perfectly for the display mounting. No bezel needed. The real work is, as always, the cutting of the rectangular hole for the display. A few holes drilled, some nibbling with a hand nibbler, and a lot of work with a big flat file. The adage “measure twice and cut once” does not describe it. I spend most of the time measuring. However, it is time well spent. I can daydream and ponder while doing the largely brainless busy work.

So a couple of bucks toward a good cause will provide a dandy cabinet and parts. A little paint and elbow grease will put your project in a stylish enclosure and your Raduino display out front and proud. You've “up-cycled” and kept those parts out of the landfill, too!

de ND6T

Wednesday, February 15, 2017

Dimming the Raduino Display

Dimming the Raduino Display
by  Don Cantrell, ND6T

The Raduino display can be a bit too bright for some folks. The backlight only needs to be bright enough to make the display readable, not to where you can read a book by it. In the sunlight it can be easily read without the internal lighting so that energy is just being wasted.

I brought the brightness of my BITX  LCD down by replacing the 110 ohm ballast resistor for that LED  with a 470 ohm unit.

On the end of the display where you can see the LED connection you should find a chip labeled R7. It will be on the end next to the USB when the Arduino board is plugged in. That is the device in the photo labeled “111” (that means a “1” followed by a “1” with “1” zero following it: 110 ohms). Replace it with a higher value to dim the light.

Now I can photograph it and read the indications. I saved 18 milliamps of current drain, too.

de ND6T

Heat Shrink Tubing for a Fatter Pot Shaft (AF Gain Control)

Heat Shrink Tubing
by Don Cantrell, ND6T

I've read stories about people replacing the volume control/power switch that was supplied with the BITX40 simply because of the 4 mm shaft! There are  ways to adapt it to your favorite knobs.

I was in a rush to get my first BITX cased and operational so I filed that nylon shaft down to size for a knob that I had. It works, but took more effort than it should have.

A friend of mine is adept at salvaging parts from the recycling bin. I had never considered using toothpaste caps and container lids for knobs but his builds look much better than mine! Some of those lids are just perfect; fluted, colorful, and large. A bit of glue, maybe an old damaged knob for a base and you can have a one-of-a-kind creation.

My second BITX was simpler. I'm learning. I used a couple of layers of heat shrink tubing to build the shaft out to fit ¼ inch knobs.

Most of us use heatshrink tubing to insulate splices,  connector pins, and component leads in tight places. I like to layer progressive lengths and diameters to use as strain reliefs on antennas and microphone cables. It is flexible stuff, to a degree, and makes for good weather proof coverings for splices. However, it can be stiff enough to form a good hard surface to support a knob on a shaft.

Cut the shaft of the control to the desired length. The small 4 mm nylon can even be cut with flush-cut wire cutters. Then cut a length of heatshrink to match. Use the dense tubing material and the size to just slide over the shaft nice and snug. Gently heat the tubing so not to distort the nylon shaft. Let it cool before slipping on the next layer. When it cools you will note that the flat side of the shaft is still evident on the new, expanded, surface. Your ¼ inch knob now slips on with a good fit and the setscrew seats firmly and with no wiggle or wobble.

de ND6T

Tuesday, January 17, 2017

ND6T's Approach to Heat Sinking the IRF-510

Heat Sinking The PA To The Cabinet
by Don Cantrell, ND6T

I am a real advocate for keeping components cool. They last longer and are generally more stable. The heat sink provided with the BITX40 is adequate for lower power levels but bigger and better heat sinks provide for longer component lives and peace of mind. If you are going to install the transceiver in a metal box then you might consider using that enclosure as a heat sink. Whenever I have done this it has  always worked very well.

In the early stages of installation I positioned the board as close to the rear of the cabinet as was convenient but it still left about 1/4” (6mm) gap between the Power Amplifier transistor (Q15) and the back panel when the original heat sink was removed. I found a small block of aluminum that fit nicely. I drilled a hole toward one end to match the hole in the tab of that IRF510.

When the mounting spacers had been attached to the BITX board (so I could tell where the transistor was located in respect to the panel) I marked and drilled the transistor's attachment hole first. I then placed the mounting bolt through the transistor, spacer block, and rear panel to temporarily hold it before marking and drilling the four holes to the attached mounting spacers of the BITX.

When mounting stuff in cabinets a bit of extra time and planning goes a long way toward preventing mistakes (and forehead slaps with cries of “OY!”). Go slow and save big washers and hole plugs.

After the board is mounted use a shouldered nylon or ceramic washer to insulate the mounting bolt from the transistor tab which is connected to the collector. Don't let it short circuit to anything. A mica or silicone pad between the spacing block and the transistor completes the insulating job while still conducting heat. If you use a mica insulator be sure to use heat sink compound on both sides of the insulator. A silicone pad does not need it.

If you cannot find a suitable aluminum block then look for any heat conductive metal. Copper, brass, even steel. I haven't tried it but I would assume that a stack of coins (with any corrosion sanded off) would work. Considering the cost of new metal stock in the hardware stores, it might be cheaper to just use money!

My BITX output transistor now runs so cool that I cannot detect any temperature increase. Just DO NOT TOUCH THE TRANSISTOR WHILE TRANSMITTING! RF burns are worse than the damage you feel from picking up the wrong end of a soldering iron. Or so I'm told.

de ND6T

Monday, January 16, 2017

A Simple Voltage Monitor for the BITX40 (from ND6T)


BITX Voltage Monitor
by Don Cantrell, ND6T

With eight pins available for reading analog signals, the little Nano® has some enticing capabilities. More useful than an “S” meter reading, a measurement of the supply voltage is another display item that you can easily include. This will allow you to keep track of your battery status if you are portable. It could also be adapted to check on any other voltage (or voltages) that you might be concerned with. Like your PA supply if you are going to feed it with a separate higher voltage.

Looking at the wiring diagram of my VFO build you will notice a couple of resistors that are in series and placed between the +12 Volt input and ground. There is a 1.8 Kohm from ground and a 5.6 Kohm on the power feed side. Power rating is unimportant since they are going to dissipate only about 25 milliwatts. I chose these values so that it could measure up to 20 Volts without subjecting the Nano® to more than 5 volts. A safe Nano® is a happy Nano®.

Whenever measuring any voltage with the Nano®, endeavor to keep the maximum voltage to the analog pin below 5 Volts but compromise to retain the best dynamic range. The measurement involves converting the range into 1024 steps and then dividing the result by whatever number will display a reading that will be closest to what the measured voltage actually is. If you have too much range then those steps will be larger and more inaccurate.

Software implementation is wonderfully simple. Here's how to read the voltage on pin A2:

    // Read supply voltage 
    int supply = analogRead(A1);

The first line is just a comment, reminding you what the next code is about. This line is optional. The second line establishes a local variable (that I named “supply”) and specifies it to be an integer type, saving a little storage space and keeping handling simple. The “=” sign assigns the result of the “analogRead” function on port A1. If the voltage applied to that pin was 2.5 Volts, for instance, the “supply” variable is now worth 512. That's because 2.5 volts is half the maximum 5 volts and, therefore, half of the maximum 1024. Make sense?

To display that as a voltage on our LCD screen we would use this line:


Why divide the supply variable by 47.7? Because that would make it correspond to the voltage that we were feeding the resistive scaling divider with. Find it by experiment. In this case you would see 10.73 on the display. Apply 10.73 volts to the top of the divider and you will see 2.5 volts on the tap where you have pin A1 attached and you will read 10.73 (or so) on the display.

Easy. Two resistors, two lines of code. You can use similar schemes to monitor any voltage or current. Rectify the voltage from a winding on a toroid that you run the antenna lead through and read output power (antenna current). Set up a VSWR bridge. Ahhh! The possibilities!

de ND6T       

Sunday, January 15, 2017

A simple S Meter for the BITX 40 (from Don ND6T)

Adding an “S” meter function
 Don Cantrell, ND6T

Aside from dressing up the display (taking up room is more like it) there is little use for a signal strength indicator (“S” meter) unless it is accurate. Admittedly this implementation is not accurate, merely a nod to the notion. That said, it does work as well as a few “store-bought” rigs that I have used. Since it is simple and easy, I will pass it on for what it's worth.

Just three components, none critical. I ran a lead from the “hot” side of the volume control through a diode to an available analog port on the Nano® and paralleled a 0.1uF capacitor with a 4.7 Megohm resistor from that port to ground. The diode can be a silicon switching diode like a 1N914 or 1N4148 but a germanium like a 1N32 will work better for the lower strength levels.

The reason for the inaccuracy is primarily due to lack of the usual AGC circuitry. Normally one just measures the AGC voltage and scales it to suit. This mod is simply using the detected audio and then using a short-period peak hold circuit to feed the micro-controller.

The code is poorly done at this point but it met my low expectations. It currently resides in lines
90 through 94 in Version 1.0 of the BITX40_VFO.ino sketch. Please feel free to change it and pass it along.
de ND6T

Saturday, January 14, 2017

ND6T's Suppression of a Pesky 2nd Harmonic

 by Don Cantrell, ND6T

No matter how I tried, I could not drop the second harmonic to make 42 dBc. Best I could get was about 39. That's not far out of FCC regulations and is probably well within the suppression required in other parts of the world. Probably it is just my particular board, the minor variations of the capacitors and inductors combining to just miss the mark. I haven't heard of anyone else reporting it but I will pass this along just in case. If you are using an external amplifier then I am certain that the filtering on that  amplifier will solve it easily. You could also use an external low-pass filter.

I ran a simulation of the filter bandpass using the AADE software. The results are shown in Figure 1(Simulation) as the upper trace. You will notice that the suppression provided at 14.4 MHz (the second harmonic) is around 26 dB. The lower trace is around 50 dB at that frequency and is the result of paralleling L7 with a 100 pF capacitor. This extra capacitor acts like a trap for the second harmonic. I find myself installing this on most of the transmitters that I design, just to save an additional filter section.

I just soldered the 100 pF disk across that inductor on the underside of the board. I don't notice any difference in operation but I am now legal. Figures 2 (Before) and 3 (After) show the before and after measurements on my spectrum analyzer.

Has anyone else experienced this problem? If so, here is a cheap and easy fix.
de ND6T


Friday, January 13, 2017

Video of ND6T's Shuttle Tuner for BITX 40

Two days ago, Don ND6T sent us a very nice description of how he used a 10K pot to tune the AD9850 DDS in his BITX 40 (scroll down).   Don sent us a video showing how it works.  Very interesting.  Thanks Don! 

ND6T's RF Gain Control (with "Tombstoning" and Feng Shui)

RF Gain Control
by Don Cantrell, ND6T
I've grown used to RF gain controls, especially on my home brewed receivers. Often it is the only gain control and serves to adjust audio output as well. When I first put the BITX on the air I called a friend who lives within shouting distance (literally!) for a test. That's when I began to miss that RF gain control on the BITX.

During my experiments with the analog VFO I installed an additional pot on the front panel that provided a precision 1 kHz tuning. The new DDS system, of course, didn't need that knob and it became available for re-purposing. What better use than a RF gain control?

My plan was to insert a 10K ohm variable resistor (rheostat) in series with the power feed to Q1, the receive RF amplifier. Original design integrity is maintained by placing it between R15 and R16. This was accomplished by “tombstoning [1]” R15 (10 ohms) on the pad next to the supply via. I attached one lead from the control to the top of the (now upright) R15 and the other lead to the (now vacated) solder pad. Done! That's it!

I now have my beloved  RF control with about 17 dB of adjustment range, and the feng shui of the front panel is restored.

de ND6T

[1] “ Tombstoning” is the term for tipping up a surface mount component. It now stands like a headstone over the site of its original position. This is the normal method of opening a circuit during trouble shooting. Very handy and one of the many reasons that surface mount construction is so much easier to work on and modify. Thanks Farhan! The BITX40 is beautifully laid out large, logical, and well-spaced. A true joy to work with. Those relatively large components make it easy compared to through-hole.