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 ND6T@ARRL.net 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

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. 

http://soldersmoke.com/GP_Metalbox_top.dxf

http://soldersmoke.com/lcd_Metalbox_bottom.dxf

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.init(SI5351_CRYSTAL_LOAD_8PF,25e6L,0);

    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

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

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

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