SECOND HARMONIC FIX
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 |
| Before |
 |
| After |
Greetings! Would the 100pF cap work on the BITX40 model.
ReplyDeleteI just wired up the kit I received two weeks ago and there was an extra smt cap in the bag of parts. I have not yet found how that was to be used. 72, Jeff K9JP
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