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Thursday, March 11, 2021

Hodgepodge: Moving the Carrier Oscillator Frequency (and a Flashback to 2002) (Video)

As explained in the video, in the course of using my RTL-SDR dongle I noticed that the signal being put out by my Hodgepodge rig had some problems.  There was poor opposite sideband rejection, and in terms of audio quality I has putting out too many lows and too few highs.  I figured the problem was the result of the carrier oscillator frequency being a bit too low, a bit too close to the flat portion of the crystal filter passband.   I needed to move that carrier oscillator frequency up a bit. 

BITX40 Module BFO 

In the actual BITX40 Modules, L5 was replaced by just a jumper wire, and the C103 trimmer was not on the board.  Farhan and his team instead selected X5 crystals to match the passband of the 12 MHz crystal filter. Mine was originally at 11.998653 MHz.  But I wanted to tweak mine a bit -- I wanted to move it up about 500 Hz.  Reducing the capacitance would move the frequency up. Putting capacitance in series with C102 would have the effect of reducing the capacitance in the circuit.  I just removed the jumper wire and used the holes for L5.  First I put in a single 30pf capacitor.  This dropped the capacitance between X5 and ground to 18 pf.  That resulted in too large a shift.  So I added another 30 pf cap in parallel with the first one.  This resulted in a total capacitance from X5 to ground of 26 pf.  This was about right -- the carrier oscillator/BFO frequency was now 11.9991 Mhz.  I had moved the carrier oscillator frequency up by 447 Hz -- just about what I was hoping for.  

This was a very satisfying fix.  it was a chance to put to use experience with other SSB rigs, to make use of the RTL-SDR dongle as a diagnostic tool, and to tinker with the BITX40 Module in the way that Farhan had intended for it to be tinkered with.  

I'd done this kind of adjustment before, but without the benefit of an SDR display.  Below is the story of one such adjustment. 


A Flashback to 2001-2002
(From my book "SolderSmoke -- Global Adventures in Wireless Electronics")

Now it was time for some debugging and fine tuning.  I needed to make sure that the frequency of the carrier oscillator was in the right spot relative to the passband of the crystal filter.  If it was set too high, the filter would be chopping off high notes in my voice that were needed for communications clarity, and it would allow too much of what remained of the carrier (residuals from the balance modulator) through. If it was set too low, the voice signal transmitted would be lacking needed base notes.  I didn’t have the test gear needed to perform this adjustment properly, but my friend Rolf, SM4FQW, up in Sweden came to my aid.

One night, during a conversation with Rolf, I explained my problem and he offered to help me make the adjustments… by ear.  Performing an electronic version of open-heart surgery, with power on and Rolf on frequency, I opened the case of the new transmitter.  The carrier oscillator has a small capacitor that allows the frequency of the crystal to be moved slightly.  With Rolf listening carefully, I would take my screwdriver and give that little capacitor a quarter turn to the right.  “Better or worse?” I would ask. 

I think this little adjustment session captures much of the allure of ham radio.  There I was, out in the North Atlantic, late at night hunched over a transmitter that had been forged from old Swans and Heathkits, from cell phone chips, and from bits of design from distant members of the fraternity of solder smoke.  Pericles, the source of many of the key parts, was gone.  So was Frank Lee, the amateur whose SPRAT article had inspired the project.  But Rolf and I carried on with the core tradition of the radio fraternity: hams help their fellow hams overcome technical difficulties.  


  1. Very cool test Bill. Great use of RTL SDR dongle to let you see what is going on. I also love your simple to understand diagram and explanation. Question - to avoid overloading the SDR are you transmitting into a dummy load? Or better yet can you describe the test setup in a little more detail?


  2. Dean: Thanks. The test setup was very rudimentary: I have an old RTL-SDR dongle that I modified for HF direct sampling. I just ran a piece of wire about 18 inches long into the antenna connector. The rig was connected to the antenna -- that is why my transmissions were so short. I think this resulted in an accurate look at the output spectrum. Another option would have been to use a nearby WEBSDR or KiwiSDR receiver such as NA5B's, but the Hodgepodge is a bit weak to reach his receiver 9 miles away. 73 Bill


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