Hacking the Hackable BITX 40 Module: VFO is the Way to Go!

I am having a lot of fun with Farhan's new BITX 40 Module.  I think I'm doing exactly what Farhan intended people to do with this rig:  work on it, modify it, improve it. 

I've been working on frequency stability.  I was, I admit, skeptical from the start about the stability of a  thumb-sized,  SMD, varactor-tuned VFO with a ferrite or iron powder toroidal coil.  Don't get me wrong -- it worked.  But it drifted. It seems to me that it would be asking too much to expect a VFO like this to be drift-free. (But I may be wrong -- are there any SMD, varactor-tuned VFOs out there that DON'T drift?)

First I thought it might be the 9 uH metallic core toroid.  So I replaced that with a 10uH choke -- no ferrite or iron powder in there.  That seemed to help a bit, but SSB QSOs would still quickly drift into Donald Duck chatter.  Then I thought it might be the varactor diode.  I let it warm up.  A lot.  Still, it drifted.  Then I thought it might be the trimmer cap, so I took it off the board.  No change.  During this process I noticed that even slight pressure on the board caused the rig to shift frequency.  I began to suspect that the drift was just structural -- a consequence of the physical characteristics of the SMD parts and the board.  To get VFOs stable I've had to build them big:  10 X 10pf  NP0 caps to make one 100 pf cap, large air-core coils, and big sturdy variable caps.  I'd isolate the frequency determining elements in a box separate from the powered components. This little VFO just looked too small to be stable.   

So faced with drift, at first I asked myself,  "What would Pete do?" I took an AD9850/Arduino combination off the shelf and plugged the output into the "DDS" jack Farhan had placed on the board.  I removed the 10uH choke.  Viola! With the DDS tuned to 4.7 - 5 MHz, the receiver worked great.  I briefly tried to updated the Arduino code to take into account the 12 MHz IF (so I could get an accurate frequency readout), but ran into the old painful Arduino IDE problems:  Now it is claiming there are library problems.  Not wanting to suffer through another round of digi-agony, I left well-enough alone.   I used the DDS with the old code for one day. 

But of course, I was not satisfied.  Attaching a DDS or PLL synthesizer to the BITX 40 Module just didn't seem right. Heck, it was kind of like just hooking up my FeelTech Chinese sig gen to the DDS jack.  Farhan's rig is simple, beautiful and ANALOG.  The parts are small, but you can see them.  You can put your scope probe on the collector of Q7 and see what is going on.  DDS or PLL.  It is a REAL HARDWARE-DEFINED RIG.  So I decided to build a VFO.   Pete calls VFO's "grief machines"  but for me, the grief machines are those little Arduino beasts.  To each his own.

When I build a VFO, I start with the variable capacitor and the reduction drive.  I found a nice one (with reduction drive) in my junk box. I tunes from 40 pf to 56 pf.   I decided to use the super-simple Hartley circuit presented by Wes Hayward W7ZOI in SSDRA (page 34, fig 7). 

I went with a 4.4 uH air core coil (wound on a cardboard tube from a coat hanger).  Consultation with on-line resonant frequency calculators showed that I'd need to put about 180 pf in parallel with the variable cap.  For this, I used a bunch (maybe 10?) of small value  NP0 caps in parallel.  This really helps keep the VFO stable.

As I did with my HROish receiver, I put the coil and the caps in one box, with the MPF-102 and associated parts in an  attached Altoids tin.  Everything was glued and bolted down very solidly.

I only built the actual oscillator stage -- I decided to use the buffer amps on Farhan's board.

The oscillator started right up.  I had to add and then take away some turns on the coil to get it to run in the desired range.  Then I plugged it into the DDS jack -- the receiver was working immediately.

I noticed, however, that it seemed a bit less sensitive than it had been with the AD9850 DDS. And when I grabbed the wire going into the DDS connector, audio output jumped dramatically.  It took me a few minutes to figure that out:  I think the output from my VFO was not adequately turning on the diodes in the diode ring.  When I grabbed the wire, I was putting a lot of noise into the mixer port, probably turning the diodes more fully on (but also letting a lot of noise through).

Fixing this problem part was fun:   Looking at the BITX 40 schematic, I saw that the two 1000pf feedback caps in the original oscillator were still in the circuit.  I figured those caps would be sending a lot of my VFO energy to ground.  So I fired up my hot air rework station and deftly removed C91, the 1000 pf cap that is connected to the base of  Q9.   Instantly the receiver started inhaling as it had with the DDS VFO.  That was a very satisfying fix.

This whole VFO project was very satisfying.  It was all done in one day, and all the parts came out of my junk box. I think I ended up with an LO frequency source that matches up in a pleasing way with the analog circuitry in Farhan's rig.  And here is bonus that I think is just what Farhan had in mind:  this kind of circuit adds a definite homebrew element to the module rig.   

I found that this external VFO improved stability significantly.  I don't know if it is as stable as the DDS, but with the external VFO the receiver no longer drifts away as I listen to SSB signals.