A couple of things came up and I was delayed in building Farhan's SMD BITX 40. With the availability and price point on the board, I decided I will stop work on the version I was designing and building. I will instead spend my time on trying some modifications to enhance the board from Farhan.
For those who are building with this board, there is a website devoted to different mods for it.
Since I haven't gotten around to build a si5351 VFO yet, I installed a 10 turn tuning pot. This made it much easier to tune, but I found there was quite a bit of drift during warm up. Looking over the bitxhacks blog, one of the first mods I made was to change some of the resistor values in the bidirectional amplifiers. Changing them from 100 to 220 ohms reduces current in those stages, should reduce heating of that part of the circuit board. I also coated the VFO coil with several coats of clear nail polish to prevent coil winding movement.
Another change I made was to cut the short trace going from switched 12 volt to the U2 (VFO 9 volt regulator) input pin. Then I ran a wire from the input pin of U2 to un-switched 12 volts. I also added a .1 uF capacitor from the regulator input pin to ground, and a 47uF and .1uF from the output to ground. This keeps the VFO circuit on at all times, and hopefully further reduces warm up drift.
After all of this it is much better, but I still have some drift, up and down in frequency even after warm up. Ordering some NPO capacitors to use in the VFO and will see if this helps.
With much of the drift problem solved, I next looked at the audio. Since the BITX does not have AGC, having to adjust audio level when going from station to station can be aggravating. Farhan posted a simple AGC circuit on the bitxhacks blog, and I decided to add it to mine.
to Eagle and laid out a simple board that can be built as mostly SMD or Muppet style.
After etching a couple boards, I built up one of each to give a try. For the SMD version I also added a solder mask to make assembly easier. Adding the AGC board to the BITX took care of leveling the audio, but the added circuitry reduced the audio level. Replacing the 1 uF. capacitor from pins 1 - 8 of the LM386 with a 10uF. brought the gain back up to where it was.
Link to Eagle files and a .pdf with mirrored top layer image for making toner transfer boards.
Next to add the frequency counter. With a little change to the gain on the counter pre-amp I had it reading correctly. Using the setup function on the counter, I measured the BFO frequency, and set the counter mode to subtract the VFO for proper frequency display. There was some noise introduced on the audio output. A 220uF capacitor across the counter power supply leads, and a 10mH. choke in series with the positive supply lead took care of that.
Except for a small amount of frequency drift the receiver portion of the BITX is working. Now to wire up the microphone and test the transmitter. It is starting to look like a transceiver.