Hello to the BITX20 community,
I'm a new happy BITX40 owner, still a SWL for now and soon to become a Ham. In the meantime, I'm fiddling with RX antennas, and now I'm trying to understand the inner workings of transceivers. I have to say, buying that BITX40 v3 and trying to overcome some of its limitations has been really instructive to me so far. I'm in the process of making some breakout boards based on the new ?BITX design (), to experiment further, try different crystal filters with relay switching for AM, SSB, CW, maybe try to use some INRAD 8-pole filters (9kHz IF or others) since sadly I don't yet have the equipment to match crystals and test DIY filters, so that will be for later.
Anyway, about my current BITX40, like a lot of you guys here, I'm trying to get the best out of it. So far I've 3D printed a faceplate, tuning knob and volume knob, I swapped the tuning pot for a 400 steps optical encoder, I'm still waiting for a couple OLED displays (one is plug and play, and the other one is 256x64 pixels). I added 4 buttons, two of which are in use for now for tuning step selection and LSB/USB switching. I added a red LED that lights up when transmitting. I also added a socket for an external mic and I use a TechnoFix electret replacement mic for Icom transceivers.
Today I spent some time disabling the BFO circuit (by removing R101 and C106) and wiring the CLK0 as the BFO using a short piece of coax from the Raduino board to the product detector transformer pins. It works a treat and with some code tinkering, I can now seemlessly switch between LSB and USB (still monoband on 40m though). I'm now using a high side VFO and I removed C91 and C92 which attenuates too much the higher VFO frequency.
I still have a few issues though:
1. First thing I noticed when listening with the low side VFO, I had quite a few noticeable birdies which was obviously to be expected. I got rid of almost all of them with the high side VFO, but the one near 7.1975MHz still remains and is quite strong. I'm not sure where this one comes from, I know there is nothing there on my SDR receiver (RSP2).
2. I know it's not the best idea to use one receive antenna for multiple receivers, I'm planning on building a simple splitter soon, but for now it works well enough for me that way. I noticed some carrier leakage on receive on my RSP2 which is S4 (my noise floor is S2 average). I tried on transmit (no worries, the SDR and BITX40 are *not* interconnected when transmitting), but I did not have any dummy load at hand today, so I just left the antenna disconnected. I had a lot of carrier leakage alongside the LSB signal around -80dBm on my SDR. What I did is, I removed the BAT54 matched pair, soldered a 10k multiturn pot, and tried a few combinaison of 1N4148 diodes I had at hand. Fortunately, I managed to find one good pair, which gave me -110dBm. It was still there, but a lot lower now. The problem is, now I had a lot of unwanted noise on receive and signals were now buried in the noise, I had to adjust the pot again to get a good reception but at the expense of more carrier leakage (-85dBm on transmit). I don't really understand what's going on here.
3. The AF amplifier, the LM386 is outputing a lot of white noise even with the trimpot disconnected, which means the noise comes from the amplifier itself and not the signal to be amplified. It's barely noticeable and well acceptable when listening on a small low quality speaker, but when I use my headphones or a higher quality speaker, I can hear higher pitch noise above the noise floor and the signal I'm listening to. I can still copy signals just fine, but it's really not pleasant when listening for more than short periods of time. I tried a few things already, including removing C113 to lower the gain, I couldn't get rid of that noise.
Do you guys have any ideas about these points?
My future plans about the BITX are to implement AGC, probably AF level AGC here (IF for the ?BITX if I can), and RF gain. I probably won't try to make my BITX40 multiband, I'll leave that to my other project which is the ?BITX. I'll go a bit further with that, and probably have someone machine an aluminum faceplate, backplate/heatsink, and panels. That will be for later though.
Thank you guys!
Etienne, from Reunion Island.
