Guys,
i was travelling the whole of two weeks. living with a thinkpad and two changes of cloths give u a lot of perspective on homebrewing.
there are a number of very interesting issues that have been raised and settled on the list.
i have some information on these.
1. T/R pop
The T/R pop was happening because the audio amplifier does not immediately disconnect. this is
because the 50 uF bypass on the LM386. there is a diode in series with the 50uf to prevent it from keeping the rest of the rx circuit powered up. check if you put that diode in. i
was troubled by this for a long time.
2. CW operation
My CW operation is a quite quirky. I generate morse code as audio on my PC and inject it into the mic input through a 20 db attenuator. (a 10K and 100 ohms in series).
There is a simpler option: An RC phase shift network coupled between the input and output of the mic amplifier will make it oscillate and CW might require just adding four or five
resistors and capacitors.
While minimalism can be a religion, i would think that semi break-in and RIT is a minimum requirement for respectable CW operation.
2.1 RIT: RIT will need a varactor. the voltages across the VFO are high, that would mean two varactors in series, facing each other. W7EL (Lewallen) had a simple ingenius RIT solution. It is found on his famous 'Optimised Transceiver' article.
2.2 Break-In: Full break-in will break down the simplicity of the design. A semi-break can still use the same relay. It will require a timing system. A 555 or even a single transistor can provide this.
3. Design revision
i think the time has come to revise the BITX20. the BITX20 is no longer 'my' design. there are people on this list who are far more informed about BITX20 then i am. the BITX20 is turning out to be a base design that can be easily adapted your personal choices.
what are the 'must have' changes that we need to make to BITX20? here are my few suggestions. please add yours.
1. change the RF mixer to doubly balanced. it takes just one more transformer. i hope it will improve the signal handling capacity.
2. replace the LM386 with a discrete stage, lower distortion amplifier. the audio amplifier it quite bad. when i swap it with my cambridge soundworks, the result is amazing.
3. the mic amplifier has open gain. if we insert a low emitter degeneration expending one resistor, it will stabilize and fix the voltage gain.
4. the audio pre-amp on the receive should be changed to the classic Lewallen design of a common base amplifier biased for 0.5ma current coupled to an emitter degenerated common emitter amplifier. this will ensure a proper match with the demodulator, better signal handling and lower noise figure.
- farhan
