Re: Modifying Bitx40 BPF band pass filter
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Fokko
If you suspect the BPF, try disconnecting it and bypassing it with a 100 pf
capacitor. This will get you lots of unwanted signals as well as the desirable
ones but it will tell you if the problem is really with the BPF or in some other
area.
You are not alone in building the BITX in a modular fashion. Hans G0UPL
built one of the earliest BITX20's using small PCB modules connected via
a motherboard arrangement.
Arv
_._
On Tue, Mar 6, 2018 at 5:52 PM, <kaapstad1@...> wrote: Arv,
Thank you very much for responding is such great detail to my question. I am not an experienced home brewer but like experimenting a lot and I will definately have a go at LTspice. The scope I use is an old Hameg and goes up to 30 MHz, the left side of the screen does not work and there is probably something wrong with the deflection yoke but I never had the time to take a look at it. I got it for free, it (kinda) works and I can see sine waves, great.
I use a modular build of the Bitx20, slightly different from the original but not much. It is described on a Dutch forum and the info is over a decade old with some images missing but the description is pretty good. I built each section on a small print starting at the AF section and working my way back towards the antenna. Luckily there is some time signal around 10 MHz over here so I could hear the reception improving and getting louder with each module I built, it is even audible through the crystal filter. I used an LM386-4 so plenty of audio power.
But after building the mixer module the audio signal in rx mode is as good as gone and the speaker is silent. I assumed there must be a great signal loss in the mixer and after building the third bidirectional amp, located between the BPF and mixer, and attaching the Arduino/AD9850 as VFO I can hear some very faint CW signals around 7.050 MHz on a piece of wire. The AD9850 has an output of only 200 mV peak-peak around 17 MHz but the signal is amplified in the ring mixer. I checked, double and triple checked but I think I wired everything correctly in the mixer so maybe this great amount of loss could be normal. But I am surprised the 3rd bi-amp does not compensate for that.
On the positive side: I can receive some signals albeit very faint. :-) If I attach a piece of wire at the base of Q5 (see schematic) the noise level goes up considerably. In the 3rd bi-amp I used the standard prefab 1.2 uH coil in combination with 200 pF instead of the original 100 pF (C25) so all frequencies around 7 MHz should be amplified. But this should only work in TX and not influence RX should it?
Going a little off topic here but you seem to have a lot of experience with the Bitx principle, maybe you have an idea of what I am doing wrong.
Mixer:
Third bi-amp:
Meanwhile I will dig through all the info in the links you provided. Maybe only two LC circuits would work best for the BPF. Or modify the original filter for 40 meters as described here:
Thanks! Fokko
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