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Hi all,

I am building the Bitx40 dead bug style. Basically changing the Bitx20 and using a different BPF, VFO frequency and low pass filter. For the BPF I used the design shown on the hfsigs website. Instead of T30-6 I used three T37-6 I had lying around, added 5 extra turns (45 in total) on each toroid which came to 6 uH as well. It turns out the signal peaks at around 7.550 MHz which is too high in frequency.?

What would be the best way to lower the center frequency of this filter? I have the toroids covered in glue so if it is possible by addjusting the capacitor values only. If the filter becomes wider that would not be a problem at all. I use an IF of 10 MHz and a VFO of 17 so there will not be many harmonics in the 40 meter band.

I would appreciate it if someone can point me in the right direction.

Thanks, PH0KKO

Thanks Arv. I only have an old oscilloscope and I use an Arduino/AD9850 combination (which will become my VFO later) as a signal generator. That' s how I tried to determine how the BPF was doing. I guess you are right, adding one or two turns might be best. But I did not take input or output impedance in accounts so that could affect the properties of the filter as well.

I kept my BFO at 3 kHz *below* the IF frequency but by using a VFO of 17 MHz the mixer should convert the LSB signals into USB and vise versa on TX. So my main concern is the CB 27 MHz band.

If you know of a simpler (wider) BPF for 40 meters, please let me know. I will experiment with LTspice, have not used it before. Appreciate your response...

Fokko PH0KKO

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

Not sure what you mean by "the signal is amplified in the ring mixer"
Diode ring mixers are lossy.

If the ring mixer is just diodes and? trifilar toroids for the transformers as on the uBitx.
it's going to take a lot more than 200mV pk-pk from the local oscillator to work efficiently.
If you are using schottky diodes, something capable of enough power to deliver
a 1 V pk-pk square wave into 50 ohms.? If using 1n4148 diodes, perhaps double that.

Injection of 200 mV pk-pk would fully explain a very weak receiver.

Jerry, KE7ER

Thank for your input Jerry. I may not have explained it very well. Yes, the ring mixer is lossy and the 200 mV sine coming from the AD9850 is pretty low at those higher frequencies. That is why, in the design I use, the ring mixer module has an extra 2N3904 buffer/amp stage which is not in Farhan's original design. Take a look at this schematic if you will. The 2N3904 seems to be used as a buffer stage before it enters the ring mixer. Maybe the signal is not amplified at all, only buffered. My knowledge is too limited. If so: it there an easy way to modify the circuit so it amplifies it a bit?

Change the amplifier to a Class A amp.

Do this by reducing R29 until you measure ~7 dBm
out of the amplifier. Use an RF probe to make the
measurement. As Satish says, that should be about
0.6v RF.

Probably R29 will end up somewhere between 10 ohms and 50 ohms.

john
AD5YE

Thanks Satish, John and Arv.

I did not have the BPF connected at the time of experimenting. But it would probably lower the signal even more. So it's best to get a strong signal before I attempt modifying the BPF. Satish makes a very good point, there is a 0.2 V loss in the 1N4841 diodes of course. Why did I not think of that myself? I did pass the exam but it's been a while. ;)

I will try John's suggestion and crank up the signal a bit to around 0.6Vp-p. Will post here how it turned out.

Arv, I use small pieces of (leftover) copper clad board, about 2.5 x 4 cm in size, around 1 by 2 inch per module and simply solder the components on dead bug style. I cut some small squares and glue them on as solder-islands where needed. The modular approach is great because you can test them individually. When I get to the final I will need a slightly bigger board and when everything is finished I will put all small prints on a larger board. Maybe put some 12V, TX and RX rails on it and I will call it the motherboard as well.

Quick update: I tried John's suggestion and changed R29 in 47 ohms. There were no voice stations but the CW stations became much more audible so it was a clear improvement. Still the audio level was very low. It seemed the 3rd amp behind the mixer was not amplifying enough. According to my cheap Chinese component tester C29 (coupling to the BPF) was unknown or defective. So I replace it with a new capacitor and presto: huge audio signal !!! It is past midnight over here so the band has gone quiet, so I will have to do some more fine tuning tomorrow. I intend to replace R29 with a 50 ohm trimpot so I can adjust the signal at the exact right level.

Thank you all! Now I can focus on the BPF.?

Final update: I restored the resistor values to their original sate and instead added an amp stage (half of Bitx bidirectional amp) and that gave me a much stronger (+15 dB) and more stable DDS/VFO signal. Instead of adding windings to the toroids in the BPF I increased the the capacitance of the 3 tuned circuits in the BPF to 125 pF (instead of the 100 pF in the original design) and that placed the 7.000 to 7.200 exactly in the peak of the filter. It was a matter of trial and error but got it working.

BPF is perfect now. Thanks all!