3 Feb 2010

Improvements to the XBM80-2 80m QRP transceiver

A few small value optimisations have improved the XBM80-2 transceiver. I've optimised the collector circuit values and this has resulted in 120mW output (was 60mW) from a 12V supply. The sensitivity is a little higher, the backwave level is lower, the amount of AM breakthrough from medium wave broadcasters is lower. At 4pm I was hearing DR2010O in the Ruhr area of Germany working all over Europe and could copy almost all the stations that he was working but I failed to break the pileup with 120mW. I'm beginning to think this little transceiver could be really useful. There is no reason why it should not work well on any band up to 28MHz (with less output) with a fundamental crystal. I'm wondering how it will perform on 50MHz with the collector tuned to x2 crystal?

5 comments:

Anonymous said...

Roger

I am thinking of building a Topbsnd version with
a two transistor modulattor,and and switching
the crystal earphone over to the input of the modulator
to act as a crytsl microphone.
I should be able to pull a 2 Mhz cermic resonator
down into the phone section of the band.
This would make an Topband Local natterbox.
Placing a carbon microphone in the emitter instead of the two transistor stage you may have the makings of a utra simple phone transciever.
When I get back in the shack I will give that idea a try.

73 Tony G4LLW

g4ilo said...

Hi Roger. I breadboarded the TX stage yesterday. Unfortunately I don't have an 80m crystal. I tried a 14.060 and got about 50mW out but very bad chirp. I then tried 10.106 and again got 50mW out and better stability but the actual frequency is more like 10.101 and I don't know what chance such a low power signal would have of being heard down there. There is quite a change in freq between key down and key up - you wouldn't need the offset capacitor!

I then tried 7.040 and got a very stable signal and about 100mW out. The change between key up and key down was quite low with this crystal, perhaps only about 200Hz.

When I next get time, I will add the audio stage and connect an antenna and see if I can hear anything.

g4ilo said...

Hi again. I seem to be getting quite a lot of harmonic energy. I don't have a spectrum analyzer so I don't know how much power is being radiated but I can hear the second harmonic of the 40m signal pretty loud. I'm wondering if a tuned circuit in place of the 3.3uH inductor might be better, and perhaps an LPF might be a good idea?

Roger G3XBM said...

Thanks for the feedback Julian. Yes, for serious use a low pass filter should be added unless the ATU itself has a lot of selectivity already. The oscillator will chirp if too much power is being drawn. Try increasing the 100 ohm resistor, but this will be at the expense of power out. The TX/RX offset depends on the difference between zero capacitance and the capacitance in series with the crystal. You may want to experiment with a small inductor in series instead.

MarkD said...

Roger

I was recovering from a minor operation so I decided to have a go at your 80m QRP transceiver.

I used a 2SC2001 for the oscillator/mixer/PA and a 2SC945 for the AF amp as I had some on hand that I salvaged from an old stereo system recently.

I used an NTSC colourburst crystal and have about 300Hz shift from Rx to Tx. I got about 80mW output and thought that I had better add a Pi filter after I saw the output on a CRO. The Rx seemed a bit deaf maybe your crystal earpiece is better than mine.

I added a second AF amp (another 2SC945) with a 1000/8 transformer so I could use walkman style earphones. That brought the AF gain up but I still think I would struggle to have anything other than a local contact here with our low density of 80m operations.

Enjoy your blog especially the home brew idea's.

73's Mark VK6WV