An 80m transceiver using just 14 parts plus crystal and earpiece   The circuit uses a simple Colpitts VXO producing around 120mW when the key is pressed. On receive, audio is derived across the 15k emitter resistor (could be higher to reduce backwave more?) and this is amplified in a single stage of audio gain. Output is to a small high impedance crystal earpiece. Sensitivity has not been accurately measured and will depend on the earpiece itself, but I was able to hear the output of my RF generator on its lowest setting and with an additional 10dB pad in the coax lead. The offset between RX and TX is around 400Hz. This is a bit too small, so more can be obtained by shorting the trimmer capacitor in series with the crystal. C1 is a fixed capacitor chosen to give 400-800Hz between switch S1 open/closed. In my case 33p was best and gave around 600Hz offset. Update 3.2.10: I've changed the values in the collector circuit and this has reduced the AM breakthrough from medium wave a lot (still get some from 75m BC band). Also, power out is now 120mW at 12V (was 60mW) and the RX is a little more sensitive too. The receiver is picking up a lot of stations now on 3.560MHz (G, GW, LA, DL etc). Making R3 82 ohms increases power still further, but I may stick with 100 ohms finally.  On the left is a suggested PCB layout for this transceiver. It fits on a small 1 inch (25mm) square single sided PCB. Please note I have NOT built the transceiver on this board; this was an exercise to see how easily it would fit on a small board. With resistors "on end" rather than laying flat it could be made even smaller - probably 20 x 25mm. With SMT parts it would be possible to make this in 12 x 25mm - tiny! Recently I received this nice email from Mike Rainey, AA1TJ: Hi Roger, I posted a quick comment on you blog last evening on the topic of your XBM80-2 transceiver. Mine worked exactly as advertised! I couldn't improve upon your
choice of Colpitts feedback divider caps. I raised my receive emitter
resistor to 15k to reduce the backwave a bit more, but your choice of
10k is FB as well. Instead of a 10uH collector inductor I used an LC
tank (3.5-6uH slug-tuned inductor/390pF) with link coupling to a
bandpass filter (similar to the one I used in my Reggie). The
output power approached 100mW with a 12.6Vdc supply potential. I backed
the supply voltage down to 11Vdc, again, in order to hold the back-wave
down. The transmitter puts out a clean 80mW at this supply voltage,
with a backwave of 360uW. I didn't use an AF amplifier stage. Instead, I connected a
step-down AF transformer (in series with an electrolytic cap) across
the 15kOhm resistor in order to use my 600Ohm magnetic headset
directly. The MDS is nearly identical to my "gain-less" Reggie receiver. My third CQ brought a reply from K1PUG, down in Connecticut; a
distance of 158 miles. I gave him a 579 and he replied with a
549. Within minutes of signing I was engaged in a rag chew with W1GUE
(112miles). He replied to my call with 100w, but as soon as he heard
that I was QRPp he dropped down to 5w. He was 579 running QRO, and 549
QRP. He gave me a solid 589. It turned out to be a very pleasant,
half-hour chat on the topic of QRP. Of course, I asked him to please
take a peek at the XBM80-2 on your blog. I think it's interesting to compare your XBM80 with G3IEE's (circa-1954) one transistor transceiver design. The lineage is striking. Your XBM80-2 is a beauty, Roger! I think one would be very
hard-pressed to come up with a simpler transceiver design. Thank you
for a very enjoyable QRPp evening. Well done, OM! 73/72,Mike, AA1TJ Well it's good to hear that Mike has had success with the XBM-80-2, although in truth with his changes I think Mike could give it a new name, HI.
Alan Yates VK2ZAY has also been having a go at this circuit (see Alan's website) and has come up with some changes which make the transceiver more useable but at the expense of added complexity. 30M CW TRANSCEIVER FROM GJ7RWT   |