20/80m CW QRP Transceiver
Oktober 2019, DL6ZB
This a 20/80m imaging CW QRP transceiver.
Imaging is a transceiver technique which was widely used by homebrewers
during the 60's and 70's. Usually a 9 MHz IF was used for imaging. A
5.0 to 5.5 MHz VFO results in 3.5/14 MHz input/output at the same time.
The desired band has to be band filtered.
The output power of the described transceiver is about 1.5 watts into a 50 ohm load.
Design goal was to make it simple and unexpensive with most parts from
the junk box and with as many BJTs and simple OPAs as possible.
Most of todays homemade projects uses micros and more or less
sophisticated frequency engines. No doubt, these projects are of great
value when it comes to learn more about modern RF technologies.
However, there still are a lot of homebrewers searching for more
traditional projects. If that's what you're looking for the you're all
right here with that project.
Here's the final design. The receiver is a single conversion with an IF
of 9 MHz. A junk box 2,7 kHz KVG XF9B SSB crystal filter was used here.
Any cheap CB radio SSB crystal filter will do the job.
That receiver has a 2,4 kHz bandwith so it isn't very selective for CW
use. Therefore I added a simple but suprisingly effective 300 Hz
bandwidth OpAmp AF filter. Since the IF amplification is not too high,
this filter works most of the time. A better solution would be a CW
crystal IF filter. However, I hadn't one in my junk box. The OP-AMP
must do the job.
A small speaker or a high impedance headphone may be used. A low
impdance headphone would require either an additional AF stage or a
plug for connecting it to the loudspeaker output.
The pictures below are from a prilaminary stage. I designed the
transceiver "manhatten style" or "dead bug style". This allows
modifications at any time.
Click on schematic for a high resolution PDF version ( version October 2019).
- VFO with a 1:36 planetary gear
- IF and AGC amplifier (open case)
- Crystal Filter
- AF amplifier
- Front Plate
- RF TX pre and power amplifiers
- AF pre-amplifier and AF filter
- 9 MHz RX&TX crystal oscillators
- Bandpass and TX low pass filters
- T/R switching