2001-11-18
Small 80m direct conversion RX
This project was inspired by my earlier mucking around with the WA6OTP PTO VFO.
In fact, I used the same unit that I built as a prototype for the LO in this project. I've added a buffer, mixer, an AF and RF amp of my own design, to make a very usable and quite compact 80m receiver. Below is a picture of the finished unit, and the block diagram of the receiver major stages. The amplifiers each have filtering not show in the block diagram.
Now let's take a peek inside:
It is a pretty cosy fit. The PTO VFO is in the middle and upper middle of the main board. The buffer the upper right corner. The mixer is quite conspicuous in the lower middle of the main board. The RF front end amp is the small board off by itself near the volume control pot, the audio amp is the long strip board at the bottom with all the large orange electros on it. The huge 470uF cap is there just to ensure the power supply is nice and clean, it can run the receiver for a good 2 seconds when the battery is disconnected.
The VFO circuit is buffered by a single transistor, a garden variety BC547, in fact all the BJT in the circuit are BC547s, the JFET is a MPF102:
Most homebrew stuff you see on the Internet uses the NE602. A great chip, but recently out of production and about as rare as hen's teeth here in Australia. Of the few places that do still stock them, most want about $AU 20.00 each for them. I decided to go with a much cheaper solution, a single balanced diode mixer. The ferrite core is one from the junk box, as are the 1N4148 diodes, the wire and the 200 ohm trimmer pot. There is no diplexers or anything fancy just a 100 ohm resistor that should be pretty pure resistance at all frequencies that are likely to be flowing into it.
To feed the RF port I cooked up a simple broadband RF amp. It is not a very high gain, its main purpose is to buffer the front end tuned circuit which helps reject strong out of band rubbish, although it was found during construction that the diode mixer (product detector?) had no problems with strong signals even fairly close ones. The RF amp module was added last as a "couldn't hurt, may even help" feature. The 820p/2u2 tuned circuit and the first 47p coupling cap is directly soldered to the RCA jack that makes up the antenna socket.
The AF strip is a fairly simple single transistor amplifier and low pass filter, followed by a LM386 power amplifier IC. I like to build without using IC where possible, and there was enough room for a 2 stage discrete power amp, but the laziness factor of the LM386 sucked me in. The device is a little noisy though, adding a 10 ohm + 100n filter network to the output should cure that.
On air the unit is very stable. Considering no real attempt was made to stabilize the VFO supply or the brass core of the PTO coil this is quite impressive. Although the receiver may appear simple, don't be fooled, it is very sensitive, even a few metres of wire will pull in the local net very well. The first night I was using the unit I listened to the 3.600 MHz net hearing many DX stations with a very poor antenna. The PTO tunes 3.4 MHz to 4.0 MHz, so it covers the complete 80m band with lots of room to spare.
Have a go at building your own. It is a tiny little receiver, and is just right for carrying around while out of town. Those of you licensed to TX on HF may like to build a CW or DSB TX from the basic skeleton presented here. It is very easy to scale the unit to your band of choice, 40m and 20m are quite easy to get going. 40 metres was achieved reasonably well just by changing the PTO coil turns, tuning up the front end, or better, offering band switched 40/80 LO and front end turned circuits would be very neat. It would make an excellent novice project as well.
10 comments.
Attachments
| title | type | size |
|---|---|---|
| PTO circuit postscript source | application/postscript | 11.065 kbytes |
| block diagram postscript source | application/postscript | 8.630 kbytes |
| VFO buffer circuit postscript source | application/postscript | 9.808 kbytes |
| mixer circuit postscript source | application/postscript | 11.170 kbytes |
| RF pre-amp circuit postscript source | application/postscript | 11.531 kbytes |
| AF amp circuit postscript source | application/postscript | 11.556 kbytes |