Simple QRP transceiver for 80m SSB

I have many rigs for QRO and QRP, old and new. But still I wanted to build myself a simple SSB rig and

feel the homebrewers satisfaction, when on the air with such a rig.

As lower frequencies are easier to work with, I decided to make it for 80m SSB. Later other bands perhaps will be added.

I like to work with CMOS as they are cheap, reliable and provide simplicity for home brew designs.

In this construction CMOS gates are used as carrier oscillator and VFO buffer. Analog switches are used in balanced modulator,

receiver demodulator, transmit-receive-switching and mixer. VFO signal is CMOS level clock.

Receiver has no RF amplifier prior to mixer. First IF amplifier is between mixer and xtal-filter.

After the xtal-filter there is only one IF amplifier stage. Receiver AGC works only on audio level.

Audio PA is the standard LM386. Audio output level is controlled with resistor in series with the speaker.

In transmitter the mic amplifier is LM386. There is no IF amplifier before xtal filter. The only IF amplification during transmit

is after xtal filter and before mixer. The amplifier in mixer module is bi-directional, contolled with a relay.

VFO is Clapp in common emitter circuit. Some people would call it Vackar. In fact they are the same.

Clapp is common collector circuit and Vackar common emitter. VFO tuning is with permeability. A 6mm brass screw

goes inside the coil. This is very practical in HB structures. Silver mica and NP0 capacitors are a must as well as

big coil. See details in VFO section.

Transmitter PA is with bipolar transistors and using push-pull principle. The output power is 5W PEP.

A suitable low pass filter in needed when using the broadband amplifier as harmonics are only 35 dB down.

In the whole rig there are only two knobs: VFO and audio level.

Sensitivity is good enough when using a half wave dipole. Band noise dominates always.

As there is no ALC in this transmitter, one must be careful with modulation. I normally monitor output with oscilloscope

in order to avoid PA clipping..

OH1TV

31.7.2007

updated

13.3.2008

AGC improved

17.8.2007

17.8.2007

24.6.2008

This PA is an alternative to the 5W amplifier.