This is my new qrp transceiver design. It uses the CS2000 clock generator circuit I use for a number of my receivers and a 74AC240 buffer as a push pull 1.5 watt final amplifier. The final unit uses my low power/low noise LCD display and is set to tune from 7.000 to 7.300 MHz.
If one changes the the output networks (L matching and half wave low pass) and simply adjusts the software for tuning range, one can use any band desired. The inductors are air wound or made using scraps of ferrite taken from junk box inductors. Tuning is accomplished by means of a cheap mechanical rotary encoder. The Micro-controller provides the control signals for T/R switching, generates a sidetone and shifts the frequency 700Hz on TX. The transceiver supports full break-in keying. The input to the receiver is isolated during TX by means of a BSS123 mosfet used as a switch. A 2N7000 will work here as well. The receiver uses a leftover inverter stage as a RF amp. It works surprisingly well and also provides the bias for the switching mixer input. There is a 2nd order op amp low pass filter for CW. The coupling capacitor C30 is normally not needed for receive, but when the TX is keyed, there is DC imbalance on the output of the mixer switches – the differential audio amp will slam to the rails creating large key clicks on the side tone. C31 is required for stability when keying, without it, occasional parasitic oscillations can develop.
The circuit could be easily modified to use a VXO or VFO. One would simply have to modify the keying logic(connect key directly to TXRX signal and invert this signal for MUTE connection) One can use the remaining two 74HC04 inverters to generate a side tone and the the logic.
Update: Changed the band to 30m for fun. The only hardware changes were: L1 & L2 (.11 uH), C21 (800pF), L3 (.6uH), C22 & C23 (270pF). The 74AC240 works beautifully at 10 MHz – outputting the same 1.5 watts and even running a few percent more efficient. I must have nailed the matching network values.
Schematic(minor edit for key clicks 8/18/2015)
Unpopulated Home Brew board
XCVR in Home Brew 3D printed Case