CW DC Transceiver, 1.8–30 MHz
Direct-conversion transceiver for amateur radio
Part 1
Diode ring mixer, broadband diplexer (750 Hz peak), 115 dB AF amplifier
Current project: a DC transceiver for 40/30 m CW operation. The AF section provides 110 dB of voltage gain. Best performance is achieved with high-impedance headphones.
Project goals:
- Fun, fun, fun
- Few (available!) parts
- Suitable for beginners, too
- Inexpensive
- Weekend project
- No alignment required
- Compliant with national and international regulations for transmitters and receivers
- Excellent receiver data for such a design
- Full break-in (QSK)
I don't recommend "ugly construction" for the VFO, mixer, and AF sections of DC receivers, because it increases microphonics. Instead, I recommend a solid PCB design: a double-sided PCB with one side routed and the other serving as a ground plane.
PCB service: I have a PCB layout for EAGLE available (single-side routed).
After a test period, I will publish the PCB data here (.brd format for the EAGLE PCB layout software). If you can't wait, you are welcome to ask for the layout by e-mail:
Circuit diagram:
1. Mixer
A double-balanced diode ring mixer is used. RF input power: approx. +7 dBm or more.
Mixer transformers: T1 = T2: 5 turns trifilar, 0.22 mm wire, on an FT37-43 core.
A version with a ready-made mixer (TUF-1, SBL-1, etc.) will be available later.
2. Diplexer
Broadband-matched to about 50 ohms, peak at approx. 750 Hz. The inductors are available from RS Components or other sources.
Diplexer inductors:
3. First AF Stage
An MPS2222A is used here, matched to about 50 ohms input/output impedance. About 20 dB of gain. A lower noise level is possible with another transistor (e.g., BC547B, 2N4124). An active voltage decoupler is inserted to prevent hum.
4. Second AF Stage
Emitter-coupled audio stage with an impedance transformer option. MPS2222A. About 20 dB of gain. The output is muted by T3 (2N3906).
5. Third AF Stage
Op-amp 1 of a TL072 dual operational amplifier. 750 Hz CW filter; change the 1 nF capacitor to change the center frequency.
6. Last AF Stage
Op-amp 2 of the TL072. Variable-gain amplifier. Best performance with high-impedance headphones. Low-impedance headphones can be used, although some hum may be noticeable.
Connect a 100 kΩ potentiometer to the "AF Gain" port.
Current status/bugs:
- VFO testing
- Receiver runs very well
- Mute clicking
To prevent hum:
- Terminate all mixer ports with an impedance of ~50 ohms!
- Shield the VFO.
- Prevent ground loops, especially at the headphone jack. Keep the headphone plug isolated from ground when possible.
- Use a power supply with good hum suppression.
To be continued.
March 23, 2011 – DL6MBI
Coming next:
- 40 m VFO
- Class C PA
- TX filter
- Preselector
- RX/TX switching unit