Direct-Conversion Receiver, 40 m (TUF-1 DBM)

What it does

A complete direct-conversion receiver back end for 40 m. RF and a local oscillator go into a diode ring mixer, and the audio difference comes out to headphones.

How it works

The TUF-1 mixer multiplies the RF with the LO. Its IF port feeds a diplexer: 0.1 µF plus 56 Ω to ground terminates the mixer at RF, and a 100 µH choke passes audio on. A low-noise NE5532 stage and further op-amp stages give about 87 dB of audio gain (EMRFD p. 6.10). The simulation gives 85.3 dB at 1 kHz (simulated).

Building it

• Drive the LO port at about +7 dBm from a clean oscillator.
• Headphone impedance isn't stated in EMRFD; the simulation assumes 600 Ω.
• Keep the audio input stage away from AC hum sources, and shield it.

Tuning and testing

Inject a weak signal at 7.0 MHz with the LO 1 kHz away and listen for a clean tone. Check for hum and microphonics.