Low Distortion/High Dynamic Range Detector |
The detector provides both low distortion and very high dynamic range. It can cleanly demodulate AM signals, even with positive peak modulation in excess of 200 percent. From Frank, WA1GFZ, "The whole point of this circuit is to compensate for the diode knee and voltage drop across the junction. The 1N34, like other germanium diodes has high leakage. This will reduce the dynamic range of the circuit. The best possible configuration is to get a matched set of diodes. HP used to sell matched sets of four, but I'm sure those days are long gone. The next best thing would be to get a lot of them and characterize them and select the closest three. The output of the I.F. strip on many radios is high enough to use silicon diodes. These are easier to come by. I have been using this setup in my home brew RX since '85 with a matched set of HP hot carrier diodes." The circuit was adapted from a circuit published in RF Design, in 1985.
For IFs below 500 kHz or so, use a good FET op-amp like a TLO72 for IC 1. For higher IFs, select an op-amp with a gain-bandwidth product well over 1 MHz for best performance. Vcc is 9 volts. As noted above, hot carrier diodes can be used for D1, D2 and D3. Also, Shottky diodes like 1N5711s will also work well. Even 1N914s work well.
Adjustment: Tune in some atmospheric noise with the AGC on the RX off. The adjust the pot (5k) for maximum noise. You will see the noise dip up and down and get distorted as the pot is moved past optimum.
In light of the distortion created by "standard" diode detectors in most receivers, you should try this circuit soon.
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