TRF III:  Long Wire

2020/4/20

Every Crystal set I start from gonna become TRF somewhat. This time I choose LM386. Even Though I did this every time, the new thing still emerge again and again.

I combine the most successful crystal receiver I ever made and a nice LM386 module, to exploring new era of TRF. 

LM386 Module

I produce this  small audio module for multi purpose,  use as audio amplifier in receiver, could be use as Radio modules for another toy project.

Short Wave Interference

Yesterday, I heard the famous shortwave interference , the  "Russian Woodpecker", all day. It may reopen/re-built since 2017.

In the night, audio quality is severely degrade. There is very obviously high frequency Hiss.  I worry about this gonna be LM386 itself very famous problem - hiss. But, my module already take all precautions, input LPF, low frequencies boost (ie, high frequency attenuation).

And then, second daytime, I found the hiss is gone completely. This is thought-provoking thing!

LM386 Hiss?

At night the signal became much dense.  By sweep small portion of them via probing the tank's signal, obviously,  the night station's side band  intersect together.

I inject a signal 80 khz apart from station, -50dbm, It generating same hiss voice, further confirm this is IMD.

Intermodulation in crystal detector

This explains everything we encounter. The most surprising thing ever meet. Even the crystal radio with passive mixer could suffer from IMD. 

• Daytime never hear  high frequency Hiss, because in daytime, the station's side band is not strong enough.

• Daytime station have same loud volume as night, but ONLY night station could hear hiss voice. same as above.

• Only few stations could hear Hiss voice, because those stations crowded together and both have strong sideband.

• Only sensitive detector could hear Hiss voice, because the intermodulation product is so small , could not be heard.

• Only high gain audio stage could hear hiss,  same as above.