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.
TRF with Long Wire antenna
I combine the most successful crystal receiver I ever made and a nice LM386 module, to exploring new era of TRF.
Successful Crystal Receiver Ever
Nice LM386 Module
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.
Audio Sample(wikipedia)
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?
Does it because LM386? I don't think so. For days observation, I could tell, the Hiss only ermerg in night, daytime is completely clean.
I change several different detector diode, include 1N60(hot carrier), 1SS86, 1SS108, 2AP8, 1N60 Point contact, HSMS 2852 etc, all perform same. Dummy sensitivity detector seems better, but it's just because i can't hear that.
I try 2 different antenna, the 15m one and 50 meter one. Both them had same problem.
I vary the couple between antenna-tuner and main-tuner don't help either.
Insert a filter which cutoff frequency is about 1.7Mhz also won't help.
Insert audio filter after detector is also not help, the hiss frequency is in audio bandwidth.
I confirmed this "Hiss" problem from a instant-message discussion group. In night, the interference became strong.
The following Spectrum photo take under 2 Khz/div Input Att:0 rbw:1khz.
50 meter antenna daytime,
50 meter antenna night
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.
day time 3 station and their side band
night 3 station and their station
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.