Part 2 of the item “VFO” (simple one). Corrections to the video: “capacity” must be “capacitance”. “Sensive” = “sensitive”. When the frequency of the VFO changes, you have to tune in again to find the radio station, this is what I mean. The item of the hand effect on the front of the VFO is perhaps not so clear. I mean this: the thin copper layer conducts electricity. Although the mass of the receiver is on minus/ground potential, it is possible that your body, touching the receiver by hand …
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Thanks for the info. Quite new for me, as I have worked only with sine wave oscillators up until now and did not work with mixer chips like the SA612A.
Just Google vlf receiver and have at it
My experience with a 555 is that it will need some output low pass filtering and a nice stable power supply.
On a 100mhz DSO I have seen the rising edge of the wave spike/overshoot to 18v on an unregulated circuit powered by a 9V battery. The harmonics were enough to jam my WIFI home network from a few feet away.
Most double balanced mixers work just fine with a square wave signal. You might even use a small op amp comparator to generate the aquare wave.
The SW radio’s I have built are completely analog (you can find the schematics in my book “Retro Radio”, publisher Elektor, Netherlands, Dutch text). Do you want to make a SDR circuit?
Hi
I’m totally new in this field of electronics though I have a GNVQ level 2 in analogue and digital electronics, but we never covered radio. I would like to build a 50Khz to 100Khz LW receiver. Would I be able to do this with a 555 timer fed into the oscillator stage of an SA612A and feed the RF stage with a 370uH coil, would the output downconvert the input frequency to an audio signal for my pc soundcard or am I barking up the wrong tree? Any help appreciated 
The metal knob is or grounded to the chassis (tuning capacitor) or insulated from the chassis (audio amp). In both cases no potential is built up.
The coils of commercial radio’s are small and shielded in metal cans, so they can not be influenced by earth effects like a hand etc. They are in metal to shield the high-frequent and low frequent electronic stages from external magnetic fields and influences.
How come—-”Cheap “radios from China-never get the “Hand effect “?—-in all my days—–even valve gear-never have had-the “Hand effect? “–maybe the tuned circuits were near the ground plane?
i.e near ground potential?
Why is it——-that ALL commercial receivers are -basically -metal?
Not a trace of-plywood?
Band tuning in the “Collector “stage-is fraught with–”Danger “?
Steve.
Just a “Obversation”-you have METAL knobs on your recveiver?
Steve.
I don’t have practical experience with this, but I suspect it would be because the tinplate is iron plated with tin. The iron, being a ferromagnetic material, would help to shield against stray magnetic fields. The copper is only conductive, so it would only help to shield from stray electric fields.
Metals have different shielding properties. I only have experience with some shielding metals for shortwave. Iron and iron like metals shield RF energy the best on shortwave, copper and aluminium shield less. (I never did tests however…). I presume the word “tin-plate” in English/American is “Weissblech” in German. Its the material foodstuff is conserved in. Its good usable to shield HF electronic stages.
now wait a minute… copper shielding is not working as good as shielding with tinplate (weißblech)? where is that coming from ? why that? never heard about that! thats interesting.