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scratch built am radio...LC frequency not making sense

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KTP:
Hi guys,

The lab kit in my circuits I course (we just finished the topic of diodes) included some 1N914 diodes so I decided to try and build a simple am radio with one of them (not part of the class or lab, just to explore tank circuits somewhat).

Anyway, I wound some 26 gauge magnet wire around a 1 inch pvc pipe, about 4 inches of windings with multiple taps.  With my "Almost All Digital Electronics" L/C Meter IIB, I measured the total inductance of my windings to be 256uH.  The middle tap is about 124uH. (I sort of lost count while winding  ;D ).  Anyway, I hooked up the full length of the inductor across a 7-100pF variable cap with one end of the parallel tank circuit grounded and the other end connected to a 20 foot antenna and the cathode of the 1N914 diode.  I then fed the demodulated output into a inverting op-amp with a gain of 100 and driving a small 32 ohm speaker.

At first I did not get any station at all, but after trying several taps I picked up a decent reception using the middle tap (about 124uH) and around 23pF on the variable cap.  When the station indentified itself, I looked it up and found out it was AM 1380 broadcasting about 30 miles away (but I am in a deep valley).  Anyway, this didn't really make sense to me.  My calculations show that with a 124uH in parallel with 23pF my LC tank should resonate at close to 3Mhz???

Since I now knew that there was a strong station at 1380khz I tried the full inductor (256uH) across 52pF but got nothing but silence.

Ideas on what is going on here?  Is there possibly hundreds of picofarads of distributed parallel capacitance across my wound inductor?  Am I picking up some harmonic of the 1380khz broadcast, and if so, why can't I get the primary signal?

Thanks for any help

Simon:
what else are you using to demodulate the AM ? you should have a capacitor after the diode as basically the AM demodulation process is to low pass the signal so that you get the varying amplitude of the audio smoothed out without the HF component. how have you physically built the circuit, I ask because my attempts at making a RIAA preamp (with more input and output leads all over the place than recommended) and ended up picking up some foreign radio station.

KTP:
I have the diode coupled to the inverting op-amp resistor with a 0.1uF capacitor

Anyway, as a slight update, I found some 3/4 inch diameter by 1 inch long ferrite rods from a scrap flyback transformer.  I wound another inductor with MUCH fewer turns (dc resistance less than 0.5 ohms vs 2 ohms) and tried to space the windings out this time.  The air core inductance of this new inductor was only 24uH, but when I insert two of the ferrite slugs, it goes up to 176uH.  I hooked this into my circuit which still had a fixed value capacitance of two 33pf in series (so about 17pf) and I was able to pull up maybe the same radio station with a stronger signal with the slugs about half way in.  I haven't heard the call letters yet so I am not 100% positive it is the same station.

I can only guess that my tightly wound air core inductor with many windings had a lot of interwinding capacitance which resulted in a total distributed capacitance of over 100pF.  I guess.

Update, the station is the same, AM 1380.  I measured the inductance of my new, more ideal inductor and it was 117uH with the slug out to a position with the strongest reception of the station.  The fixed capacitance in my tank circuit is 16.5pF.  These numbers are not what I was expecting...they seem to indicate resonance would be at about 3mhz. :-(

KTP:
Yay.  Ok, I figured it out I think.

With an inductance of 117uH I *knew* that my capacitance should be around 120pF to resonate at about 1.38Mhz, so I ignored the working 17pF capacitance and replaced the two 33 pf series caps with two 220pF series caps (so 110pF).  Moving the ferrite slug slightly and I pulled in the same AM 1380 station.  This time the inductance and capacitance make sense for that frequency.

So this is interesting.  I was picking up the third harmonic with the old values, and it was coming in almost as strong as the base frequency!  I would have thought the third harmonic would be significantly down in power.

Ah well, now I at least have a start on a circuit that makes sense to me.

Simon:
apart from the coupling cap to the op amp you should have a cap from the diode output to ground, but then as a an op amps input is considered virtually ground sometimes I'm not sure if you have achieved the same result, hopefully someone else knows more on that

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