My latest project, AM receiver finished! (w/ pics)

[hamfiles]

I just finished building an AM receiver. I'm very happy with how it turned out. Basically, its a reflex receiver with an amplified output. The receiver uses 2 3904 BJT's, the preamp uses 1 MPF 102 FET, and the final output is fed to an LM 386. I did not design the orignal receiver portion, but I modified it somewhat to improve the selectivity and lower the distortion.

Details and pics are on my website here:

http://www.geocities.com/hamfiles/Page_2.htm

 

[Mosfet]

That's a nice radio :!:
I find small boxes the hardest part of a project.
Could you give some information on the tuning cap and xformer :?:

 

[pebe]

reflex rx

Hi Hamfiles,
I've looked at your circuit - nice one - but I can't see how it's a reflex rx. My understanding of a reflex is where demoded audio is fed back to the rf stage for further amplification, though with the cheapness of transistors these days I should think the disadvantages outweigh the advantages. Any comments?

 

[hamfiles]

I remember measuring the capacitor at 0 - 130 picofarads, but it may have been a little different, standard size. I'm not sure what the standard is for AM, maybe 1/2 of the old style 365 pfd, which would be 182.5 pfd. I measured 130 max, but my meter has long been out of calibration.

The ferrite loopstick for the coil, it is 4 1/2 inches long, 1/4 inch wide on the flat side of it. Its not round, more of an oval; standard ferrite coil.


Primary coil wound with about 60 - 100 turns of wire, loosely wound, with double the width of the wire spaced between the turns, if that makes sense. Used solid 22 guage wire. Secondary coil is 7 turns same guage wire, wound in the center of, and over top of the primary. It's even more loosely wrapped 1/4 inch space between the turns on the loopstick.

If I can get my friend to snap some more digital pics, I'll try to post a decent pic of a close-up on the loopstick.

The 60 - 100 turns on the primary was me being lazy. I started w/ about 100 turns, and kept taking coils off until I could tune the full AM band.

Otherwise, resonant frequency = 1 divided by the quantity of 2 pie times square root of product of inductance of coil and capacitance.

Formula calculating inductance of coil is much worse; too many variables to account for.

The easiest way is to get a matched coil and tuner from an old radio, or a crystal kit. I had built another prototype a few months ago, and that is what I did. A good tweak if you use a standard loopstick is to make your own secondary, and feed one end of the lead back thru the center, pressed against the loopstick. Details about this technique are here:
http://www.geocities.com/hamfiles/

 

[hamfiles]

The RF signal passes thru the secondary coil, to the base of Q1, and thru the 10 nanofarad cap to ground. Signal is demodulated and converted to audio signal by the diode, and immediately fed back to the bottom lead of the secondary coil. AF Signal then goes back thru coil to base of Q1 again. There is no AF at the collector of Q2 until it is demodulated by the diode and passed thru both transistors, and so it is a reflex receiver.

 

[hamfiles]

The original circuit for this was from a design published in "The Radio Constructor" mag, in 1969. David Hoult has an article on the circuit.
https://www.saburchill.com/tech/electronics/elect025.html

 

[hamfiles]

Pebe, yes it's not a real practical circuit, with the advent of superhets, and cheap dual gated FETs for RF stages. A good superhet with several IF stages is really impossible to beat for performance. But it was a fun project, and something that I built actually works very well for a change :wink: . I was kind of suprised by it.

Thing about superheteorodyne receivers is they are difficult to work with IMO. If anyone has a good but simple (oxymoron?) superhet receiver, please post it.

I saw a nice superhet receiver someone posted once, that used the AN 602 for the mixer stage, but I couldn't find it again, don't remember the link, maybe it went away.