Mystery reggen - simple(st) FM receiver?

Trash parts FM receiver, no antenna required.

The circuit used in this video (tuning depends on parasitics, tuning C was here 10pF and output C was 100nF):

http://www.niksula.cs.hut.fi/~plahteen/misc/old_projects/s1.png

You can also try to make the resonant circuit with the inductor at the transistor base instead of the collector. I used 2N3904 transistor.

Circuit that is easier to get working:

http://www.niksula.cs.hut.fi/~plahteen/misc/old_projects/s2.png

Tuned to around 100MHz (or maybe some harmonic). Essentially consists of 1 transistor, 1 resistor, 1 capacitor and some wire (coils).

The idea in the first schematic is to make a positive feedback and an oscillator by coupling the two coils. The second schematic doesn't depends on that.

Now why does this works as a receiver? Regenerative feedback gives high gain for a narrow band of frequencies and tuned in a slope will change FM to AM and the nonlinearities in the transistor detect the signal (original idea about product detection was incorrect, this would require slightly more complicated circuit).

The same circuit could be used in reverse as a transmitter although it should be amplified significantly and the signal filtered since the spectral purity would be awful.

The reverse will work because the transistor junction capacitance is a function of the bias voltage resulting in different oscillation frequency and so archiving the frequency modulation. It also causes some amplitude modulation which can be removed by amplification.

Tuning is done by deforming the coil and/or using the screwdriver as a slide. Surprisingly it works and sound quality isn't too bad. Needs significant amplification though (might want to use opamp) and exact tuning is next to impossible.

Category:

Howto & Style

Tags:

• radio
• receiver
• FM
• transistor
• rig
• simple

[hecanet]

great work...

I see you have about 3 resistors and a capacitor at one end of the breadboard...what are the values and how did you connect them - in series/parallel and why?

are those the 10k you have in the schematic and why 3 of them?

[decod31]

@hecanet Schematically there is only 1 resistor. Nothing more really than the original schematic I drew. In this video I think I just had 2 resistors in series with the voltage supply to make the 10k or maybe 20k. The 3rd one is not doing anything, both of its legs are just connected to ground and used for connecting the external ground wires. The extra capacitor is equally doing nothing.

[hecanet]

@decod31 Thanks for the quick response.

Did you try using electrolytic cap as the output cap? I am just curios. I guess my question would be 'how did you determine the values of the caps you used?'

Lastly, the 2 coils used, which of them is connected to the base and which is connector to the collector and the resistor - the loop/jumper-like wire or the 3-turn wire? It wasn't very obvious from the video nor the schematics

Thanks for all the tips. I really appreciate it all.

[decod31]

@hecanet Didn't try electrolytic, but it should work as AC-block if the polarity is right. The cap isn't critical if it is large enough. The shorter coil is connected to the base, but values are guess work as the environment is not well controlled. In principle the tank coil inductance can be calculated from the frequency and cap. Simulating the circuit as an amplifier will tell you if it's going to work, you just need to adjust the feedback etc. to get a proper gain and bandwidth.

[decod31]

@hecanet Mean to say DC-block of course about the electrolytic cap...

[decod31]

@dmctactic Give me break, where's your sense of humor? This is more or less the standard regenerative receiver circuit known for ages. Tuning it is simply done by deforming the coil and/or adding some capacitance to the tank circuit. I can easily tune to 3 stations where I live. Besides, it wasn't even the point of the circuit to make a fully usable receiver, just to demonstrate something simple. I never claimed to be expert and couldn't care less, although I do hold M.Sc. of relevant field.

[decod31]

@dmctactic Of course I can, you just need to be good at what you do. Tuning knobs are for mortals. ;-D

[doctormcgoveran]

neat going on the reciever, decod31 it is great to see it work, don't let every body who shops at walmart and bought a better one than you built and has to run you in the ground bother you.

[EIectroMania]

@decod31 holy shit you do have a Masters degree? hey that's awesome, it inspires me to study hard !!! :D i wanna get my BS in EE as soon as possible yay! :D

[ChiIdKiIIer]

@dmctactic change the stupid resistance and viola!

[ChiIdKiIIer]

@dmctactic i think he changes the resistors everytime he wants to change the radio station hehe until he gets to his favorite station :D

[rockangel1967]

bla bla...you uys should see my valve tube FM radio i have built :-)

[hecanet]

@decod31 I made this FM receiver last night. A little modification was introduced though... a small trimmer was added in parallel with the tank coil and the 10pf was flipped around to connect between collector and emitter (I adopted the schematic I used for my fm transmitter) and it works very great. Was able to tune in to a couple of stations - 97.3 etc.

Thanks for the tips...I will make and upload my video later.

[hecanet]

@decod31 Thanks....I will try this at home....Just ordered GOOIT GY560 portable FREQUENCY COUNTER from ebay for more fun....$24.00 for greater RF fun...ha ha ha ha

Thanks for all the tips...keep in touch...most of my youtube videos are under hecanet.