Ok, /diy/ I'm trying to make myself a tiny portable FM radio, inspired by foxhole radios, with an added generic amplifier circuit. Pic related it's the schematic.
So on the left is an inductor L1=100uH with 2 caps. A C2=22pF and a variable C1=(2-10)pF cap to keep the frequency between ~103-89 MHz. The limiting part being the 10pF variable cap available locally (the only one available).
Since I can't find a germanium diode like the 1N34 I'm settling for a silicone alternative a 1n41448.
C3's are high pass filter capacitors 0.1uF
The rest of the circuit is a generic amplifier.
The core component being a transistor I chose to use a 2N4904. Now biasing it was a bit of a challenge. So I would be glad if someone could check this part for me. R1 = 5k6 R2=1k R3=10k R4=20k.
The battery is a standard 9V battery, since I'd like to fit the whole thing in a small tin box I have laying around.
So my question is will it work?
Will the diode let the signal through? What will it's voltage be? I looked at the data sheet for the 1N4148 it's min voltage seems to be 0.68V with a pretty long antenna (175-200cm) will it be enough? With the 0.68V assumed I biased the transistor but is it correct?
A 100uH on the VHF band? How did you get such a high value? With the formula f=159/(sqrt(L*C)) [MHz, uH, pF] I`m getting between 2.8 to 3.2MHz and that is HF band.
Also the detector in your circuit is for [A]mplitude [M]odulated signals, you will only catch international stations from around the world or some strong local ones. The diode is good but only for the HF bands.
The bias for the transistor is quite high and the tranz will be wide open. 3 to 6V ratio smells like magic smoke.
Please, before you continue to the depths of despair as why it`s not working, atleast read on some [F]requency [M]odulation recievers theory.
>>909943
Ty for the info anon I'll re check my equations...
Isn't the formula f= 1 / ( 2*3.14(sqrt(L*C)) ?
Still that's very high for AM.
You wouldn’t happen to know where I could read up on the theory from a reputable source... I'm scrapping this together form old physics books from university and most stuff I look up online isn't reputable or reliable or is very basic to the point of being useless.
>>909966
1/2pi = 0.159 and your equation uses F, H and Hz.
The radio in your pic is what can be used to receive AM. It's basically a crystal set with an amplifier and both the circuit topology and the component choices suck at 100MHz FM.
You can try your luck with regenerative receivers or one-chip-does-it-all type IC receivers (TDA7000 or whatever is its current iteration). Or you can build a superhet, but it requires much more work and skill.
Usually people building radios start from simple AM receivers, but better check what stations are near you before wasting time on it.
>>909915
How does that demodulate the signal?
>>910356
Badly. The detector diode also needs some load to function properly.
https://en.wikipedia.org/wiki/Detector_(radio)#Slope_detection
>>910360
Not really sure I understand that. I have a "theoretical" understanding of FM (including synthesis).
Don't you use a high-Q bandpass filter to zero in on the carrier, then do some Fourier type trickery to detect the deviations of frequency which are then converted to a voltage?
Or is that how the "proper" gear does it, whereas this is some kind of quick and dirty "hack"?
>>910413
Yeah, that's the basic idea, except that you tune it slightly off so that (say) increasing frequency produces higher output and decreasing frequency lower output.
>>910421
Yeah that makes sense. If it was tuned too precisely then both raising and lowering the frequency would lower the amplitude.
Wow I might just get the hang of this stuff yet...
OP here with an update
I tested the amplifier circuit before hand... had a transistor bow up :D first time that happened
Re-biased it to
R1= 2.2K
R2= 1K
R3= 33K
R4=10K
It lived so I continued the build
Added the antenna(white wire in headphone jack connected to an old radio antenna I salvaged)
and the LC circuit
and POW it works I'm mostly getting static and its very hard to tune with the trimmer since its so sensitive.
I did mange to tune in to a station at around 94ish Mhz that I confirmed was at 94.6 with a radio that I tuned to the same station.
There is however a faint buzz of sqeek coming out of the speaker it's very weak but annoying... wonder what it is...
the diode is somehow demodulating FM signals, but not the way they're supposed to be demodulated, so you can never expect to hear proper sound unless you tune to the AM band instead, where the diode is sufficient to get clear sound.
>>909915
I have been modding some FM receivers, such that i could replace the bulky and some what expensive mechanical variable cap with a varicap or varactor.
You actually don't need a dedicated varactor diode, you can use any Schottky diode or in worst case a Y5V ceramic capacitor.
I did something similar to
http://mysite.du.edu/~etuttle/electron/circ359.gif
You see the capacitor in series with the diode, the one noted .1uF. depending on frequency an such, this will vary is size.
but a rule of thumb is to keep this 10 times larger than the maximum capacitance om the varactor or diode junction capacitance.
>>910729
Oh, do share...
This is somewhat a failure since I wanted this assembly to fin in a box slightly larger than a 9v (with the 9V inside it) had to scrap that idea since due to the mechanical cap(which are getting pretty rare these days).
So for version 2.0 of this I would like to integrate your suggestion
The original Foxhole Radio, although it only gets the AM. Doesn't get any simpler.
>just move the pencil lead along the rust spots until you pick up a station
>>910802
I dont have much to share, as i did not take any pictures.
But google around for FM tuner varicap or AM tuner varicap to get ideas, i got mine from this image and my knowledge of component behavior.
>http://circuit-zone.com/ediy_blog/25/TDA7000_FM_Receiver.jpg
The reason why it is possible to use schottky diodes, is due to the large change in junction capacitance, when they are reversed biased (blocking a voltage). this source must be high impedance (10k or more) to prevent this from affecting the resonant tank.
Make sure to keep bias voltage constant and the temperature of the diode steady, otherwise the tuning will drift. A battery is not a steady source, use some voltage regulator.
The reason why a Y5V ceramic capacitor can be used, is due to the HIGH non linear capacitance change when a voltage is applied, this capacitance will also change with temperature, pressure and time, so this is not useful for more than test and experiments.
