FM receiver

[zachtheterrible]

I know ive asked this before, but last time it was about getting something pre-built. Now I'm asking for a schematic.

I'm not just looking for a schematic that will let me pick up regular radio broadcast (i'd just buy a regular radio for that). I want an EXTREMELY sensitive receiver. It would be modified to work just outside of the FM band so that it wouldn't be overloaded because of it's sensitivity.

It is for use with a small FM bug. I WANT MORE RANGE :twisted:

thanx :lol:

 

[Russlk]

The noise level can be reduces by reducing the bandwidth. Regular FM is designed for high fidelity, not maximum range. You might be able to re-tune a standard FM receiver for narrow bandwidth and add a low noise preamp.

 

[audioguru]

Hi Zach,
A good FM radio is extremely sensitive but doesn't overload on a crowded band because it uses a tuned circuit or two at its input ahead of its input RF amplifier.
A cheap radio overloads because its antenna connects directly to its input RF amplifier without tuning, so it amplifies all stations including strong ones causing intermodulation of them all. Because of this overload, cheap radios are made with reduced sensitivity so that overloading is also reduced. Some cheap radios have automatic-gain-control so that they are less sensitive in the city but fairly sensitive out in the sticks. How do you find one like that? Ask the bears out in the sticks!
Maybe you could add your own tuned circuit to the input of a cheap radio, but it would be difficult to tune it to the same frequency as the rest of the radio and you still have to figure out how to increase the radio's sensitivity.

He, he. Why don't you try an obsolete TDA7000 single IC FM tuner that doesn't tune its input nor use proper IF tuned circuits. Some people on the web say they are (choke) wonderful!

Where are you going to find all the special parts needed to make a good radio? I know where! Buy a good radio, take it all apart and use its parts to make your own. :lol:

What you need is an RF power amplifier (maybe illegal) at the output of your transmitter, and a good radio.

 

[zachtheterrible]

Russ, what you are saying is very very interesting. It sounds like what ive been reading up on in my ARRL handbook I just got. In order to decrease the bandwidth and increase the sensitivity, I would need to decrease the Q of the tuned circuit, correct? Would there be a resistor that I would need to lower in order to do this? The low noise preamp would probably be pretty easy.

Audio, I ALREADY MADE A TDA7000 (choke) like u said, it was supposed to be some wonderful glorious super sensitive radio but it turned out 2 be CRAP!! :evil:

I don't have to worry about tuning it because it will stay on one frequency (the frequency of my bug).

Now I'm on to something here, im excited :lol: . I just need to know how to do what russ said.

 

[audioguru]

Hi Zach,
You got it backwards. To reduce the bandwidth you need a higher Q which is difficult to do. To reduce Q just use a resistor in parallel with the tuned circuit.
A higher Q would make a transistor using it as a collector load more sensitive because it would have a higher impedance.

The bandwidth of an FM radio must match the transmitter or the loudest sounds will be horribly distorted.

 

[zachtheterrible]

I REEEALY want to do this, it would be a huge breakthrough for me, like a dream come true :lol: . so how do i do it??

You got it backwards. To reduce the bandwidth you need a higher Q which is difficult to do. To reduce Q just use a resistor in parallel with the tuned circuit.

You know thats what i meant :lol: :roll:

The bandwidth of an FM radio must match the transmitter or the loudest sounds will be horribly distorted.

This I do know. I guess I was talking about the bandwidth of the tuned circuit that lets 88-108 MHZ through . . . if that makes sense?

PLEASE TELL ME HOW I DO IT!!

 

[audioguru]

Hi Zach,
An FM radio, even a cheap one, doesn't have only one or two tuned circuits for the signal:
1) Input tuned circuit. Maybe.
2) Collector load of input transistor. Probably.
3) 1st 10.7MHZ IF. Yes.
4) 2nd 10.7MHZ IF. Probably.
5) 3rd 10.7MHZ IF. Maybe.
6) 4th 10.7MHz IF. Probably not in a cheap radio. In a good radio, probably.
7) Discriminator, ratio detector or quad-coil. Yes.

Show us a schematic of the radio you want to modify and we will show how to increase the Q on each of the above circuits individually. It won't be easy and they would need to be tuned with special test equipment. The whole bunch will probably end-up oscillating.

He, he. My very 1st job (a long time ago) was repairing car-radios on the production line. The radios with the "best" IF transistors oscillated. Instead of replacing the transistors until I found one that worked OK, I just tacked-in a resistor across the IF transformer to reduce its Q and therefore the gain. I got paid a nice "quantity bonus" for that trick. I didn't let the other guys know about it, they just kept changing transistors all day long! I was also quickly promoted to engineering and design. ICs were just beginning to be made at that time. Lots of fun!
The guy at the end of the production line also got a "quantity bonus". There was a shaker there and he just scewed-in lightbulbs and tacked-in all the electrolytic caps that fell out (that was before hot-melt-glue and LEDs)!

If the radio uses ceramic filters for the IF then it is impossible to increase their Q.
A broadcast band FM radio needs a bandwidth of at least 150KHz:
1) Digikey sells ceramic filters for FM communications radios with a 3.75KHZ, 7.5KHz or 15KHz bandwidth. Way too low.
2) They also sell ceramic filters for FM broadcast band radios with a minimum bandwidth of 150KHz which the radio probably already has.
3) They also sell a tiny surface-mount ceramic quad-coil with a 500KHz bandwidth.

Do you still REEELY want to modify a cheap radio to be another "wonderful glorious super sensitive radio"?

 

[zachtheterrible]

Sounds a bit complicated, but i still RREEEALY want to do it.

Instead of modifying my radio that I already have, which would mean tracing the tracks, which would be a huge pain in the ***, how about if there is a good schematic that you already know about?

I'm willing to spend money on this because as you know, I am obsessed with range :twisted:

I like your story :lol:

 

[Russlk]

You will need an FM sweep generator to re-tune the radio. Find a radio that has tunable IF transformers and peak the IF to the max. You may be limited by oscillation, but any narrowing of the IF will help. The bandwidth for voice is less than 3000 Hz.

You are right, Audioguru, mea culpa.

 

[Sebi]

I have an idea:try to find a cordless-phone base station, it have a nice double-mixed (i don't know the exact term) superheterodyne receiver. The first two IF stages work on 10,7MHz, the next two on 455kHz.
As front-end (tuner) use a car-radio tuner, it have three tuned coils with very good sensitivty.

 

[Nigel Goodwin]

I have an idea:try to find a cordless-phone base station, it have a nice double-mixed (i don't know the exact term) superheterodyne receiver. The first two IF stages work on 10,7MHz, the next two on 455kHz.

It's called 'dual conversion', it's common practice for narrow band FM receivers.

 

[audioguru]

The signal bandwidth is not dependent on volume, only on frequency and the bandwidth for voice is less than 3000 Hz.

No Russ,
The loudness of an FM broadcast determines the amount of its deviation, which requires a 150KHz bandwidth for full modulation. That's why a narrow bandwidth receiver would sound very distorted on the loudest sounds that are broadcast by a wideband transmitter.
The audio frequency just determines how often the carrier frequency swings.

 

[k7elp60]

I know ive asked this before, but last time it was about getting something pre-built. Now I'm asking for a schematic.

I'm not just looking for a schematic that will let me pick up regular radio broadcast (i'd just buy a regular radio for that). I want an EXTREMELY sensitive receiver. It would be modified to work just outside of the FM band so that it wouldn't be overloaded because of it's sensitivity.

It is for use with a small FM bug. I WANT MORE RANGE :twisted:

thanx :lol:

It has been my experience that all the senstivity and the selectivity of a receiver is obtained in the IF strip. If the deviation from the bug is less than about 5Khz then a dual conversion receiver with the second IF frequency of 455Khz would be the thing to do. With an dual gate mosfet transistor as a RF amplifer it would no doubt perform in a remarkable manner and not respond to overload. I have several extra MC3362P IC's that I woul be happy to donate to you.

 

[zachtheterrible]

Thanx for all the replies.

I guess what it comes down to is which will be the easiest?

I have a phone receiver in my electronics junk box that could be used.

I also have an old radio that only has one TDA1083 IC, so it would probably be pretty easy to tweak, I don't see any tranformers on it though. Russ, how old of a radio would have those transformers? And how big are the transformers?

k7elp60, what you are saying sounds pretty easy, is it?.

Oh yeah, and I would like it to be portable.

EDIT: Actually, the radio that i have may have tunable transformers. There are 6 metal cubes with a round hole in the top that can be tuned with a standard screwdriver. I've always wondered what those are.

 

[TheOne]

FM Bandwidth relates to the number of "effective" sidebands generated from the modulation signal. The number of sidebands generated depends on the Modulation Index. MI = (deviation produced by the modulating signal level or amplitude)/(frequency of the modulating signal)

Example: If a 5kHz signal produces a +-75kHz deviation of the carrier the MI (75/5 = 15) from Bessel function tables tells us that we will have 19 effective sidebands (effective in that it will contain 1% or more of the main carrier power when unmodulated). Thus the total bandwidth required will be 2 X 5000 X 19 = 190kHz. So the bandwidth will be a function of the amplitude (%modulation) and the frequency of the modulating signal.

 

[zachtheterrible]

I typed in IF transformer on google and found a picture of one. These are the things that I have in my radio. Its an old one, there are 6 of them. Will doing what you said Russ have as good of results as the things everyone else is suggesting? Because this is probably the easiest.

I could "reverse engineer" my radio if it is needed, I'm a bit bored lately. Here's the picture I found:

 

[Nigel Goodwin]

I typed in IF transformer on google and found a picture of one. These are the things that I have in my radio. Its an old one, there are 6 of them. Will doing what you said Russ have as good of results as the things everyone else is suggesting? Because this is probably the easiest.

I could "reverse engineer" my radio if it is needed, I'm a bit bored lately. Here's the picture I found:

They will be standard IF transformers, some will be for the AM side (455KHz), and some for the FM side (10.7MHz).

You may be able to peak the 10.7MHz ones very slightly, but I doubt it will make much difference - as has already been mentioned, the audio bandwidth required for speech frequencies doesn't have any bearing on the FM bandwidth, which is dependent on the amount of deviation applied to the transmitter.

If you're wanting to receive narrow-band FM (rather than the commercial wide-band FM), you really do need dual-conversion - the level of audio from the demodulator is dependent on the percentage of deviation of the demodulated frequency. Using a 455KHz IF will give around twenty times as much output for the same deviation!

 

[audioguru]

Yeah, but then the crude, extremely simple transmitter would seriously over-deviate the thingy. How can we restrict the deviation of the transmitter? Turn down its input by 20 times? What do you gain?

Don't forget that the transmitter drifts its frequency all over the place!

 

[zachtheterrible]

So what russ said isn't a very good thing to do? I should do either what sebi said or k7elp60. Which will be easier?

audio, the bug barely drifts. If it does, it will only drift like 1 MHZ.

 

[Russlk]

1 mHz drift will put the signal outside the bandwidth of even the unmodified receiver. You might do better with a crystal controlled transmitter and AM modulation. You could then have a very selective receiver.