FM strength Measurement???

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buts101

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I want to build a simple strength meter which can measure the signal

strength of FM stations in my city(different places in the city) for project, which is in a week.

So, please help me with the circuit and tell me how can i display the

strength reading digitally. Please tell me how can i vary the frequecy

so that i can measured the strength for different FM stations.
 
Good luck!
It is very difficult to build an FM tuner. Buy a fairly good FM radio with a digital frequency display so that you can accurately identify stations apart.
Hopefully it will be good enough to include AGC for overload prevention. Then add a digital multimeter to read digitally the voltage of the AGC.
 
audioguru said:
.
Hopefully it will be good enough to include AGC for overload prevention.
Click to expand...

It is unusual (almost unheared of) to have AGC in an FM receiver.
Generally it is not necessary. The last IF stage is usually a limiter (overdriven amplifier), so overload in the later stages is not a problem as it is with AM modes.

As for the original poster, If you have to ask this question, I dont think you will do it in one week.
You need an AM receiver with a wide IF bandwidth and a signal strength meter usually driven from the AGC line.
There is little point in making the signal strength meter digital, you wont be able to read it as the signal strength varies, and you dont need high resolution anyway. The meter scaling will be inherently non-linear due to the action of the AGC circuits anyway.

As Audioguru said - goodluck.

JimB
 
Chips like CA3089 and LM3189, FM IF and demodulator, have build in input level metering. They can be used for field strength measurements. You will need ceramics filter in front of it and a tuner.
 
Hi Jim,
Years ago I was designing car radios that had AGC for the AM and FM RF amps to prevent overload when you drove past a high power station's transmitter.
Cheap radios overloaded and either were muted if the RF amp was cutoff by the overload, or they got only that strong station all across the dial.

An NE602 IC has a linear wide range AGC output that could be used with an accurate FM tuner. :lol:
 

Without actually checking the number, isn't an NE602 a front end balanced mixer IC?, and has an AGC input (not an output).
 
That AGC for FM is usual in the tuner and not in IF. Limitting of IF signal is neccessary for stable audio output and good S/N so no AGC is needed.
 
bloki said:
That AGC for FM is usual in the tuner and not in IF. Limitting of IF signal is neccessary for stable audio output and good S/N so no AGC is needed.
Click to expand...

Quite right, although the strength signal comes from the IF, so it's not really much different to AM AGC, except it comes from an earlier stage in the IF. It's usually called 'delayed AGC' by the way, the AGC control is 'delayed' until a specific high signal level is reached, then the front end gain is reduced - which is what Audioguru was talking about.
 
Audioguru
I cant comment on "domestic" type radios but certainly "communications" type VHF and UHF FM radios do not have AGC.
If they are close to the base station they just keep on working OK.
However, if they are close to a transmitter on another frequency there can be various overload effects and no AGC in the world will sort that out, only a redesign of the RF amp and mixer stages.

JimB
 
JimB said:
Audioguru
I cant comment on "domestic" type radios but certainly "communications" type VHF and UHF FM radios do not have AGC.
If they are close to the base station they just keep on working OK.
Click to expand...

Generally they are designed for use with much lower power transmitters in mind - its no big deal being next to a 50W transmitter, but being next to a 500KW transmitter is a bit different 8)

However, if they are close to a transmitter on another frequency there can be various overload effects and no AGC in the world will sort that out, only a redesign of the RF amp and mixer stages.
Click to expand...

Again, it's quality design that counts - a GOOD quality design would be fully screened, so signal can only enter via the aerial feed. In such high quality receivers you would normally have an RF attenuator system either manually via a control, or automatically via delayed AGC - most use manual controls though. Also, a manual pre-selector system could be sharply tuned to reduce the unwanted frequency - but all this is far above normal domestic FM radios!.
 
Nigel and Audio

Our areas of experience are different.
True there is a difference between a main area FM broadcast station and the average mobile radio system.
I have also just dug out from my archive the circuit diagram which came with a car I bought some years ago.
The radio is/was a Blaupunkt "London" and indeed there is an AGC line generated by the IF amp chip and controlling gate 2 of the VHF RF amp MOSFET.
I have learned something today.

JimB
 
JimB said:
The radio is/was a Blaupunkt "London" and indeed there is an AGC line generated by the IF amp chip and controlling gate 2 of the VHF RF amp MOSFET.
I have learned something today.
Click to expand...

Due to the very nature of car radios they are subjected to huge ranges of signal strength, so it's more likely that a car radio would have such AGC, whereas home systems are less likely to.

Also of course, Blaupunkt are a quality manufacturer, which could explain it as well?.

I've just checked a couple of Sharp car stereo manuals, one had a DX/Local switch, and the other a DX/Local gain control - neither had automatic control of the front end gain, just manual - both operated on the dual gate MOSFET as your Blaupunkt.
 
Nigel Goodwin said:
Blaupunkt are a quality manufacturer
Click to expand...
I bought a "quality" Blaupunkt car radio for my '75 VW Rabbit because it fit and it was the only one I found that recorded cassette tapes. I ordered its service manual for me to soup it up (I got its cassette R/P flat to 15kHz with very low distortion!) and a Chinese manufacturer's name was crossed out with a felt tip pen and Blaupunkt was stamped on it.
It was a quality unit 'cause it lasted 6 years and recorded millions of great-sounding tapes.

Sorry, my old brain messed-up my posted part number. The NE604 IF amp IC has an AGC output with a 90dB range. It has many pins but I still remember an 8-pin NExxx IC with a good AGC output that folks use as an audio sound level meter.
 

I'm with Overclocked on this one.

Just get yourself a receiver, rip the speaker out, and couple it (using a capacitor and resistor) to the stereo VU meter shown at talkingelectronics.com

The capacitor and resistor needs to be modified to match the wanted strength and fall time. To start, try a 150pF capacitor and a large resistor (> 50K).

Now take the LED outputs on the VU meter, and convert them to binary result. You will need a combination of resistors and comparators.

take your binary result, hook it up to a 74LS47, and connect it to a 7-segment LED.

Do a search for Rick Anderson. He has a page of radio receivers he has built. I like his ideas, but some other well known users around here think his schematics are substandard (a.k.a. TOY).
 
MStechca,
You are confusing the strength of a transmitter's RF carrier with its amount of audio modulation.

How can the audio output of an FM radio possibly provide an indication of the RF field strength received by its antenna? The FM transmitter could be unmodulated and broadcasting silence, but its carrier is still at its max level. The speaker output of a radio tuned to it would be zero.

A good FM radio is so sensitive that the volume of its sound doesn't change with received field strength. A very distant station's 4uV signal of field strength at a radio's antenna will be just as loud at its speaker as if it was 500mV from a local station, except that the distant low level signal will be noisier in stereo. In mono they would sound and measure the same at the speaker. Only if the received field strength at an FM radio's antenna drops below about 1uV will the sound level drop.
 
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