How to Detect What Frequency a Radio Has Tuned

[SaurusX]

Anyone have any clue how to do this? For example, say I want to be able to point a device at a passing vehicle and determine what radio station is being listened to. Is this possible? What components would be needed?

[Wall Street]

There are electronic devices for detecting oscillators like the ones used in car radios. For example, some police radar detector-detectors use similar circuits. Finding the exact station would be nearly impossible though.

[PottedMeat]

you'd have to detect the generated PLL frequency mixed into the FM signal inside a metal box. i would think any radio that generated that kind of spurious emission would fail some FCC test.

[C1]

This is just a notion.

What if the "device" outputed (via RF scan) a full modulated single chirp (square wave) scanned across all the available local (station) frequencies (shouldnt be that many of them for a given city location) then picked up the sound (thru the subject car's speakers) off the car's window glass (use optical means - might require an IR laser or something not visible).

(I recall that a device was used by the CIA to listen in on interior office building conversations by monitoring vibration
of the window glass.)

[Paperdoc]

There could be another way to make use of the technology of laser detection of window vibrations to listen to sound inside the car. If you can detect the sound stream, then digitize it and compare to similar signals picked up via a second radio that scans all local stations - find the matching station. This way you're not transmitting anything, and should not be detected yourself. Of course, it they're watching for laser signals outside visible light frequencies, you're busted!

[SaurusX]

So there's no "field", as it were, to detect whether a receiving radio is tuned or even active? Is there no change in the antenna doing something when it's actually receiving the signals and then when the radio's tuner selects a particular frequency?

I recall reading about the US spying on soviet underwater cable communications by attaching a ring of sensors around the cable. With that the US was able to read the signals as they were transmitted along the cable without actually tapping the cable and being detected. I guess I was thinking there'd be a theoretical way of doing something similar when it came to detecting an active radio reception.

[Raizinman]

Yes, a neighbor of mine was stealing cable doing this. He did not touch or attach anythign to the main cable. He put a ring around the cable and was able to receive all the cable channels. He called it a highly directional antenna that pulled the signals from the cable.

[Paperdoc]

Radio receivers used to work (and I believe still do) on the heterodyne principle at the tuner end. This means the system to tune a particular frequency first generates a frequency signal just slightly different from the signal desired. (Today's PLL oscillators are much better in many ways than the older types.) This is then mixed with the whole mishmash of incoming signals picked up by the antenna and amplified by a low-noise broad-band stage. It is then run through a series of narrow passband filters and amplifiers tuned to a specific centre frequency. (Again, today's designs use quartz crystals to achieve very narrow passbands with sharp cutoff cliffs.) This section is called the "IF strip" (IF = Intermodulation Frequency), and it is tuned to the exact difference between the broadcast frequency and the receiver's internal tuning oscillator. Thus only the radio's very first stage, an amplifier, needs to be high gain and wide bandwidth. From there on the amplification is done on a narrow bandwidth at a fixed centre frequency, allowing very high gain with low noise. After this is done, the resulting strong modulated IF carrier is processed with the appropriate type of detector to extract the original audio signal that was broadcast.

So OP's basic question is a reasonable first thought - if the local oscillator at the front end MUST be different from the broadcaster's frequency by some exact and known amount, you can determine what station is being listened to. Wall Street and Potted Meat pointed out the flaws in the logic - the frequency determination would have to be VERY exact, and the signal generated by the internal oscillator should NEVER be leaked out of the receiver so an outside antenna could pick it up. Even if you could mount a pickup coil around the car's antenna (which is certainly NOT the subtle detection system OP was dreaming of!), I doubt the signal from the local oscillator would be strong enough to detect and analyze.

[C1]

Well, if your going to be able to walk up to a vehicle and put an inductive coupler on the antenna, then why go thru the trouble? Just look thru the window and see what station the radio is tuned to.

BTW, regarding antenna couplers, one of the tips from the auto repair guys was to use such a method to determine if car stalling was fuel or spark related. That is, just wrap one end of a piece of wire aound a spark plug wire and the other end around the radio antenna, tune the radio to listen to the spark and drive around. If or when the auto begins to stall, determine if it is because of ignition failure (ie, spark sound thru the radio cuts out).

[SaurusX]

Thanks for the thoughts, people. For some reason my mind had latched onto this idea a while back and it seemed like it might be possible using some kind of Nikola Tesla-like wizardry. Cest la vie.

[OSULugan]

Easy solution to the problem:

Broadcast through the valid FM frequency range a conflicting signal. When you see the person reach for the dial to "fix" the glitch, you've determined the radio station they were listening to.

[GWestphal]

Originally Posted by C1

This is just a notion.

What if the "device" outputed (via RF scan) a full modulated single chirp (square wave) scanned across all the available local (station) frequencies (shouldnt be that many of them for a given city location) then picked up the sound (thru the subject car's speakers) off the car's window glass (use optical means - might require an IR laser or something not visible).

(I recall that a device was used by the CIA to listen in on interior office building conversations by monitoring vibration
of the window glass.)

A laser mic, they are actually pretty easy to make with modern gadgets you have around your house.

[Colt45]

In old, leaky stuff, you'd measure the freq. of the local oscillator. Which for FM was usually 10.7MHz away from the received frequency. (for AM it was usually 455kHz, and for NTSC - 45.75MHz.)

I'm not sure how much the LO on something modern leaks, definitely less.



That's how the BBC knew you were lying when you didn't pay the TV tax "because you don't have a TV".

Well, I suppose they may have just measured the IF, as what they are watching is irrelevant...

[Abwx]

Originally Posted by Paperdoc

This section is called the "IF strip" (IF = Intermodulation Frequency), and it is tuned to the exact difference between the broadcast frequency and the receiver's internal tuning oscillator. .

Actually it is intermediate frequency , even if it s produced
using intermodulation.

Otherwise , your post is accurate.

[exdeath]

That only works if the tuner is an analog superheterodyne design.

Heterodyning was a way to get around the bandwidth limitations of the early tuning and amplifying electronics of the era by employing signal addition/subtraction early on in the detector to downmix any carrier to a fixed constant low bandwidth frequency. It's completely not necessary in today's high speed digital solid state world.

Modern digital systems work with bit streams and don't have any sort of RF emitting local oscillator that would have any correlation to the current station.

[exdeath]

Originally Posted by SaurusX

So there's no "field", as it were, to detect whether a receiving radio is tuned or even active? Is there no change in the antenna doing something when it's actually receiving the signals and then when the radio's tuner selects a particular frequency?

I recall reading about the US spying on soviet underwater cable communications by attaching a ring of sensors around the cable. With that the US was able to read the signals as they were transmitted along the cable without actually tapping the cable and being detected. I guess I was thinking there'd be a theoretical way of doing something similar when it came to detecting an active radio reception.

The only RF source from a radio receiver is a local oscillator that is used to mix with the tuned antenna circuit to down mix any received signal to a constant fixed frequency. It would be extremely weak in the first place, shielded by metal in the radio, dash, and car.

Radar detectors and inductive/capacitive transmission line tapping works because you are picking up an an active signal from a high power transmitter.

Radar detector detectors in fact do work by picking up the local herterodyne oscillator in the radar detector receiver, but we are talking much higher frequencies that are exclusive to radar and a device with a direct line of sight through a windshield. In this case, you don't need to know an exact frequency, just picking up a weak 5 GHz +/- RF emission for example in and of itself is evidence of the existence of the device and that's good enough.

[jumpncrash]

Originally Posted by GWestphal

A laser mic, they are actually pretty easy to make with modern gadgets you have around your house.

I think that might be a problem in a car due to the large amount of vibration already present

[MrDudeMan]

Originally Posted by jumpncrash

I think that might be a problem in a car due to the large amount of vibration already present

I don't see why that would matter unless inherent vibrations from the engine are in the frequency range of interest. It's like saying you wouldn't be able to hear a jet engine from a seat in a crowded football stadium.

[WildW]

In the UK, where we need to pay an annual fee to the government to be allowed a television, we have the "TV Detector Van" that can drive down the street, detect if a television is in use inside an unlicenced home, and even allegedly which channel is being watched - according to the government adverts designed to remind you to pay.

http://www.youtube.com/watch?v=8NmdUcmLFkw . . . this was back in the 70's.

Rumour over the years has given rise to competing theories of "it's all true" and "there's no such technology" . . . Of course, TV and radio and basically the same thing, just a matter of signal content.

[John Conner]

I was just going to post about the UK's TV detector vans. Now I have a question about that. What if someone has cable or or satellite TV? How do they detect that?

I read in Popular Science that malls would use a device to detect what radio station you were listening to and somehow transmit specific ads. That I read years ago.

[tynopik]

this is already done

http://www.sfgate.com/bayarea/articl...es-2744204.php

http://www.businessweek.com/bwdaily/...8/nf80306e.htm

http://www.washingtonpost.com/wp-dyn...2004Oct24.html

[SaurusX]

So it is possible(maybe)! These articles are way old, so it makes me wonder if anything ever happened with this tech. Did it turn out to be vaporware or what? A lot of the discussion in this thread indicates that the "leaked antenna emissions" that MobilTrak was supposed to pick up on is practically non-existent in modern tuners.