FM IF limiter

Yes. It also has the very nice side effect of almost completely eliminating interference from weaker signals and noise on the same frequency--the *capture effect*. Most amplifiers don't limit cleanly unless they're specifically designed to.

Cheers,

Phil Hobbs

Thanks, Phil! I did a bit of skimming regarding the capture effect. What would be a solution to the possible fading in and out of two similar frequencies with equal strengths as picked up by the FM receiver?

Sounds like listening to FM while in your car. Nothing can be done about that. As one signal droops, another signal can capture your receiver. Satellite radio?

Learning to like both kinds of music, or using a directional antenna.

Cheers,

Phil Hobbs

yeah, but everything you said is true at the input to the limiter as well...

Look at the zeroe crossings of a combined signal even if it is not limited.,,,you get the same thing..

what if you fed the unlimited IF signal directly to a ratio detector (that resppnds only to FM and not AM)?

Mark

No, it isn't. If you draw yourself a phasor picture of two signals at different frequencies, like this,

Im(V) ^ | V2 exp(jw_2t) | *------>

| / | / | / V1 exp(jw_1t) | / |/

-------+-------------> Re(V) |0 | | | V

the frequency at the output of the limiter is set by the zero-crossings of the sum of the two signals, V1(t)+V2(t). From the picture, it's obvious that V1 will set the frequency of the rotation of the resultant whenever |V1| > |V2|, because then V2 is too short to reach the origin.

In this case, the only way V2 can change the number of zero crossings is for it to make the sum phasor cross and recross the imaginary axis, which can't happen as long as

V2*w2 > V1*w1.

This is the origin of the capture effect in a good limiter.

Cheers,

Phil Hobbs

The limiter is not what gives you the "capture effect". It's the FM demodulator its self.

Jim

No, it isn't true at the input. The key is that only the zero crossings are preserved by the limiter--otherwise it would apply to AM radios as well as FM, which it doesn't.

The slight limiting action of a ratio detector is a help, but nothing like as good as a proper limiter. Ratio detectors came in in the tube days, where they saved an IF stage, but that's entirely moot now.

Cheers,

Phil Hobbs

Yes, my point exactly....

the zero crossings at the input to the limiter are the same as the zero crossings at the output of the limiter

and the FM information is contained in the zero crossings, so the capture effect is inherent in FM modulation and exists at the input to the limiter as well as at the output of the limiter.

A limiter is only one way to make the detector ignore the AM component but not the only way.

What I am saying is that any detector that responds to FM and not to AM (using a limiter or not) will exhibit the capture effect because it is inherent to FM. It is not a function of the limiter. A PLL based FM demodulator for example without any limiter will have a capture effect.

Mark

Phil Hobbs

True, but also maybe not true.

In FM broadcast radios, the stage really doesn't have excessive gain, but is designed to go into limiting only when a strong enough signal comes in for noise free reception. If it had more gain, then the thermal noise of the receiver front end would be amplified to the clipping point and cause loud 'white' noise to come out of the speakers when tuning between stations.

In narowband FM two way radio, there is a lot more gain in the IF and limiter to guarantee the white noise under 'no signal' conditions. This noise is looked for by the squelch or muting circuit.

Re capture effect - This effect is most pronounced in wide bandwidth receivers. Typically FM broadcast uses receiver bandwidth of more than 200kHz and has a capture effect of less than 3dB. This is how close in amplitude an interfering signal has to be before the desired signal suffers at 1000uV. In narrow-band FM using 9-15kHz BW, the number is in the 20-30 dB range. Spark/lightning interference rejection is also better with wide bandwidth, since the duration of the received pulse is inverse proportional to the bandwidth.

Ratio detectors have some impulse/AM rejection, but it's less than perfect. I used to measure 20-30dB, but this was 25 years ago. Maybe today's circuits work better.

I think "oversampling" is another one and have nothing to do with subject here. The comparator simply looks at the zero-crossings. If you oversample there is a signal generated with a higher frequency band.

As I recall the theoretical limit is 3dB for the capture effect (for an ideal detector).

- Henry

"Phil Hobbs" schrieb im Newsbeitrag news: snipped-for-privacy@SpamMeSenseless.pergamos.net...

Hi Jim

I remember ( a long time ago) when my boss told me to connect two audio generators to a scope and turn the gain way up and look at the zero crossings...

seeing is believing

thanks

Mark

Please, Phil knows what he knows and you can't confuse him with the facts. After all, Phil can draw phasor diagrams. The fact that the phasor diagrams don't support his argument is irrelevant.

Jim

Seems that you know more than me. Where can I get more infos especially about the performance of various fm demodulator techniques? Preferable on the Net.

- Henry

"Gandalf" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

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