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From: HardySpicer on 18 May 2010 02:03
Used in FM before demodulation. Do they actually do any filtering
though apart from removing amplitude variations?
What I have seen is that for low SNRs (carrier to noise ratios) you
are better off without it! Many people have told me that too. For high
SNRs they are fine ie if you are connected to the ariel.


Hardy
From: Tim Wescott on 18 May 2010 02:30
HardySpicer wrote:
> Used in FM before demodulation. Do they actually do any filtering
> though apart from removing amplitude variations?
> What I have seen is that for low SNRs (carrier to noise ratios) you
> are better off without it! Many people have told me that too. For high
> SNRs they are fine ie if you are connected to the ariel.

Ideally a hard limiter does no filtering.

Yes, they make things worse at low SNR -- at least in a NBFM system; I
don't know if they'd make much difference in a WBFM system.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Vladimir Vassilevsky on 18 May 2010 08:43


HardySpicer wrote:

> Used in FM before demodulation. Do they actually do any filtering
> though apart from removing amplitude variations?
> What I have seen is that for low SNRs (carrier to noise ratios) you
> are better off without it! Many people have told me that too. For high
> SNRs they are fine ie if you are connected to the ariel.

Cretin. This is what the recent tread of phase-amplitude detectors was
about. You didn't bother to understand a thing, although you stick your
stupid noze into it.

VLV

From: Jerry Avins on 18 May 2010 12:42
On 5/18/2010 2:30 AM, Tim Wescott wrote:
> HardySpicer wrote:
>> Used in FM before demodulation. Do they actually do any filtering
>> though apart from removing amplitude variations?
>> What I have seen is that for low SNRs (carrier to noise ratios) you
>> are better off without it! Many people have told me that too. For high
>> SNRs they are fine ie if you are connected to the ariel.
>
> Ideally a hard limiter does no filtering.

Well, they "filter out" AM. As implemented, they add to the selectivity
of the IF, being coupled through selective interstage transformers. In
the good old days, quality FM receivers had two limiters following one
or two AGC'd IF amplifiers. The signal chain then had four or five tuned
transformers providing twice that number of core slugs to twiddle. That
made it easy to completely screw up the IF response or, if you
understood the theory, to stagger tune for a broad, flat response. You
could trade away bandwidth for gain if it was needed.

Ratio detectors did away with the need to limit. In general,
ratio-detector receivers performed more poorly than the
limiter-discriminator combination. That turned out to be due more to
transformer count than anything else.

> Yes, they make things worse at low SNR -- at least in a NBFM system; I
> don't know if they'd make much difference in a WBFM system.

Most WBFM systems want at least 20 dB suppression of noise and 30 is
better. That puts their operation far enough above threshold for such a
difference to rarely matter.

Jerry
--
"I view the progress of science as ... the slow erosion of the tendency
to dichotomize." --Barbara Smuts, U. Mich.
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From: Tim Wescott on 18 May 2010 13:04
Jerry Avins wrote:
> On 5/18/2010 2:30 AM, Tim Wescott wrote:
>> HardySpicer wrote:
>>> Used in FM before demodulation. Do they actually do any filtering
>>> though apart from removing amplitude variations?
>>> What I have seen is that for low SNRs (carrier to noise ratios) you
>>> are better off without it! Many people have told me that too. For high
>>> SNRs they are fine ie if you are connected to the ariel.
>>
>> Ideally a hard limiter does no filtering.
>
> Well, they "filter out" AM. As implemented, they add to the selectivity

I was going to say "is memoryless", but I didn't want to confuse things.

Gee, thanks.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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