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RE: [linrad] AM and FM
This kind of instantaneous attack on FM would be a pretty severe
I wish to comment on Leif's AM detector page.
I am not sure since I have yet to look at the code, but
it appears that Leif is saying he uses a narrow filter
at the carrier to extract the carrier for basebanding
before the final detection is done. A thing one could
do instead of a PLL is to take a high-Q resonator (IIR
filter) which has three terms, at the carrier.
This works well on good signals quite well if the follow on
processing takes advantage of the independence of the
sidebands. However, all of us have heard a selective
fade take the carrier and one of the sidebands out for
seconds. A 2nd order PLL based carrier recovery algorithm
with a fairly narrow loop filter will "flywheel" for
many seconds during a severe fade and it will have to be
a deep fade indeed for the 2nd order PLL to lose lock on
the "pure tone" that is the carrier. Another thing
PLL's can easily do is gear shift the gain or loop
bandwidth depending on the lock condition. This means
that in a lock condition, one can REALLY narrow the
bandwidth (down 1 Hz if you like) and this will take
flywheel through a really nasty fade.
In the best coherent detector, one then makes an SNR
estimation of each sideband and then votes in a weighted
way, which sideband to believe. If both are high SNR,
pick your favorite, if both are a bit ratty, then take
an average. If one fades, take the other, etc.
This last procedure is where the really good coherent
detectors gain a significant advantage. This is the
implementation in the software I have written for the
SDR-1000. I also do PLL for the FM detector since I
like the threshold depression that the PLL gives. It
is worth the extra complexity.
[mailto:owner-linrad@xxxxxxxxxxxxxxxxxxxxxx]On Behalf Of
Sent: Friday, January 16, 2004 3:02 AM
Subject: Re: [linrad] AM and FM
It will be interesting to see if you make a noise blanker that
works on FM. The Ham rigs fail badly at that.
One idea comes to mind, perhaps down the road on FM.
When you expect a 5Khz deviation FM signal, you could set
the maximum IF bandwidth properly as an initial setting.
The dynamic bandwidth of the signal would then be measured
and the IF filter bandwidth would be changed quickly to match
the instantanious deviation level. The result should be an improvment
in S/N especially at times that the transmitter is not fully modulated.
This would feature would be much easier in software then in hardware.
A noise operated squelch would then have to operate with a fixed IF
bandwidth on a separate detector.
I found your text on AM. It is interesting reading.
73, Jim Shaffer, WB9UWA.
On 14 Jan 2004 at 20:46, Leif Åsbrink wrote:
> Hi all,
> Linrad01-11 is now uploaded.
> This version has detectors for AM an FM.
> Coherent processing is an advantage in AM and I
> have written a page about that.
> The FM mode is just a conventional FM detector,
> enhancements are possible, but I have not yet any
> suitable (marginal) FM signals to play with....
> Leif / SM5BSZ