Yes, but the analog AGC is significantly helped by the roofing filter since
far fewer signals are in the analog AGC passband. So the Orion roofing
filter helps tremendously with overload by narrowing the analog AGC
passband.
Carl Moreschi N4PY
121 Little Bell Drive
Bell Mountain
Hays, NC 28635
----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <tentec@contesting.com>
Sent: Monday, May 29, 2006 9:52 PM
Subject: [TenTec] Roofing Filters (LONG)
>
>
> Hi Carl,
>
> >I don't think that definition works for a single conversion receiver
that
> has just one filter in the IF. This filter would certainly not be called
a
> roofing filter. The purpose of the roofing filters in the Orion's are to
> keep the DSP stage from overloading on signals that are not in the DSP
> passband. A roofing filter always has another stage of filtering in the
AGC
> loop following it.
>
> I'm not sure it makes any difference...you are
> limiting IMD products passed to both Orion's 2nd IF at
> 455 kHz and 3rd IF (DSP at 14 kHz). Regarding direct
> conversion, I was just listening to a recording of Gerald
> Youngblood (Flex-Radio) refer to the "25 Hz roofing filter"
> in his direct conversion SDR.
>
> As far as overload, I believe you might be thinking of
> the function of analog AGC in Orion, which limits inputs
> to the DSP's ADC to S9+30.
>
> http://www.doug-smith.net/orion.htm
>
> "ADC overload can never be allowed to occur because when that
> happens, signals are irrevocably corrupted. Signals that are larger
> than the full-scale range of the ADC must force reduction of gain in
> the analog section of the receiver. So both analog and digital AGCs
> are used in the Orion. Resort to analog AGC only need be made when
> signals inside the roofing bandwidth exceed about S-9 plus 30 dB. "
>
> 73, Bill W4ZV
>
>
>
>
>
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