[TenTec] Roofing Filters (LONG)

[Bill Tippett]

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