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Re: [Discuss-gnuradio] Quadrature demodulator update


From: Dave Emery
Subject: Re: [Discuss-gnuradio] Quadrature demodulator update
Date: Wed, 12 Feb 2003 02:29:07 -0500
User-agent: Mutt/1.4i

On Tue, Feb 11, 2003 at 03:30:36PM -0800, John Turner wrote:
> Hi Philip,
> 
> 
> Depending on the input signal level and the gain of the unit the amplifiers
> could still saturate or generate some undesired intermodulation products. In
> the AOR information they quote a high level signal of 5uV, this is most
> likely for a narrowband signal not broadcast FM, i.e. 20uV (-80dBm) is a
> typical limit for stereo reception with 50dB quieting, and you may find that
> typical signals are 20 to 30dB above this limit. As an example, we are
> located quite close to some local FM transmitters and experience levels
> of -47dBm, these levels are sufficient to casue overloading of commercial FM
> radios.

        My experiance with ICOM radios is that the 10.7 mhz IF output
is located ahead of most of the AGC action of the radio so it maybe
sees 20 to 30 db of AGC action at the most when the radio is tuned to
a strong signal.   And indeed gain to the 10.7 mhz output is not
huge - maybe 20-25 db or so - it is not a constant level AGC controlled
high power output as are the more traditional IF outputs of many
other receivers.

        This does indeed have implications for the dynamic range of
quadrature downconverters and A/Ds.   Given that that dynamic range
of signals of interest is from maybe -125 dbm to perhaps as high as
-25 dbm (sitting under the antennas) - 100 db or more - one does
have to think about either using some kind of software controlled
AGC (gain controlled by a DAC), hardware AGC, or careful tweeking
to make the hardware work over the required dynamic range.

        RF modem systems I am familiar with actually adjust gain ahead
of the A/D by looking at the data coming out.   This, however works
better if one is digitizing only the signal of interest and not
a whole band of stuff with carriers coming and going unpredictably
including perhaps some strong nearby ones that are much more powerful
than the desired signal.

        

-- 
        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18





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