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Re: Converter gain and noise..



The main problem with "high noise levels" or more appropriately
high levels of gain is exceeding the IP3 levels of all of the stages of a 
receiving system.

This means that the amps, mixers, etc. in the rig are meant to operate 
linearlly at certain power levels, and beyond that they lose the linearity.  
When this happens, mixers become "sloppy", amps become mixers and harmonic 
multipliers, etc.

This is a problem, for example, if another signal somewhere is strong enough 
to mix with this mess, and degrade what your trying to listen to.  (This is 
an issue as well with phase noise of a LO as well, and that is why there is 
a certain minimum level that is acceptable per unit bandwidth.. if there is 
a strong signal on the channel next to you, and the phase noise is broad, 
you'll pick up part of the energy in the adjacent channel...)

Attenuation will help.  The only situation where is won't is if the
front end of the converter saturates, then the only real solution is
to either use a "power device" with a higher IP3 spec, or accept
front end attenuation which will increase "noise figure".

Many commercial (and I mean non-ham, not Kenwood, Yaesu, etc) rigs have 
lousy noise figure when compared to ham rigs.  This is usually because of 
the trade-off when running co-located transmitters/receivers-- the higher 
IP3 specs are more important than the extra sensitivity.

A viable ham alternative is a high quality, silver plated, low loss
cavity filter in front of a converter-- at least out of band sigs
won't be as much of a factor....  But then a minimum NF is a few dB instead 
of 0.3 or whatever.  Nice thing is at microwaves, a little larger dish will 
more than make up for that.


Fred W0FMS


>From: Douglas Braun NA1DB <doug@dougbraun.com>
>To: Estes Wayne-W10191 <W10191@motorola.com>, amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] Converter gain and noise..
>Date: Fri, 27 Apr 2001 14:35:47 -0700
>
>At 09:44 AM 4/27/01 , you wrote:
>
> >Wayne replies:
> >
> >An extremely high noise level effectively widens your IF bandwidth, 
>allowing more noise and interference to get through relative to the desired 
>signal.  For best signal-to-noise ratio, the noise level should be very low 
>so that the desired signal is at an amplitude where the IF filter has its 
>narrowest bandwidth.  Perhaps this is less of a problem with radios that 
>use brick-wall IF DSP filtering.  But surely even those radios have crystal 
>or ceramic filters in the first IF.
>
>
>But wouldn't the receiver's AGC take care of this?  If the signal
>is S9+20, isn't the AGC is reducing its amplitude a lot, and the IF
>is actually seeing roughly the same signal level it would on a S1 signal?
>
>Doug
>NA1DB
>
>----
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