From: a-kevinp@microsoft.COM (Kevin Purcell, Rho)
Newsgroups: rec.radio.amateur.misc
Subject: Is the ICOMR1 a good receiver
Date: 20 Dec 92 02:00:10 GMT
Organization: UCSD Usenet Gateway
NNTP-Posting-Host: ucsd.edu
Originator: daemon@ucsd.edu

Gary requested a justification for the value of an attenuator. And yes, 
LA was the sort of ugly RF environment I had in mind.

The reason an attenuator improves the signal is that the 
intermodulation products grow as the cube of the input power of the 
"wanted" signals.

If you think in terms of decibel power scale the wanted signal output 
is linear with the wanted signal input. For example, the output power 
is input power plus the gain of the amplifier: -10dBm into a 12dB 
amplifier gives 2dBm out. The third order (normally the ones of most 
interest) intermodulation products grow at a 3 times rate on these 
log-log scales. This is a result of how the distortion products are 
generated. See ARRL handbook or other similar texts for more info.

If you plot the power out versus the power in the slope of the 
undistorted signal is one and that of the intermodulation products has 
a slope of three. Eventually these two lines intersect. This is the 
third order intercept point. The (input or output) power that 
corresponds to this interception (the point at which the 
intermodulation products and the wanted signal have the same power) is 
a "figure of merit" for the amplifier (or mixer or whatever). The 
larger this number the more power you have to put into it to get it to 
produce distortion products of a given level.

So back to attenuators. Why does putting an attenuator improve 
reception when you have problems with distortion products. Because 
although it reduces the strength of the wanted signal by N dB, it 
reduces the distortion products by 3N dB. Giving an improvement of 2N 
dB in signal to distortion products ratio.

The dynamic range (DR) of the receiver stays the same, but you have 
reduced its minimum discernible signal (MDS) by N dB. Or you can say 
you've raised its noise figure (NF) by N dB. By doing this you have 
made it better able to cope with large signals.

What is the optimum value of attenuation? That depends on the MDS, DR 
and IP of your receiver and how large the signals are in the operating 
environment and what level of distortion products you are willing to 
tolerate. Try values from 6dB to 36dB for a reasonable range. In fact a 
switchable attenuator box with 3 or 6dB steps can be useful for this 
application.

Of course if you're trying to copy weak signals in the presence of loud 
signals the adding the attenuator may push the wanted signal below the 
noise floor. In which case you need a better receiver front end!

Another way to improve reception is to add some form of preselector 
filter to filter out strong out of band signals. A series of helical 
resonators can make for a narrow front end but one with good 
performance with respect to out of band signals.

This sort of effect can be very important on the HF bands too. A useful 
way to set you input attenuator (or variable RF gain control) is such 
that you can just hear the band noise. This sets you MDS (of the NF) to 
be that of the band, so you are not throwing away any of your DR with 
needless sensitivity, and you can better cope with large signals.

73 de Kevin Purcell N7WIM/G8UDP (Seattle, WA)          a-kevinp@microsoft.com
Do your interests match mine? If so call me. I'd like to hear from you.
QRP, Packet, TCP/IP, Digital voice, Macintosh, Spread Spectrum, Homebrew,
Satellites, Antenna design, Propagation, VHF/UHF weak signal, Microwave, QEX