Yo! Bill:
Monday September 09 1996 14:59, Bill Funk wrote to Bill Cheek:
 BF>>> Neat, for sure. I just do it another way. Although, to be honest,
 BF>>> I've never found a birdie on the R-7100. It's neat to hook it up to
 BF>>> Grove's spectrum analyzer, and watch internal signals march up and
 BF>>> down when searching. But the radio never stops on them. *Yet*
 BC>> Does the radio hit them?  Why won't it stop on 'em?  And what happens
 BC>> if you manually stop on one?
 BF> I can (and have) tried to get the radio to stop on the travelling
 BF> signals, but it won't, so far. The output to the Grove SDU 100 is out
 BF> of the IF. I don't know, and can't read the schematic well enough to
 BF> tell (and it sure isn't mentioned in the service manual!), but there
 BF> may be a discriminator of some sort that will allow the radio to tell
 BF> the spurious signals from the real ones. Using the tuning wheel,
 BF> tuning to one of the internal signals, the S-meter won't move, I get
 BF> nothing with the squelch turned all the way down except the normal
 BF> hash, the radio just doesn't seem to know the signal's there.
Ahhhhhh....... I think I know what's going on there.  The better radios, 
PRO-2004/5/6 included, and no doubt, Icom, employ effective little circuits 
called 'traps' to block known pseudo-signals.  You could call these "notch 
filters", I suppose......but 'traps' are the correct term.
The thing is, those flakey signals do exist up there in the early IF stages 
where the spectrum display is connected.  But down the IF strip a ways are 
the traps, so that in effect, the detector/discriminator and metering 
circuits simply do not see these signals.
 BF> But, I can't see any reason for it to be skipped, otherwise. I try
 BF> changing modes,and the result is the same.) Like I said, I'm no tech,
 BF> so I may not be describing this right.
Well, you described what you see..... and now I understand.  If it weren't 
for the 'traps' downstream, you WOULD see and hear those birdies or whatever 
the hell they are.  Actually, they're more than birdies.  The strongest and 
most predictable ones are simply trapped out.  The IF and mixer scheme of the 
radio actually allows REAL signals of the same frequency to pass right on 
through, quite unawares of the trapped interferant.
Bill Cheek | Internet: bcheek@cts.com | Compu$erve: 74107,1176
Windows 95 Juggernaut Team | Microsoft MVP
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