From: Rob & Terri Sherwood <rob@sherweng.com>
To: <topband@contesting.com>
Date: 2001/08/17 Fri PM 10:37:21 GMT+00:00
To: crg8ibq@rmi.net, topband@contesting.com
Subject: Re: [Fwd: Topband: Collins 75A4]
REPLY from Rob Sherwood
I have never tested a 75A4. If I had, it would be on my web site.
(www.sherweng.com or
www.sherwood-engineering.com. Either URL goes to the same web site.)
The noise floor sounds believable if you compare it to the S-Lines on my page.
I don't quote IP3, though one can calculate it from the data on the site.
Third order intercept goes up
and down when you switch on a preamp or a pad, so I just stick with dynamic
range. That is basically
constant, assuming the preamp is designed correctly.
I would expect that a 2 kHz reading could be done, since it can with an R-390A
or an S-Line, especially
with a 500 Hz filter.
I only measure 1 dB blocking at 100 kHz due to phase noise on modern radios.
Most modern up-conversion
radios do block at lower levels inside the first IF window, but I don't measure
that because phase noise
often predominates. The really bad ones block noticeably in the 5 to 10 kHz
range. While slowing tuning
away from your test signal, you notice the background noise or phase noise drop
way down, the come back up
as your test signal goes outside the up conversion filter. Dead giveaway of
window blocking.
An AGC threshold of 1.5 uV would be in line with other Collins, but one also
has to quote the number of dB
drop at that level. Radios like the Drake have a very stiff AGC, and are
usually measured at a 3 dB drop
in audio level referenced 5 mV rms (about S9 +40). But most Collins have a
less stiff AGC by design, and
drop about 12 dB before the AGC quits working. Neither design is "right", just
different.
Some military radios like the Racal 6790GM have so much IF gain that the AGC
threshold is a tiny fraction
of a uV. When a signal drops down into the noise, the room tends to fill with
band noise as loud as the
signal was. I don't really like that, but it does keep you from having to ride
the audio gain under weak
signal conditions. This can be a dramatic difference when comparing the Racal
to an Icom 781 with the
preamp off or the R-9000. What do I prefer? If there is no switchable preamp,
then about 0.7 uV is good.
If it has a preamp, then about 1.5 with it off and about 0.5 with it on is
nice. Note: this is only my
preference, not a statement of what is "right".
73, Rob Sherwood
NC0B
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