One more thought...
The NPR test applies best when the interfering signals are on all the time, all
at the same time. This would be the case in, for example, a cable TV system, or
a satellite downlink system. That's what the notched noise is trying to
replicate.
In a ham receiver, the interfering signals aren't usually on 100% of the time
(in other words they have a duty cycle less than 100%), and they aren't usually
packed solid across a band-- although a crowded contest is the closest we come
to that condition. Therefore, the NPR test would really be a
much-worse-than-worst-case test, and we would probably be appalled by the
measurement results of our receivers.
The NPR signal really re-creates the conditions that we would have if every ham
in the world picked a different frequency on 20 meters (except for 14.025 MHz)
and then went key down for 60 seconds while you tried to listen for a weak
signal on 14.025 MHz. Can you imagine? Not very realistic.
Regards,
Al W6LX
-----Original Message-----
From: Eric Scace K3NA [mailto:eric@k3na.org]
Sent: Tuesday, March 18, 2003 1:06 PM
To: Cq-Contest
Subject: [CQ-Contest] receiver evaluations
For some time now, ARRL and others have included in their collection of
receiver performance test the blocking dynamic range and
two-tone 3rd-order IMD, using two signals with some spacing such as 5 or 20
kHz. Of course, two strong signals doesn't emulate much
of the real world.
I recall a variation of this test that was used by AT&T to evaluate
performance of multi-channel radio receivers used to carry
large quantities of telephone channels. Naturally, one did not want a strong
signal in one telephone channel to contaminate the
signals being carried in other channels on the route. The test was performed
as follows:
-- instead of two signals being applied to the receiver under test, a
broadband noise was applied. The noise was modified by
notching out the bandwidth for one channel; i.e., essentially no noise in the
notched channel.
-- measurements were made in the channel corresponding to the notch.
-- noise power to the receiver was increased until the point at which the
measured channel started to exhibit degradation (e.g.,
increase in the noise floor).
This seems to be a more general test that corresponds more closely to what a
contest receiver experiences on a crowded band;
i.e., LOTS of signals attacking the receiver across the band.
Could those who are knowledgeable about receiver evaluation methodology
comment as to whether such a test would be more likely to
accurately characterize the ability of a receiver to hold up against strong
signals outside of the operating passband than the
two-signal test method?
Thanks.
-- Eric K3NA
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