[CQ-Contest] receiver evaluations

[al_lorona at agilent.com]

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 at 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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