[CQ-Contest] receiver evaluations

al_lorona at agilent.com al_lorona at agilent.com
Wed Mar 19 11:39:07 EST 2003

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.


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?


-- Eric K3NA

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