[CQ-Contest] RE: Receiver tests

[George Cutsogeorge]

Eric and the list,

The testing you have described is called "Noise Power Testing" and the
figure of merit is the NP Ratio or NPR.  This type of testing is many
decades old and has been used in the communications industry for testing
multiple channel equipment, including radio receivers.  Basically, the band
of interest is filled with noise while one channel is notched out, or free
of noise.  The radio is tuned to the clear channel and the noise power is
then increased until the noise floor of the radio is degraded.  Some simple
arithmetic then calculates the figure of merit.

In the early 70s I was involved with the design and construction of low
power, channelized SSB receivers.  One of the specifications which my
customer required was a certain NPR.  I obtained the equipment and performed
the test many times as each receiver design progressed.  The NPR test is an
excellent way to optimize such parameters as the mixer and product detector
drive levels and waveforms.  Also, these receivers used 3rd order PLLs for
the LO injection and the loop parameters could be optimized by watching the
NPR as break points were moved.

A couple of years ago I was able to purchase a noise generator designed for
NPR testing on eBay.  They are made by the Marconi Company and can be picked
up for $200 or so.  HP also makes equipment, but it's rather expensive.  The
band stop filters used for notching out the noise are much too wide for our
purposes, but it's a simple matter to build a quartz narrow band stop filter
with crystals.

I have done this and have tested a few radios.  The big problem here is that
it's not easy to relate NPR to 3rd order IMD tests.  Maybe some
mathematician on here could work that out.  I'd be happy to give him the
references needed to do the math.  There are books and articles. etc.

One day when there is not so much to do, I'll continue this study of radio
NPR measurements.

George, W2VJN