Pete KE9OA wrote:

The only way that this should happen is if either the receiver is very
conservatively rated (do we know exactly what method was use to make this
measurement?), or if the measurements were incorrectly done. I am not sure
that real world performance would reflect those measurements, unless you are
in an area where there are several strong signals that are only 5kHz apart.
When I am measuring the overload point on the receiver that I am developing,
it is very easy to drive the system into overload with a signal generator,
yet with a 100 foot longwire in the presence of three 50kW MW broadcasters,
no overload is present.
I think that specs do tell the story, if the measurement system is properly
set up.
As an example, on one project, I needed to make some desense measurements
from 5kHz to several hundred MHz away from the desired signal. The desired
signal level was -140dBm. Using an HP8657 or an 8640B, the broadband noise
from these two units was so high, even a 300MHz away from the desired
signal, that I had to run the generators through a K&L tunable filter. The
only generator that was slightly usable was an HP8642B. This is the one that
uses the Modulated Fractional Divider, with the Sigma-Delta modulation.
In reference to you statement about the receiver working better than its
rated specs, I just don't think so, unless as I said earlier, the
measurements were done incorrectly. The only way to really to a close-in IP3
measurement is to run the interfering signal through a very selective, deep
skirted crystal filter. You need the interfering signal to have almost
non-existant close-in phase noise; otherwise, the measurement is
meaningless.

Pete,

As you know the receiver testing for 'Passport' is done by Sherwood
Engineering. Do you know anything about the equipment they use and how
the test specs are generated?


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