I believe I understand the low level IMD values not matching the line.
Looking at the Sherwood table, the K2 2kHz DR is "Phase Noise" limited.
Makes sense that phase noise would muck with the low values. Within
Ron's presentations it is mentioned that ARRL tried to solve the problem
with very narrow filters to remove the phase noise.
The 3rd order line slope is defined as two! Seams to me, the simple
approach is to find the line, as was done in the plot (notice the line
equation has "2*x"), and simply extend the line to the noise floor.
If the page is reloaded, there is a new "S3" line in the K2 plot to
demonstrate where the phase noise is an issue, about 12 dB below S3, or
S1 signals. Don't know about others, but the noise floor of my antennas
is well above S1, closer to S3 on a good day.
John KN5L
On 10/29/2018 08:38 AM, John Oppenheimer wrote:
> While evaluating a receiver design, a K2 Narrow Spaced Dynamic Range
> measurement was performed to validate the test bench. The K2 plot is on
> the page:
https://www.kn5l.net/newReceiverProject/plot-IMD/>
> If the two line equations are used, the Noise floor is -138 dBm and 2kHz
> dynamic range is 87 dB. These are close to the Sherwood values of -136
> and 80. The K2 settings are my normal operation IF/AF BW settings, which
> is a little narrower than the Sherwood measurement parameters.
>
> The question is the low level IMD measurement points do not follow the
> line equation. Is this expected?
>
> Guess I need to order a $60 Mini-Circuits ZFSC-2-6+
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