Tom says:
>You guys should see how difficult it is to correct an error
>in a review, and your tune would change!
I've done a fair number of reviews myself. One item I went to some trouble doing
was internal spurs in receivers - which are rarely mentioned. Trouble is,
measurement techniques differ - even in professional Type Approval labs. Times
were that a duty free bottle of Scotch got them to measure your way! Having also
been a national society President, I also know how reliant society magazines are
on advertising, and the delicate line that has to be drawn - and I lost the
argument a couple of times. The penalties of being an engineer - makes me feel
like Dilbert!
>Receivers are pretty poor for close-spaced dynamic range.
> DSP based radios are generally the worse of the lot!
I worked out that you need about 24 bits worth of ADC dynamic range for a good
HF receiver. You could use 27, but practical phase noise limits mean about 24 is
all that's currently useable. A major intermodulation contribution close in is
often crystal filters - e.g. the old KVG XF9B is about +23dBm input intercept at
5 and 10kHz off centre. Two Motorola guys called Malinowski and Smythe brought
that up in a paper at the Frequency Control Symposium in 1973. LF filters are
better, because the crystal is thicker; therefor the stress is less for any
given input, and the departure from Hookes' law less. Crystal filters are not
always reciprocal linear network that we like to think - quite often, the IMD
is better if you turn the filter round (no, Jeeves, that doesn't give the
opposite sideband out!).
And of course, the IMD limited dynamic range should be matched by equally good
phase noise limited dynamic range, or you're wasting your time anyway.
73
Peter G3RZP
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