ARRL

AB4OJ

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Two Tone 3rd Order IMD Testing W1RFIAdmin on 2/9/11
The ITU-R have made many of their Recommendations freely downloadable from their website, so here is the link to SM.1837:

http://www.itu.int/dms_pubrec/itu-r/rec/sm/R-REC-SM.1837-0-200712-I!!PDF-E.pdf

Section 3, 3rd paragraph (p. 6) calls for a "realistic choice" of RBW:
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The IF filter bandwidth or resolution bandwidth (RBW) for the measurement can be chosen by the manufacturer but must be a realistic choice for the type of receiver and intended application. The chosen bandwidth (BW) for the frequency range 9 kHz to 30 MHz must be ≤ 5 kHz and for the frequency range 20 MHz to 3 000 MHz it must be ≤ 30 kHz.
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However, Section 2, penultimate paragraph (p. 5) speaks to the case where phase noise is dominant:
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Phase noise

If the levels of the IM products at f3 and f4 are near the noise floor or phase noise skirts, the measured levels will represent the IM product level plus noise. The actual IM product level can be obtained by subtracting out the noise contribution.

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This appears to validate the ARRL's current DR3 test method (at 3 Hz RBW) within the narrow context of the test engineer wishing to determine DR3 and thence IP3, with a view to comparing the usable sensitivity attributable to IMD3 products with that traceable to phase noise (reciprocal mixing noise, RM). One could say that the ARRL test method would be more universally applicable if the DR3 test results were placed side-by-side with those of the RM test, with a clear statement as to which of the two degrading factors, IMD3 or phase noise, is dominant at each spacing tested. The RM data should be presented as ranking equally in importance to the DR3 data.

I believe that one could conceive of presenting the two sets of data -
DR3 or IMD-free dynamic range measured at 3 Hz RBW, and reciprocal mixing noise - side-by-side in a table, with a clear and unequivocal statement as to which of the two parameters determines the receiver's usable sensitivity.
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Example: IC-7410. 14.1 MHz, 500 Hz CW, 3 kHz roofing filter.

2 kHz spacing/offset, DR3* = 88 dB, RM = 78 dB, RM dominant

5 kHz spacing/offset, DR3* = 98 dB, RM = 88 dB, RM dominant

20 kHz spacing/offset, DR3* = 103 dB, RM = 101 dB, RM dominant

* DR3 is measured at 3 Hz resolution bandwidth (RBW) to subtract out the noise contribution.

It will be seen from the above comparison that for the frequency spacings tested, synthesiser phase noise is the dominant factor in determining the receiver's usable sensitivity. To avoid confusion on the part of the reader/radio shopper, it will be necessary to present and explain this data comparison very clearly. This will also increase the incentive to the OEM's to improve the phase-noise performance of their synthesisers.

73, Adam Farson VA7OJ/AB4OJ

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