[Amps] Duty cycle for processed SSB in contest conditions?

peter chadwick g8on at fsmail.net
Sun Feb 23 02:00:02 EST 2014

Roger said:

>Only recently have SS rigs began to approach the IM figures of the old 
Collins S-Line.  Even some of the mighty expensive rigs have pretty 
crappy IM figures.<  

SM5BSZ has done some work on this, and let me use some of his data for my presentation at the RSGB Convention last year - which I called 'Spreading the Sewage' as 'Slinging the s**t' wouldn't have been acceptable! In it, I used an analysis of published equipment reviews back to 1972 (123 transmitters) to look at how the IMD performance of transmitters has degraded with the move to solid state. What is especially bad is the big increase in high order IMD, in some cases up to the 11th order. The rigs that were really clean were those Yaesu rigs where the PA could operate in a heat producing, lower power, very low IMD, Class A!

>They have receivers that have passed the point of 
practical numbers for intercept points, dynamic range, selectivity, and 
have a sensitivity that is far, far below the band noise, yet they seem 
to be ignoring the transmitted signal which is one of my pet peeves.<

I am somewhat amazed that the manufacturers appear not to realise that the intermodulation limited dynamic range and the phase noise limited dynamic range need to be comparable, although the phase noise limited one can arguably be required to be better by about 10dB. This is because the phase noise moves linearly with signal level, dB for dB, while IMD drops faster with reduction in signal level - often not the theoretical 3dB/dB  but usually more than 2dB/dB. The analysis I did that were published in QEX and NCJ some years apart (sunspot cycle peak and trough) showed that in the UK at least, in a situation with noise meeting ITU 'rural' levels, about 95 to 100dB of DR (phase noise or IMD) was all that was needed. I did a paper on this subject at RF Expo in Anaheim in 1986 (Phase Noise Intermodulation and Dynamic Range in Receivers), so it's hardly new. There was a ham radio reception at that RF Expo with door prizes and I was a lucky guy - a nice new Bird 43 with 1kW HF slug came my way!

So, Roger, you aren't a lone voice crying in the wilderness, but the number of voices is very low.....


Peter G3RZP

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