On 2/23/2014 2:00 AM, peter chadwick wrote:
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
Thanks Peter.
What amazes me is the number of hams who purchase these top end rigs,
then complain that they are nowhere as good a claimed/advertized. One
ham complained bitterly that he was trying to copy an S2 or 3 signal and
was swamped by a station about 2 or 3 KHz above him. The station was 20,
or 30 over and if you do the figures, he still had at least an S8 signal
"in his receiver's pass band". I don't think we ever convinced him that
his receiver was not at fault Get one of those many rigs with less than
30 db IM3 and high 5th and 7th order IM and they can swamp an S3 or 4
signal for quite a ways.
Take one of these rigs and compress the crap out of it and add a bit of
clipping...no, they compress the crap into the signal,(maybe with a bit
of clipping) and then run it through an over rated amp that is over
driven that adds even more IM products to a signal already loaded with them.
Really annoying are the rigs that use the ALC to set the output power.
At reduced power they need a signal fed back to limit the output. That
generally results in a leading edge spike which really over drives a
high gain Tetrode. Mis-tuned Tetrodes that are over driven plus a
leading edge spike. No wonder Tetrodes get a bad rap!
73,
Roger (K8RI)
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 sl
ug came my way!
So, Roger, you aren't a lone voice crying in the wilderness, but the number of
voices is very low.....
73
Peter G3RZP
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|