> The context that I was using is that nonlinear front ends cause a lot of
> low level mixing products in the pass band in the presence of multiple
> large signals in the front end.
With a front end problem, the non-linear artifact is almost always easily
identifiable spurious signals. All sorts of odd signals (like mixes of BC
stations) are present that "aren't really there" when listening on a better
receiver. I suppose there are rare cases where an overloaded receiver might
generate something that sounds like noise, but signal levels would have to
be phenominal...like multiple local extermely strong transmitters.
The common effect of poor IM3 dynamic range in CW pileups are musical bleeps
and bloops that sound like real CW signals with sloppy sending. This is best
described as like listening to the Novice bands of the 60's and 70's with
the receiver BFO turned off. All sorts of uncipherable CW signals with
otherwise good tone are heard, they sound exactly like normal CW signals
otherwise.
Drake R4C's under SN 18000 or so had a very bad case of this, probably the
worse in history of modern receivers. The most problematic Drake (with FET
second mixer) had the narrowest front end used in any commercially
manufactured receiver, and was one of the worse ever for overload problems.
> I agree that the noise we all hear should be external to the receiver.
> But in many cases it is not. A prime example is a heavy DX pile up.
> When many people are calling the band noise appears to rise.
Most composite noises I have tracked down actually comes from low-level
transmitter stages, not synthesizers. I spent some time looking at various
rigs because I like to duplex (transmit while receiving) even with close
splits.
Some rigs have heavy composite transmitter noise. In the FT1000D, the noise
is mostly from a stage back in the early transmitter IF system. The same is
true for a few Ten Tecs I have tested.
On occasion I've seen dominant synthesizer noises. The rare FT1000MP can
have broad rough hissing noise when transmitting, occasionally with many
buzzy spurious signals. (Someone posted something a few years ago on this
reflector about a cure.) Pileups generally wind up with at least a few nasty
transmitters causing problems, but I've never heard it as "noise" There
commonly are clicks, occasional hissing rigs, and spurious thumps and bumps
from poor VCO switching (most commonly from 775DSP's).
> partially due to nonlinearities in the transmitters outside the
> receiver, but is also partly due to nonlinearities in the receiver front
> end including phase noise.
CW transmitters can be as non-linear as we like, as long as rise and fall
times are good. Non-linearity aggrivates key clicks, not noise.
Non-linearity can actually reduce AM noise!
> the line receivers such as Ten-Tec and K2 as a lack of gain even though
> the weak signals pop up with
> a better signal to noise ratio than many other receivers.
The K2's and TT's I have looked at actually did have "low gain" compared to
other receivers. While that is prefectly OK in noisy locations or when using
transmitter antennas for receiving, it makes them poor performers in quiet
locations (especially those with very directive receiving antennas). One odd
thing with the only K2 I tested was even though gain increased with the
internal preamp, noise figure did not improve. I think I measured around
10dB or more NF, about what the mixer and post mixer amp should do, and it
didn't change significantly with the preamp on. That meant K2 IM3 DR was
significantly worse with the internal preamp on.
There is a human effect that, if the signal is over a certain S/N threshold,
attenuation (less gain) appears to make a signal "stand out" more. This only
works with signals that are already above noise a reasonable amount. It
gives a false operator impression S/N has increased, because the removal of
AGC action reduces noise fill between signal-on periods. Some of us don't do
as well at ignoring noise when reading weak signals, so less gain (and the
resulting reduction in AGC) helps.
> By the way, a receiver front end must not only be super linear for the
> power range of the desired signal but must be able to maintain that
> linearity for all of the power presented by all other (large) signals in
> the front end pass band.
That's a good point. It is the peak power of all the signals present that
pushes a receiver towards overload. That's why contests are a good test.
Many receivers that roll-along perfectly fine on SSB or CW moderate strength
nearly clear channel operation are horrible in contests or for weak signal
work.
I think what we will see is operators who want to reach down into the noise
and dig the weakest possible signal out with the best possible copy will
disagree with the choice of operators who like a receiver to "sound
quieter". Some operators will disagree with other operators choices.
If a receiver of the same bandwidth as another sounds quieter with noise, it
almost certainly isn't going to make copy better on noise floor signals.
Unless you have the very rare grossly overloaded or saturated receiver,
noise is all external to the receiver. It is all inside the filter passband
of the narrowest filter in the system. Birdies, IM, and spurious *signals*
(which do not sound like noise) are different, of course.
My opinion is a receiver has to let me copy weak signals very well. Then it
has to not overload, and generate phantom signals. All other things are far
down the list.
73 Tom
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