Tom,
Thanks for the tutorial that helps to clarify the various sources of
noise and interference. I absolutely agree with your conclusion that
the goal is to be able to copy weak signals without artifacts caused by
overload.
I believe that I was guilty of sloppily using the term noise to refer to
all unwanted stuff showing up in my audio while trying to listen to weak
signals instead of correctly separating antenna noise, external spurious
noise, receiver noise
and receiver generated intermod products.
I agree that much of the "DX pileup noise" is probably due to
transmitters." However, I have noticed that the noise rise effect is
noticeably less and signal to noise ratio is better when using K2,
Omni, Orion than when using IC 735, TS120, TS530 and others. I think
this is more than the an operator perceived effect described below.
So my conclusion remains that some of the apparent noise is generated in
the receiver in the presence of multiple strong signals. Of course the
artifacts you describe are there also. Perhaps your ears and
instrumentation are more discriminating that mine. Also perhaps some of
the noise that shows up as bleeps and bloops in the narrow CW case
appear more similar to random noise when generated by a large number of
mixed ssb signals.
A difference in our observation base is that you appear to be looking at
narrow band cw and I have been looking primarily at SSB bandwidths. I
also agree that my K2 does not have the excess gain necessary for a very
quiet receive antenna. I think this was a design goal to minimize
"excess" gain before the mixers and filter consistent with adequate
noise figure. It is fine for use with conventional transceive antennas.
I have been looking at 40 meters and up and with dipole and beams.
Your explanations are logical with good separation of the various
effects. And perhaps we are both attacking the "I want to hear a loud
background noise that must show my receiver has sufficient gain" folklore.
Is it possible that my "noise" rise in a strong signal power environment
observation is a phase noise in the receiver fault? Again not the same
as a front end amplifier/mixer non linearity cause.
Aloha,
John KH7T
I did not Snip Tom's text here as I believe his words deserve careful
reading.
The initial quotes are from my original message. -- John
Tom Rauch wrote:
>>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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