--- Original Message -----
From: "Brad Rehm" <brehm@ptitest.com>
>
> This is a perfect example of what concerns me about the off-the-air
> testing we're doing. And please understand that I'm not criticizing
> your work. What you have done is very much like the testing I've done
> with equipment from my company's EMC lab.
>
Brad,
Actually that is one reason why I posted my comments.
I have the same concern. Recently I have been doing a
lot of so-called "off-the-air testing" and each time I do
it, I come away with the same nagging feeling that
something doesn't smell quite right. Specifically, the rigs
I have tested "off-the-air" always seem to sound a lot
more dirty than I would expect based on what I generally
hear "on-the-air". So I ask myself, why is this? One
possible answer is that in the artificial lab environment,
we have eliminated atmospheric noise, so we are able to
listen down to the receiver MDS where we can hear the
broadband noise from a strong signal, whereas in real life
this broadband TX noise gets covered up by atmospheric
noise which can easily be several orders of magnitude
stronger than a receiver's input thermal noise (especially
on the lower HF bands).
Another explanation (which Tom has offered) is that the
signal we normally receive just don't get that strong (e.g.
a real S9+50dB signal is rare). That bears consideration.
When for example the guy down hill from me is calling
DX (he is probably 1/4 to 1/2 mile from here), I do hear
his broadband noise sidebands. They are not real loud,
and he isn't on the air too much, so its not a really big
problem for me, but it can be annoying if you are trying
to hear a weak signal. Likewise, during the CQ WW CW
contest, I had some trouble on 15 meters with a guy
who was a mile or so from the local club station. For the
short time we were on the band together, I could hear
his noise sidebands from one end of the band to the
other. But most of the time, signals aren't that strong
here (and noise is high), so I don't hear noise sidebands
on most signals. I do hear key clicks from time-to-time,
but that is another subject.
Another possible explanation for the seemingly dirty
signal is measurement error. One difference between a
real RF link and a simulated link is proximity. In a real
RF link, the stations are miles apart. There are no sneak
paths. In a simulated link, however, the transmitter and
receiver are in some cases right next to each other.
They probably share a ground connection vis-a-vis
the shield of the coaxes and attenuators connecting
them. There is also the common connection to
the AC mains.
I saw what I think is a good example of this when I tried
to monitor myself with a nearby rig while transmitting
CW through a 1KW linear amplifier. I tried this on three
different setups recently using a nearby receiver with a
short piece of wire stuck in its antenna input to sample off
a little of the transmitted signal. The first time I tried this I
was surprised by how much 60 Hz hum was present on
the transmitted CW signal. Even with the received signal
attenuated to a pretty low level I saw this. At first I thought
there was really something wrong with the rig under test,
but after seeing this same thing on three different setups
each of which has never received any complaints, it
occured to me that I was probably seeing some test set
effect (common mode conduction through the AC line -
perhaps?). Anyone else ever notice this?
> If the IMD and BDR measurements mean something, why have I been able
> to do what I've described here? Working anything within 5 kHz of a 50
> or 60 dB over S9 signal should have been very difficult. Do our
> measurements really predict how our equipment will behave under actual
> band conditions?
>
Well in the case of a single really strong adjacent signal,
it seems to me that the BDR would be the biggest concern
(for IMD to be a factor you have to have at least two signals
mixing with each other to create a product on your receive
frequency). If the signals you are describing really were
S9+60dB, then that would imply a receiver input level of
-10dBm (a very very strong signal). If we go out on a limb
and assume an accurate s-meter, then at 6dB per S-unit,
your S3 noise floor would be at -106 dBm. This implies
a blocking range of 96dB which is nothing special at all
(at least at 5 KHz spacing). In the example you gave,
the signal you were working was S7, which in the case
of an S9+60dB adjacent signal would require a
dynamic range of 72dB. Again, nothing extraordinary, so
I don't think the examples you cite make a strong case
that IMD and BDR specs are not good predictors of
real world performance.
In terms of TX noise, at a composite noise level
of -100dBc/Hz and an effective receiver noise bandwidth
of 300Hz, the noise sideband power would be -75dBc.
In the case of a close-by S9+60dB signal (-10dBm), that
would put the received noise sideband power at -85dBm
or ~S6 which would be 3 units above your S3 noise floor.
On the other hand, if the signal you were getting
from N5TW or K5NA was really only S9+50dB (-20dBm)
and their TX noise was down around -110dBc/Hz (very
typical of many modern transceivers), then their
composite noise sidebands could be easily have been
down around your -106 dBm noise floor where they
wouldn't be all that noticeable. I guess the upshot of this
is that in most cases -100dBc/Hz TX noise will probably
not produce noticeable adjacent channel interference, but
certainly in the extreme case (two stations very close
together) it can. I certainly wouldn't want to be using
anything worse than -100dBc/Hz. Also, one might ask
why strive for receivers with >120dB blocking dynamic
range if we are all transmitting -100dBc/Hz broadband
noise? Seems like a wasted effort. On the other hand,
if everyone followed suite with the Ten-Tec Orion and
reduced TX noise to -140dBc/Hz, then all that BDR
everyone seems think they want might actually be
useful.
> 73,
> Brad, KV5V
73 de Mike, W4EF..........................
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|