francesco messineo wrote:
On 9/19/10, jimlux <jim...@earthlink.net> wrote:
francesco messineo wrote:
It's hard to explain why to ones not familiar with weak signal
operation between broadcasting signals, but really the noise floor
raise a lot when you have some 5 or 6 broadcasts signals in 500 KHz of
band (all with power levels of at least 10 dB more than the levels
used in amateur radio, often +20 dB more)
I would need some more fundamental understanding of the system and needs
to be able to understand how you come up with the above noise level at
100 Hz.
as I said, if it's not possible or not practical, of course I'll take
what I can get. The receiver will be limited by its phase noise and
not for example by its IMD3.
I think already -110 dBc/Hz at 100 Hz is better than any LO in
commercial receivers (for ham radio at least).
But why at 100 Hz offset. Are you looking for weak signals 100 Hz away
from the strong interferer? (and are worried about reciprocal mixing
from the LO) That would imply that the interferer has equally good
phase noise, and that's not particularly likely?
the first scenario. Even if the interferer has poor phase noise, why
folding back another share of interference on my side?
As for the 100 Hz offset, it's just a practical measure, if @100 Hz
things are good, @ 1KHz or more they must be even better, right?
In real life though, interferers are often much closer than 100 Hz to
the wanted signal and this often means no contact made (it happened
two times to me this year already).
On the other side of the contact however, the band was much more clean.
Sure.. but you might find yourself struggling to get good 100Hz
performance, and if your real need is at 1000Hz offset, it might be
counterproductive..
Consider this..a PLL based LO and you have a fairly quiet VCO and you're
locking it to a quiet quartz oscillator to clean up the close in noise.
If the loop bandwidth is 500 Hz, then it's the VCO that's setting the
1kHz offset noise and the XO that's setting the 100 Hz noise.
In very narrow band applications (e.g. coherent transponders used in
deep space), we might have a crystal locking a DRO (or, these days, a
GaAs VCO), but the crystal is locked to the received signal. The
crystal to DRO loop might have a BW of a few tens of kHz, but the
receiver to crystal loop might have a BW of 1 Hz. Since the measurement
of the round trip signal is made by averaging over many seconds, we
aren't as worried about the 100Hz and farther out noise, except insofar
as it adds a bit of noise to the received signal.
On the other hand, in this sort of application, there's no interfering
signals.
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