Minus the 6 dB L(f) conversion factor,
so we get theoretical L(f) noise floors of -176 dBc/Hz and -191 dBc/Hz.
Adrian
Adrian schrieb:
Thanks to Rick's great hint, I'm now awaiting some AM123 amps to arrive.
Why is it that I just don't get better than -165...-170 dBc/Hz?
Let me try some simple math:
At L and R levels of +7.5 dBm (that's what come out of my 10811's),
subtract some 7.5 dB mixer conversion loss to see that 0 dBc would be
exactly 0 dBm at the mixer output.
(The calibration is performed with the R signal decreased by 40 db. At
the 40 dB LNA output, I'm measuring pretty exactly 0 dBm.)
At room temperature, the thermal noise is -174 dBm/Hz.
Add some 4 dB for the LNA noise figure and LPF insertion loss, so the
system noise floor is at -170 dBm/Hz.
Give or take a dB, but that's pretty much about it.
Now, with the calibrated carrier level of 0 dBm at the LNA input, the
residual noise floor is -170 dBm/Hz -(+0 dBm) = -170 dBc/Hz
A modern RF spectrum anylyzer has a noise figure in the 15 dB range,
add 10 dB of 'safety' attenuation, so the analyzer noise floor is -174
dBm/Hz -(+25 dB) = -149 dBm/Hz. Remember that the mixer output / LNA
input of -170 dBm/Hz is amplified by the LNA by 40 dB, so the analyzer
input 'noise signal' from the test set is -170 dBm/Hz + 40 dB = -130
dBm/Hz, which is 19 dB above the analyzer's noise.
If we increase the L and R levels by 15 dB, we need a +23 dBm mixer,
but we will add 15 dB to the system dynamic range.
The mixer conversion loss, LNA noise figure and thermal noise floor
haven't changed, so the noise floor in dBm is still the same -170
dBm/Hz, but we're now refering to a carrier level of +22.5 dBm instead
of the above +7.5 dBm (+15 dBm versus 0 dBm at the mixer output).
That's why we can now measure down to -170 dBm/Hz -(+15 dBm) = -185
dBc/Hz.
Adrian
John Miles schrieb:
That sounds about right to me. I was guessing you meant 40 dB and
not 30 dB
in the previous message, or there was something else causing about 10
dB of
loss. Lots of things to go wrong in this process!
-- john, KE5FX
-----Original Message-----
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com]on
Behalf Of Adrian
Sent: Friday, May 07, 2010 10:38 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] HP 11729C versus 11848A
Just the roughly 16 dB of insertion loss caused by the 562 ohm resistor
at the input don't make that filter such a great solution ;-)
So, I brewed something better together...
The 10 kHz beat note is now near 0 dBm when the R input signal is
decreased by 40 dB, which makes a lot more sense than before...
And, I have a noise floor of some -165 to -169 dBc/Hz at +7.5 dBm
input,
and I'm seeing the 10811A's some 5 dB above that.
Adrian
John Miles schrieb:
John,
I'd say you nailed it.
After some more testing, I can confirm that the limiter amp and
the LPF
are the culprit.
I opened the box and plugged directly into the mixer LO port.
And, for the LPF, as a quick 'n dirty solution, I connected the <1
MHz
front panel output with the LNA input.
Now, at 10 dBm each into the mixer ports, I'm getting a noise
floor of
<-145 dBc/Hz at 100 Hz and about -170 dBc/Hz at 10 kHz and above.
That's pretty aggressive for 10811s. The floor on those is usually
around -165 dBc/Hz. How's your calibration process -- are you
accounting
for the 600 ohm output Z of the <1 MHz output port? It'll lose
a few dB if
you try to drive 50 ohms with it, and/or the filter response won't be
correct.
-- john, KE5FX
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