Hi Bob,
I know that for signals well above the noise level, the calibration between
the 5000A and the HP 4396A were virtually identical (I'm using the 4396A in
network analyzer mode with the frequency span set to zero, so that it
generates a single frequency output). I don't remember exactly how close to
the noise floor I could get and still have agreement, but it was down to
less than 10 dB above the noise.
Clay
----- Original Message -----
From: "Bob McGwier" <rwmcgw...@gmail.com>
To: "Clay W7CE" <w...@curtiss.net>
Cc: <flexradio@flex-radio.biz>
Sent: Wednesday, April 15, 2009 9:08 PM
Subject: Re: [Flexradio] PowerSDR S-Meter Power Readings
The meter is calculated on a single filter with a large signal in its path
and for the rest of the time, it is assumed that noise is negligible
compared to the tones in the calibration. This assumption appears to
break down near the noise floor if your measurements are correct. This is
useful information. I will be in Austin Friday - Wednesday and we will
simply make several measurements to see what gives. I do clearly recall
making the meter measurements at all sorts of levels with my (calibrated)
8640B (as accurate as that can be with its analog level set) and it was
definitely close but I cannot state emphatically that it was not (3.1 -
1.8) = 1.3 dB off since we just didn't have test equipment capable of
that accuracy then.
Bob
Clay W7CE wrote:
Hi Bob,
It's certainly possible that I'm doing this wrong, so let me explain my
procedure and then you can tell me if I'm making a mistake. All of the
following is performed on 6M at 50.200 MHz with a 50 ohm termination
connected to antenna port 1 and the bandwidth set to 500 Hz (mode CWU).
Also, the multimeter average time is increased to 5000 mS and the Digital
Refresh is increased to 2000 mS. The external preamp I am using is an
ARR P50VDG preamp with a spec'd noise figure of 0.5 dB and a measured
gain of 26 dB. Gain was measured on an HP 4396A Spectrum/Network
Analyzer and confirmed using the ADC L and ADC R meters.
First, I measured the receiver noise floor using the RX1 meter set to
"Sig Avg". With the internal preamp turned on and no external preamp,
the average noise level is -125.0 dBm (the value fluctuates some, but
seems to be centered here). This indicates a receiver noise figure of
22.0 dB. Now if I enable the external preamp the average noise level
decreases to -143.9 dBm, corresponding to a receiver noise figure of 3.1
dB. However if I do the math, adding my preamp and assuming connector,
coax and internal relay losses of 0.3 dB in addition to the 0.5 dB preamp
noise figure, I should see a receiver noise figure of 2.0 dB. For the
receiver noise figure (including external preamp) to be 3.1 dB the
external preamp noise figure and passive losses would need to add up to
2.2 dB which is much larger than I would expect.
Next, I computed the receiver noise figure by measuring the receiver MDS
using an external HP RMS volt meter at the speaker connector and the HP
4396A as a signal generator. Without the external preamp the measured
MDS is -126 dBm and with the preamp the MDS is -145 dBm, corresponding to
noise figures of 21 dB and 2 dB, respectively. If I do the math, adding
my preamp in front of a 21 dB NF receiver, should yield a system NF of
about 1.8 dB, so the measurements agree closely with the math.
Based on the results, my MDS measurements (using the voltmeter) with the
preamp enabled are more consistent with the calculated values than those
using the internal meter. I suspect that measurements this close to the
noise floor will not be as accurate and that I'm asking too much. I'm
certainly not complaining though. The accuracy of the metering rivals
that of my HP spectrum analyzer on measurements that I've made from -100
dBm to -30 dBm.
73,
Clay W7CE
--
(Co)Author: DttSP, Quiktrak, PowerSDR, GnuRadio Member: ARRL, AMSAT,
AMSAT-DL, TAPR, Packrats,
NJQRP, QRP ARCI, QCWA, FRC.
"It is human nature to think wisely and act in
an absurd fashion.", Anatole France.
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