Hi,
I have had my new RX-340 only since Wednesday.
Am learning more about it each today, and today
was interested in the accuracy/absolute and relative
of the radio's S meter, which is also calibrated in dBm.
Being in dBm, I though perhaps Ten Tec had done
a particular job in ensuring the meter accuracy.
My approach to checking the meter was to first use
thermal noise and the wide bandwidth selections
available from the 340's DSP filters.
Per text books, a formula, kTB, can be used to
determine the noise power at a given temperature,
T, and in a bandwidth, B. The k is Boltzman's
constant. When it is all worked out, the answer
is that -114 dBm (-113.86 dBm precisely!) of
thermal noise power ( 0 dBm = 1 mW) is contained
in 1 mHz of spectrum bandwidth, independent of the
absolute frequency. If the bandwidth is cut 1000x's to
only 1 kHz, the noise power contained falls to -144 dBm,
or by 30 dB.
However, at HF, the thermal noise of our rcvr's is
not really important, as so much noise exits in our bands
because of natural causes -- galactic noise, solar noise,
and man-made noise.
My reason was, to check the accuracy of my S/dBm
meter on my new rcvr -- a Ten Tec RX-340. With
the antenna connector terminated in a 50 ohm load
resistor, I found the meter read something below
-140 dBm with a BW of 100 cy, the most narrow
DSP filter position and about the same with 150 cy
bandwidth; increasing BW 10x's, to 1500 cycle bw,
the reading was "jiggly" around -133 dBm +/- about 3 dB;
increasing about another 10x's to 15,200 cycles, the reading
increased to -126 dBm with the same "jiggle" spread.
So with 10x's increased in BW, the noise level deltas
do seem to be in the 10 db range as expected. So,
I guess Ten Tec did a reasonable job with the meter
calibration, at least at low levels.
Connecting the rcvr to my 20 meter antenna, and tuned
near 14100, the readings were: 150 BW, -125 dBm,
+/- 5 dB; 1500 BW, -119 dBm, +/- 3 dB; and at
15,200 BW, noise power increased to about -107 dBm,
and again +/- 2 or 3 dB randomly.
And going from the load resistor at ambient temperature,
to the 20 meter antenna, and recognizing that out here
on this island, there is little to no man-made noise
because of my pretty rural QTH, that the roughly 15 dB
to 20 dB noise increase from "natural" sources is probably
just about right when connected to the antenna and checked
at 20 meters.
Will do some more S meter accuracy checks using
my "precision resistor" attenuator (MFJ-762). The
WWVH 2.5 mHz omni signal here on Kauai is quite
constant in recv'd amplitude at my QTH, so I can
check the dBm scale on the meter vs. the attenuator
setting, at least at 2.5 mHz. If interested, will report.
73, Jim KH7M
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