Elecraft XG3 RF - Manual (Page 16)

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6. Turn on the XG3 and tune the receiver to its frequency. Watching your DMM or the receiver S-
meter, peak the XG3 signal in the receiver passband. Note: If the peak response does not occur at
the expected beat frequency tone, the receiver’s I.F. filter may need to be re-aligned. If the filter
peak does not correspond with the expected beat note, the sensitivity will suffer.
7. Set the XG3 output level to
-107 dBm 1 µV
.
8. Set the receiver audio gain to maximum. If this results in enough gain that it may damage the
loudspeaker or that causes distortion, reduce the gain setting as needed, but note the setting so you
can repeat it to properly compare future tests on the same receiver.
9. Note the DMM’s reading: _________VAC. This is the signal + noise (S+N) reading.
10. Turn the XG3 off and note the new reading without touching any other controls: _________VAC.
This is the noise (N) reading.
Perform the Calculations as follows:
1. Divide the S+N by N; call the result R.
2. Take the base-10 logarithm of R (“log” key on most calculators).
3. Multiply the result by 20 to obtain the S+N/N ratio in dB for a 1 microvolt signal.
4. If the S+N/N ratio is greater than 10 dB, you can approximate the minimum discernable signal
(MDS) by subtracting the S+N/N ratio from -107 dBm.
Sample Calculation:
Assume the DMM reads 1.0 VAC with the XG3 supplying 1µV and 0.030 VAC with no signal applied.
1 / 0.03 = 33
Log 33 = 1.52
1.52 x 20 = 30 dB This is the Signal+Noise/Noise (S+N/N) Ratio.
Since the S+N/N ratio is greater than 10 dB , the minimum discernable signal is about:
-30-107 = -137 dBm
S-Meter Calibration
Many S-meter readings vary according to whether a preamp or attenuator is switched in (e.g. the Elecraft
K2), while others compensate for the preamp and attenuator to produce consistent signal readings (e.g.
Elecraft K3 when S-meter is set for absolute mode). A long-accepted standard is for a 50µV signal to
produce an S-9 reading on the S-meter. However, S-meter calibration varies from manufacturer to
manufacturer. If in doubt consult your receiver owner’s manual. You’ll also need to consult your owner’s
manual for how to adjust your receiver to produce a specific S-meter reading.
1. Set up your XG3 and receiver as shown in steps 1, 2 and 3 under Measuring Receiver Sensitivity
on pg 15 except leave (or turn) the receiver AGC on.