Electro-Metrics EM-2115, Instruction Manual

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ELECTRO-METRICS EM-2115 2.0 OPERATING INSTRUCTIONS
(EM2115S2-6)
2.4.1 Amplitude And Frequency Measurement
The digital meter of the EM-2115 is used to determine the amplitude of an input signal.
However, since the response time of a digital meter is slow compared to an analog meter, the fol-
lowing procedure is employed:
a. Tune the receiver for a maximum indication on the analog meter, increasing or
decreasing receiver attenuation as required to keep the signal on scale.
b. After the signal is peaked for maximum on the analog meter, read the level indi-
cated by the digital meter.
This procedure can also be used when determining the frequency of an unknown signal.
However, in order to obtain the best possible accuracy in this indirect measurement of the signal
frequency, the narrowest possible bandwidth should be used. The
analog meter is meant for rel-
ative amplitude indication only .
2.4.2 Amplitude Correction Factor
For signals close to the noise level of the receiver, a correction factor must be added to
the meter reading to obtain an accurate amplitude indication. Use the Correction Factor Graph,
Figure 2.1 on Page 7, for this purpose.
The X-Y axes of the graph are denoted as follows:
X axis: Signal and Noise to Signal ratio.
Y axis: Signal and Noise to Noise ratio.
To use the graph, note the increase in dB of the digital meter reading over the noise level
when an input signal is applied. This is the signal and noise to noise ratio which is found on the
Y-axis of the graph. From the X-axis, find the corresponding value for the signal and signal to
noise ratio. This value, which is the correction factor, is then subtracted from the digital meter
reading to determine the actual signal level.
Example:
Internal Noise .............................................................-30 dB(µV)
Digital Meter indication with signal applied..............-25 dB(µV)
Increase in meter indication .......................................+5 dB(µV), locate Y-axis.
From X-axis, amount to be subtracted .......................1.7 dB
Actual signal level: ....................................................(-25) - 1.7 = -26.7 dB(µV).