noise ratio, SNR are made. A calibrated RF signal generator is used as a signal
source for the receiver. It must have an accurate method of setting the output level
down to very low signal levels. Then at the output of the receiver a true RMS AC
voltmeter is used to measure the output level.
S/N and (S+N)/N: When measuring signal to noise ratio there are two basic
elements to the measurement. One is the noise level and the other is the signal. As
a result of the way measurements are made, often the signal measurement also
includes noise as well, i.e. it is a signal plus noise measurement. This is not normally
too much of a problem because the signal level is assumed to be much larger than
the noise. In view of this some receiver manufacturers will specify a slightly different
ratio: namely signal plus noise to noise (S+N/N). In practice the difference is not
large, but the S+N/N ratio is more correct.
PD and EMF: Occasionally the signal generator level in the specification will mention
that it is either PD or EMF. This is actually very important because there is a factor
of 2:1 between the two levels. For example, 1 microvolt EMF and 0.5 microvolt PD
are the same. The EMF (electro-motive force) is the open circuit voltage, whereas
the PD (potential difference) is measured when the generator is loaded. As a result
of the way in which the generator level circuitry works it assumes that a correct
(50) load has been applied. If the load is not this value then there will be an error.
Despite this most equipment will assume values in PD unless otherwise stated.
While there are many parameters that are used for specifying the sensitivity
performance of radio receivers, the signal to noise ratio is one of the most basic and
easy to comprehend. It is therefore widely used for many radio receivers used in
applications ranging from broadcast reception to fixed or mobile radio communications.
w.2. Measuring Signal to Noise Ratio (SNR)
We will make a very simple measurement--how much signal is required for a specified
signal-to-noise ratio at the headphone output. More precisely, we will measure the
required signal for a specified (Signal+Noise)/Noise ratio, but with a target of 10 or 20
dB, the difference between S/N and (S+N)/N isn't too great and we will call the result the
sensitivity.
w.2.1. Required Equipment
The required equipment is:
A signal generator with a calibrated, variable output covering the frequency range of
interest (I used the tracking generator on a Rigol 815 Spectrum Analyzer and a step
attenuator).
A multimeter (analog or digital) capable of reading audio output of your receiver. I
used a Fluke digital voltmeter, but in fact a good analog meter is preferable as you
have to do some "eyeball averaging" when reading the noise level.
Signal to Noise Ratio (SNR) Page 54
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