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Keysight Technologies
SINAD Measurements Using the
Keysight U8903A Audio Analyzer
Application Note
02 | Keysight | SINAD Measurements Using the Keysight U8903A Audio Analyzer - Application Note
This application note discusses the use of the Keysight Technologies, Inc U8903A Audio
Analyzer in characterizing radio receiver sensitivity by means of measuring SINAD. An
overview of the measurement is given and potential differences in results between the
modern U8903A and older generation instruments like the 8903B are discussed.
SINAD SINAD is an acronym for SIgnal, Noise And Distortion. It is an audio quality value that
is typically used to specify the RF sensitivity of radio receivers. The ratio is usually
expressed in dB and is calculated as per the equation:
ms value of signal, noise, and distortion
SINAD = 20Log ( )
rms value of noise and distortion
There are other forms of this equation, depending on the exact form the values take. For
example, if the values are powers then the equation would use 10Log. On the other hand,
if the values are already in dB then the two components are simply subtracted. A higher
SINAD value indicates higher quality audio.
03 | Keysight | SINAD Measurements Using the Keysight U8903A Audio Analyzer - Application Note
Measuring SINAD The classic block diagram of a SINAD measurement is shown in Figure 1. An RF signal
of appropriate frequency is applied to the receiver under test. This RF signal is generally
modulated with a 1-kHz tone. On the top path, the audio signal from the receiver is
passed straight through to the level measurement block which measures the combined
level of the signal, noise and any distortion.
Audio
Signal Radio level
generator receiver meter
(S+N+D)
Audio
Analog
level
notch
meter
filter
(N+D)
RF signal modulated Audio Noise and distortion
with a tone with
noise and distortion
Figure 1. Typical block diagram of a SINAD measurement
In the lower path, the audio signal from the receiver is applied to a notch filter whose
purpose is to remove the signal but leave any noise or distortion components for
measurement. The two measurements are then used in the above equation to compute
SINAD. Modern instruments tend to make more use of digital techniques to achieve
higher resolution. The audio signal is sampled as early as possible and the computation is
performed in DSP as per the block diagram in Figure 2.
Signal Radio
ADC DSP
generator receiver
RF signal modulated Audio
with a tone with
noise and distortion
Figure 2. Block diagram showing the sampling of an audio signal and computation in DSP
These modern techniques offer increased accuracy and repeatability in many respects,
which will be discussed further throughout this application note.
04 | Keysight | SINAD Measurements Using the Keysight U8903A Audio Analyzer - Application Note
Receiver Sensitivity This is the minimum RF level specification of a radio receiver. The task is to find the
minimum RF level which results in a particular SINAD level. This level is generally 12
dB for a communications receiver and 23 dB (mono) or 26 dB (stereo) for a broadcast
receiver such as a car radio or Hi-Fi tuner.
At first it may seem a little strange that an audio quantity is used to quantify the
RF performance of a radio receiver, however, this is quite logical once the subject is
explored. The basic receiver block diagram shown in Figure 3 will be used to describe
how the two quantities are related.
Let's start at the right hand side of the diagram where we have the audio detector. This
block has the job of extracting the audio signal from the IF carrier. From a measurement
point of view, it converts an IF Signal/Noise Ratio into an Audio SINAD Ratio. The block
will, of course, have imperfections of its own that will be ignored for the purposes of this
example.
When considering a 2-way communications radio, the general minimum standard for
SINAD is 12 dB. This amounts to audio distortion of 25%, which is the minimum deemed
intelligible. The detector will require a particular IF S/N Ratio in order to provide this 12
dB SINAD. The particular value of S/N required will depend very much on the detector
employed.
For this example we will assume we have an analog detector of some description and
that it requires 10 dB S/N to give 12 dB SINAD.
Antenna
3 dB noise figure
RF filter Low noise amplifier Mixer IF filter Loudspeaker
15 kHz Detector
Theoretical
RF noise floor