Receiver sensitivity basics     Signal to noise ratio     SINAD     Noise Figure, NF     Noise floor     Reciprocal mixing

SINAD, Signal to Noise and Distortion is a widely used measure of radio receiver sensitivity performance. It is often used for FM and VHF / UHF based systems but also used for many other radio communications systems as well.

SINAD is a very convenient method of including a variety of signal degradation issues into a single measurement so that an overall measurement of the system performance can be assessed.

As SINAD is widely used as a parameter for measuring radio sensitivity, and it is tested during the RF circuit design stages, specialist test instruments for measuring SINAD are available.

SINAD is a measurement that can be used for any radio communication device to look at the degradation of the signal by unwanted or extraneous signals - in particular noise and distortion. However the SINAD measurement is most widely used for measuring and specifying the sensitivity of a radio receiver.

The actual definition of SINAD is quite straightforward. It can be summarised as the ratio of the total signal power level (Signal + Noise + Distortion) to unwanted signal power (Noise + Distortion). Accordingly, the higher the figure for SINAD, the better the quality of the audio signal.

The SINAD figure is expressed in decibels (dB) and can be determined from the simple SINAD formula or equation:

$\mathrm{SINAD}=10{\mathrm{log}}_{10}\left(\frac{\mathrm{SND}}{\mathrm{ND}}\right)$

Where:
SND = combined Signal + Noise + Distortion power level
ND = combined Noise + Distortion power level

It is worth noting that SINAD is a power ratio and not a voltage ratio for this calculation.

To make the measurement a signal modulated with an audio tone is entered into the radio receiver. A frequency of 1 kHz is normally taken as the standard as it falls in the middle of the audio bandwidth.

A measurement of the whole signal, i.e. the signal plus noise plus distortion is made. As the frequency of the tone is known, the regenerated audio is passed through a notch filter to remove the tone. The remaining noise and distortion is then measured.

Although it is most common to measure the electrical output at the radio receiver audio output terminals, another approach that is not as widely used, is to pass the signal into the loudspeaker and then use a transducer connected to SINAD meter to convert the audio back into an electrical signal. This will ensure that any distortion included by the speaker is incorporated, and it may overcome problems with gaining access to the speaker connections in certain circumstances where this may not be possible.

Obtaining the figures for the signal plus noise plus distortion and the noise plus distortion it is then possible to calculate the value of SINAD for the radio receiver of other piece of equipment.

While the measurements for SINAD can be made using individual items of test equipment, a number of SINAD meters are made commercially. These test instruments incorporate all the required circuitry and can be connected directly to radio receivers to make the measurements.

Accordingly SINAD meters are a particularly convenient test instruments for making these measurements.

A procedure for testing the SINAD of a radio might follow a procedure like that given below:

• With no signal present on the input, the noise and distortion of the receiver are measured at a convenient level.
• A signal is applied to the input such that the output increases by 12 dB.
• The level of the signal needed to produce this is noted. In this case, it was found to be 0.25 microvolts.

It can be seen that a filter is required to notch out the tone in the SINAD measurement. As might be expected this filter shape and performance have an effect on any measurements that are made.

In an ideal world the filter would have an infinitely sharp notch so that only the modulating tone is removed. However in the real world the filter will have a finite bandwidth. As its bandwidth increases, so it will remove noise and distortion as well as the tone. As the distortion products will typically result from the second and third harmonics of the tone, the filter will not have a noticeable effect on this element of the reading, but it will affect the noise level readings.

In view of this problem some standards set down specifications or guidelines for the filter used in the SINAD measurement. ETSI -European Telecommunications Standards Institute - defines a notch filter (ETR 027). With the standard tone frequency of 1 kHz, it states that a filter used for SINAD measurements shall be such that the output the 1000 Hz tone shall be attenuated by at least 40 dB and at 2000 Hz the attenuation shall not exceed 0.6 dB. The filter characteristic shall be flat within 0.6 dB over the ranges 20 Hz to 500 Hz and 2000 Hz to 4000 Hz. In the absence of modulation the filter shall not cause more than 1 dB attenuation of the total noise power of the audio frequency output of the receiver under test.

In addition to the filter performance another critical area of a SINAD measurement is the measurement of the output signal power levels. These have to be a true power measurements that accommodate the different form factors of the variety of waveforms, i.e. sine wave for the 1 kHz tone and its harmonics, but the noise will be random and have a different form factor.

SINAD specifications are found in many specifications for items like VHF radios, VHF / UHF walkie talkies, other radio communications system, particularly those using FM, but SINAD can also be used for AM and SSB.

In many respects the SINAD specifications are given in a similar format to that for signal to noise ratio, SNR.

The SINAD specification is typically couched in terms of a given input voltage at the antenna terminal to provide a particular SINAD measurement.

A typical specification may appear with the following format: Receiver sensitivity = 0.3 µV at 12 dB SINAD. Typically a VHF radio may have a SINAD specification of 0.25µV for 12 dB SINAD, but a UHF one may be slightly less sensitive at 0.35 µV.

Although SINAD measurements are most widely used to give an assessment of receiver performance, they can be used in a variety of manners to provide useful system performance information.

• Receiver sensitivity:   The most common use of the SINAD measurement is to assess the sensitivity performance of a radio receiver. This may be measured as a routine test during the acceptance testing, for inclusion in a data sheet, but it is also used during the RF circuit design stages to ensure that the performance of the radio meets its requirements.

To achieve this the sensitivity can be assessed by determining the RF input level at the antenna that is required to achieve a given figure of SINAD. Normally a SINAD value of 12 dB is taken as this corresponds to a distortion factor of 25%, and a modulating tone of 1 kHz is used. It is also necessary to determine other conditions. For AM it is necessary to specify the depth of modulation and for FM the level of deviation is required. For FM analogue systems ETSI specifies the use of a deviation level of 12.5% of the channel spacing.

A typical specification might be that a radio receiver has a sensitivity of 0.25 µV for a 12 dB SINAD. Obviously the lower the input voltage needed to achieve the given level of SINAD, the better the receiver performance. The figure of a 12dB SINAD is normally used for a given input voltage. The input voltage required for this tends to be the comparison factor.

• Receiver blocking:   SINAD can be used to form the basis of a receiver blocking measurement. Again, this is very useful for acceptance testing, for inclusion in a data sheet, and more importantly early in the RF circuit design stages to ensure the design meets its requirements. It often occurs in a radio communications system, that there may be other transmitters operating close by, and it is important that any radio receiver is not unduly affected by strong, or very strong signals that are off channel.

As with other similar measurements a reference SINAD sensitivity level is found. The level of the SINAD signal is increased by 3 dB at the antenna. An un-modulated off channel signal is then added and its level raised until the receiver desensitises to an extent whereby the reference SINAD level is reached.

• Adjacent channel rejection:   This parameter is a measure of the ability of the receiver to reject signals on a nearby channel. As the adjacent channel performance degrades, so the levels of noise and extraneous signals will increase, thereby degrading the SINAD performance.