User guide

DETAILED INFORMATION

6.40 Advanced SINAD Information

What is SINAD?

SINAD is a parameter which provides a convenient measurement of the quality of an audio

signal from a radio or other communications device. SINAD is a simple measurement of the

power ratio between signal + noise + distortion to just noise + distortion. To remove the

signal to calculate the SINAD, a narrow band-stop filter is employed. Then an RMS value is

found for the unaltered signal, and another RMS value for the filtered signal. Then these two

values are compared using the formula:

SINADdb := 20 * log10( RMS(S+N+D) / RMS(N + D))

In many applications, a standard 1kHz tone and band-stop filter is used for the SINAD test,

primarily since it is in the middle of most narrow-band audio transmissions and it happens to

be roughly the center of the standard voice band. In the COM3010 this is provided though FM

modulation at 1kHz, or AM modulation at 1kHz, when set in the modulation fields (using

SHIFT, 8 sets up the COM3010 automatically). On rare occasions other tones are used, such

as 400Hz on very narrow-band transmissions. The COM3010 allows the flexibility of any tone,

and the band-stop filter coefficients are calculated on-the-fly for the chosen generated tone

frequency.

When the 1kHz tone is generated by the COM3010, a fairly distortion-free modulated signal is

created due to the digital modulation methods used. This allows the COM3010 to accurately

measure the SINAD in a receiver under test, including the COM3010’s own receiver. If you

want to test the SINAD meter out, connect a cable between GEN OUT and RECEIVE IN, and

another between MONITOR and EXT IN. Set the generator and receiver to the same frequency,

say 146.52MHz (Hint: After setting receive and generate to the same value, you can LINK the

receive frequency to the generate frequency to test other frequencies). Now press SHIFT, 8

and SINAD mode will be activated. Now press [LEVEL] and step the level down until the

SINAD meter shows roughly 12dB. Note the value will go around a bit since that is the nature

of measuring a noisy signal. Some averaging is applied to the SINAD measurement, but in the

interest of speedy meter updates you will need to average the rest of the way by eye. Now

you can switch the level setting from dBm to uV by pressing either dBm or mV/uV. At the

point where the SINAD is reading 12dB, it is considered the SINAD rating of the radio under

test.

A widely accepted value of 12dB of SINAD is considered the useable threshold of sensitivity

for a radio, but that depends on the manufacturer specifications. Values less than 12dB of

SINAD are considered to be unintelligible for many people. Some manufactures like to use

20dB of SINAD instead, typically meaning their radios have very conservative sensitivity

specifications, and are typically very sensitive. Remember that the higher the dB of SINAD, the

better the quality of signal. You will need to review the radio’s specifications for sensitivity to

know what level of SINAD is acceptable.

The External modulation input used during the SINAD test has a digitally controlled gain with

customizable AGC which is controlled by the main microcontroller. The AGC makes sure to