sinad in fm radio

Receiver specifications usually mention a '12dB Sinad', SINAD meaning Signal over Interference, Noise And Distortion - 12dB of difference between DEMODULATED signal and other noises, gives comfortable readability. Other specs may use a 20dB SINAD, or specify a number of microvolts intead of -dBm, just to confuse! UHF receivers generally resolve signals as weak as -120dBm or less. 

Sensitivity equivilances 
 (probably in this range for a 12dB SINAD   or several dB higher for a 20dB SINAD)
dBm   ---(50 Ohms)---  (75Ohm)         dBW
-115  0.398uV  -8dBuV  0.487uV  5dBf  -1445
-116  0.354uV  -9dBuV  0.434uV  4dBf  -1446
-117  0.316uV -10dBuV  0.387uV  3dBf  -1447
-118  0.282uV -11dBuV  0.345uV  2dBf  -1448
-119  0.25uV  -12dBuV  0.306uV  1dBf  -1449
-120  0.223uV -13dBuV  0.273uV  0dBf  -1550 (femtowatt)
-121  0.2uV   -14dBuV  0.245uV -1dBf  -1551
-122  0.178uV -15dBuV  0.218uV -2dBf  -1552
-123  0.158uV -16dBuV  0.194uV -3dBf  -1553
-124  0.141uV -17dBuV  0.173uV -4dBf  -1554
-125  0.126uV -18dBuV  0.154uV -5dBf  -1555
-126  0.112uV -19dBuV  0.137uV -6dBf  -1556

SINAD means Signal over Interference, Noise And Distortion.

To measure the SINAD.
1. Feed the RF signal modulated with 1,000 Hz or 800 Hz and with the specified modulation index (depends on the test standard).
2. Connect the distortion meter to the AF output of the receiver under test.
3. Set the AF volunme of the receiver under test to about 1/3 (one third).
4. Set the distortion meter filter to pass only the modulated AF and mark the level in dB scale.
5. et the distortion meter filter to reject only the modulated AF and mark the level in dB scale.
6. the difference of item 4. and 5. is the SINAD.
7. To do this first you have to increase the RF siganal and measring according to item 4. and 5. until the SINAD value is as the mentioned is the specification (12 dB), then read the RF level that's the sensitivity of the receiver for 12 dB SINAD.

Hope this is clear enough for you.

Of course it will be very easy to just use a SINAD calibrated meter. 12dB SINAD represents 25% distortion in the measured signal.

Thanks Pico.

Can I know what instrument built in "distortion meter".
I got audio analyzer here. Can I use it for SINAD measurement ?
What does "AF" stand for ?
Thanks.

1. Distortion meter is a AF level meter with a selectable filter to for as a very narrow band-pass or band-reject function. In the band-pass mode we'll measure the wanted AF level (S) and in the band-reject mode we'll measure the unwanted noise level (INAD) => SINAD.
2. "AF" stands for "Audio Frequency" and "RF" stands for "Radio Frequency".

Hope this will explain what you want.

so "12dB SINAD 0.158uV (SINAD 12dB) "

means RF input level >=0.158uV , output can get 12dB SINAD.
Is that right ? Thanks.

Receiver's sensitivity means how less of the RF level that the receiver can meet the SINAD value.
In this case it means that
RF input level <=0.158uV , output can get 12dB SINAD

Hope this can clear your understanding.

I think it means that with a minimum RF level of
0.158 uV you have at least 12 dB SINAD. In
others words 0.158 is the minimum level that can
guarantee 12 dB SINAD