Measuring FM Receiver Sensitivity
By Jeff DePolo  WN3A

To measure 20 dB quieting, connect an AC voltmeter across the speaker leads.  With no signal into the receiver, open the squelch.  Adjust the volume control to produce something around 1 or 2 volts on the voltmeter.  This is the base noise reading, make a note of it.  Then turn on the signal generator and adjust the RF output level to get a reading on the voltmeter that is 1/10th of what it was initially.

At this point you've quieted the receiver by a factor of 10:1, or 20 dB.

To measure 12 dB SINAD, connect the SINAD meter to the speaker leads.  Set the volume control on the radio to a level that is compatible with your SINAD meter (most SINAD meters are auto-ranging and auto-nulling, so this usually isn't too critical).  Generate a signal on the receive channel, modulated with a 1 kHz tone at 3 kHz deviation.  Adjust the RF output level of the signal generator until the SINAD meter reads 12 dB (25% if that's how your meter is calibrated).

Keep in mind that you have to do these measurements across the speaker terminals, for two main reasons:

1:  The audio signal has to be de-emphasized.  If you take these measurements off the discriminator, they will typically come out 6 to 10 dB worse than what they should be because the higher frequencies are not being attenuated (i.e. there will be more high frequency noise).

2:  This is how the manufacturer specs the radio, i.e. the sensitivity specs are based on the quieting and distortion yielded after going through all of the stock audio circuitry, including the speaker amplifier which usually contributes a few percent of distortion in most radios.  So in order to compare apples to apples, you have to do it the same way they specified it.

  So, you see, it's pretty simple.  20 dB quieting is just what it says - how much RF you have to put in to quiet the speaker output by 20 dB.  12 dB SINAD is how much modulated signal you have feed the receiver in order to reduce the noise and distortion by 12 dB.  SINAD stands for Signal to Noise And Distortion.  SINAD is preferred because it takes into account distortion, while 20 dBQ only deals with noise.  A SINAD meter is a pretty simple device.  It's basically an AC voltmeter that has a notch filter to remove the 1 kHz fundamental audio tone, leaving just the noise and distortion.  The meter is driven by a differential amplifier such that it displays the difference between the audio with the tone, and the audio without the tone (i.e. just the noise and distortion).  Pretty straightforward once you understand what's going on behind the scenes.

  The EIA standard amount of deviation used to measure SINAD is 3 kHz.  Some Service Monitors like IFR and some SINAD meters like Helper Instruments specify the use of 3.3 kHz deviation.  If you use another deviation value, such as 2 kHz or 5 kHz, the SINAD reading will probably change slightly.  Generally speaking, the higher the deviation, the more distortion will be induced in the receiver, i.e. your SINAD will be degraded.  The lower the deviation, the worse the signal to noise ratio is, again degrading the SINAD measurement.  For most receivers, deviation values in the 3 to 4 kHz range will result in the most optimistic SINAD measurement, with 3 kHz being the standard value.

  The 1 kHz tone was chosen presumably because it is the highest modulating tone (and therefore the highest overall modulation bandwidth for a given deviation) that would have both its second and third harmonics fall within the standard 300-3000 Hz audio passband.  Also, 1 kHz at 3 kHz deviation produces a signal with 8 kHz bandwidth, which is half the standard 16 kHz channel bandwidth, i.e. it's a good middle range value.

                                                                      --- Jeff

Original Copyright April 2, 2001 Jeff DePolo  WN3A.
HTML Copyright April 3, 2001 Kevin K. Custer  W3KKC.