The versatile sinad meter
I remember, back when I first started working in the land mobile radio field, that we were taught how to measure receiver sensitivity using the 20dB quieting method. We were also taught to “tune” a receiver using the quieting method along with the plug-in test sets that were generally used for troubleshooting and alignment work. Whatever happened to receiver and transmitter metering jacks and test sets? At some point in the ’70s my boss bought a sinad meter for me to try out. It was Helper Instruments’ model S103. My first thought was the old paradigm: “What’s wrong with the way we have always done it?” After using the sinad meter for a short time, I discovered what was wrong was the way we had always done it.
It was a real eye-opener to go back and check receivers that were aligned using the 20dB quieting method against the 12dB sinad method. Practically all of the receivers that were aligned using the 20dB quieting method had to be “touched up” with the sinad method to meet specifications.
One of the biggest advantages of the sinad method is in tuning adjustments that affect the bandwidth of filter circuits. Most bandpass filter circuits in receivers have an impedance-matching adjustment at their input and output. This adjustment also affects bandwidth. If the adjustment is made using a continuous wave (CW) signal (as it is with the 20dB quieting method), then it is possible to tune the filter for a frequency response that is too narrow to allow the sidebands of a fully modulated FM signal to pass without severe attenuation. The resulting distortion degrades the sinad sensitivity of the receiver.
What is sinad? The acronym sinad stands for signal, noise and distortion. The signal portion of the composite sinad signal is specified to be 1,000Hz. The sinad meter contains a sharp notch filter that is (or should be) tuned to notch out the 1,000Hz signal, leaving only thenoise and distortion components to be measured. The setup for performing the sinad sensitivity test on an FM receiver is shown in Figure 1, below.
If the squelch control is set to unsquelch the receiver with no signal input to the receiver, then the sinad meter will automatically set the noise output to 0dB sinad. If the FM signal generator is modulated by an accurate 1,000Hz tone (to about 660% system deviation), the composite signal (1,000Hz 1 noise 1 distortion) will be present at the input to the notch filter inside the sinad meter.
An AGC amplifier is used to keep the composite signal at a constant level at the input to the notch filter, regardless of the makeup of the individual components of the composite signal (see Figure 2, below). As the signal level to the receiver input is increased, the signal component (1,000Hz) becomes a greater part of the composite signal at the input to the filter. At the same time, the noise component is reduced. The notch filter removes the 1,000Hz component, and the remaining noise and distortion components appear at the output of the notch filter and are measured by the metering circuit.
The point where the remaining noise and distortion components are reduced by 12dB below the composite signal is the point where the 212dB sinad sensitivity of the receiver is taken. At this point, the RF level of the signal generator is the 212dB sinad sensitivity of the receiver.
Once the 212dB sinad point is determined, the modulation acceptance bandwidth should be measured. The RF level from the generator is increased by 6dB (double the microvolts). This will improve the sinad reading to something better than 212dB. Now, increase the deviation until the sinad meter again indicates 212dB sinad. At this point, the deviation level of the generator is taken as the modulation acceptance bandwidth of the receiver. It is stated as 6″X”kHz. Check the level against the specifications for the particular receiver.
Measuring distortion The sinad reading can be converted to percent distortion. To check the distortion figure of a receiver, the signal generator is set to a high RF level (500mV-1,000mV). This reduces any noise in the receiver output so that the sinad meter is predominately looking at distortion components. The generator deviation should be set to 50% to 60% system deviation at 1,000Hz. If the receiver is producing 10% distortion, the sinad reading will be 220dB. The standard 212dB sinad point represents 25% distortion. To convert from sinad to percent distortion, use the following formula:
D = antilog(dB/20) x 100
Where D = distortion in percent dB = sinad reading entered as a negative figure. To convert percent distortion to sinad, use the following formula:
DB = 20log(D/100)
The following table converts between percent distortion and sinad: On some sinad meters, the scale doesn’t allow measurements of sinad readings below 220dB sinad. Other instruments allow readings better than 230dB sinad. A receiver’s distortion level should be measured at its rated audio output. It is a good idea to check the distortion level of a receiver before performing the sinad sensitivity check. If the distortion level is high, the 212dB sinad point may not be achievable.
Audio power output Because the sinad meter has a root-mean-square (RMS) voltmeter, it can be used to set or measure the audio output power from a receiver. If we know the load impedance and RMS voltage, the power can be calculated from
P = E^2/R where P = power
E = rms voltage
R = load impedance
The formula can be rearranged to calculate power from voltage as
E = square root of PR
The above table converts power to voltage at several values of impedance.
A non-inductive audio load should be used at the output of the receiver with the sinad meter connected across the load resistor. The sinad meter can be switched from RMS voltage measurement to sinad measurement to measure the distortion at any power level within the receiver’s audio output range. “Technically speaking,” the 212dB sinad measurement should be made with the audio level from the receiver at least half of the radio audio output power level. This is in accordance with EIA specifications.
Additional applications Other uses of the sinad meter include signal tracing in audio stages of the receiver or transmitter. Stage gain measurements can be made easily because the sinad meter is capable of measuring audio voltage below 10mV. The sinad meter also can be used to send a tone over a line (remote control line), while a second sinad meter at the other end measures the tone level. The far end should have a 600V termination. If a 0dBm tone is injected in one end, and the sinad meter on the other end indicates 216dBm, then the line has a 16dB loss. The measuring instrument can be switched to the sinad position to measure any distortion on the line. Be sure to use 600V/600V isolation transformer between the instrument and line on both the generate and measure ends of the line. This prevents unbalancing the line, causing a hum problem.
Calibration: a final detail It is important to make sure that the 1,000Hz tone that is used for any tests with the sinad meter is accurate. If the tone does not fall deep into the notch of the instrument, valid sinad measurements cannot be made because the tone itself will be seen as noise and distortion. Make sure that the sinad meter automatically zeroes with only noise at the input. Internal adjustments are provided to properly zero the sinad meter and to properly adjust the notch or set the audio tone to exactly match the notch filter.
These are just a few examples of the sinad meter’s versatility. There are many more uses for this instrument, limited only by the imagination of the user.
Until next time-stay tuned!
Contributing Editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, With Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301.
Kinley’s email address is [email protected].
