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AMATEUR RADIO By GARRY CRATT, VK2YBX What's all this talk about SINAD? SINAD measurements (signal plus noise and distortion) are now the accepted method of evaluating FM receiver sensitivity, having replaced the old "20d.B quieting" method. Here's a quick rundown on what SINAD measurements are all about. Many amateurs will be familiar with valve transceivers. They were used in large numbers across Australia by amateurs keen to operate on the VHF and UHF bands. To keep costs low, some amateurs converted retired commercial equipment, some of which is still in use today. When those transceivers were originally manufactured, receiver sensitivity was measured using the "20dB quieting" method. Basically, this involved connecting an AC voltmeter across the loudspeaker output terminals, opening the receiver squelch, and measuring the noise voltage with the volume control set to some convenient level. An unmodulated carrier was then applied to the receiver and the signal level adjusted until the out- put noise was reduced by 20dB (ie, by a factor of 10). When the - 20dB level was reached, the signal generator level was noted and this became the "20dB quieting point". Receiver sensitivity was then specified as so many microvolts (µ V) for 20dB quieting. A typical figure was 5µ V. This method was popular because it was simple but it could sometimes give misleading results. Let's consider an example - a receiver that has a very narrow bandwidth due to misalignment. In this case, the 20dB quieting method will still indicate normal sensitivity. In practice, the receiver would be unable to receive intelligible signals due to severe distortion. To receive FM correctly, the receiver must have adequate band- width to include all the important FM signal components. For example, if the receiver is required to correctly demodulate a ± 5kHz signal, then it must have a bandwidth of at least 10kHz. If this bandwidth is reduced by misalignment, the receiver will not recover intelligible audio. This was the major disadvantage of the "20dB quieting" method. SINAD measurements SINAD measurements do not suffer from the above problem. Basically, SINAD is a measure of the quality of a communications signal. When a signal is introduced into a receiver, the output of the receiver will consist of the original modulating signal plus system noise and receiver distortion. Thus SINAD is defined as: Signal + Noise + Distortion Noise + Distortion Expressed in dB, the equation becomes: 20.Log Signal + Noise + Distortion Noise + Distortion Because distortion can be considered as a type of noise for this application, the equation simplifies to: 20.Log[Signal/Noise) The "Sinadder Linear 5" is part of a range of SINAD meters made by Helper Instruments Company, USA. The local distributor is RF Devices Pty Ltd, 9 Lyn Pde, Hoxton Park 2171. 92 SILTCON CHIP The big advantage of the SINAD method is that it measures both signal and noise, unlike the 20dB quieting method which only measures noise. In fact, SINAD measurements in RF equipment are similar to THD (total harmonic distortion) measurements in audio gear. To measure SINAD, we apply an FM SIGNAL GENERATIJR ANTENNA RECEIVER SPi::ER With an RF input signal of 20µ,V, the noise and distortion should be low. So when the meter is switched to the SINAD mode, the reading will be very low, say - 40dB or more. Reducing the RF signal will progressively increase the noise and distortion so the SINAD reading will rise. For today's amateur gear, the RF input will have to be reduced to around 1µ,V or less to obtain a reading of - 12dB SINAD. Fig.2 shows the test setup if no SINAD meter is available. \ SINAD METER lkHz MODULATION 3kHz PEAK DEVIATION Fig.1: basic test setup for measuring SINAD. Initially, the set level control on the meter is adjusted for a reading of OdB, then the FM signal generator output is reduced until a reading of - 12db is obtained. o-------FM SIGNAL GENERATOR ANTENNA RECEIVER 1kHz MODULATION 3kHz PEAK DEVIATION lkHz NOTCH FILTER v Receiver alignment Fig.2: this test setup can be used if no SINAD meter is available. SINAD measurements are similar to total harmonic distortion (THD) . measurements in audio gear. RF signal modulated at lkHz to the receiver and connect a SINAD meter to the receiver output. Fig.1 shows the basic setup. The peak deviation (of the lkHz test signal) should be set to 3kHz while the RF signal level should be adjusted so that the receiver hard limits. Initially, the meter should be switched to the "AC volts" range and the set level control adjusted for a reading of OdB. After that, it's simply a matter of switching to "SINAD" and reducing the generator output until a reading of - 1ZdB is obtained. The sensitivity of the receiver can now be determined simply by noting the generator output level. What actually happens inside the SINAD meter is this: when the instrument is switched to "AC volts", we get a reading of the combined signal plus noise output. When the unit is switched to "SINAD", a lkHz notch filter is switched into circuit. This filter removes the lkHz fundamental but allows all other frequencies to pass. So, in the SINAD mode, we simply get a reading of the noise and distortion components. If no noise is present, the reading is simply a measure of the distortion. Dynamic balancing for fans rite magnet from an old louspeaker. With one side sitting firmly on the centre of the piezo transducer, and the other side resting on the cabinet, it gave the transducer quite a workout when the fan was turned on. The voltage reading with a 10MO load was a steady 5V AC (straight into the input of an AC voltmeter). Next I mixed up a small amount of 5-minute epoxy and coated the underweight joint on the fan. The "vibration" reading decreased to about 1VAC. Had I gone too far, or not enough? Attaching a small piece of sticky tape dropped the reading to 0.9V, so I mixed up some more 5-minute expoxy and applied what I guessed to be ten times the mass of the piece ctd from p.87 of sticky tape. The next reading was quite a surprise - only 30mV AC. Bingo! I had hit near perfect balance after only a couple of tries - admittedly more by luck than anything else - but it was reassuring to find 0 Manufacturers often specify various test points on their circuits to assist alignment. To align the transceiver in the conventional way, a suitable signal is injected and the circuit is tuned for maximum output. However, maximum output does not necessarily indicate the optimum tuning point, particularly when considering RF amplifier stages and mixers: Tuning for maximum gain is not the same as tuning for best signal-to-noise ratio. In some cases, it's possible to improve receiver sensitivity by as much as 3dB by realigning the circuit to obtain the best SINAD reading. This is equivalent to doubling the transmitter power at the other end! ·~ that putting a small piece of tape anywhere on the fan rotor only increased the vibration, regardless of where I put it. All's well that ends well - in fact, the cabinet vibration of this particular air cleaner is now an order of magnitude better than one straight out of the factory! ~ ,, P ~ ~ -~ . ao_1 o c,ILPf ~~ ·co ~~ RCS Radio Pty Ltd is the only company which manufa.c tures and sells every PCB & front panel published in SILICON CHIP, ETI and EA. ,.'Jc 651 Forest Road, Bexley, NSW 2207 Phone (02) 587 3491 for instant prices NOVEMBER 1988 93