# Is your instrument fooling you?

Have you ever had times when you just couldn't believe what your test equipment was telling you? If not, you probably haven't been working in the field

Have you ever had times when you just couldn’t believe what your test equipment was telling you? If not, you probably haven’t been working in the field of electronics very long. Test equipment is getting more and more reliable as far as accuracy is concerned. However, the point has not yet been reached where test equipment is totally infallible. There are many things that you can do to keep your equipment more or less calibrated. We will take a look at some of the methods that can keep your test gear from taking you on a wild goose chase.

Bessel-zero method The Bessel-zero method is great for checking the calibration of your deviation-measuring instruments. According to Bessel functions, when a carrier is frequency-modulated at various values of modulation index the carrier will be completely nulled out. For our purposes in land mobile radio we need only be concerned with the 1st and 2nd nulls occurring at modulation indices of 2.405 and 5.52.

Before going further into this method, let’s look at the definition of modulation index, which can be determined mathematically by dividing the frequency deviation by the modulating frequency. If a modulating tone of 1kHz is applied to an FM transmitter and causes a deviation of 2kHz, then the modulation index is 2.0. As long as we use a 1kHz modulating tone, the modulation index is easy to calculate. It is equal to the frequency deviation in kilohertz. This is one good reason for using a 1kHz modulating tone for such a test. The other good reason is that an accurate 1kHz tone is easy to come by because it is used so frequently in transceiver testing. Most service monitors, as well as sinad meters, provide an accurate 1kHz tone source.

Suppose you want to check the accuracy of your service monitor’s deviation calibration. See Figure 1, above. Start with an unmodulated carrier and, using a 1kHz modulating tone, gradually increase the deviation level until the carrier disappears, leaving only the 1kHz-spaced sidebands. At this point, the deviation level is 2.405kHz, and this is what the service monitor should indicate, as shown in photos 1 through 4 on pages 48-49.

The spectrum analyzer must have sufficient resolution to resolve the 1kHz sidebands into separate and distinct displays on the spectrum analyzer. Without sufficient resolution, you will not be able to tell when the carrier has been nulled out.

Once the deviation increases above 2.405kHz, the carrier will reappear and will null again at 5.52kHz. The first two nulls are all we need to be concerned with for checking the calibration of instruments for land mobile radio. If your deviation meter indicates anything other than 2.4kHz at the first null, then your instrument needs calibrating. Double-check this at 5.52kHz for the 2nd null.

What about the accuracy of the 1kHz modulating tone? It is important that the modulating tone be accurate. The accuracy of the modulation index will follow the accuracy of the tone frequency. For example, if the 1kHz tone is actually 995Hz, then this represents an error of 0.5%, and your results from using this modulating tone will be in error by 0.5%. This represents an accurate calibration for a deviation meter, one that I will gladly accept for servicing purposes.

sinad meter It is a good practice to check your sinad meter frequently to make sure that the tone used for sinad measurements is centered in the notch of the sinad meter. If the 1kHz tone is not accurate, or if the notch filter is not properly tuned to the tone frequency used for sinad measurements, the accuracy of the measurement will suffer. To ensure that the tone you are using for sinad measurements is centered in the notch of the filter, feed the tone directly into the sinad meter and observe the sinad reading, which should indicate 24dB sinad or better. More is better! Adjust the frequency of the tone up or down a few hertz to see if any improvement can be made. If so, check the tone frequency against one known to be accurate. If the tone frequency is accurate, adjust the notch of the sinad meter for best sinad reading (maximum null of the 1kHz tone). Tone generators built into sinad meters are accurate. For example, the Helper Instruments’ S105 tone generator is accurate to +/-1Hz.

If you are ever in doubt about the accuracy of your service monitor frequency measurements, there is a simple way in the field to get a quick rough check on the calibration. Through the years, I have noticed that one of the television stations in my area seems to keep tighter than other stations. I have come to rely on this station to occasionally check my service monitor’s frequency calibration. This particular station has its visual carrier at 175.26MHz. I have never seen more than a 40Hz disagreement between my service monitor and this television signal. If I am out in the field and have a need to check on my frequency monitor, I can easily tune in this television signal for reference.

Summary I don’t mean to imply that by using these methods you will never have to send your service monitor off to the factory for recalibration to factory specifications. But on occasion you can use these methods to keep on track and avoid a frustrating and often time-consuming wild-goose chase. By making frequent checks on your test equipment, you can also avoid the embarrassment of improperly adjusting customers’ radios to your inaccurate test equipment.

Maxtrac repeater response First, a couple of notes about the Maxtrac repeater combo column that appeared in November 1998. Jack Tibbetts of Cellular One in Boston pointed out that the diode labeled “2N4148” should be “1N4148.” That’s correct Jack and thanks for pointing it out. I hope that didn’t confuse too many of you. Rob George wanted to know if the same kind of arrangement could be made with GM300 model radios. Coincidentally, Earl S. Garber emailed to say that such an arrangement could be done with Radius or GM300 models using the 16-pin connector on the back without making any internal modifications to the radio. Earl also said that a 4.7k resistor seems to reproduce the deviation better than a 1.8k resistor. Thanks to all who emailed comments, and good luck with your individual projects.

Until next time-stay tuned!

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