ARTICLE FROM MARCH 1997

TUNING FM RADIOS, PART 1

At some point every ham working with FM radios has need to do some tuning of the transmitter and receiver. If you are like me and lucky enough to have professional test equipment, this task is easy. If you don't have access to professional equipment, you just have to be a little more clever.

Note that a good deal of what I have to say here applies equally to the popular Mitrk and Maxar Motorola radios along with most other crystal controlled amateur and commercial radios. The basic principles are the same. If you become confused by what I have to say about local oscillators, mixing and "IF strips," I strongly recommend reading about those subjects in the ARRL handbook. The knowledge you will gain you will find extremely valuable in the future.

EQUIPMENT

Setting the frequency of both the transmitter and receiver in a crystal controlled radio is pretty straight forward. Frequency counters that work on the 2 meter and 70 cm bands are common place and affordable. A decent counter in calibration gives you a good test equipment foundation that is hard to beat for the cost.

With a counter transmitter frequency alignment is pretty simple and straight forward. First off allow all of your equipment (this includes the radio to be serviced) time to warm up so all of the oscillators are stabilized. I would wait at least 20 to 30 minutes.

Make sure that YOUR test equipment is in calibration! It sure does not hurt to run a calibration check of your own gear before working on something as sensitive as a 9600 baud data radio or a repeater. If you are using a ham type counter keep in mind that most inexpensive counters do not have crystal oven. This means that it is very easy for your counter to be off frequency and lead you astray.

If you are lucky, your counter will use a time base crystal that is on one of the WWV standard frequencies. This makes zero beating the counters time base crystal against the standard very easy. Before you buy a counter, check what the calibration proceeder is. Check the calibration of all of your test equipment before doing any critical work.

SETTING THE FREQUENCIES

Setting the frequency of a FM transmitter is easy with a counter as you can read out the frequency directly on the digital display. You can also set the frequency of receivers using the old two way service techs trick of measuring the local oscillator frequency with your service monitor or counter.

When you are confidant that your test gear has stopped drifting, loosely couple your counter/service monitor into the local oscillator section of the radio. Note that this trick works on ANY FM radio. I use a patch cable with a BNC connector on one end and a pair of alligator clips on the other for this type of testing. Connect the cable to the frequency meter and ground the cables shield to the radio chassis.

Fish around the L.O. section of the radio with the clip connected to the center of the coax. At some point (like on a shield can) your going to find enough RF to make a measurement. If you can't you might have to consult the radio's schematic and temporarily connect the counter into the L.O. output with a small value capacitor (5-100pF).

LOCAL OSCILLATOR FREQUENCY

When you get a stable reading, calculate the correct L.O. frequency. For example on the Maxar you subtract the I.F. frequency (10.7Mhz is the most common) from the desired receive frequency. The result is the frequency that the L.O. signal should be on. Lets use the example of a data radio on a receive frequency of 445.000Mhz:

445.000 - 10.700 = 434.300Mhz L.O. frequency

NOTE: Most FM radios use 10.7 as the I.F. frequency. Odd exceptions are Motorola Micor radios that use 11.7Mhz and old Bearcat scanners that use 10.8Mhz.

After you've set your receiver local oscillator on frequency you might want to check the discriminator tuning. Proper discriminator tuning is critical to obtaining good audio recovery and low audio distortion. The Motorola radios use a quadrature detector whose only adjustment is a single slug tuned coil. Rarely will you need to adjust this coil. If you do, you will need a signal source exactly on 10.7Mhz ( or what ever the detector frequency is).

If you lack a signal generator, you can build yourself a simple crystal oscillator for the popular I.F. frequencies. JAN Crystals will make a 10.7Mhz crystal for $11-$15 depending on tolerance. With a crystal and a few junk box parts, you have yourself a very useful alignment oscillator. The schematic shown has a simple circuit using a common 7400 TTL gate that can be easily adapted to for the job.

DUAL CONVERSION

Many radios are dual conversion. That is they convert the frequency of interest (say 146.52 for example) first to 10.7Mhz and then convert the 10.7Mhz IF frequency down to 455Khz for detection. In all cases I have seen the second IF conversion is crystal controlled. Unless in the very unlikely case this oscillator crystal has drifted, you can use your 10.7Mhz signal generator to align the entire IF. Remember that the 10.7Mhz signal you inject into the IF strip is converted down to 455Khz.

This system of double conversion allows the designer to use less expensive ceramic filters to determine the radios selectivity. In the single conversion design where all amplification and detection is done on 10.7Mhz, expensive multipole crystal filters must be used. Double conversion is used in most of the low cost radios using the popular Motorola MC3357 series IF/detector IC's where all of the IF and detector circuitry is combined into one IC.

Getting back to setting discriminators, keep in mind that there are many different types of FM detector (discriminator) designs. The Maxar, for example, has you tune the detector coil (L23) for a specific voltage (5.1 volts +/- 0.2 volts) at a test point (pin 13) on the rear chassis connector. Other radios will require different to different specifications. For this reason it is vital to have the service manual for instructions when working on any radios IF and detector.

If you choose to build a 10.7Mhz test oscillator it has another use as well. You can use it to "zero beat" your radios local oscillator crystal on frequency. Let's say for example you want to set a crystal for 145.01 receive on frequency. Have a friend transmit a carrier with no modulation on that frequency. Loosely couple the signal from your 10.7Mhz alignment oscillator into the radio. Vary the coupling of the 10.7Mhz signal till you can clearly hear a beat note. Then adjust the receive crystals trimmer for zero beat.

HERE'S HOW IT WORKS:

What you are doing is beating the signal of your 10.7Mhz oscillator against the incoming received signal after it has been converted to 10.7Mhz by the radios mixer and local oscillator. If there is any difference in frequency between the two signals after mixing, a beat note will be generated. For example, lets say that your new crystal for 145.01 receive is 1Khz low in frequency.

145.01Mhz - 134.309 = 10.701 Mhz

That is, the signal in the IF is now 1Khz HIGH in frequency. You mix that with your alignment oscillator signal and:

10.701Mhz - 10.700 = 1Khz tone heard as the beat note!

As you trim the local oscillator crystal on frequency, the error decreases and the frequency of the beat note you will hear will decrease. This method works so well that many professional service monitors use it in preference to digital displays. In their case the error frequency is displayed on an analog meter as well as hearing the beat note on a meter.

CHALLENGE

Next month I'll discuss some methods of tuning receivers for best sensitivity. Till then I'll issue this challenge: Is there a cheap microprocessor crystal available that can be divided down to 10.7Mhz or 445Khz? Look through the lists of available frequencies in the parts catalogs and let me know if you find one.

David Metz, WA0AUQ
Comments to davemetz@muscanet.com
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