FM TUNE UP

Description: At some point, every amateur radio operator working with FM radios needs to tune the transmitter and receiver. For those with access to professional test equipment, this task is straightforward. However, for those without such equipment, a more inventive approach is required. The principles discussed here are applicable to popular Motorola radios, such as Mitrk and Maxar, as well as most other crystal-controlled amateur and commercial radios. Understanding local oscillators, mixing, and intermediate frequency (IF) strips is crucial; it is advisable to consult the ARRL handbook for further information on these subjects. Setting the frequency of both the transmitter and receiver in a crystal-controlled radio is generally simple. Frequency counters suitable for 2-meter and 70-cm bands are widely available and affordable. A reliable counter, when calibrated, provides an excellent foundation for test equipment at a reasonable cost. With a counter, aligning the transmitter frequency is relatively easy. It is important to allow all equipment, including the radio being serviced, sufficient time to warm up for oscillator stabilization, ideally 20 to 30 minutes. Ensuring that all test equipment is calibrated is essential. A calibration check of personal gear is advisable before working on sensitive devices like a 9600 baud data radio or a repeater. It should be noted that many inexpensive ham-type counters lack a crystal oven, making them susceptible to frequency drift. If the counter utilizes a time base crystal that aligns with one of the WWV standard frequencies, zero-beating the counter's time base against the standard becomes simpler. Before purchasing a counter, verify the calibration procedure. Confirm the calibration of all test equipment prior to performing critical tasks. Setting the frequency of an FM transmitter is straightforward with a counter, as the frequency can be read directly from the digital display. Receivers can also be tuned using the technique of measuring the local oscillator frequency with a service monitor or counter. Once confident that the test equipment has stabilized, loosely couple the counter or service monitor into the local oscillator section of the radio. This technique is applicable to any FM radio. A patch cable with a BNC connector on one end and alligator clips on the other is suitable for this testing. The cable should be connected to the frequency meter, grounding the cable's shield to the radio chassis. The center conductor of the coax can be used to probe the local oscillator section of the radio until sufficient RF for measurement is detected. If RF is not found, consulting the radio's schematic to temporarily connect the counter to the local oscillator output with a small capacitor (5-100 pF) may be necessary. Once a stable reading is achieved, calculate the correct local oscillator frequency by subtracting the IF frequency (commonly 10.7 MHz) from the desired receive frequency. For instance, if the receive frequency is 445.000 MHz, the local oscillator frequency can be determined. After setting the receiver local oscillator frequency, checking the discriminator tuning is advisable. Proper tuning of the discriminator is critical for optimal audio recovery and minimal audio distortion. Motorola radios utilize a quadrature detector, which typically requires adjustment of a single slug-tuned coil. Adjustments to this coil are infrequent, but if necessary, a signal source precisely at 10.7 MHz (or the relevant detector frequency) is required.

The tuning process for FM radios involves several critical steps to ensure optimal performance. The first step is to prepare the equipment, ensuring that all components, including the radio and test equipment, have adequately warmed up. This is crucial for the stability of the oscillators within the devices being tested. Following this, verifying the calibration of the frequency counter is essential, especially if the counter lacks a crystal oven, as this can lead to inaccuracies.

Next, the procedure to align the transmitter frequency can be initiated. The counter should be connected to the transmitter output, and adjustments can be made until the desired frequency is achieved, as indicated on the counter's display. For receiver alignment, the local oscillator frequency must be measured. This is accomplished by coupling the counter into the local oscillator circuit, which may require the use of a schematic for proper connections if the RF signal is not readily accessible.

Once the local oscillator frequency is determined, it is vital to ensure that it is correctly set by performing the necessary calculations with the IF frequency. After achieving the correct local oscillator frequency, the discriminator tuning should be checked. This involves ensuring that the quadrature detector is properly adjusted to minimize distortion and enhance audio clarity.

Overall, meticulous attention to calibration, measurement techniques, and adjustments is required to ensure that FM radios operate effectively and reliably.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. 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 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). 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: 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