The MICOR UHF Mobile - Theory and Operation

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UHF Mobile
An explanation of this
weird radio configuration
By Kevin Custer W3KKC
Comments by Mike Morris WA6ILQ

Most people that I help in the conversion of the MICOR UHF Mobile to repeater don't understand the operation of the radio set. The MICOR UHF Station actually uses a VHF exciter, with its own channel element, that is tripled to UHF, but the MICOR UHF Mobile uses a sample of the receiver channel element's signal, which has been multiplied, and is heterodyned with an offset oscillator to create the transmitter frequency. One Channel Element is required for each channel, but not a separate one for the transmitter and receiver. The frequency of the Channel Element is determined by the receive frequency and the transmitter frequency is determined by it and the amount of difference the offset oscillator produces. Frequency modulation is applied to the offset oscillator and not to the channel element oscillator. Crystal frequencies to cover the 430 to 450 MHz. range would run from about 17.43 MHz. to 18.26 MHz.

Since the receive channel element (the KXN1024A, 5 ppm, AFC capable and very common or the KXN1029A, 2 ppm without AFC and rare) is used to create the receiver local oscillator frequency, and because the radio could be set-up for transmit above or transmit below the receiver by the amount determined by the offset oscillator, this is difficult to comprehend, and is likely to cause confusion. To compound things, the tuning procedure in the manual is complicated and it is very difficult to move a 460.xxx unit to 440.xxx using the instructions supplied in the manual.

Units were also produced for 450-470 MHz split, and for the 470-512 MHz split. These may use 5 MHz, 3 MHz or in the 406-420 MHz range almost any offset (the repeater input-to-output offset).

The radio can be configured to transmit either above or below the receiver frequency. You select the appropriate "side" injection of the receiver to obtain a transmit frequency that is either above or below the receiver. The UHF MICOR normally operates on "low-side" injection.

Confusing? Well read on as this is the difficult part for most....

OK, So a sample of the receiver local oscillator is used to beat with an offset oscillator to create the transmitter frequency. If the offset oscillator is 11.7 MHz (the frequency of the receiver I-F) the radio will transmit on the receiver's operating frequency. If the offset oscillator is 16.7 (receiver I-F + 5 MHz) [most common] the transmitter frequency will fall five MHz above the receiver's operating frequency. Some radios have BOTH an 11.7 and 16.7 MHz offset oscillators; these are referred to as "Wide Spaced" radios from Motorola. Wide Spaced radios are capable of operating either simplex (talk around) or duplex (through a repeater). Wide Spaced radios are fine for building a repeater from, you will simply strap the 16.7 MHz offset oscillator on all the time and make sure the 11.7 MHz oscillator is disabled (pull the crystal?). In reality, you really want to use a regular radio for the repeater and save the wide spaced radio for use as a mobile - the ability to flip a switch on the control head and go from repeat to simplex on the output is very useful.

High and Low Side Injection, What's all this?
First, let's say you buy two channel elements on 444.000 MHz, one for high side injection and one for low side injection. The receiver actually hears on the same frequency (444.000), even though the crystals in the elements are operating on two different frequencies 23.4 MHz apart. So, when you order a crystal for either high or low side injection, the receiver frequency doesn't change... Why?

The MICOR receiver uses an 11.7 MHz I-F {intermediate frequency). Since the mixer would accept, and convert, a frequency that is 11.7 MHz above [higher], or 11.7 MHz below [lower] than the injection frequency, a means of selecting which image, either high side or low side, is to be received must be provided. If no selection is afforded, the receiver will copy signals on both frequencies with equal sensitivity. The preselector provides this function. In quality FM receivers, like the MICOR, a "front end" preselector is used ahead of the mixer to only allow the desired frequency, and a wee little on each side, to be processed. The design characteristics of the front end determines how well the receiver will deal with out of band signals. In cheap receivers, with little preselection, strong out of band signals, or adjacent in band signals, or both, could overload the mixer, or RF preamplifier, producing multiple undesired signals to be demodulated.

In the case of this radio set, we can determine what frequency the receiver hears on plus the side in which the transmitter frequency appears! How?

For repeaters with the transmit frequency 5 MHz HIGHER than the receive frequency (sometimes called "in low, out high" systems), this formula is used:

Xtal freq. (MHz) = (R - 11.7)/24 where R is the receive frequency in MHz.

In other words if your repeater is transmitting above its receiver frequency you need to buy a crystal for Low Side Injection.

For repeaters with the transmit frequency 5 MHz LOWER than the receive frequency (sometimes called "in high, out low" systems), this is the formula:

Xtal freq. (MHz) = (R + 11.7)/24 where R is the receive frequency in MHz.

In other words if your repeater is transmitting below its receiver frequency you need to buy a crystal for High Side Injection.

Since in Commercial Service almost all repeaters listen above their transmitters and the radios were used to operate INTO a repeater, they used low side injection (for the most part) from the factory. Since you may be building a repeater from this radio, it needs to be configured to operate the way you want the transmitter to appear.

Note from WA6ILQ:
If you are going to use a duplexed MICOR mobile either as a repeater or as a mobile, AND you "flip it over" and make it transmit on the other side from its original factory configuration AND you chose to use the receiver's Automatic Frequency Control function (i.e. use a common KXN-1024 element) THEN you need to "flip over" the AFC diodes - they are used only to generate the AFC (DC) voltage only, they are not part of the discriminator (or just do it the easy way and disable the AFC).

Here's why:

If the received signal drifts off frequency, the AFC diodes generate a varying DC voltage that "pushes" the channel element back onto the center of the signal. In the normal environment the tracking action of the AFC voltage (about ± 0.7vDC) keeps the receiver channel element on frequency, and when the user keys the transmitter with the PTT button the AFC voltage immediately is clamped to zero volts DC and the transmitter transmits on the center frequency of the channel. This works beautifully in a simplex or as a user radio in a normal repeat environment.

But when you duplex the radio (either as a duplex mobile or as a repeater) the AFC voltage is either grounded or stays active when the transmitter is keyed, depending on which duplex mod you followed. If the AFC voltage is left active then as the receiver tracks an off frequency carrier the AFC voltage will pull the transmitter off frequency in lock-step. This is not a good thing if the repeater transmitter is off frequency and the repeater receiver is OK. It gets worse if the radio is flipped over and the AFC diodes aren't, then when the receiver hears an off frequency signal the AFC voltage pushes the transmitter off frequency in the wrong direction !! The simple solution: either ground the AFC line or swap the KXN1024 channel elemnt for a KXN1029 (which ignores the AFC voltage). The correct solution (if you plan on using AFC): flip the DC voltage over (by flipping the diodes over).

Well, you say, unsoldering and removing diodes and reinstalling them in the opposite direction isn't that difficult! True, in theory. But in reality, there is a problem with reversing the diodes in a UHF MICOR mobile: Motorola used very fragile (electrically and mechanically) glass diodes and in about ten different radios I have NEVER been able to remove even one of them without either breaking it in half or destroying the diode action (open or shorted) no matter how careful I was. In four cases where I was building full duplexed wide-spaced 90 watt 12-channel UHF mobiles I simply ordered new diodes before I even benched the first radio, and after the new diodes arrived I pulled the old ones with zero effort expended at salvage, then installed the new ones in the opposite direction. In the previous cases I either removed the diodes (destroying them in the process) and then grounded the AFC line, or I simply ignored the diodes, saved my money (i.e. not buying any replacement diodes), and my time by just grounding the AFC line. By the way, the easiest way to ground the AFC line is to short C115, a 10uf cap, which is located in one corner of the receiver board near the channel element selection diode matrix. I just took a piece of a resistor lead, bent it into a horseshoe shape and soldered it down AFTER I laced the string from a shipping tag through the loop - with a black fine-tip Sharpie I wrote on the tag "Field Mod: AFC disabled (grounded) by shorting C115 when radio converted to low-TX/high-RX on (calendar date) by WA6ILQ".

Yes, just switching to a KXN1029 high-accuracy receiver channel element disables the AFC, but my own suggestion is to ground the AFC line no matter what - someone later on may switch elements on you not knowing if the AFC has been disabled or not (and besides, the KXN1024 is much, much, much more common than the KXN1029... save the rare high-accuracy elements for the hilltop repeater receiver). And I'd consider disabling the AFC in any MICOR mobile used in point-to-point amateur link or repeater service - if a radio drifts off frequency I want to know about it BEFORE it drifts too far (i.e. the link receiver could track the drifting transmitter and hide the drift). Or you can make it a switchable function from the repeater controller: take a leftover repeater controller function that goes to ground and wire it to close a reed relay mounted in the receiver chassis (with the contacts wired to short out C115). With the function off the AFC is normal, with the function on the AFC is grounded. Reed relays are simple, reliable units and when using one as an AFC on/off test you aren't liable to wear it out very soon.

The designer of the "real" MICOR base stations and repeaters used a separate highband exciter followed by a tripler which precludes the receiver AFC from pushing the transmitter off frequency - there is no connection between the two chassis - so I'd either leave the repeater receiver AFC enabled or make it a controllable function (as mentioned above).
Note that the channel element used in the UHF station (in the highband exciter) is NOT the normal KXN1019 high band element - the one specified for the transmitter in a UHF station is the KXN1052. It's essentially the same as a KXN1029 except that the modulator in the element is modified to generate 1/3 of the deviation (compared to the high band equivalent) to compensate for the tripler needed to go from highband to UHF.
End of note from WA6ILQ

So how does the MICOR mobile UHF transmitter work? The third harmonic of the receiver channel element is applied to the multiplier in the receiver. This multiplier has two outputs, one feeding the receiver mixer and one is applied to a jack on the receiver casting to be connected to the transmitter mixer. The offset oscillator described above is also applied to the transmitter mixer along with the injection produced by the receiver oscillator / multiplier chain. The mixer output is amplified and the transmitter frequency is selected from the mixer spectrum by the exciter output filter. Many harmonics are produced by the mixer so it is essential to align the output mixer filter properly otherwise the radio set will not be operating on the intended frequency. The output filter will attenuate all undesired harmonics to 85 dB below the desired carrier signal. As a function of mixing the relatively high injection frequency with the low frequency of the offset oscillator, a highly stable transmitter frequency is produced even though the stability of the offset oscillator(s) are approximately 10 to 15 ppm. The offset oscillator's contribution to the transmitter frequency error is never more than 0.5 ppm. The offset oscillator is modulated true FM so wide audio bandwidth is available and the modulator is capable of direct PL or DPL modulation. The IDC pot does affect the PL deviation as the PL signal is mixed into the audio chain before the IDC control.

Bomar Crystal Manufacturing can make you a crystal for your KXN1024A (5 ppm accuracy), or your KXN1029 (2 ppm accuracy) receive element, or your KXN1052 station transmit element.

Here are articles to help in the conversion of the MICOR UHF Mobile to repeater service:
Conversion of the UHF MICOR Mobile to a repeater, by Jim Reese WD5IYT
Modification of the Low Level Amplifier for dependability, by Jeff DePolo WN3A

Final note from Mike WA6ILQ:
The MICOR Mobile has a heat sink rated at only 35 watts - yes, that fact is in the MICOR manual - and therefore radios higher than 45 watts are best left in mobile service, or you can "downgrade" a higher power mobile to a lower power level by removing the final sections of the PA deck to lower the output power to the 45 or 25 watt level. You want to do this rather than just turning the power level down since the radio does not run cooler at a lower power level setting. Just study the manual and compare the assembly numbers from the higher power radios and the 45 and 25 watt radios. You will discover that the driver for the 75w radio is the same board used in the 25 watt radio and the driver for the 100 watt mobile is the same as the 45 watt radio. And besides, the removed sections will provide spare parts for the repeater's high power PA deck.
End of note from WA6ILQ

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This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.