Concept:
This page explains a procedure for converting a MICOR high-band Phase Modulated (PM) exciter to True Frequency Modulation (FM).  The procedure involves modifying the exciter and installing a KXN1019B True FM channel element.  The PM exciter is easily identified by the existence of a DIP chip.  Many of the PM exciters used only 3 pins to interface to the channel element but some later PM exciters had 4 pins, we commonly refer to these as 4 pin phase modulated exciters here in the shop.  This procedure requires either the 4 pin model or a 3 pin model modified to allow connection to the fourth pin in some way.  The KXN1019B element was designed originally for the MICOR Mobile that was used in DPL operation.  This channel element differs from the original element (K1007) in that it utilizes a True Frequency Modulator inside the channel element itself.  Modulating audio is applied to this extra pin of the channel element.  The crystal in the FM element is different, so you won't be able to use the one from your K1007.  The crystal is much smaller to accommodate the extra modulator parts inside the element, and it is cut so it can be modulated with a varactor diode and have reasonable deviation linearity.

Note:
For simplicity, only the later model 4 pin PM exciter is explained in this modification.

An overview of the PM exciter:
From the factory, audio going into the Mic High pin goes through a lot of changes before it is applied to the PM modulator.  First, the mic audio is is pre-emphasized to be applied to the Clipper (the limiter).  Audio is then limited in amplitude by the clipper and low pass filtered by the Splatter Filter.  The output of the limiter/filter is controlled by the deviation (IDC) potentiometer and then it is de-emphasized.  This audio is applied to the Phase Modulator (PM) where it is yet again pre-emphasized.  The PM modulator automatically pre-emphasizes the applied audio as a function of the modulator.

How the FM modification works:
Repeaters that de-emphasize the receiver audio:
Audio that has been de-emphasized somewhere in the path will need to be pre-emphasized to be transmitted.  PM modulation adds this pre-emphasis as a function of the modulator, however, the new FM modulator doesn't automatically add its own pre-emphasis.  If your repeater de-emphasizes the audio at some point it will need to be re-emphasized.  For this type of repeater, audio from the 'processing' (blob) (the tall blue chip) which has been pre-emphasized, limited and filtered is applied to the modulation input pin (pin 4) of the FM channel element.  Since this exciter pre-emphasizes the audio prior to its application to the peak limiter stage, (the blob) the output result can be applied directly to the FM channel element.  (Normally the audio would be de-emphasized again only to be re-emphasized by the PM modulator method)  The transmitted audio will follow the 6 dB/octave pre-emphasis slope up to the limit of the Splatter Filter which is about 2500 Hz.  The Splatter Filter is also in the blue limiter chip and cannot be changed (by individual part substitution) to add audio bandwidth.  If this bandwidth is not acceptable, the internal path/processing can be bypassed and a better outboard limiter/filter application should be fitted.  Such Audio Processing can be obtained from a few sources.

It was found that the pre-emphasis curve in the PM exciter poses a low end bump.  This was likely done to compensate for the lack of modulation ability on the part of the PM method in this exciter.  We believe the Serrasoid Modulator doesn't follow the ideal pre-emphasis curve so Motorola designed in some circuitry to help correct for this.  This correction circuitry needs to be removed to allow the exciter to follow the pre-emphasis curve exactly.  This is done by removing R404 and C420 completely, and changing C408 from .068 uF to .047 uF.  The response curves of a stock exciter and the modification as described can be viewed on the link below.
Curves for comparison (click here)

Modification Procedure:
Remove the following:
R401 - 330 ohm.  (if mic bias is not needed)
R404 - 5.6 K.
C420 - .033 uF.
R412 - 22 K.
C415 - .33 uF
C421 - 4.7 uF Electrolytic, (disables PM modulator)

Install a 4.7 uF electrolytic non polarized capacitor (can be the old C421) between the non common ends of where R412 and C415 were located. (See figure below)

Short the trace from IC401 Pin 15 to the trace that runs next to it. (See figure below)

Cut keyed 9.6v trace as shown below.

Jumper from marked point to constant 9.6v

(The above procedure will stop frequency drift by keeping the first stages of the transmitter running continuously.)

Install a KXN-1019B Element on your frequency.

You should now be true FM'ing with the advantages of peak limiting, and BPF'ing.

Repeaters that don't de-emphasize the receiver audio:
If "flat audio" throughput is going to be conveyed through the repeater, there is no need for the exciter to add its own pre-emphasis, as pre-emphasized audio has been used throughout the repeater audio path.  The exciter can be modified to use the limiter/filter in the blue blob, but not add the pre-emphasis or de-emphasis that was needed in the PM scheme.  Since pre-emphasized audio is going to be applied to the blue blob, it will still do the same job it was intended for.  Audio will not need to be de-emphasized after the processing because the FM modulator doesn't automatically pre-emphasize it.  The transmitted audio will follow the pre-emphasis slope of the user up to the limit of the Splatter Filter which is about 2500 Hz.  The limiter in the blue blob will still be functional and will amplitude limit the audio so over deviation doesn't result.  The Splatter Filter is also in the blue limiter chip and cannot be changed (with individual part substitution) to add audio bandwidth.  If this bandwidth is not acceptable, the internal path/processing can be bypassed and a better outboard limiter/filter application should be fitted.  Such Audio Processing can be obtained from a few sources.

Modification Procedure:
Remove the following:
R401 - 330 ohm.  (if mic bias is not needed)
C408 - .068 uF.
R412 - 22 K.
C421 - 4.7 uF Electrolytic, (disables PM modulator)

Change the following:
R404 from 5.6 K to a 1 K.
C415 from .33 uF to a jumper wire.
Switch polarity on the 4, 4.7 uF tantalum caps that feed the modulation pin.
Jump between the wiper of the IDC pot and the modulation pin bus.
Install a KXN-1019B Element on your frequency.

You should now be true FM'ing with the advantages of peak limiting, and BPF'ing.
 
 

HTML Copyright September 19 2002, Kevin K. Custer
All Rights Reserved.