Programming the MC145170 PLL with a PC


When I first got my Technician Class Ham License (similar to Limited in the USA)  I was keen to get started and build some basic VHF equipment as my interest was more in the construction and experimentation than  operating 'off the shelf' equipment.

I found out fairly quickly that building a stable VHF oscillator was harder than it first appeared (to me at least).  I built a couple of FM radio microphone type transmitters that worked but drifted terribly.... and that was at 100mhz.... what was going to happen at 144mhz which was that target?

After lots of reading I finally decided I needed to go with the PLL controlled VCO to get the stability and precise frequency selection that I wanted.  The Motorola web site had lots of information on PLL's and application notes but the loop filter calculations and design looked a little out of my league without any real test equipment.

Note:  In the next section I refer to the EXP-1 kit from DC Electronics.  The software available on this page is not restricted to the EXP-1 kit.  It can be used with any MC145170 PLL controlled oscillator.

After a lot of browsing the internet a solution appeared in a kit from DC Electronics called the EXP-1.  The EXP-1 is a PLL experimenters kit.  It comprises a Motorola MC145170 PLL with loop filter and a preprogrammed PIC chip allowing selection of any frequency from 0..185mhz using the on-board keypad.

To build a complete oscillator the constructor needs to construct a VCO on the development section of the EXP-1 board.  DC Electronics provide some good oscillator examples in the application notes that come with the kit.

Constructing the EXP-1 kit should be straight forward for anyone with basic soldering skills and a reasonably fine soldering iron.  Comparing the parts with the circuit diagram helped prevent a couple of errors due to incorrect component references on the PCB and explains the couple of 'spare' parts that are left at the end.... a resister and a capacitor which the circuit shows are used in the connections between the loop filter/PLL and the audio input respectively. 

Having completed the kit is was time to build an oscillator. 

The application notes provide some good examples (although a little light on the theory of operation..... particularly calculating L/C resonant frequency with varactor diodes providing the capacitance) of VHF VCO's with recommended components for selected frequencies.  I picked the lowest component count option which required a Motorola MC1648, a shielded coil, 2 capacitors and a dual varactor diode (all of which can be ordered from DC Electronics with the EXP-1).

I built the oscillator on a small 'daughter board' rather than directly on the prototyping area.  This probably has an effect on the overall performance but surprisingly everything worked first time and after keying 14400 on the keypad and setting the appropriate frequency on my scanner there was the carrier......


Programming the Motorola MC145170 PLL

In the EXP-1 kit the MC145170 is configured by a preprogrammed Microchip PIC.  This works well to get the kit running but if you want a different frequency step from the supplied 10khz (as I required in NZ as I want to use the kit as a basis for an 144mhz transmitter) you are a little stuck because the code for the PIC is not supplied and writing new programs for the PIC is more of a challenge than most people would want to attempt.

Reading the Motorola data sheets and looking at the connections in the EXP-1 kit it appeared that if a PIC could program that chip so could a PC with the appropriate software and an interface providing the required signals.

The beauty of the MC145170 is that the programming involves toggling 3 lines between 0 and 5 volts.... something that PC's do with the parallel printer port.

The interface requires 3 data lines from the PC's parallel port connecting to the ENB, DIN and CLK pins of the MC145170.  The parallel port ground also needs to be connected to the PLL circuit ground.

Suggested PC to PLL connections are as follows:

PC Parallel Port MC145170 PLL
Pin 2 (Data 0) --- Pin 5 (DIN)
Pin 3 (Data 1) --- Pin 6 (ENB)
Pin 4 (Data 2) --- Pin 7 (CLK)
Pin 28 (Ground) --- Pin 12 (VSS)

Connecting the PLL to the PC will probably require that you remove the existing PIC used to program the PLL.  This is because many hold the ENB pin at +5v which masks the level changes from the PC.