Using the MC2000X4-001W 10MHz OCXO

Dave Robinson, WW2R

Introduction

These oscillators were available at last years microwave update from Ted Buell at Tesco. From the datasheet they are ovenised,  electronically tunable and operate from 27V. The difference with these units are that instead of applying a tuning voltage to one of the oscillator pins to do the fine tuning Digital signals must be applied to 3 of the pins. There are a number of ways of generating the appropriate signals but the PIC microchip was chosen again.  Fortunately the signals are the standard SPI signals which were previously  encountered when designing the Z3801 digital clock. (1) The 3 signals required are read/write, clock and data.

From the datasheet the digital oscillator control value (the “data” signal) is 16 bits 0000 to FFFF. The nominal 10MHz occurs with a value of 7FFF. No PICS are made with 16 bits, and if they were the noise problems would be troublesome. For this reason 2 pots are employed; a “coarse tune” and a “fine tune”, each pot being used to generate 8 bits of the full 16 bit signal. Measurements showed that the oscillator can be tuned +-5Hz over the range of the 16 bits. It was also found that the units would operate from 12V.

Design.

The first thing to note is that when the oscillator powers up it will not have remembered the previous digital control word. The PIC must therefore write this control word to put the oscillator onto frequency.

After considering many options (and trying a few of them), the following two modes of operation of the controller were implemented in the software, controlled by jumper  JP1.

1. No jumper in place on power up

The PIC will read the 2 pots and send the corresponding control word to the oscillator. It will then continually read the pot values and send them to the oscillator. This mode is used for trimming the oscillator onto 10MHz. When the alignment is done inserting the jumper will stop the digital signals being sent to the oscillator, very useful in eliminating digital noise

2. Jumper in place at power up

The PIC will read the 2 pots and send the corresponding control word to the oscillator then stop, eliminating digital noise. This mode is useful when calibration has been done and the oscillator is to be left “unattended”.

The PIC chosen was the 8 pin, flash memory, 12F675, which has four 10 Bit  A to D converters and can run off 3.3V (2). The software was written in assembler using the free MPLAB IDE software from Microchip (3)

Circuit

The circuit diagram of the controller is shown in Figure 1. The component listing is shown in Table 1. The prototype was built dead bug style on a piece of strip board, mounted centrally on the oscillator can; see Figure 2 for the interconnections. However a PCB has subsequently designed, the PCB layout is shown in Fig 3 and a photograph of the completed unit, showing component locations,  is shown in Figure 4.  Two wire links are required, one for the ground connection of JP1, one for the +3.3V supply to VR2. Note the oscillator requires 3.3V digital signals, not 5.0V, hence the reason for running the PIC off an LM317 variable regulator set to generate 3.3V.  Both variable resistors need to be 10 turn devices to enable accurate resetting.  VR1 is the coarse adjustment.

Figure 1. Circuit Diagram

 

 

 

 

 

 

 

 

 

 

Table 1. Component listing

Figure 2. Interconnections

Figure 3. PCB Layout;

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Figure 4: Component positions on PCB

 

Figure 5: Populated PCB showing Component positions

 

     

Figure 6. Top view of Oscillator on PCB

As an example of the performance of the oscillator, the following shows the output frequency of a Qualcom 1152MHz  synthesiser using the oscillator as a reference. The offset from 1152MHz is automatically measured as a function of time by a GPS locked counter. The following plot is when the unit is running off 13V, when the unit is run off 24V the frequency stabilizes in 20% less time

As can be seen by the results the units will operate from 13V but they take twice as long to stabilise.

References

1.    http://g4fre.com/z3801clock.htm

2.    http://ww1.microchip.com/downloads/en/DeviceDoc/41190c.pdf

3.    http://www.microchip.com

 

Last modified 24 May 2006