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GPS-Disciplined 10 MHz Frequency Standard with Atmel SAM D20

Status: Proposal
February 28, 2015

I bought an old PM 6622 80MHz frequency counter, but it contains a simple oscillator. I'd like to improve its accuracy by creating a 10MHz frequency standard as external source for the counter. Since Rb-references are quite expensive I want to make my own reference oscillator synchronized by the 1pps signal of a GPS receiver. The 1pps signal is supposed to be 10ns accurate, which is 0.1Hz at 10MHz. I'll use a secondhand oscilloquartz 10MHz OCXO as the main oscillator and use an Atmel SAM D20 ARM microcontroller to do the processing and adjusting the OCXO output. I chose this microcontroller because I just bought it via the elektor shop with discount and want to do something with it. I think this is a perfect project for it. Any other microcontroller can be used ofcoarse as long as it can count a 10MHz signal.

How will it work: The microcontroller will count the rising edges of the OCXO between 1 or more 1pps signals from the GPS. Then it will change the control voltage of the OCXO using a PID system until the frequency is exactly 10.000.000 MHz.

I'd also like to add an lcd for status info (OVEN,GPS,OFFSET) and also some extra outputs with 1MHz, 5MHz, 1Hz and serial gps output for other purposes.

The reference will be built in a seperate case with multiple outputs, so it can be used for other instruments at the same time. I hope to keep the total cost under $100 for the complete project.

See here for details about the Atmel SAM D20J18:

I've added the schema, but its not completed yet I'm still working on it. Just to give you an idea.

OCXO Measurements
I received my Oscilloquartz 8663-XS OXCO this week and did some measurements.

Current draw @ 12V:
measured: I(cold) = 490mA       datasheet: <8W --> I(cold) <666mA
measured: I(warm) = 200mA     datasheet: <2.5W --> I(warm) <208mA

Uout = 2,12Vpp sinewave     datasheet: >+4dBm / 50 Ohm

Frequency control:
f(0v) = 10000091.5 Hz
f(10V) = 10000099.0 Hz
measured: Δf = 7.5 Hz       datasheet: > 3Hz

Oscilloquartz datasheet

Note: I used a 74LVC245 as a level converter. A zenerdiode or resistor devider will probably not work correctly because of the high frequency. See here why:

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Schematic diagram
Schematic diagram
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