The availability of $10 OCXO modules on eBay recently prompted me to upgrade the reference oscillator in my HP53131A counter.
I looked at HP’s original design, and also the excellent work done by Gerry Sweeney to extend that design to support multiple types of OCXOs. But I decided to go in a slightly different direction.
So I designed a new version of the OCXO board, using a Microchip DAC to replace the discontinued part from Analog Devices that was used originally.
The DAC that I chose supports an external reference voltage, which made it possible to use the OCXO’s own reference voltage in place of the separate chip that supplied that voltage in the original design. This simplifies the design. And since the reference voltage source is now inside the ovenized oscillator, the overall temperature stability of the system should be improved a bit as compared to a system with an external reference chip.
The Microchip DAC has a simple interface that is similar to, but not identical to, the original DAC’s interface. Both DACs accept a 16-bit SPI transfer and regard the last 12 bits as the data for the converter. The new part uses the first 4 bits for control signals, however; furthermore, it uses a different edge of SCLK to sample the incoming data line. So some glue logic is needed.
Referring to the schematic, the glue logic amounts to three chips: a counter (U4), a multiplexor (U3), and an inverter package (U5). The counter/multiplexor select hard-wired values for the first 4 bit cells in the incoming SPI data stream, then switch to that data stream for the remaining bits. The inverter corrects the sense of the clock and chip select signals, and also stops the counter when it reaches the fifth bit cell.
The timing diagram shows more detail about the interface. The “MOSI DAC” data sent to the DAC has the first bit held low (selects Channel A of the DAC), as well as the second bit (disables the internal reference voltage buffer); then the third and fourth bits are held high to select unity gain in the output buffer and to hold the DAC in active mode. After that, the bits are simply the input data sent by the counter (“MOSI”).
The schematic shows the circuit as built. I used the dual channel MCP4922 DAC because I had some on hand; the single channel MCP4921 should work as well. I also added three LEDs to show the presence of supply voltages, just to remind me that the board is always live when the counter is plugged in.
The OCXO used in the prototype was a CTS 970-2187-46; I have another unit that uses a Bliley NV47A1282 OCXO. Both are 5-volt, “Euro package” units that were available on eBay for $10. These appear to be telecom grade units, but work fine for the purpose at hand.
After installing the prototype in the counter I was able to perform auto calibration against an external Rubidium standard, and wind up measuring 10 MHz as 10 000 000.00 Hz, as expected.