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HP5328 Counter OCXO Replacement

My HP 5328 Universal Counter OCXO failed, so I made a replacement using an off-then-shelf OCXO and this PCB

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The HP 10544 OCXO is used in the HP 5328 Universal Counter. My 5328's OCXO failed a while ago, and used replacement HP 10544 OCXO units were pretty expensive. I used the counter's internal xtal oscillator very briefly, and was absolutely disgusted with its performance. I used the counter for a long time with an external OCXO and then later an Rb Osc. I finally got motivated enough to take a look at the HP 5328 and 10544 details, and realized that the 10544 OCXO unit could easily be replaced with an off the shelf OCXO and a small amount of support circuitry.

The OCXO that I used was an Oscilloquartz 8663-XS unit. There are lots of units that share this same physical size and footprint. I imagine that any comparable unit would work just as well. YMMV.

The HP 5328 has a 30 pin edge connector for the OCXO option. The HP 10544 unit is in two pieces - a PCB and the insulated oven oscillator itself. This retrofit simply plugs into the 30 pin edge connector and emulates the behavior of the HP 10544. I looked online for the HP 5328 and HP 10544 schematic diagrams, and quickly reverse engineered the interface.

The HP10544 retrofit circuit consists of three sections:

  1. OCXO voltage regulator
  2. OCXO output conditioning
  3. OCXO fine tuning

The OCXO that I used was an Oscilloquartz 8663-XS unit. There are lots of units that share this same physical size and footprint. I imagine that any comparable unit would work just as well. If you find a unit that has a square wave output, you will likely have to simplify/remove the output conditioning circuitry. Also be sure to verify the supply voltage and current requirements of your OCXO if it is of a different type than what I used!

I chose an LM2675-12 to regulate the (nominal) 28VDC supply down to the 12VDC for the OCXO. The HP 5328 mainboard provides the 28V rail continuously, so it made the most sense to me to use a DC-DC switching regulator to keep the PCB small and simple. My observation is that the OCXO does not seem to have any measurable dependence upon supply voltage 'cleanliness'. (DC-DC vs. LDO vs. Lab Supply). An LED indicates that the 12V rail is up. I used the TI Web Bench software to design the regulator for a 1.2A maximum output current, and chose components based upon its recommendations.

The OCXO output is a clipped sine 10 MHz analog output at +4 dBm, which needs to be shifted and squared up to emulate the output of the HP 10544 OCXO. I used a squaring circuit from the Efratom LPRO 101 Rb oscillator datasheet which has worked very well for me in the past. The sine signal is AC coupled, biased to mid-rail, and applied to the input of a fast CMOS inverter. I used a 74AC04. The HP 5328 mainboard provides a switched 5V supply that is active whenever the counter is on, and this is used to power the 74AC04, as well as a second indicator LED. See the LPRO 101 Rb oscillator datasheet for a great comparison of several ways to convert a 10 MHz sine into a 10 MHz square wave.

The fine tuning is simply a 20-turn 10K potentiometer connected across the VREF and GND pins of the OCXO, with the wiper connected to the VFC terminal.

Mechanical design is pretty straightforward. I sized my retrofit PCB to be the same as that of the PCB that interfaces the HP 5328 to the HP 10544. ( HP P/N 05328-60018 ) The Oscilloquartz 8663 OCXO is much smaller and lighter than the old HP 10544, so I am for the time being running without a support post from the retrofit PCB to the internal wall of the HP 5328. This post provided mechanical support to the original OCXO option.

  • 1 × Oscilloquartz 8663 OCXO Ovenized 10 MHz oscillator unit
  • 1 × LM2675-12 Power Management ICs / Switching Regulators and ControllersDC-DC Switching Regulator
  • 1 × 74AC04 High Speed CMOS Inverter

  • Fine Tuning

    tomcircuit06/21/2015 at 00:01 0 comments

    I let the HP 5328 counter warm up for a couple of days and then tweaked in the fine tuning on the OCXO. I used my Rb standard as a reference, and got the display to settle on 10000.000 KHz after a few turns of the adjust pot. Seems to be working as anticipated.

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tomcircuit wrote 12/03/2015 at 19:52 point

Thanks!  I'll take a look and see what I have in the way of PCB.  One I sacrificed for another project already, but I suspect I still have one blank PCB remaining. 

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Dashie Val wrote 12/03/2015 at 18:50 point

Great mod, thanks ! Do you have any spare PCBs to sell maybe ? I have a HP 5328A here too without TXCO unfortunately, but i will see with the OSHpark project if not.

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