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HP E1938A Quartz Oscillator



The HP E1938A is a very modern ovenized SC-cut quartz oscillator of radical design that had little or no product release due to project cancellation. As far as I know it was the first new oscillator designed by HP since the venerable 10811A. The performance of the E1938A is truly outstanding. I'm not sure which product it was intended for -- perhaps something Telecom or GPS related. HP sold that product line to Symmetricom in the late 1990's -- perhaps that had something to do with it.

Several years ago one of the developers of the oscillator graciously donated several surplus specimens for the museum which is how I was able to make this report.


This highly unusual oscillator has a pancake or hockey puck appearance. It looks like it would work in a 1U instrument. You can see several PIC microcontrollers on board.

E1938A board with E1938-60201 ovenized oscillator assembly


For scale, the E1938A with a 10811A

Power on

The E1938A requires +12 VDC at about 100 mA and +5 VDC at about 3 A at startup dropping to 1.5 A after a 10 minute warmup. The output is 10 MHz. There is an EFC input but no manual fine or coarse frequency adjustment. Here is a connection diagram:

E1938A connector diagram

Pin number   Pin number   Pin number
   of special   of DB-25S    of DA-15 on
   sub-D:       (equiv.):    puck:               Description:

         1         1           10              ACOM/DCOM
         2         2            2              VREF common
         3         3            9 (indirect)   EFC shield
         4         4           none            TX, serial port
         5         5           none            Data from PIC
        none       6           none            not used
        none       7           none            not used
        none       8           none            not used
         6         9           none            Data to PIC
         7        10           none            PIC data ready
         8        11            7              VREF, +2.5V
         9        12           none            10 MHz. common
        10        13           none            10 MHz. (signal)
        11        14           10              ACOM/DCOM
        12        15           10              ACOM/DCOM
        13        16            1 (indirect)   EFC+
        14        17           none            +5V
        15        18           none            +5V
        none      19           none            not used
        none      20           none            not used
        16        21           none            RX, serial port
        17        22           none            +12V
        18        23           none            PIC clock
        19        24           none            not used
        20        25           none            not used
       shell     shell         shell           ACOM/DCOM

                     Signals on puck only:
                                1              RF/EFC+
                                3              +4.5V
                                4              M/L heater
                                5              Heater return
                                6              Rim heater
                                8              Thermistor bridge
                                9              RF return/EFC-
                                11             Warmup heater
                                12             M/L htr sense
                                13             Heater rtn sense
                                14             Rim htr sense
                                15             Therm. bridge ref.
                              shell            ACOM/outer can
                                               (does not connect to puck)


For those E1938A with a DB25P (male) connector on the top of the PCB you can make a matching connector using a DB25S such as this:


The performance of this E1938A is ok but not stellar...

Allan Deviation plot out to tau 10k seconds (S/N d11013)

And another E1938A, with better performance, getting down in the 13's from tau 1 to 1000 seconds.

Allan Deviation plot out to tau 10k seconds (S/N d11014)

An example of frequency scale, ...

Allan Deviation plot out to tau 10k seconds (S/N d11014)


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