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| HP E1938A Quartz Oscillator |
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20-Sep-2003
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 |
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| For scale, the E1938A with a 10811A |
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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)
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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) |
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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) |
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An example of frequency scale, ...
| Allan Deviation plot out to tau 10k seconds (S/N d11014) |
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