[time-nuts] Adding adjustment pot to 5680

Arthur Dent golgarfrincham at yahoo.com
Mon Jan 16 04:16:53 UTC 2012


>Rex:
>"Question:
>Has anyone measured or calculated the tuning sensitivity of the pin-5 
>EFC voltage? (Hz/V number)."
++++++++++++++++++++++++I'm not sure what HEX number was loaded into my 5680A but the frequency
was just slightly off from 10Mhz. Pin 5 on my 27M4BI quad opamp 'floats" at
2.599V and to get 10Mhz out it needs to be at 2.445V. I guess I could correct
the frequency digitally so it is at 10Mhz with pin 5 floating but I don't feel it is
necessary. The output frequency I measured with zero volts on pin 5 (through
the 100K resistor) was 10,000,000.028Hz  and with 5V it was pretty close to
9,999,999.969Hz or .031Hz low so it's close to centered. I tried using a 2K
10-turn pot with about 10K on each side to restrict the tuning range further and
give me finer control. After the 5680A had been on for a while I tried setting
the frequency  as close to 10Mhz as I could and watch the drift on my scope
using the 10Mhz Rb from my Datum 9390 GPS receiver as the reference.
The 5680A seemed to stay within 5ns for 45 minutes so the resolution on the pot
is quite good  and the 5680A seems quite stable.

The pin 5 IC connection is the non-inverting input to the opamp and I'm not
sure if connecting the 100K resistor to the inverting input, pin 6, would give
you a positive change in frequency for a positive change in voltage or not.
If you're using a pot for adjustment or if the controller can be programmed
to change the polarity, it doesn't really make any difference and pin 5 works
just great. I'm glad Bill Riches found this input and posted the information.

-Arthur 


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