[time-nuts] FE-5680A decoded - another piece of the puzzle

Arthur Dent golgarfrincham at yahoo.com
Tue Jan 31 10:25:13 EST 2012

>Tue Jan 31 14:15:57 UTC 2012
>I did the mod and getting to pin 5 was tough. However Doug mentioned that
>there is another point to the left of the chip thats easier to get to.
>Don't recall the point. But if the lead ever comes off of 5 I can assure
>you I will find it. :-)
>My unit zeroed about at about 2 V. So I am using a 10 T 10 K bourns pot
>with 1000 counter. Then a 3.3V reg directly attached with bypass caps
>driven by 8 V from the regulated supply in the RB.
>Very clean, 3 leads into the RB. Using shielded cable through the pot hole
>that actually does nothing.

In case you missed it I had posted the information and a photo of the spot
to more easily solder the 100K resistor to give you EFC. I'd go with Bill Riches'
suggestion to have the 100K in series with the EFC line. I connected the lead
from the 100K to a pin on the DB-9 connector that was freed up when I put the
5V regulator inside so I could run off a single supply. Here's what I did.

>Sun Jan 15 18:31:39 UTC 2012
>I didn't solder the resistor directly to pin 5 of the IC. I found that pin 5
>was connected to a nearby SMD capacitor that was a little easier to
>solder to. Attached is a photo of the correct location if you want to try
>bringing the EFC out for analog control.
> http://farm8.staticflickr.com/7170/6702435071_f684967719_b.jpg

>Mon Jan 16 04:16:53 UTC 2012
>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.


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