[time-nuts] Fury Interface Board: 5MHz needed?

[Bernd T-Online]

Hi all,

being more than 30 years in the Frequency Control business, I want to 
comment on what Hal Murray wrote:
> My first guess is that it's a backwards compatibility mess.  Ages ago 
> somebody made a part with a single ground pin and somebody used it in some 
> telco gear and everybody has been using the same footprint ever since.  If 
> that's the case, I'd expect there would also be a variation with a second 
> ground pin to get better performance.

Look for example to the very popular "Eurocase" package CO 08, as used 
in the MTI 260 (and many OCXO from other manufaturers - like the AXIOM40 
  & 45 series of AXTAL). This package originated from a TCXO package, 
and was first used to make OCXO (as TCXO replacements) in the early 
seventies. The connection scheme for the 5 pins was given and included 
only one ground pin.
Next generation in OCXO miniaturization (in Europe) was the 20x20 mm 
package CO15 (as AXIOM30 & 35), which has the similar legacy, i.e. the 5 
pins were defined  for TCXO, i.e. with one ground pin, and later (in the 
late 80's) used for OCXO.
The next smaller THD OCXO was in the 4-pin DIL14 package (AXIOM20 & 25), 
and it's clear that the 4 pins do not allow 2 GND pins.

Any additional pin means more cost - and as such special versions would 
run in much smaller quantities - the cost are even higher.
And - the accuracy and stability requirements for those small OCXO was 
(and is) less stringent in most applications such as for telecomms.

The cost situation changes, when it comes to SMD packages (like the 
22x25 mm package CO 28 - AXIOM10). For those packages you can indeed 
find a few specs on the market, which have two separate ground terminals 
for oscillator and heater circuit. Such specs are written by people who 
know about the effects of ground loops.
However I do not know any "modern" catalog or standard parts on the 
market with two ground terminals.
In fact the impact on stability is mostly neglectable under stationary 
operation conditions with rather constant environmental temperature.
For applications where temperature changes are an issue _and_ highest 
stability is required, there is no other way than having two separate 
grounds.

> Are they running temperature tests with the EFC pin open so they don't see 
> any problem as the heater current changes?  (or something like that)
Indeed, many (if not most) OCXO manufacturers run OCXO temp tests (and 
aging tests with the EFC pin grounded. The undesired ground loop effect 
can be reduced if the connection is made very close to the pin, and if 
the inside connection to the PCB ground has low impedance.

Regards

Bernd  DK1AG
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