[time-nuts] Fury Interface Board: 5MHz needed?
BNeubig at t-online.de
Wed Nov 7 18:04:19 EST 2007
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
> 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.
AXTAL GmbH & Co. KG
D-74821 Mosbach / Germany
fon: +49 (6261) 939834
fax: +49 (6261) 939836
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