Hi
A few generic Rb information points:
1) All the telecom Rb’s need an external heatsink. To just mount them on
a PC board, you need a lot of airflow (like fans …).
2) Heat sinking improves the lifespan of these devices. Without a heatsink,
a year or two is doing pretty well. With a heatsink that gets the base to 40C
(possibly heatsink + small fan) a decade or more is possible.
3) If your Rb has a PPS output, cal is pretty easy. Just compare the pps
to a < $20 GPS module with a scope. A day or three of tweaking should
get you pretty close.
4) With very few exceptions telecom Rb’s either have rotten phase noise / spurs
or they have horrible phase noise / spurs. If you are going to do anything
“fancy” (like microwaves) with one, you will need a cleanup loop.
5) Many outfits produced a wide range of parts, all with the same part number.
What you have with a XXXX model number on it may be *very* different than
what I have with the same … errr ... FE-5680 model number on it.
6) Most Rb’s have some sort of crystal oscillator in them. There often is an
adjustment (re-centering) needed on surplus parts.
7) Like OCXO’s Rb’s do have a warmup / retrace process that runs into
days. They also are sensitive to temperature, voltage, pressure, and
humidity. Compared to an OCXO, most of these sensitivities are pretty
small.
As mentioned in a number of posts, in a lot of ways, an Rb makes a pretty
good basement lab standard. Compared to a GPSDO, there are a lot
fewer things to go wrong.
Bob
> On May 15, 2020, at 12:19 AM, Jeff Woolsey <j...@jlw.com> wrote:
>
>> Hmmm...
>> You mention varying suply voltage by 1.5V, but from where as a starting
>> point?
>
> Sorry, that is a red herring. The regulator on the support board needs
> enough headroom to regulate to 15V for the FE5680A. Thus anything less
> than about 17V input will drag down the voltage into the unit, and the
> frequency rises. I got down to 14V--the unit remains locked but 10MHz
> doesn't. (It also doesn't help that the voltage display on the power
> supply is about half a volt off.) On the other hand, the regulator has
> a high-temp cutoff which I managed to hit at around 20V--there is no
> real heat sink on the support card. Usually it's running around 80C;
> cutoff at 125C.
>
>
>> It's been a while since I calibrated an FE5680A but looking back through my
>> notes, doing it "properly" is, or was for me anyway, a non trivial
>> exercise.I'm not familiar with the Windows software mentioned but the
>> approach I remember was first to determine what, if any, offset was
>> programmed into the unit as received,
>
>
> The software I'm using doesn't seem to be able to read and display the
> previous offset.... Sigh.
>
>
>> then to measure the frequency of the unit as received, then to calibrate the
>> tuning itself by setting positive and negative tuning extremes and measuring
>> the frequency range before interpolating to find an initial tuning word,
>> followed by calculating a further approximation, and so on and so fifth, and
>> of course eventually programming the FE5680A accordingly.
>
>
> Which is what I should be doing instead of winging it....
>
>
>> At that time I was using a similar test setup of a 53132A referenced to a
>> Thunderbolt, although I did use a second Thunderbolt feeding the second
>> channel of the 53132A as a confidence check.
>
>
> Somebody should push me to pulling the trigger on buying a little ublox
> LEA M8F-based GPSDO (VCTCXO) (currently on that auction site). It
> would replace a wireless cellphone eval kit (with ublox LEON chip) I
> picked up MAD-magazine-cheap at a flea market that I managed to short
> out the TIMEPULSE on... My tools are too big for soldering a wire to
> surface-mount, let alone replacing the chip.
>
>
>> Much as I love Lady Heather, hmmm just how kinky is that?:-), I don't rely
>> too much on her reported offsets etc,
>
> The particular figure I think I'm looking at is straight out of the
> TBolt. It's "10MHz offset" bytes 20-23 in the 0x8F-AC report packet.
> All that LH does is multiply it by 1000 to report it as parts per
> trillion. The TBolt is measuring the frequency offset of the 10MHz
> output relative to GPS/UTC in parts per billion. Positive values
> indicate the 10MHz clock is running slow relative to GPS/UTC. Watching
> this value in LH shows a lot of jitter. I try to take measurements on
> the counter when this value is closest to 0. This is impossible to
> predict, of course. Next best would be to correlate this value with the
> readings I take (via GPIB) from the counter. My impression is that the
> Rb may be more stable over the short term than the GPSDO, but I'd like
> to be more certain.
>
>
>> preferring to trust hardware measurement for that, but would suggest that if
>> she is showing your thunderbolt as locked and tracking a reasonable number
>> of sats then experience suggests you should be able to trust your
>> Thunderbolt as being on frequency.
>
>
> Trust but verify...
>
>
>> Experience also suggests, at least with all the units I've seen, that the
>> FE5680A generally reached the surplus market with a programmed offset of
>> zero, presumably because that was good enough for its intended
>> purpose.Soooo, I would suggest that if you have any doubts at all the first
>> obvious thing to do is to reprogram the offset to zero, and start again from
>> there.
>> I'd be happy to share my programming notes, but must admit I'm having a bit
>> of fun understanding them myself right now:-)
>>
>>
> I have to pay more attention to whether increasing the offset I send
> makes the frequency rise or fall. And it has to be a fairly large
> offset to make the output change obvious. The manual isn't very clear
> whether this offset is for the 50.5MHz oscillator or for the divided
> result (to give 10MHz independently). I'd expect the latter.
>
> I suppose I should re-learn how linear regression works on my calculator.
>
> --
> Jeff Woolsey {{woolsey,jlw}@jlw,first.last@{gmail,jlw}}.com
> Nature abhors straight antennas, clean lenses, and empty storage.
> "Delete! Delete! OK!" -Dr. Bronner on disk space management
> Card-sorting, Joel. -Crow on solitaire
>
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