[time-nuts] FE-5680A tuning vs resonant peaks

[Bob kb8tq]

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 <jlw at 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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