[time-nuts] FE 5680A Warning
Chuck Forsberg WA7KGX N2469R
caf at omen.com
Sun Nov 13 17:21:50 UTC 2011
Seems like a lot of folks selling these lately.
Curious minds would like to know why.
On 11/13/2011 07:15 AM, EWKehren at aol.com wrote:
> Pete
> That is exactly the same math I did, there is no other choice but it would
> be nice if some one can help on the C field. I just finished mounting it on
> a heat sink added a Toshiba 15 V 3 A power supply and a 7805 and will run
> it for a month with out changing any thing. Will read it twice a day and
> see how it behaves.
> Bert
>
>
> In a message dated 11/13/2011 9:24:47 A.M. Eastern Standard Time,
> bell.peter at gmail.com writes:
>
> Yes, I'm quite sure this is how it works - if you scope the snap diode
> drive you can quite clearly see both the 60MHz and the 5.3MHz from the
> DDS chip mixed together. My assumption is that the DDS output is
> subtractively mixed with the 114th harmonic of the 60Mhz inside the
> cavity: 60*114 = 6.840Ghz - ~5.3Mhz = ~6.8347 GHz clearly puts you in
> exactly the right ballpark.
>
> This would also explain the very fine tuning steps - a 1Hz change on
> the DDS output would only need a ~1.45mHz change of the unit output
> frequency (at 10MHz) to bring the loop back into lock.
>
> I still have the unit with the bad Rb cell in bits, so I will have a
> poke around it and see if I can figure out how the drive on the
> C-Field coil is derived.
>
> Regards,
>
> Pete
>
>
>
>
> On Sun, Nov 13, 2011 at 7:40 PM,<EWKehren at aol.com> wrote:
>> Pete
>> Thank you for your information. I agree with your findings and as I
> pointed
>> out previously it support the math that the AD 9832 is used for tuning.
>> That allows them to set the Rb at the optimum operating point based on
> their
>> gas mixture. What I like to know is how they control the C field.
> Again I
>> think they picked an optimum point and only vary it slightly for
>> temperature compensation. I would like to use the C field to fine tune
> using a
>> modified Shera controller. Latest voltage measurements on the C field
> across the
>> coil are 2.5110 and 2.5238 down from 2.5134 and 2.5256.
>> Bert Kehren
>>
>>
>> In a message dated 11/13/2011 4:38:09 A.M. Eastern Standard Time,
>> bell.peter at gmail.com writes:
>>
>> I can let you know what I've found out about these units - I got given
>> a couple of dead ones and did a bit of poking around trying to get
>> them working. I didn't get that far, since one of the units was
>> easily fixed (the MAX882 3.3V regulator had failed) and the other
>> seems to have a faulty Rb absorption cell (it works if you swap the
>> cell from the other unit into it).
>>
>> The first is, as you noticed, that they have 3 voltage regulators -
>> but apparently couldn't spare one to generate the +5V line - they seem
>> to run (1) The Rb lamp / heater (the one nearest the connector) (2)
>> the cell heater (the middle one) and (3) the analog electronics @8.8V
>> (the last one).
>>
>> The quickest way to verify this is to apply +15V / GND to pins 1 and 2
>> of the d-type and then check the voltage on the VCC pin (20) of the
>> 74ACT240 buffer chip next to the connector - there is no 5V there.
>> There is also no 5V on pin 5 of the MAX882 - and hence no 3.3V line.
>>
>> There is a connection between these points and pin 4 of the d-type -
>> and applying an external +5V to this pin will bring up the 3.3V line
>> and allow the unit to lock. When locked, pin 3 on the d-type goes
>> low, and will sink enough current to operate an LED.
>>
>> Pin 5 is also connected to GND, and was (on the original PCB) also
>> connected to the ground plane along with pin 2. It doesn't seem to be
>> actually necessary, as the unit will operate with a single ground
>> connection, but would it would seem to be good practice to use this
>> pin, too.
>>
>> It's been alleged that pin 6 is a 1PPS output - but I can't see
>> anything on it at all except a constant logic 1 - I tried changing the
>> 'ACT240 in case the buffer was damaged, but this had no effect, and
>> the pin is still a constant 1. I suppose it's possible that there is
>> an extremely narrow -ve going pulse on it, but if there is it's too
>> narrow for the trigger circuit on my (100MHz) scope to see it.
>>
>> Pin 7 outputs a 10MHz sine wave - about 800mV p-p unloaded. I haven't
>> put it into an analyzer, but it looks pretty good.
>>
>> Pin 8 and 9 are connected to the MAX3232 on the board, and from them
>> onto the serial port on the DS80C323 CPU. Pin 8 is RXD, pin 9 is TXD
>> (from the unit's point of view). This serial port accepts the commands
>> described in the FEI document that's floating around the net for
>> trimming the oscillator. They appear to operate as described.
>>
>> Internally, the main VXCO runs at 60MHz - it's buffered and fed into
>> the Xilinx XC9572XL PLD on the back of the board. This chip appears
>> to contain mostly frequency dividers - it generates the 10MHz output,
>> 20MHz for the MCLK to the DDS chip (although with a slightly nasty
>> 40:60 duty cycle) a 416.6666Hz drive for the fselect pin on the DDS
>> chip, a 833.33333Hz drive for the lock in amp. There are also a bunch
>> of signals that go to the test connector, and are presumably used for
>> factory programming.
>>
>> The output of the DDS is about 5.3MHz with FM on it at a 416.666Hz
>> rate - this is mixed in with the 60MHz output from the VCXO to give
>> the drive signal to the snap diode in the physics package.
>>
>> There are 3 pots on the board - the one that's normally accessible via
>> a hole in the case seems to be intended as a C-field adjust, but does
>> nothing on the units I have - the other one near it appears to trim
>> something in the photocell amplifier circuit. The one on the other
>> side of the board (near the VCXO) sets the bias on the snap diode
>>
>> That's all I've got, since I then worked out that I didn't have an
>> electronics problem - hope it helps.
>>
>> Regards,
>>
>> Pete Bell
>>
>>
>> On Sun, Nov 13, 2011 at 1:46 AM, Don Latham<djl at montana.com> wrote:
>>> I just bought a couple of 5680A's that have the same FEI p/n. They are
>>> pop-riveted to a heavy double-sided board., obviously cut off from a
>>> longer board. I've removed one of these from it's board/heatsink.
> There
>>> are two pots on the board, only one of them can be trimmed with the
> case
>>> on. The sinewave forming filter and maybe countdown chips are not
> there,
>>> and never were. There are three identical voltage regulators on the
>>> heatsink spine.
>>> I'll be doing some more reverse engineering on these units; they were
> so
>>> cheap I had to have a couple! If only FEI was more helpful! hey must
>>> have moved on from these, and could put one manual out there?
>>> Oh Well!
>>> Don
>>>
>>> EWKehren at aol.com
>>>> The FE 5680A's advertised as NEW are not NEW! A friend that made me
>>>> aware of the sale did notice clear signs of use, so I did a closer
>>>> inspection
>>>> under a Microscope and found clear indications that the unit has been
>>>> bolted down in the past. Does work fine and I am looking at ways to go
>>>> directly
>>>> to the C field to disciplin the Rb. Does any one have info that would
>>>> help
>>>> to do so.
>>>> Bert Kehren Miami
>>>> _______________________________________________
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>>>
>>> --
>>> "Neither the voice of authority nor the weight of reason and argument
>>> are as significant as experiment, for thence comes quiet to the mind."
>>> R. Bacon
>>> "If you don't know what it is, don't poke it."
>>> Ghost in the Shell
>>>
>>>
>>> Dr. Don Latham AJ7LL
>>> Six Mile Systems LLP
>>> 17850 Six Mile Road
>>> POB 134
>>> Huson, MT, 59846
>>> VOX 406-626-4304
>>> www.lightningforensics.com
>>> www.sixmilesystems.com
>>>
>>>
>>>
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--
Chuck Forsberg WA7KGX N2469R caf at omen.com www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
Omen Technology Inc "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231 503-614-0430
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