[GPS_Standard] Fw: Odd voltage VFC for OCXO

[Bob Stewart]

Should have included the board with this reply.




----- Forwarded Message -----
>From: Bob Stewart <bob at evoria.net>
>To: Mark G0MGX <mark.g0mgx at gmail.com> 
>Sent: Monday, November 11, 2013 12:11 PM
>Subject: Re: [GPS_Standard] Odd voltage VFC for OCXO
> 
>
>
>Hi Mark,
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>
>Getting information about Trimble oscillators is extremely hard to do.  But, usually what that means, is that they have used a pot to tune it to 10MHz on their counter and the voltage required was 2.54V in your oscillator's case.  I would feed it with 5V if it were me.  Take a look at the VREF pin while it's running.  That is the actual max voltage you should apply to the EFC line, I believe.  It's the center pin on the side that has 3 pins.  Another thing is that these oscillators, even though they are already old, burn in after they are on for awhile.  It takes a few days or even weeks of "on" time before they reach their stable point.
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>Also, what type of GPS receiver do you have?  You may or may not know that I am in the process of rewriting Bert's code.  I have done a lot of testing with Oncore timing receivers, and they are just not suitable for Bert's board due to having the possibility of more than 360 degrees of phase shift on occasion.  They also have periods where the phase can remain about 180 degrees off in one direction or the other.  I've bought an Adafruit Ultimate Breakout board and it is great.  I've also heard good things about the "ublox" I believe it's called.
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>And be sure you have your antenna up high and unobstructed.  Throughout most of my testing I didn't have this, and it has made me draw some inaccurate conclusions.  It doesn't have to be an expensive antenna.  It just needs to be as high as you can get it.  I bought a $5 ebay puck antenna and with the Adafruit, it's showing less than 3 meters of movement, which translates to very good 1PPS stability.
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>Bob
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>>________________________________
>> From: Mark G0MGX <mark.g0mgx at gmail.com>
>>To: Bob Stewart <bob at evoria.net> 
>>Sent: Monday, November 11, 2013 11:56 AM
>>Subject: Re: [GPS_Standard] Odd voltage VFC for OCXO
>> 
>>
>>
>>Hi Bob
>>
>>It's a OCXO surplus eBay job - a Trimble 65256. I have got another
      one here somewhere which I should hunt out.
>>
>>I'm only going on what someone has written on the bottom of the
      unit "VFC = 2.54V" - so I am not even sure that's correct!
>>
>>I've built the board today and was planning, in the absence of a
      1pps GPS signal which is still en route, of generating a 1pps
      signal from an Arduino board to test the set up.
>>
>>Mark
>>G0MGX
>>
>>On 11/11/2013 17:24, Bob Stewart wrote:
>>
>>It should work fine.  But I would suggest that you put the divider right at the EFC pin on you VCXO, as this will reduce the effects of noise pickup.  I don't imagine the values are all that critical, as the current flow is on the order of microamps through the EFC pin.
>>>
>>>What type of VCXO are you using?  Is it an inexpensive VCXO, or
        is it a surplus quality OCXO?
>>>
>>>Bob
>>>
>>>
>>>
>>>
>>>
>>>
>>>>________________________________
>>>> From: Mark G0MGX <mark.g0mgx at gmail.com>
>>>>To: gps_standard at mailman.qth.net 
>>>>Sent: Monday, November 11, 2013 10:55 AM
>>>>Subject: [GPS_Standard] Odd voltage VFC for OCXO
>>>> 
>>>>
>>>>Hi
>>>>
>>>>I'm starting my build today and I wanted to ask about
                  the VCXO tuning range. My VCXO has a 0V to 2.54V
                  variable control voltage and as the U5 op-amp has a
                  minimum voltage rail requirement of 4V clearly I can't
                  use an external 2.54V supply.
>>>>
>>>>I was planning to simply add a 50-50 potential divider
                  to the output and configure the board as a 0 to 5V
                  adjustment.
>>>>
>>>>Does anyone have any suggestions?
>>>>
>>>>Thanks,
>>>>
>>>>Mark.
>>>>G0MGX
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>>
>>
>>
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