[GPS_Standard] Odd voltage VFC for OCXO

[Bob Stewart]

Hi Mark,

The Adafruit uses an MTK3339, which is essentially the new and improved version of the 3301.  The one you're getting is Version 3, which is the latest.  It is a very nice board.  When it comes in, use some tracking program, such as foxtrotgps, connect it to your antenna and let it run for awhile to see what kind of track it's taking.  That will give you a strong indication of what kind of timing accuracy you can expect from it.

Bob




>________________________________
> From: Mark G0MGX <mark.g0mgx at gmail.com>
>To: Bob Stewart <bob at evoria.net> 
>Sent: Monday, November 11, 2013 12:18 PM
>Subject: Re: [GPS_Standard] Odd voltage VFC for OCXO
> 
>
>
>Hi Bob
>
>Interesting - thanks for the pointers on the voltage - I will do
      some measurements and see what I find. I will also apply come
      power and leave it alone for a while!
>
>I started with one of these, but failed completely to get a 1pps
      signal from it. It's now on it's way back as the 1pps output
      requires a firmware update!
>
>http://www.ebay.co.uk/itm/171151843286?ssPageName=STRK:MEWNXhappy":IT&_trksid=p3984.m1497.l2649
>
>I'm now waiting for  one of these:
>
>http://www.ebay.co.uk/itm/111208323128?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
>
>which, strangely, is exactly the board you mention!
>
>My GPS antenna is on my porch roof so it's pointing very up but
      slightly East - seems to work OK but I guess time will tell!
>
>Mark
>G0MGX
>
>On 11/11/2013 18:11, Bob Stewart wrote:
>
>Hi Mark,
>>
>>
>>
>>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.
>>
>>
>>
>>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.
>>
>>
>>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.
>>
>>
>>Bob
>>
>>
>>
>>
>>>________________________________
>>> 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
>>>>>______________________________________________________________
>>>>>GPS_Standard mailing list
>>>>>Home: http://mailman.qth.net/mailman/listinfo/gps_standard
>>>>>Help: http://mailman.qth.net/mmfaq.htm
>>>>>Post: mailto:GPS_Standard at mailman.qth.net
>>>>>
>>>>>This list hosted by: http://www.qsl.net
>>>>>Please help support this email list: http://www.qsl.net/donate.html
>>>>>
>>>>>
>>>>>
>>>
>>>
>>>
>
>
>