[time-nuts] TCVCXO Adjustment

Bob kb8tq kb8tq at n1k.org
Tue Apr 10 21:12:34 EDT 2018


Hi

In a commercial setting calibration would be done against a local standard. 
It might be checked with a counter. It also might be checked against something
more complex. 

A reasonable level of calibration would be 0.1 ppm. Anything much more accurate
than that would be quickly swamped by the temperature coefficient of a typical TCXO. 

Simply put, 0.1 ppm is 1 microsecond over 10 seconds. If your time code is capable of
resolving 1 us, monitor the output of the board for 10 seconds. If it is lower accuracy, 
the time period would be longer. It does get a bit impractical on something like a 1 ms
code ….

The sensitivity of the TCXO should be reasonably well understood. That should give 
you a ppm / bit sort of number. As an example, a 10 ppm trim range might not be 
that crazy. Your 12 bit DAC would give you roughly 0.01 ppm per bit. 

Since your typical PC does not have anything in it that is accurate to 0.1 ppm, you 
still need something as a reference to compare things to. A GPS module or a GPSDO
are probably the easiest things to get ahold of. 

Bob

> On Apr 10, 2018, at 7:10 PM, Wayne Holder <wayne.holder at gmail.com> wrote:
> 
> I'm designing a small, portable, SMPTE LTC Timecode Generator
> <https://en.wikipedia.org/wiki/SMPTE_timecode> as an open source/hardware
> project for amateur filmmakers and videographers.  LTC Timecode is
> typically recorded on the audio tracks of cameras and sound recorders so
> the video and sound comments can be automatically sync'd later.  I'm
> planing on using a small, SMD TCVCXO such as the LFTVXO075806Cutt
> <https://www.mouser.com/ProductDetail/IQD/LFTVXO075806Cutt?qs=sGAEpiMZZMscy%2f6qMaFHY0htnjNN0iZ6XRXaS1jehPSKkjjKOKqbkg%3d%3d>,
> which is spec'd at a frequency tolerance or +/- 1 PPM and a frequency
> stability of 0.28 PPM and a yearly aging of +/- 1 PPM max/year which, to
> me, seems pretty impressive for a part that costs about $8.
> 
> Since the TCVCXO includes a voltage control input, my plan is to also add
> a 12-Bit Digital-to-Analog Converter with EEPROM Memory, such as the
> Microchip MCP4725
> <https://www.mouser.com/ProductDetail/Microchip-Technology/MCP4725A0T-E-CH?qs=sGAEpiMZZMvfFCidbTccA97L6UsE6%2fky>
> to
> provide a way to initially check and calibrate the frequency after surface
> mount soldering and also later to compensate for aging.  But since this is
> intended as an open source/hardware project rather than a commercially
> manufactured one, I've been pondering how someone building the device would
> be able to easily and reliably calibrate it.
> 
> I'm basing the design around the Arduino, so the device could, in theory,
> use the USB Serial connection as a way to connect to a calibration program
> running on a PC.  I have a few idea on how to attempt to do this, but this
> is new territory for me, so I'm asking for advice and/or thoughts on how
> feasible this might be.  Is this a crazy, impractical idea given that all
> the builder will probably have available to perform the calibration is a
> regular PC and an Internet connection, or is there a way to make it work?
> 
> Wayne
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