[time-nuts] TCVCXO Adjustment

Wayne Holder wayne.holder at gmail.com
Tue Apr 10 19:10:32 EDT 2018


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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