If you have an old “Racal-Dana 199x” frequency counter or similar 10 MHz internally referenced gear with a poor tolerance “standard quartz crystal oscillator” or bit better “temperature compensated crystal oscillator” (TCXO) you could upgrade to a high stability timebase “oven controlled crystal oscillator” (OCXO) for under $25. [Gerry Sweeney] shares his design and fabrication instructions for a DIY OCXO circuit he made for his Racal-Dana frequency counter. We have seen [Gerry] perform a similar upgrade to his HP 53151A, however, this circuit is more generic and can be lashed up on a small section of solderable perf board.
Oven controlled oscillators keep the crystal at a stable temperature which in turn improves frequency stability. Depending on where you’re starting, adding an OCXO could improve your frequency tolerance by 1 to 3 orders of magnitude. Sure, this isn’t as good as a rubidium frequency standard build like we have seen in the past, but as [Gerry] states it is nice to have a transportable standalone frequency counter that doesn’t have to be plugged into his rubidium frequency standard.
[Gerry’s] instructions, schematics and datasheets can be used to upgrade any lab gear which depends on a simple 10 MHz reference (crystal or TXCO). He purchased the OCXO off eBay for about $20 — it might be very old, yet we are assured they get more stable with age. Many OCXO’s require 5 V, 12 V or 24 V so your gear needs to accommodate the correct voltage and current load. To calibrate the OCXO you need a temperature stable variable voltage reference that can be adjusted from 1 to 4 volts. The MAX6198A he had on hand fit the bill at 5 ppm/°C temperature coefficient. Also of importance was to keep the voltage reference and trim pot just above the oven for added temperature stability as well as removing any heat transfer through the mounting screw.
You can watch the video and get more details after the break.
[Dave Jones] gives an excellent overview of different crystal oscillators and their characteristics in EEVBlog episode about Rubidium Frequency Standards.