User Manual
UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
11
OCXO131-40
This IsoTemp OCXO module is available at low cost from various
eBay sellers. These units are probably pulled from working
frequency standards, or from other equipment. It can be assumed
that they are well aged. Measuring 36.3 x 27.2 x 19.1mm, they are
a tight fit on the compatible PCB. They are SC-cut, sinewave
output with a fast warmup, and have frequency stability
specifications that are on par with the MTI 220-series OCXO that
was originally in S/N ‘142. Hard to beat for $21 each … units
purchased from eBay often have date codes in the early 2000’s …
Barrel Jack and MMCX Connector
By placing a 2.1mm/5.5mm barrel jack on the OCXO PCB, it’s
trivial to power the OCXO separately from the Model 3xxx
frequency counter. Similarly, by connecting an MMCX jack to the
OCXO’s output, it’s trivial to connect the OCXO output to an
oscilloscope or frequency counter, via an MCX-to-BNC adapter
cable. These two features facilitate initial measurements and
trimming of the 10.000MHz OCXO while on a lab bench. The
barrel jack and MMCX upright jack are located on the PCB so that
they are accessible even when the PCB is installed into the Model
3xxx.
High-quality Trimpot
We chose the Vishay Accutrim™ 1280G 26-turn trimpot with its
15ppm/ºC temperature coefficient (tempco). At $15, it is not
inexpensive, but it is similar to what was on the MTI-based OCXO
originally in the Model 3080.
Ultra-low Tempco Resistors
In an attempt to have the finest possible trim voltage adjustability,
we chose to augment the trimpot with fixed resistors. We chose the
Vishay VCFP series ultra-high-precision resistor with a 0.2ppm/ºC
temperature coefficient. These are 0.01% resistors in a 1206-size
package. At $10 each, they are also not inexpensive, and they are
available only in a limited range of values.
It has been suggested that resistors grouped in single packages
provide optimal tracking and stability, better than (separate)
discrete resistors. One option would be a non-divider version of
Vishay’s SOT-23 MPM-series divider networks.
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