User Manual
12
UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Circuit Design
DJW reverse-engineered the original OCXO PCB to create a
reference schematic and design a new PCB. The new schematic
and PCB layers are attached to this manual.
12V LDO
A 12V TO-220 linear regulator accepts 12V power from the Model
3xxx and provides power to the OCXO and an op-amp. The choice
of a 12V regulator is a little odd, as its input is only 12V, and it
cannot therefore regulate its output to 12V. However, most
OCXOs run at 12V or 5V, and we’d be dissipating over half a Watt
if we used a 5V-supply oscillator, so we stuck with this topology,
for use with the 12V IsoTemp OCXO131-40.
Based on our preference for 12V linear regulators, we chose the
LM2940SX-12 low-dropout (LDO) regulator. This part has
overvoltage (up to 26V) and reverse-voltage protection, and has a
low dropout at low currents, in a surface-mount package.
Op-amp Circuit
The original circuit included an op-amp that detected when the
OCXO was no longer in warm-up mode, and drove an output to
+5V to indicate that warmup was complete. While warming up, the
op-amp detected the increased current draw and drove a small FET
to pull the
–WARMING
signal down to 0V.
We replicated this circuit, with a 180mA trip point, using an
LT1716 precision rail-to-rail comparator.
OCXO
The original circuit included a filter on the OCXO’s +VDC
terminal, as well as a pi filter on the output. We verified that the
trim voltage on the OCXO131-40 is effective over a range of 0V to
5V, centered at 2.5V.
We replicated the earlier circuit’s filters, added an MMCX jack on
the OCXO’s output, and planned for a VCO trim voltage centered
at 2.5V.
VCO Trim
Based on Gerry Sweeney’s OCXO board’s circuit design, we
selected the high-precision Maxim MAX6350 5V regulator to
generate a stable voltage for the trim voltage from 12V. Its tempco
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