User Manual
10
UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Pumpkin purchased multiple IsoTemp OCXO131-40 (12V, sine-
wave output) from an eBay seller for around $21 each and set out
to create a drop-in replacement OCXO for S/N ‘142, and for other
XL Microwave Model 3xxx units.
OCXO Design
S/N ‘142s MTI-based OCXO is mounted on a 2” x 2” 0.062” PCB
using SAE dimensions and fasteners. Four 4-40 captive threaded
standoffs on a 1.5” bolt square enable the module to be mounted to
the Model 3xxx’s chassis via 4-40” flathead screws.
The assembly has a 0.100” pitch 4-position rectangular receptacle
connector TE Connectivity / AMP P/N 3-640440-4 for a few
inches of 22AWG wiring, as follows:
1. O Purple 10MHz output
2. G Black Ground
3. + Red
+12V
4. S Yellow Heater status
5
Table 2: MTA100 connector pinout to mate to XL
Microwave 3xxx
Note
Our 705-01667 PCB’s connector pinout is opposite from
the above … the 22AWG wires are connected to the PCB based on
the O/G/+/S labels.
Critical Components
We decided to create a replacement module with a few
enhancements compared to the original:
•
Compatibility with inexpensive (used) IsoTemp OCXO131-40
10.000MHz OCXOs
•
An ex12V barrel jack for desktop calibration / testing
•
An MMCX connector on the output, for desktop calibration / testing
•
A high-quality multi-turn trimpot
•
Precision resistors in the trimpot circuit
These components are expanded upon in detail below.
5
Not present / not used on the fancier OCXO36-44 and units originally equipped
with them.
Summary of Contents for UM-20
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