Item Description:
DIY HP538xA OCXO Assembly
For time‑base modification of HP‑5384A / 5385A / 5386A universal frequency counters to improve accuracy and stability.
This OCXO is a standalone assembly, designed to fit the reserved mounting space inside the instrument. Its core component is a used oven‑controlled crystal oscillator, augmented with fine‑frequency adjustment and anti‑interference circuits, assembled on a custom‑designed circuit board.
It has been pre‑calibrated to ±0.1 Hz using a GPSDO and subjected to a 24‑hour burn‑in test prior to shipment. Ready for direct installation. Its accuracy and stability are sufficient for general‑purpose use. For higher‑grade performance requirements, further calibration against a high‑precision frequency reference may be performed by the user.
Modification Procedure
WARNING! The HP‑5384A / 5385A / 5386A operates from high‑voltage mains power. Always disconnect the mains power cord before performing any modification work.
Remove the four screws on the bottom of the HP‑5384A / 5385A / 5386A and take off the top cover. Two rear screws require removing the rear rubber feet for access.
Remove the HP‑IL interface board.
Undo the two screws securing the main board and pull the circuit board out of the chassis.
4a. For HP5384A: Remove components C61‑66, R96‑99, R110, Q13, Q14, Y1C and jumper W2.
4b. For HP5385A or TCXO‑equipped HP5384A: Remove Y1B, R95 and jumper W2.
Fit an inductor at position L5 on the main board, or install a direct wire link.
Insert the four pins of this OCXO assembly into the main‑board footprint and solder securely.
Reinstall the main board into the chassis and refit the interface board.
This OCXO assembly is a hobbyist DIY project built for recreational purposes only. Due to variations in materials, workmanship and other factors, the modified frequency counter may not meet official metrological standards.
The HP‑538xA series frequency counters can display up to 11 digits. Under a 10‑second gate time, the theoretical resolution reaches 1 mHz (0.001 Hz). This is a theoretical specification only; the least‑significant digits will not remain stable under typical operating‑environment conditions. Minor temperature fluctuations, electromagnetic interference, vibration and other effects may cause reading jumps of ten or even dozens of counts. This OCXO assembly cannot deliver absolute reading stability either.
Users with professional metrology requirements should purchase brand‑new compliant commercial instruments separately.

Package Included:
1X OCXO Assembly (Do not include other)