Fixing with the Flu - Keysight 53230A 15-Digit, 20ps Frequency Counter Repair, Teardown & Upgrade
In this episode Shahriar repairs a Keysight 53230A frequency counter with an unusual problem. The instrument reports OVERLOAD for both high & low thresholds on both Channel 1 & Channel 2 with or without an input signal. With this problem, the instrument also fails self-test.
There is no schematic or even a block diagram for this instrument. But since the unit is equipped with both the Microwave Pulse Measurement as well as the 15GHz Microwave input option (as Channel 3), it is certainly worth repairing. Channel 3 of the instrument is fully functional, which is a good sign for the health of the latter DSP portion of the unit.
Teardown reveals two parallel & identical Channel 1 & Channel 2. The circuits are examined in details and the primary comparators are identified. After some testing, it becomes clear that the comparator thresholds are controlled via some ADI DACs which are maxed out due to inactivity at the output of the comparators. Interestingly, the latch inputs ports of the comparator ICs are also controlled by a different DAC. This is the first part which is in common with both channels. After replacing this DAC, the instrument no longer reports the OVERLOAD error.
The instrument is also upgraded with an OCXO which should bring much better frequency stability. The unit is then calibrated against a Rubidium standard. The microwave pulse measurement is demonstrated first using a R&S SMBV100B Vector Signal Generator & Tektronix MSO-6B Mixed-Domain Oscilloscope. The Keysight 53230A is easily able to measure pulse duration, repetition as well as the carrier frequency. The frequency counter’s trend chart, histogram & other advanced DSP functions are also demonstrated.
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