Agilent HP 53131A Universal Frequency Counter: Comprehensive Technical Guide and Applications

What this delivery is: one Agilent (HP) 53131A 225 MHz universal counter, used in exceptional condition — the record calls it 成色极新, and the photos back it: unmarked case, original Agilent "win a prize" feedback sticker still on the top lid, screen protector film half-peeled but present — received 2026-02-10, 4 kg, three-month warranty. The serial plate reads 53131A, SN: MY40019425, MADE IN MALAYSIA. This is the dual-channel instrument: channel 1 measures DC to 225 MHz, channel 2 runs 100 MHz to 3 GHz, and the rear panel carries the input 3 BNC (with its protective foam cap in place), external arm, reference input and the 10 MHz output, plus GPIB and RS-232 for automated benches.

What the front panel offers

The 53131A is the counter most labs standardised on: frequency and ratio, time interval and period, plus totalize — with the MEASURE block giving one-key access to Freq & Ratio, Time & Period and Other Meas, LIMITS for go/no-go screening with Upper & Lower and Limit Modes, and MATH for Scale & Offset and statistics on live readings. Each input channel has its own trigger controls: 50 Ω / 1 MΩ coupling, DC/AC, X10 attenuate and a 100 kHz filter, with the damage-level warning printed at each BNC — 5 V rms MAX at 50 Ω. The arrow-and-Enter key cluster drives the menu, and the yellow POWER switch is the HP bench signature. The OPTIONS block on the rear shows the three oven-timebase dots — 001 MS Oven, 010 HS Oven, 012 US Oven — all unfilled, so this unit runs the standard timebase; the Osc Adjust hole beside the RS-232 port is where a cal lab trims it.

Checking a used counter

A frequency counter either counts accurately or it is furniture, and the good news is that the check takes one reference signal: feed the instrument's own 10 MHz output into channel 1 (a short BNC cable, no level worries) and read the display — a healthy 53131A counts its own timebase as 10 000 000.000 Hz without flicker, every digit. Then step a known source through channel 2 to exercise the 3 GHz path. The YF warranty seals on this unit sit over the case joints, so the instrument has not been opened since the reseller serviced it; breaking a seal voids the three-month cover. For bench work that needs better long-term stability, the oven options listed on the rear are the upgrade path — they install as a module, and the Options dots get marked accordingly.

Photos

Video walkthrough of the counter and its leads.

Related products on KOEED

Bench-side neighbours in the delivery records: the Agilent 82357B USB/GPIB interface — the cable that puts this counter on a PC-controlled bench — and the Dell H825EF-00 workstation PSU from this batch.