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Abstract:
This video documents an attempt to measure the relative frequency stability of two vintage HP 5061A cesium atomic clocks as a preliminary step towards potentially observing relativistic time dilation effects, similar to the Hafele-Keating experiment. The measurement setup utilizes HP timing generators to derive 1-pulse-per-second (1PPS) signals from each clock's 5 MHz output, an HP 5334B counter to measure the time interval between these pulses, and a restored HP 5087A distribution amplifier with a frequency doubler providing the necessary 10 MHz reference for the counter. Allan Variance is discussed as the relevant stability metric. Initial measurements revealed significant relative drift (tens of nanoseconds overnight), far exceeding the stability required for relativistic measurements. Attempts were made to improve stability by finely adjusting the C-field (magnetic field) of one clock. A vintage HP-IB logging system, comprising an HP 85 computer, HP 59501B DAC, and HP 7132A chart recorder, was implemented to track the drift over time. Despite the successful operation of the all-HP measurement chain, the logged data showed continued drift and discrete jumps in the time difference, indicating persistent instability likely originating from one of the clocks. The experiment concludes that further clock repair is necessary before achieving the required stability, while highlighting the successful performance of the vintage test equipment setup.
Comparing Two Atomic Clocks: Stability Test for a Relativity Experiment
0:00:32 Goal: The primary aim is to compare two HP 5061A atomic clocks to assess their relative stability, as a precursor to attempting a measurement of relativistic time dilation (inspired by the Hafele-Keating experiment).
0:01:02 Initial Check: The first step is confirming the clocks' stability relative to each other while stationary in the lab, aiming for sub-nanosecond drift over days.
0:01:50 Measurement Setup: Comparing two imperfect clocks means only their relative difference can be measured. The setup aims to precisely track this difference.
0:02:13 Clock Outputs: The HP 5061A units are frequency standards providing 5 MHz, 1 MHz, and 100 kHz outputs, unlike the time-of-day clocks used in the original Hafele-Keating experiment.
0:02:43 1PPS Generation: HP 59308A Timing Generators are used to divide the 5 MHz output from each clock down to a precise 1 pulse-per-second (1PPS) signal.
0:03:31 Time Interval Measurement: An HP 5334B counter measures the time interval between the 1PPS signals from the two clocks with 0.1 ns resolution. The counter's math function zeros the initial arbitrary delay.
0:04:04 Counter Reference Challenge: The counter requires a 10 MHz reference, but the clocks output 5 MHz.
0:04:17 Frequency Doubling Solution: A restored HP 5087A Distribution Amplifier with its frequency doubler option converts the 5 MHz output from one clock to the required 10 MHz reference for the counter.
0:04:56 Stability Metric (Allan Variance): Allan Variance is introduced as the standard measure for clock stability over different time scales. The target stability for the relativity experiment (around 1 part in 10^13) is noted as challenging for these specific clocks.
0:06:52 First Overnight Result: The initial overnight test shows a significant relative drift of ~35 nanoseconds, far too unstable for the intended experiment.
0:08:31 Second Overnight Result: After restarting, the drift is lower (~6.7 nanoseconds) but still too high.
0:08:58 C-Field Adjustment: An attempt is made to finely tune the frequency of one clock relative to the other using its C-field adjustment (tuning the magnetic field affecting the cesium resonance), aiming for minimal relative drift.
0:10:31 Characterization vs. Tuning: It's noted that the original Hafele-Keating experiment characterized clock drift against a master standard rather than attempting to tune the clocks perfectly relative to each other.
0:11:04 Long-Term Logging: A vintage, all-HP logging system is assembled to record the drift continuously.
0:11:08 Logging Components: The system uses an HP 7132A chart recorder driven by an HP 59501B DAC, controlled by an HP 85 computer via the HP-IB bus. An HP 3478A DVM monitors the DAC output.
0:11:56 Control Program: A simple 13-line BASIC program on the HP 85 reads the time interval from the counter, scales it, and sends the value to the DAC for plotting.
0:14:10 Third Result (Logged): The chart recording shows ~14 nanoseconds of drift overnight.
0:14:20 Drift Instability: Crucially, the recording reveals the drift isn't smooth but includes significant, abrupt jumps, indicating underlying instability.
0:14:44 Conclusion: The clocks (or at least one of them) are currently too unstable, exhibiting both drift and jumps ("atomic gremlins").
0:15:03 Measurement System Success: Despite the clock instability, the entire vintage HP measurement and logging system performed flawlessly.
0:15:11 Next Steps: More atomic clock repair work is required before the desired stability measurements and the subsequent relativity experiment can be successfully performed.