Phase Comparator

Monitors Electronic Pendulum Clock

Indicates fine-grain drift rate in near real time.

Read "Electronic Pendulum Driver" for the background on this project.

Max Carter

The Phase Comparator provides a means to electronically compare the beat of the Electronic Pendulum Driver (available at the scope TP) to an external 1 Hz standard. The phase comparator indicates the relative phase of the pendulum's beat, compressing the time required to measure and adjust it's rate. Short-term drift can be determined in 15 minutes or less.

Phase Comparator with Reference Source

The 5-inch (127mm) meter and associated circuitry are mounted in an old computer power supply case. The meter was surplus, obviously designed originally for some other application. To achieve second-per-week accuracy, the external standard must be accurate to better than one part per million (10-6). (One second/week = 1.6 parts/million.) A quartz-based function generator that has been calibrated against a source traceable to NIST would meet the requirement. It's output level must be 4 volts peak-to-peak or higher. The one shown is calibrated against NIST radio station WWVB.

Control Panel

Circuit Board

Block Diagram


Figure 1
In addition to the +12V and ground connections shown,
  • Connect U1 pin 14, U2 pin 14 and U3 pin 8 to +12Vdc
  • Connect U1 pin 7, 12, 13 to ground; U2 pin 7 to ground; and U3 pin 4 to ground
**The scaling resistor (Rscale) calibrates the meter to 1 volt full-scale. Calculate its value in ohms per the equation:

Rscale = 1/Ifs - Rmtr

Ifs is the specified (or measured) full-scale meter current in amps,
Rmtr is the measured resistance of the meter movement in ohms.

How it Works

NOR gates U1a and U1b condition the input from the external reference. The reference signal can be a squarewave or a sinewave. The conditioned signal is applied to D-type flip-flop U2b. The low-amplitude pulses from the pendulum are amplified by the two transistors and applied to U2a. The two signals are phase compared by U2a and U2b.

The output pulses from the phase comparator are integrated (LP filtered) into a (more or less) steady DC voltage by the 10-meg resistor and 2.2 µF capacitor. The resistor and capacitor provide a time constant of 22 seconds. A 10µF capacitor can be switched in from the front panel (DAMPING High) to increase the time constant to 122 seconds.

The voltage from the integrator appears on pin 3 of U3a, the meter driver. The output voltage from U3a (pin 1) will vary from about +1 volt to +11 volts, depending on the phase relationship of the two input signals. This voltage is attenuated by the 8.2k and 12k resistors to 1/10 its original value. The SENS (sensitivity) switch bypasses the attenuator in the High position and applies full voltage to the meter. The 866-ohm resistor scales the 1 mA meter movement to 1 volt full scale.

U3b, under control of the SCALE CENTER potentiometer on the front panel, provides a variable reference voltage to the negative terminal of the meter. This allows the phase offset reading to be placed on-scale.

Regulating the Electronic Pendulum Clock

As with all pendulum clocks, regulating the electronically-driven pendulum clock is an iterative process.

  1. Pre-Adjust

    1. Pre-adjust the pendulum clock to within a few seconds per hour by other means, such a comparing the clock's second hand to an accurate time source.

  2. Initialize, Connect and Power-up.

    1. Set DAMPING switch to Low, SENS (sensitivity) switch to Low, SCALE CENTER control to the center of its range.
    2. Connect the comparator to the clock motor driver (Scope TP) and external 1 Hz source. Set external source output level to 4Vp-p or higher. Power the comparator.
    3. Wait for the integrator to settle, about 2 minutes. Adjust the SCALE CENTER control as necessary to obtain and maintain an on-scale reading.

  3. Monitor and Adjust

    1. Allow the comparator to run long enough to determine if the clock is running fast or slow. A fast-running clock will cause the meter to drift to the right; a slow-running clock will cause the meter to drift to the left. The meter's response time depends on the settings of the DAMPING and SENS switches. If the pendulum is maladjusted, the meter deflection will be easily noticed, especially in the SENS High mode. The DAMPING switch can be placed in the HIGH position after a few minutes to dampen the pointer movement if you like. Adjust the SCALE CENTER control as necessary to obtain and maintain an on-scale reading.
    2. Tweak the pendulum longer to correct a fast-running clock. Tweak the pendulum shorter to correct a slow-running clock. The aim is to produce a stationary reading on the meter.
    3. Repeat step 3.

    • As the pendulum comes closer and closer to being on time, the tweaks should become smaller and smaller, becoming very small near the end of the effort. A correctly adjusted pendulum will maintain a stationary meter reading, or very slow drift, for hours.
    • Some intra-day variation may be noticed, depending on how the ups and downs of ambient temperature affect the pendulum's length during the day. Monitor the pendulum for 24 hours in Low SENS mode to allow intra-day variations to average out.
    • You may notice the pointer occasionally reverse direction and quickly return to it's starting point. This happens when the pendulum drifts out of the meter's (±1 cycle) range, causing a "cycle slip". After the pointer returns its starting point (wherever that may be) it will resume its previous direction of drift.
    • When set to High SENS, the meter is capable of discerning pendulum maladjustment to a second per week in a few minutes.
    • Allow the integrator time to settle after any changes.
"We must not be hasty."  - Treebeard

Meter Calibration

Full-scale on the as-built meter represents two (2) cumulative cycles or 720 degrees.
  1. Divide the scale into equal major divisions. The easiest way to do this is to simply use the existing scale markings (assuming the scale is linear).
  2. Convert the number of major divisions (Ndiv) into a fraction (Sfrac) as follows:

    Sfrac = 2/Ndiv (Low SENS).

    When the SENS switch is in the High position, the meter is 10 times more sensitive, so the fraction becomes a tenth as great:

    Sfrac = 2/(10 x Ndiv) (High SENS)

    For example, the number of major divisions on the scale of the huge meter in the as-built device is 8. So,

    for Low SENS,   Sfrac = 2/8 = 1/4 = 0.25 (at 1 Hz, this corresponds to 250 milliseconds),

    for High SENS,   Sfrac = 2/(10 x 8) = 2/80 = 1/40 = 0.025 (at 1 Hz, this corresponds to 25 milliseconds).

    Mark the scale directly or put a sticker on the meter indicating Sfrac per division.
Meter Scale on the As-Built Phase Comparator

Low SENS range, Sfrac = 0.25 per major division (250 mS at 1 Hz)
High SENS range, Sfrac = 0.025 per major division (25mS at 1 Hz)

Calculate Seconds-per-Week or Seconds-per-Month

  1. Adjust SCALE CENTER to a major division mark at or near the center of the scale.
  2. Monitor the meter and note the number of seconds (Nsec) required for the meter to deflect 1 major division.
  3. Calculate as follows:

    Seconds/week = (Sfrac / Nsec) x 604800

    Seconds/month = (Sfrac / Nsec) x 2592000

    So, for example, if Sfrac is 0.250 and Nsec is 86400 (24 hours),

    Seconds/week = (0.250 / 86400) x 604800 = 1.75

    Seconds/month = (0.250 / 86400) x 2592000 = 7.5


  • The accuracy of the pendulum phase meter depends on the accuracy of the 1 Hz standard used with the device.
  • The meter reads phase. The pendulum need not run at any particular phase relationship to the reference, only that the relationship not change over time. A perfectly timed pendulum will produce a stationary meter reading.
  • The meter is sensitive to the slightest deviation from "on-time" in the High SENS mode. If the pendulum is not on-time, it will be noticed in the meter reading in 15 minutes or (usually) less.
  • To project long-term performance, monitor the pendulum for 24 hours in Low SENS mode. This averages out intra-day variations.

The phase meter is usable at beat rates from 1/2 Hz to about 10 Hz without modification with a suitable reference source.

Schematics produced with DCCAD.


2N2222 (PN2222):Futurlec/PN2222
2N2907A (PN2907A):Futurlec/PN2907A
Resistors, capacitors, diodes:Futurlec/components

Futurlec is cheap, but they're a little slow. Allow a couple of weeks for delivery.

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Electronic Pendulum Driver