Measure and record 60 Hz power line frequency using GPS 1 pps as a reference.
Use an Arduino Teensy to accurately measure the 60 Hz AC powerline frequency using 1 pps calibration from a GPS and 60 Hz interrupts from an AC transformer (which also powers the system). A Raspberry Pi Zero W running Python reads the serial data stream from the Teensy every 5 s and appends it to a log file; it also outputs to a terminal.
A 9 VAC 300 mA transformer powers the system. The transformer powers a bridge rectifier, filter capacitor and simple DCDC converter to deliver 5 V to the Raspberry Pi and the Teensy. Thje PCB link between the USB and 5 V pin on the Teensy is cut to allow the Teensy to be programmed via USB even while the system is powered.
A 2-stage R.C filter with a 3.3V zener clamp is driven from one AC terminal of the transformer. The other is connected to DC GND via 1 kohm. This is important as while the AC waveform is less than the bridge rectifier output, the common-mode level of the transformer is undefined. The resultant signal (looks like a smoothed half-wave rectified signal) drives the comparator in the Teensy which squares it up (with hysteresis) and drives the Teensy's FTM (Flex Timer) module interrupts. These are counted against the Teensy's internal 48 MHz crystal.
Separately, a precise 1 pulse-per-second (pps) signal from the GPS (about 20 ns jitter) also drives interrupts to another FTM channel. These 1 Hz interrupts calibrate the 48 MHz clock to give a precsise timebase. A slow filter ( 16 s time constant) filters this. If the 1 pps is lost, the last good calibration is used until the 1 pps is restored.
The Teensy outputs serial ASCII data on USB and its serial port. Each line contains the number of 48 MHz clocks, number of power line cycles, calculated 60Hz frequency, calibrated 48 MHz, cumulated cycles, filter and 60 Hz phase information in response to a 'request' (a single blank line). If the 'request' contains any additional character, the Teensy outputs a header and resets the cumulative counter.
The Teensy's serial port is connected to the Raspberry Pi. A small Python program sends a request every 5 s (based on the Raspberry Pi's clock), and logs the time, Teensy's output and a simple ASCII plot to the terminal. Data is also appended to a (networked) log file. This program logs continuously in the background as a service; it can be monitored by logging in to the Pi and using TMUX to attach.
The Pi's clock is only used to timestamp the data; the precision of 60 Hz measurement comes from the 1 pps GPS signal. Care is taken in the interrupt and processing routines that NO 60 Hz cycles are lost. This can result in some 5 s intervals containing 299, 300 or 301 cycles. Irrespectively, the 60 Hz calculation is correct over that precise number of cycles.
The initial Python log output looks like this: 2022-02-12 21:01:50.072 # 2022-02-12 21:01:50.072 # Ticks, Cycles, Frequency, FBUS, Total Cyc, Flt, Phase, 1Hz 2022-02-12 21:01:50.151 4002125, 5, 59.966893838, 47999001, 5, 16, 122, 118 2022-02-12 21:01:55.005 232924802, 291, 59.966602440, 47999000, 296, 16, 62, 10 2022-02-12 21:02:00.005 240125542, 300, 59.967381562, 47999000, 596, 16, 3, 12 2022-02-12 21:02:05.005 240109872, 300, 59.971295141, 47999000, 896, 16, 311, 14 2022-02-12 21:02:10.005 239281816, 299, 59.978235036, 47999000, 1195, 16, 272, 3 2022-02-12 21:02:15.005 240051332, 300, 59.985921261, 47999001, 1495, 16, 247, 4 2022-02-12 21:02:20.005 240035730, 300, 59.989820266, 47999001, 1795, 16, 228, 5
The first few measurements are not over 5 s as the Python program synchs to 5 s intervals.
The R Pi uses systemd to mount a network recorder drive, and to start the 60Hz service. TMUX on the Pi doesn't work with a simple systemd service, so is wrapper in a shell script.
Here's an example of the terminal data also showing the plot (the plot itself is not logged) This correlates precisely with University of Tennessee's FNET service at http://fnetpublic.utk.edu/tabledisplay.html#WECC
2022-02-12 22:03:55.005 239995033, 300, 59.999993000, 47999001, 223465, 16, 169, 7, -·························/·························+ 2022-02-12 22:04:00.005 239997705, 300, 59.999326244, 47999002, 223765, 16, 168, 7, -························/0·························+ 2022-02-12 22:04:05.005 239999145, 300, 59.998966246, 47999002, 224065, 16, 166, 7, -························|0·························+ 2022-02-12 22:04:10.005 240002613, 300, 59.998099271, 47999002, 224365, 16, 162, 7, -·······················/·0·························+ 2022-02-12 22:04:15.005 240023214, 300, 59.992948432, 47999001, 224665, 16, 150, 8, -··················/······0·························+ 2022-02-12 22:04:20.005 240031158, 300, 59.990964173, 47999002, 224965, 16, 134, 8, -················/········0·························+ 2022-02-12 22:04:25.005 240037041, 300, 59.989492622, 47999001, 225265, 16, 115, 9, -··············/··········0·························+ 2022-02-12 22:04:30.005 240047995, 300, 59.986755149, 47999001, 225565, 16, 91, 9, -············/············0·························+ 2022-02-12 22:04:35.005 240059397, 300, 59.983905983, 47999001, 225865, 16, 62, 10, -·········/···············0·························+ 2022-02-12 22:04:40.005 240028170, 300, 59.991710973, 47999002, 226165, 16, 47, 11, -·················\·······0·························+ 2022-02-12 22:04:45.005 240006390, 300, 59.997153826, 47999001, 226465, 16, 42, 11, -······················\··0·························+ 2022-02-12 22:04:50.005 239998958, 300, 59.999012996, 47999002, 226765, 16, 40, 11, -························\0·························+ 2022-02-12 22:04:55.005 240000788, 300, 59.998555505, 47999002, 227065, 16, 37, 11, -························|0·························+ 2022-02-12 22:05:00.005 239995227, 300, 59.999945749, 47999002, 227365, 16, 37, 11, -·························\·························+ 2022-02-12 22:05:05.005 239995759, 300, 59.999812747, 47999002, 227665, 16, 37, 11, -·························|·························+ 2022-02-12 22:05:10.005 240001501, 300, 59.998376010, 47999001, 227965, 16, 34, 11, -·······················/·0·························+ 2022-02-12 22:05:15.005 239989974, 300, 60.001257803, 47999001, 228265, 16, 36, 11, -·························0\························+ 2022-02-12 22:05:20.005 239963438, 300, 60.007894203, 47999002, 228565, 16, 50, 11, -·························0·······\·················+ 2022-02-12 22:05:25.005 239950235, 300, 60.011194821, 47999001, 228865, 16, 71, 10, -·························0··········\··············+ 2022-02-12 22:05:30.005 239959655, 300, 60.008838986, 47999001, 229165, 16, 86, 10, -·························0········/················+ 2022-02-12 22:05:35.005 239963278, 300, 60.007932964, 47999001, 229465, 16, 101, 9, -·························0·······/·················+ 2022-02-12 22:05:40.005 239947895, 300, 60.011780057, 47999001, 229765, 16, 122, 8, -·························0···········\·············+ 2022-02-12 22:05:45.005 239929564, 300, 60.016365053, 47999001, 230065, 16, 151, 7, -·························0···············\·········+ 2022-02-12 22:05:50.005 239919301, 300, 60.018933616, 47999002, 230365, 16, 185, 6, -·························0··················\······+ 2022-02-12 22:05:55.005 239916644, 300, 60.019598307, 47999002, 230665, 16, 221, 5, -·························0···················\·····+ 2022-02-12 22:06:00.005 239934105, 300, 60.015230432, 47999002, 230965, 16, 248, 4, -·························0··············/··········+ 2022-02-12 22:06:05.005 239971193, 300, 60.005954965, 47999002, 231265, 16, 259, 4, -·························0·····/···················+ 2022-02-12 22:06:10.005 239984642, 300, 60.002592166, 47999002, 231565, 16, 263, 4, -·························0··/······················+ 2022-02-12 22:06:15.005 239961928, 300, 60.008271812, 47999002, 231865, 16, 278, 3, -·························0·······\·················+