# Presentation: 177375

## Testing the time-stamp accuracy of a digital variometer and its data logger

- by
- J. Rasson,
- Royal Meteorological Institute of Belgium

## It is important that an Observatory be able to test the timing accuracy of its geomagnetic field observations and not solely rely on the manufacturer claims. Therefore we propose:

- A black-box testing procedure for variometric data acquisition chain.
- The specialized equipment necessary for performing the test
- The data processing used to derive the tests result

- Black-box testing should be performed by injecting a magnetic signal
- to the variometer sensor and analyzing the resulting standard
- output file

- The testing is possible on any geomagnetic variometer
- The testing is simple enough to be carried out in any magnetic observatory. A voltmeter and oscilloscope may be necessary
- No necessity to move or modify the variometer installation, nor to dismantle nor stop the magnetometer electronics or data acquisition.

- The input will be provided in part by a coil system sitting around the variometer sensor and producing a magnetic field there.
- This field will be sensed by the variometer in addition to the geomagnetic field.

## Test signal generated by the coil
The test signal S(t) is a periodically variable magnetic field of constant period T (frequency f=1/T) and amplitude A. T within the pass-band of the variometer.
The phase Φ of this test signal will be referred to an UTC epoch t0 = hh:mm:ss.000 fixed beforehand in each testing experiment. Ideally Φ=0.
The waveform of the test signal may be a sine, triangle or square wave. Only the first harmonic is used in the test. A square wave is the easiest to generate with accuracy. 
Amplitude of the test signal as big as possible but avoid saturation of the variometer. Attention to perturbations inflicted on nearby geomagnetic equipment.
!!!  the current in the coil must be without delay nor phase lags
can be monitored with an oscilloscope measuring the voltage across a resistor in series with the coil.

- The test signal S(t) is a periodically variable magnetic field of constant period T (frequency f=1/T) and amplitude A. T within the pass-band of the variometer.
- The phase Φ of this test signal will be referred to an UTC epoch t0 = hh:mm:ss.000 fixed beforehand in each testing experiment. Ideally Φ=0.
- The waveform of the test signal may be a sine, triangle or square wave. Only the first harmonic is used in the test. A square wave is the easiest to generate with accuracy.
- Amplitude of the test signal as big as possible but avoid saturation of the variometer. Attention to perturbations inflicted on nearby geomagnetic equipment.
- !!!  the current in the coil must be without delay nor phase lags
- can be monitored with an oscilloscope measuring the voltage across a resistor in series with the coil.

## Manual Reset fixes t0
M adjusts T, the test signal period
Long cable to variometer hut
N points selected in the standard variometer record starting at t0

- M adjusts T, the test signal period
- Long cable to variometer hut
- N points selected in the standard variometer record starting at t0

## (linear phase filter has constant delay E)

- (T is known)

## Actual measurements

## Slide 10

## Delays                at various frequencies from which the time-stamp errors can be deduced. 
Amplitudes

Testing the test:
Artificial delays

- Amplitudes
- Testing the test:
- Artificial delays

- Artificial delay

- What is the test going to measure?

## There is no significant delay introduced by the variometer under test: the time-stamp error is < 10 ms

- When artificial delay is inserted in the excitation square wave signal, it is measured with good accuracy.
- Even with relatively low amplitudes of the test signal, down to 44 nT, the test performs well and the delays are determined with a 10% accuracy.

## Thank you for your attention

**Conclusion**
- This timing test is able to evaluate the variometers time stamp accuracy to 10 milliseconds or less in the case of a 1s variometer.
- The "black-box" test is simple to perform and does not perturb the variometer installation. There is no need to perform specialised measurements on the variometer electronics.
- A magnetic signal is injected in the sensor via a coil and a robust statistical procedure extracts the timing errors from the standard variometer record.

## Slide 15

## A band-limited measurement cannot provide data without introducing a phase-lag. A real world variometer is necessarily band-limited. 
For a periodic signal of period T, the relationship between timing t and phase lag Φ in degrees is given by: t = T×(Φ/360)
The procedure of sampling allows shifting the timestamp on the data. 
Therefore if the analogue and digital part of the variometer use filters of the linear phase type, and the resulting constant delay is a multiple of the sampling rate, an adequate timestamp shift can theoretically result in a data acquisition with the required timing accuracy.

- For a periodic signal of period T, the relationship between timing t and phase lag Φ in degrees is given by: t = T×(Φ/360)
- The procedure of sampling allows shifting the timestamp on the data.
- Therefore if the analogue and digital part of the variometer use filters of the linear phase type, and the resulting constant delay is a multiple of the sampling rate, an adequate timestamp shift can theoretically result in a data acquisition with the required timing accuracy.