Potomac Instruments PI 4100 - Calibration; PI 4100 Calibration Verification Test

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5.3 Calibration
The PI4100 normally holds its calibration for a very long period with minimal change.
Nevertheless, it is prudent to recheck the calibration periodically, especially in situations where
repeatability of measurements is critically important. It is therefore recommended that a calibration check
be done at two-year intervals. For the greatest certainty a factory recalibration is preferable. When return
of a unit to the factory is not practical, a field test to verify the calibration can be done; a detailed
procedure for such a verification test appears below. If it is planned to use this test at regular intervals
without returning the PI4100 to the factory for calibration, it is recommended to perform the test
according to this procedure on the unit when it is first received, using the same test equipment and test
setup which will be used for all future tests. This will establish a local basis for judging whether the
calibration has changed. For every verification test done it is recommended to keep a copy of the test data
sheet together with the factory certificate of calibration in order to maintain a record of calibration test
results.
5.4 PI 4100 Calibration Verification Test
General: The purpose of this procedure is to verify that a particular PI 4100 Field Strength Meter
continues to perform in accordance with its published specifications and its factory calibration. The test
makes use of the 4100’s external connectors and loop antenna and requires an external signal generator
and a precision voltmeter. It is a detailed version of the test described on the certificate of calibration.
Test Method: The CAL In BNC jack provides a way to check the 4100’s calibration using an
external signal generator. An RF voltage source at this jack produces an RF current in a conductor at the
outside edge of the loop antenna pc board. This current generates an RF magnetic field around the loop
antenna which simulates a received radio station field. It induces a voltage in the loop antenna just as a
radio station transmission does, and the 4100 measures its value and displays the result as an equivalent
received field strength value. The calibration circuit is so designed that the displayed value in dBuV/m
(or mV/m) is within ±10 per cent of the RF voltage in dBuV (or mV) at the CAL In jack. This test using
an external signal generator is done at the factory after all adjustments and calibration of the 4100 are
completed. The test results are recorded on the certificate of calibration supplied with the 4100 and are
kept on file at the factory. At any future time, therefore, the overall performance of the 4100 from
antenna to display can be checked by connecting a signal generator to the CAL In jack and repeating this
factory test. A change in the displayed field strength for the same generator voltage as indicated by the dc
output at RF Out would be cause for concern.
Accuracy - the calibration detector: The accuracy of this verification test depends on knowing
precisely the calibrating signal generator voltage at the CAL In jack. If the generator output accuracy is
specified at +/- 0.5 dB, for example, the resulting 4100 reading could deviate as much as +/- 0.5 dB from
the expected value when its calibration has not changed from the factory setting. To prevent generator
inaccuracy from affecting calibration, the 4100 includes a precision detector circuit at the CAL In jack
which produces a dc voltage equal to the RF rms voltage over the range of 114 – 117 dBuV (500 – 750
mV). This dc voltage can be measured at the RF Out jack center pin using an acccurate, high-impedance
voltmeter and is used to adjust the generator output level to the required value in both factory and
verification tests. A small uncertainty remains when a different generator is used for verification because
the detector dc output for a given generator voltage depends slightly on the generator's output harmonic
content.
Low signal level tests: Checking the 4100’s accuracy at low signal levels requires a precision 50
Ohm attenuator between the generator output and the Cal In jack. This can be the generator’s internal
attenuator. Again, test accuracy is limited by the attenuator accuracy. An uncertainty of ± 1 dB in the
attenuation value results in ±1 dB uncertainty in the expected 4100 reading, which must be added to the
generator output uncertainty.