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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
LIVE, Web-Based RF Safety Training
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Contents
Overview Calibration methods depend on several factors:
Probe Calibration Probes calibration begins with establishing either an electric field or a magnetic field of known intensity. The probe is then precisely positioned in the field. Some type of fixture is often used. A thorough calibration should take into account Isotropic Response by either manually or mechanically rotating the probe about its axis. The value used should be the average value obtained throughout the 360° rotation of the probe. See Selecting Equipment for more information on isotropic response. The magnitude of the calibration field should be somewhere in the middle of the probe's dynamic range. This will minimize the effects of any linearity error. Linearity errors refer to the errors that occur at the same frequency and probe position when only the magnitude of the field is changed. In a good probe, linearity errors are normally small—typically ±¼ dB. Calibration Equipment Several different types of hardware are used to establish a precise field. They include:
The description of this equipment and their use is beyond the scope of this web site. In general, horn antennas are used at the higher frequencies, typically at frequencies above 1 GHz. Some calibration systems use antennas down to 500 MHz. Sliding waveguide fixtures are most useful from 500 MHz to 1000 MHz. TEM cells are used at frequencies below 500 MHz. Single Frequency Calibration If a probe is only going to be calibrated at a single frequency, an adjustment is made within the probe if it has an amplifier. If the probe does not have an amplifier where gain can be adjusted, an adjustment is made in the meter so that the survey set reads correctly at the calibration frequency. Single frequency calibration is fine if the instrument has a dedicated application or is only rated for a narrow bandwidth. A microwave oven leakage instrument is a good example of a narrow band instrument. It only has to work at a very narrow band centered at 2450 MHz. Another example would be a survey set that is dedicated to checking a specific systems that operates at a single frequency or a narrow band of frequencies. So, even if the equipment is broadband in design, a single frequency calibration is perfectly adequate. The major problem with single frequency calibration is that the sensitivity of broadband probes can vary dramatically over its rated frequency range. So there is automatically an unknown error every time the instrument is used at any frequency other than the calibration frequency. The largest component of measurement uncertainty is normally frequency deviation. See Selecting Equipment for more information on frequency sensitivity. Multiple Frequency Calibration Multiple frequency calibration is the only way to guarantee accuracy with broadband probes. Most manufacturers calibrate at 10 to 20 frequencies, depending on the frequency range of the probe and whether the probe has a flat frequency response or shaped frequency response. See Shaped frequency response versus flat response for a description of a how shaped response sensors work and why they can be very important. Calibration frequencies should normally include the band ends (the highest and lowest rated frequencies for the probe) and frequencies no more that about an octave (2:1 ratio) apart. For shaped frequency response probes, there should be calibration points at the "break points" in the particular standard that the probe in attempting to conform to. For example, in the FCC Regulations the break points are at 3 MHz, 30 MHz, 300 MHz, and 1500 MHz. Since it is impossible to accurately mimic these sharp breaks in the standard, these are the regions within the rated band of the probe where the frequency sensitivity is the greatest. Therefore, a multiple frequency probe calibration involves the following steps:
Meter Calibration Meters normally have a single adjustment for "gain" assuming that all probes use the same input. Some meter designs have two inputs. In these cases, each of the inputs must be adjusted separately. Older meters have use potentiometers that are manually adjusted so that the meter reads correctly with a particular input level. Modern digital meters store the "gain" as value that is used by the microprocessor to automatically compensate for errors that are based on the levels coming from the probe. For example, one manufacturer's meters are designed so that a one volt input represents full scale from any probe (the probes all have amplifiers). Thus, it is important that the meter accurately display a value equal to half the probe's rating when there is an input of 0.500 Volts. Calibration of Survey Sets Some survey instruments are calibrated as a set. The probe is connected to the meter and placed in test fixture at a known field intensity. The adjustments, either mechanical or digital, are then made inside the meter. This is a less expensive way of calibrating equipment. The downside is that calibration is lost if the probe and meter are separated. Even identical models of the same probe or meter cannot be substituted without calibrating the equipment again. |
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