8. ALIGNMENT AND ADJUSTMENT PROCEDURE This section describes the procedure for making the (initial) adjustments of the internal potentiometers in the EFM-5 metering unit. In addition to op-amp zeroing, gainsetting controls, and logarithm circuit adjustments, a "shaping" operation is performed in order to produce the correct meter indication as a function of field intensity. This nonlinear processing is required for field strength levels above 10 V/m. The frequency chosen at NBS to perform the signal shaping is 100 MHz. As mentioned previously, the standard field setup is accurate and convenient at this frequency and the EFM-5 response curve is essentially flat. In essence, the shaping procedure involves an iterative adjustment of the non-linear adder circuitry, for each channel, to achieve an optimum overall readout accuracy. That is, potentiometers are set to achieve minimum error in indicated field strength over the total measurement range of 1 to 1000 V/m. The first stage in the shaping procedure (and also in a calibration of the radiation monitor) is to "zero" all the op-amps in the metering unit. The probe antenna must be in a zero-field (shielded) environment for this step, for example within a TEM cell or anechoic chamber. The basic alignment procedure then involves adjustment of the receiving gain controls and shaping controls until the meter indicates the correct field strength value, for each of the three dipoles in the isotropic antenna. The shaping adjustments are made with the EFM-5 probe inserted in the standard electric field of a TEM cell, using the instrumentation shown in figure 14 and described in section 6.1. For an rf power of 200 W, a 0.6 x 1 m TEM cell will produce field intensities up to about 50 dBV/m (316 V/m), and a 0.2 x 0.3 m cell will produce fields up to 60 dBV/m (1000 V/m). There are three removable electronic circuit boards in the metering unit. They have been designated in this report as: (a) preamplifier board, (b) shaping board, and (c) logarithm board. Figures 24, 26 and 28 give photographs of these three main boards showing the internal alignment potentiometers and voltage test points. The following procedure is used to make the amplitude/shaping adjustments within the metering unit, producing a meter indication which is proportional to the RSS value of the measured E field as expressed in dB above 1 V/m. NOTE: Refer to figure 11 for a sketch of the EFM-5 front panel. (a) Mechanical zero. With the OFF/CHARGE/ON power switch in the OFF position, check the mechanical zero of the meter movement. If required, adjust the screw located on the meter housing to obtain a zero dBV/m indication. (b) Battery check. With the power switch in the ON position and no 115 V cord plugged into the back panel, check the battery voltage. If the warning LED located above the power switch does not glow, it indicates that the battery voltage is sufficient for proper operation of the EFM-5 radiation monitor. Do not leave the power switch in the ON position when the instrument is not in use because the battery life is only about seven hours. (NOTE: If the batteries are discharged, there may not be enough potential to light the LED.) (c) Preliminary setting of the front panel switches and controls. (d) (2) Field intensity RANGE selector switch in the + 0 dBV/m step, corresponding to the maximum sensitivity range of 0 to 20 dBV/m (1 to 10 V/m). PEAK/AVE switch in the AVE position. (3) (4) ANTENNA channel-selector switch in the TOTAL position. (5) TIME CONSTANT switch in the 0.1 SEC position. TEM cell setup for zeroing the EFM-5 and producing standard fields. (1) (2) Mount the EFM-5 probe in the 0.6 x 1 m TEM cell at the analytic angle, with the sensor sphere located as shown in figure 14. Connect the probe to the metering unit via the flexible resistance line. Insert a low-pass filter having a cutoff frequency between 100 and 175 MHz between the rf source and the TEM cell. (4) (5) Turn on the rf source at a frequency of 100 MHz. Increase the cell voltage to obtain an indication on the EFM-5, recalling that 3 volts corresponds to a field strength of 10 V/m = 20 dBV/m. Note: if this metering unit has not been aligned previously, it may be necessary to do steps (e) and (f) before completing step (d). Turn the ANTENNA switch to the X-channel position. (6) Set the antenna rotator and X-Y plotter to an angle of 0°. (7) (8) Rotate the probe axially to align the X-channel dipole parallel to the E Set the field strength to zero (TEM cell voltage = zero) and turn off the (e) Zeroing the op-amps on the preamplifier board. (See figures 23 and 24). Connect an external high-impedance dc voltmeter between test point number 1 (2) Turn the ANTENNA switch to the X position. NOTE: for accurate zeroing, the instrument should be turned ON for at least 15 minutes before final adjustment of the zeroing potentiometers. (4) Adjust the X-ZERO potentiometer on the pre-amp board for minimum indication Connect the external voltmeter to TP-2 for measurement of the Y-channel preamp output. (6) (7) Adjust the Y-ZERO potentiometer to obtain a voltmeter indication of 0 ± 1 mV. (9) Connect the voltmeter to TP-3 for measurement of the Z-channel pre-amp (f) Zeroing the op-amps on the shaping board. (See figures 25 and 26). (1) If this metering unit has not been aligned previously, insert an extender board between the shaping board and back plane of the metering unit. Set all six varistor potentiometers to approximately their center positions. These are the 100 ks potentiometers (R1 through R6) shown near the center of each figure. (2) (3) (4) (5) (6) (7) Connect the voltmeter to TP-4 (X-channel mid-amp output). Adjust the X-ZERO potentiometer to obtain a voltmeter indication of 0 Connect the voltmeter to TP-5 (Y-channel output) and adjust the Y-ZERO Connect the voltmeter to TP-6 (Z-channel output) and adjust the Z-ZERO Turn the front panel ANTENNA switch to the TOTAL position. Connect the voltmeter to TP-7 (adder output). Adjust the front panel ZERO control with a screwdriver for 0 ± 1 mV on the voltmeter. (g) Alignment of the shaping board. (See figures 25 and 26). Turn the ANTENNA switch to the X position. Connect the voltmeter to TP-7 and adjust the front panel ZERO control for 0 ± 1 mV on the voltmeter. At a frequency of 100 MHz, set the field strength in the TEM cell to 20 dBV/m. (1) (2) (3) (4) (5) Check to insure that the probe is positioned at the analytic angle and (6) (7) Set the field strength to 10 dBV/m. Connect the voltmeter to TP-7 and adjust the adder LO-RANGE gain (9) Set the field strength to 30 dBV/m. (10) Adjust the adder MID-RANGE gain potentiometer for 100 ± 5 mV. (11) Set the field strength to 40 dBV/m (12) Adjust the R1 potentiometer for 1 volt± 50 mV. (13) Repeat steps (6) through (12), if required, until the voltmeter readings are within the specified values. NOTE: this completes the X-channel shaping and alignment for the lower two fieldstrength ranges. (14) Set the field strength to zero. (15) Turn the RANGE switch to the 0 dBV/m step. (16) Turn the ANTENNA switch to the Y position. (17) With the voltmeter connected to TP-7, adjust the front panel ZERO control for 0 ± 1 mV indication. (18) Set the field strength to 20 dBV/m. (19) Rotate the probe axially to align the Y-axis dipole with the E field, that is, to produce a maximum indication on the voltmeter. This should occur at an angle of 120° on the antenna rotator. (20) Set the field strength to 10 dBV/m. (21) Adjust the Y-GAIN potentiometer for 100 ± 5 mV. (22) Turn the RANGE switch to the 20 dBV/m step. (23) Set the field strength to 40 dBV/m. (24) Adjust the R2 potentiometer for 1V ± 50 mV. (25) Set the field strength to 10 dBV/m and turn the RANGE switch to the 0 dBV/m step. (26) Repeat steps (21) through (24) until the voltmeter readings are within the above specified values. NOTE: this completes the Y-channel shaping and alignment for the lower two ranges. (27) Set the field strength to zero. (28) Turn the RANGE switch to the 0 dBV/m step. (29) Turn the ANTENNA switch to the Z position. (30) Adjust the front panel ZERO control for 0 ± 1 mV. (31) Set the field strength to 20 dBV/m. (32) Rotate the probe axially to align the Z-axis dipole with the E field. The maximum voltmeter indication should occur at an angle of 240° on the antenna rotator. (33) Set the field strength to 10 dBV/m. (34) Adjust the Z-GAIN potentiometer for 100 ± 5 mV. (35) Turn the RANGE switch to the 20 dBV/m step. (36) Set the field strength to 40 dBV/m. (37) Adjust the R3 potentiometer for 1 V ± 50 mV. (38) Set the field strength to 10 dBV/m and turn the RANGE switch to the 0 dBV/m step. (39) Repeat steps (34) through (37) until the voltmeter readings are within the above specified values. NOTE: this completes the shaping and alignment of all three channels for the lower two ranges. (40) Set the field strength to zero. (41) Turn the RANGE switch to the 0 dBV/m step. (42) Turn the ANTENNA switch to the X position. |