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Peak mode: This is a dBc plot like the normal mode but with peak hold added. Each point
remains at the highest value it reaches during any sweep since the start of the plot, and a timer shows the
time elapsed since the plot was started.
Absolute mode: This is a plot of field strength value in dBuV/m vs. frequency, with 20 dBuV/m
(10 uV/m) at the bottom of the screen and 148 dBuV/m (25 V/m) at the top, without peak hold. This
mode is the most useful for showing what signals are present whether or not there is a signal at the center
frequency.
Marker: All modes have a marker to give precise amplitude and frequency values. The marker
is a short vertical line which extends above the plot and is moved across the plot by the right and left
arrow keys. As it is moved, the frequency difference from center and the amplitude value is displayed.
There is also a very short cursor that moves across the plot as the frequency is stepped.
Span (sweep width): In the menu the span can be shortened from 128 kHz to 45 kHz, which
gives 2.8 times as many sweeps per minute of the central part of the plot. The central part occupies the
same width on the display space as it does for the 128 kHz span. To change the span to 45 kHz, press
MENU, go to Spectrum kHz: 128 45, and use the right arrow key to move the underline cursor to 45,
or the left arrow key to change back to128. Press MENU a second time to return to the spectrum screen.
4.2.8 Measuring RF voltage: the RF In BNC jack
General: The PI 4100 can serve as a tuned RF voltmeter to measure the level of a signal
connected to its RF In BNC jack, with approximately ±5 per cent accuracy over a range of 30 microvolts
(32 dBuV) to 40 Volts (151 dBuV). Voltages greater than 40 V can damage the input circuits. The input
resistance is 2500 Ohms, therefore an external termination must be used when measuring a transmission
line that requires correct termination. This is essential when measuring the output voltage of an antenna
current sampling line, especially one driven by a sampling loop; the line must be terminated before the
RF In jack is connected to it to prevent 4100 damage caused by high voltage. The bandwidth for
measurements is 1.0 kHz. Measurement units are both voltage units and dBuV, according to the units
selected for field strength.
Bridge detector use: The 4100 can serve very well as a detector for an RF bridge, with the
bridge output connected to the RF In jack. The measuring range is very large with no range switching
and the analog level indicator serves as the null indicator. In the voltage measuring mode the output of
the loop antenna due to external fields is well isolated from the measuring circuits and is not likely to
cause errors.
Voltage measurement procedure: To measure voltage press MENU and move the cursor to
Input. Move the underline cursor from LoopAnt to RF in and press MENU again to cause switching to
take place. On the display the two legends FS1 and FS2 change to RF1 and RF2 with voltage units
related to the units selected for field strength. Manual calibration is not needed and does not function.
4.2.9 RF Out BNC jack use
General: The RF Out BNC connector provides a broadband output from an amplifier driven by
the loop antenna. The source impedance at RF Out is 50 Ω. The output voltage for a given field
strength is nearly constant over the frequency range 200 kHz – 5 MHz, which makes the output useful as
an input to a spectrum analyzer. For a field strength of 1 V/m the output level is approximately 28 mV
rms (89 dBμV) with a 50 Ω load. If the spectrum analyzer displays amplitude in dBuV, add 31 dB to the
displayed value to get field strength in dBuV/m. If it displays amplitude in dBm, add 138 dB to get
dBuV/m. At field strengths above approximately 21 V/m (146 dBuV/m) an attenuator is switched in,
reducing the output level by 31 dB. The noise floor of the spectrum display will be determined by the
analyzer rather than by the 4100 unless analyzer has a low-noise input amplifier, or a separate low-noise
amplifier is used at the RF Out jack.
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