Boonton 9240 RF Voltmeter - Manual (Page 47)

Boonton 9240 Series RF Voltmeter
4. Operation
This section provides detailed background information on various aspects of operation of the Model 9240. It is assumed that
the reader is familiar with the basic operating procedures covered in Section 3. This section covers the following topics:
Voltmeter Probe calibration, Zeroing, Filtering, Noise, Dynamic range, Measurement time, Chart recorder operation and
Waveform sensitivity.
4.1 Zeroing the Instrument
For greatest accuracy, especially on the more sensitive ranges, the instrument must be zeroed. To eliminate the need for
tedious and often inaccurate manual zeroing, the instrument incorporates an automatic zeroing capability. When automatic
zeroing is initiated, the microprocessor reads, averages and stores zero corrections for each measurement range of the
instrument and applies the proper zero correction for the range in use for all subsequent measurements. Zero corrections are
most important on the more sensitive ranges of the instrument. During instrument warm-up periods and during use in
environments with varying ambient temperatures, the instrument should be zeroed frequently if measurements are being
made on the lower ranges. To zero the instrument, proceed as follows:
Never press the ZERO key with a signal applied to the probe. To do so will result in erroneous zero
corrections and inaccurate subsequent measurements. If the input exceeds normal zero offsets, an error
message will be displayed when automatic zeroing is initiated; remove the input signal and rezero the
instrument.
1. Remove All Input Signal to the Probe. This can be done by unscrewing the probe tip until the tip just breaks contact with
the internal connection, leaving the metal shell engaged with the probe body threads. Alternatively, the probe tip can be
removed and a Model 952002 50
Ω adapter connected in its place.
2. Press the ZERO key. The automatic zeroing cycle takes approximately 9 seconds; the microprocessor computes and
stores zero corrections during this period. When zeroing is complete, the instrument display typically shows -99.99 in all dB
modes, indicating input underrange. In the mV mode, the underrange indication does not appear; the instrument display
shows the "zero condition". This is composed of residual noise and offsets and, ideally, should show + and - excursions of
similar amplitude less than 100
µV.
4.2 Making Measurements
Once the instrument has been programmed and zeroed, it is ready for voltage or dB level measurements. Merely connect the
probe to the source whose voltage or dB level is to be measured; the measured level will be displayed directly.
4.3 Overload Limits
1. The Model 952001B RF Probe supplied with the instrument is overload protected to 10 volts AC and 200 volts DC.
Exceeding these limits may result in permanent damage.
2. The Model 952002 50
Ω BNC Adapter supplied with the instrument should not be subjected to continuous overload of
more than 10 volts (DC + RMS AC) in order to avoid excessive heating of the terminating resistor.
3. Where voltages above the specified overload limits are likely to be encountered, use the Model 952005 100:1 Voltage
Divider. Maximum rating of the voltage divider is 1000 volts (DC + peak AC).
Operation
4-1