BNC 630 - Manual (Page 37)

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5.18 Power & Voltage Measurement Mode
Introduction
The Power & Voltage Measurement mode measures the signal level on the Ext Mod In connector and displays the power and
voltage level on the LCD display.
The Power Measurement mode applies to frequencies in the DC to 50 KHz range. All signals outside this frequency range are
attenuated by an internal lowpass filter. A point-by-point true RMS power is calculated by the internal Digital Signal Processor
(DSP). For the voltage measurement, an averaging (low pass filter) algorithm is used.
To calculate power, a "system impedance" must be specified. If the input signal were to be applied across a load resistor then that
resistor would dissipate power. The power dissipated in the resistor is a function of the resistor's value. The value of this load
resistor is referred to here as the "system impedance" which must be specified by the user. As opposed to instruments which use a
fixed value for this impedance (i.e. 50 or 600 ohms) the 630 allows any value from 1 to 999 ohms to be used for power calculations.
The Ext Mod In jack is high impedance (about 30 K ohms). If your system expects this signal to be terminated, place a resistor (of
the appropriate terminating resistance) across this input.
Power & Voltage Measurement Mode Parameters
Figure 5.18-1: Power & Voltage Measurement mode display
1. Power Level
In this field the true RMS power level is displayed with .1 dBm resolution. The calculated power level is calculated using the RMS
input voltage and the specified system impedance.
2. Voltage Level
In this field the average DC voltage of the input signal is displayed. If an AC signal is applied to the input, this field will display
the DC component of that signal.
3. System Impedance
In this field specify the value of the load resistance used to calculate power. You may enter a value between 1 and 999 ohms.
To enter a value, type 1 to 3 numeric digits. If you type 3 digits, the value is automatically entered. If you only type 1 or 2 digits,
you can press the MHz key or the KHz key or the Hz key (all are equivalent) to enter the impedance value.
4. External Modulation In Connector
(rear panel)
Apply the signal to be measured to this connector. For this mode, the input signal may be in the -5.0V to +5.0V range. Levels
outside this range will be clipped by limiting circuitry. *** Caution : Levels outside -25V to +25V may damage the 630.
5. RS232 Port
You can have the power level value continuously sent to the terminal port by issuing the remote control command "enable LCD
echo." See chapter 6.0 for further details on this command.
Detected Level on mod input: -23.5 dBm
Volts: 1,652 mV System impedance: 600 ohm 2
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