4.2.14
Frequency Modulation Tests
Specifications
Maximum Deviation
(wide mode):
2.5 MHz peak (<2 GHz);
10 MHz peak (>2 GHz).
For 2-20 or 2-26.5 GHz instruments:
10 MHz peak (>2 GHz).
Deviation Flatness (wide
mode):
+2dB for rates from 10 Hz to 1 MHz;
+3 dB for rates from 1 to 5 MHz.
External Sensitivity:
Settable; calibrated for 1 Vp input (see
specification, Chapter 1, for further information).
Description
The output from the UUT is connected to a discriminator. The discriminator output is monitored
on an oscilloscope to determine deviation and bandwidth.
Procedure
A special test fixture must be assembled to calibrate and test the frequency modulation circuits.
The recommended FM test fixture includes an RF splitter, two lines of equal length, and an RF
mixer. The RF output of the UTT is divided by the splitter into two signals, unequally delayed.
The required differential delay can be obtained by using two coaxial cables of, for example, 5
and 10 inches in length. The outputs of these delay lines are furnished to the two inputs of the
RF mixer. For certain frequencies which may be identified by experiment, the mixer will
produce a DC voltage near zero. The number of null frequencies can be increased by making
the delay lines longer in absolute terms or by increasing the ratio between their lengths. If the
output becomes frequency modulated, the mixer output voltage will change, and the voltage
variation will be proportional to FM deviation. The polarity of the voltage change may be either
directly or inversely related to the direction of frequency deviation. When voltage levels have
been established for different frequencies, the mixer output can be monitored on an oscilloscope
to provide a continuous display of FM deviation.
Equipment Required
Function Generator
Oscilloscope
Digital Voltmeter
FM Test Fixture
Figure 4-10. Frequency Modulation Test
Calibration and Testing
Manual No. 120AM00250, Rev C, September 1998
4-25
Summary of Contents for GT 9000
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