SWEPT-FREQUENCY TESTING EQUIPMENT

SWEPT-FREQUENCY testing is used to determine the bandwidth, alignment, frequency response, impedance matching, and attenuation in various circuits, systems, and components. Swept-frequency testing can be used to quickly determine the broadband response of a device that otherwise would require a number of separate measurements and manual plotting of the response curve. Swept-frequency techniques are applicable over the entire electronic spectrum from vlf to ehf and are generally limited only by your resourcefulness and the basic limitation of the equipment employed. The basic swept-frequency arrangement is shown in figure 5-28.

Figure 5-28. - Frequency-response test.

The swept-frequency technique can effectively determine the Frequency response of an amplifier or filter and is useful in the alignment or bandwidth determination of an IF or rf stage. The test equipment permits direct visual readout on the crt of the spectrum analyzer. The spectrum analyzer can also be connected to an X-Y chart recorder if a permanent record or print is desired. Figure 5-29 shows a spectrum analyzer crt display of the Frequency response of a multicoupler. The tracking generator used must be capable of sweeping the desired frequency range of the device under test.

Figure 5-29. - Typical spectrum analyzer frequency-response display.

Q.10 What is the purpose of swept-frequency testing?

TRACKING GENERATOR

Figure 5-30 shows a typical tracking generator used with the Hewlett-Packard 141 T spectrum analyzer. A TRACKING GENERATOR is basically a sweep generator in which the sweep rate is matched to that of the spectrum analyzer. The output circuitry of the tracking generator contains a network that ensures a constant output amplitude over the entire range being swept. When the fm signal produced by the tracking generator is applied to a device or circuit under test, the instantaneous output amplitude is always proportional to the response of the circuit to the frequency at that instant. Thus, the original fm input signal is changed in passing through the circuit under test. The output signal, therefore, would consist of an fm signal that is also amplitude-modulated. For equal deviations, the positive and negative portions of this envelope are symmetrical, making it necessary to observe only one side of the envelope. After the detection stage in the spectrum analyzer, only the modulation remains to appear on the face of the crt. This presentation will appear as a continuous curve because of the persistence of vision and the phosphor characteristic of the crt. The polarity of the detector determines whether a positive or a negative output is displayed. The frequency at any point on the crt display can be analyzed by arresting the scan of the spectrum analyzer either electronically or manually at the point of interest. For greater accuracy in frequency determination, a frequency counter may be attached to the output of the tracking generator at the point of the arrested scan.

Figure 5-30. - Tracking generator used with a spectrum analyzer.

IMPEDANCE MATCHING

Conventional tuners cannot be used successfully to cancel source or load reflections in swept-frequency measurements. This is because the tuning is effective only at single frequencies; therefore, pads or isolators are required. However, by the use of automatic-level control, the power output of the sweep generator can be maintained relatively constant at the point of measurement. The source impedance may thus be maintained very close to the nominal value. With this arrangement, any impedance variation in the connecting cables, connectors, and adapters is effectively cancelled since these components are within the leveling loop. The attenuation of a device under test will be displayed on the associated crt as a continuous response curve as it is scanned. This will result in an attenuation versus frequency plot of the device under test only.

IMPEDANCE

Circuit impedance is measured conveniently by using the reflectometer principle. The individual values of the incident and reflected signals (swr) in a transmission line feeding an unknown impedance are measured. The ratio between these signals indicates how closely the load impedance matches that of the transmission line.

Another method is the use of an auto-mechanical load control to hold the forward power at a constant level while the return load of a specific load is measured. A short is then placed in the circuit, and 100% reflected power is measured. The loss detected is then calculated to obtain swr figures.

NOISE FIGURE

By using a frequency-sweeping receiver and an automatic noise-figure meter, you can make noise-figure measurements on broadband microwave devices, such as a traveling-wave-tube amplifier. To conduct such a test properly, you must first check the receiver noise figure.

Q.11 In swept-frequency testing the impedance of a transmission line, what electrical characteristic is actually being measured?