The HP-431C power meter measures rf power from 10 microwatts (-20 dBm) to 10 milliwatts (+10 dBm) full scale in the 10 MHz to 18 GHz for a 50-ohm coaxial system and 2.6 GHz to 40 GHz for a waveguide system.

SIGNAL GENERATORS

Standard sources of ac energy, both audio frequency (af) and radio frequency (rf), are often used in the maintenance of electronic equipment. These sources, called SIGNAL GENERATORS, are used to test and align all types of transmitters and receivers. They are also used to troubleshoot various electronic devices and to measure frequency.

The function of a signal generator is to produce alternating current (ac) of the desired frequencies and amplitudes with the necessary modulation for testing or measuring circuits. (Modulation was discussed in NEETS, Module 12, Modulation Principles.) It is important that the amplitude of the signal generated by the signal generator be correct. In many signal generators, output meters are included in the equipment to adjust and maintain the output at standard levels over wide ranges of frequencies.

When using the signal generator, you connect the output test signal into the circuit being tested. You can then trace the progress of the test signal through the equipment by using electronic voltmeters or oscilloscopes. In many signal generators, calibrated networks of resistors, called ATTENUATORS, are provided. You use attenuators in signal generators to regulate the voltage of the output signal. Only accurately calibrated attenuators can be used because the signal strength of the generators must be regulated to avoid overloading the circuit receiving the signal.

Q.3 In signal generators, what device is used to regulate the voltage of the output signal?

There are many types of signal generators. They are classified by use and the frequency range covered as AUDIO-FREQUENCY (AF) GENERATORS, VIDEO SIGNAL GENERATORS, RADIO-FREQUENCY (RF) GENERATORS, FREQUENCY-MODULATED RF GENERATORS, and other special types, which combine frequency ranges.

AUDIO AND VIDEO SIGNAL GENERATORS

AUDIO SIGNAL GENERATORS produce stable af signals used for testing audio equipment. VIDEO SIGNAL GENERATORS produce signals that include the audio range and extend into the rf range. These signal generators are used to test video amplifiers and other wideband circuits. In both audio and video signal generators (figure 5-5), major components include a POWER SUPPLY, an OSCILLATOR (or oscillators), one or more AMPLIFIERS, and an OUTPUT CONTROL.

Figure 5-5. - Af and video signal generator block diagram.

In the audio and video generators that produce a beat-frequency, the output frequency is produced by mixing the signals of two separate rf oscillators. One is fixed in frequency, and the other is variable. The difference between the frequencies of the two oscillators is equal to the desired audio or video frequency.

Audio signal generators often include resistance-capacitance (rc) oscillators in which the af is directly produced. In these signal generators, a resistance-capacitance circuit is the frequency-determining part of the oscillator. The frequency varies when either the resistance or the capacitance is changed in value.

In other signal generators, however, the capacitance alone is often chosen as the only variable element. The change in frequency that can be produced by this method is limited, and it is usually necessary to cover the entire range of the generator in frequency steps. This is usually accomplished by providing several rc circuits, each corresponding to a specific portion of the entire range of frequency values. The circuits in the oscillator are switched one at a time to provide the desired portion of the af range.

The amplifier section of the block diagram (figure 5-5) usually consists of a voltage amplifier and one or two power amplifiers, which are coupled by means of rc networks. The output of the final power amplifier is often coupled to the output control (attenuator) by means of an output transformer.

The output control section regulates the amplitude of the signal. A commonly used af signal generator is the model SPN audio oscillator shown in figure 5-6. The model SPN is a programmable synthesized signal generator designed to provide a stable, low-distortion, wide-amplitude range signal over a 1-Hz to 1.3-MHz frequency range. Additionally, the equipment provides a square wave output and means for swept operation with an external signal and can be remotely controlled via an IEEE-488 bus. For the proper operation of any test equipment, you should always refer to the specific technical manual that describes its use.

Figure 5-6. - Model SPN audio oscillator.

RADIO-FREQUENCY (RF) SIGNAL GENERATORS

In addition to the necessary power supply, a typical rf signal generator contains three other main sections: an OSCILLATOR CIRCUIT, a MODULATOR, and an OUTPUT CONTROL CIRCUIT. The modulator modulates the rf signal of the oscillator. In addition, most rf generators are provided with connections through which an external source of modulation of any desired waveform can be applied to the generated signal. Metal shielding surrounds the unit to prevent signals from the oscillator from affecting the circuit under test.

Q.4 Name the three main sections of a typical rf signal generator.

A block diagram of a representative rf signal generator is shown in figure 5-7. The function of the oscillator stage is to produce a signal that can be accurately set in frequency at any point within the range of the generator. The type of oscillator circuit used depends on the range of frequencies for which the generator is designed. In lower frequency rf signal generators, the oscillating circuit consists of one of a group of coils combined with a variable capacitor. One of the coils is selected by the position of a range selector switch that connects the coil to a capacitor to provide an inductance-capacitance circuit. The inductive-capacitance circuit then has the correct range of resonant frequencies.

Figure 5-7. - Rf signal generator block diagram.

The function of the modulator is to produce an audio (or video) modulating signal that can be superimposed on the rf signal produced by the oscillator. The modulating signal may be provided by an audio oscillator within the generator. This is termed INTERNAL MODULATION. It may also be derived from an external source. This is termed EXTERNAL MODULATION. In some signal generators, either of these two methods of modulation can be employed. In addition, a means of disabling the modulator section is available so that the pure, unmodulated signal from the oscillator can be used when desired.

Q.5 What is the function of the modulating circuit?

The type of modulation selected depends on the application of the particular signal generator. The modulating signal may be a sine wave, a square wave, or pulses of varying duration. In some special generators, provision is made for pulses over a wide range of repetition rates and widths.

The output circuit of the rf signal generator usually contains a calibrated attenuator and an output level meter. The output level meter provides an indication and permits control of the output voltage of the generator. The attenuator allows you to select the amount of this output. The attenuator is made up of a group of resistors that form a voltage-dropping circuit.

It is controlled by a control calibrated in microvolts. When the control is adjusted so that the output meter reads unity (1.0), the reading on the attenuator control gives the exact value (no multiplication factor) of the output in microvolts. If an output voltage at a lower value is desired, the attenuator control is varied until the meter indicates some decimal value less than 1. This decimal is multiplied by the attenuator reading to give the actual output in microvolts.

An rf signal generator currently in wide use by the Navy is the HP 8640B (figure 5-8). The HP 8640B signal generator covers the frequency range of 500 kilohertz to 512 megahertz, and can be extended to 1,024 MHz by using adapters.

Figure 5-8. - Rf signal generator (HP-model 8640B).

This completes our discussion of signal generators. The following section deals with an instrument that measures frequency - the FREQUENCY COUNTER.

FREQUENCY COUNTERS

The signal generators you studied in the previous section provide signals for use in testing, aligning, and troubleshooting electronic equipment. Now, we will study the FREQUENCY COUNTER, an instrument that measures frequencies. Frequency counters are used to measure frequencies already in existence. An example of a typical frequency counter, the model 5328A, is shown in figure 5-9.

Figure 5-9. - Model 5328A 500 MHz universal frequency counter.

GENERAL DESCRIPTION OF THE FREQUENCY COUNTER

The following description is for the model 5328A counter only. Other counters use different techniques to derive the displayed frequency. The outputs available vary from counter to counter. For further information on other types of counters and their uses, refer to EIMB, Test Equipment, NAVSEA 0967-LP-000-0040.

The model 5328A is a portable, solid-state electronic frequency counter. It is used to precisely measure and display, using a nine-digit LED readout, frequency, period, period average, time interval, time interval average, and ratio of electronic frequency signals. This frequency counter can also provide a 1-MHz and 10-MHz output signal through the back-panel BNC.

Q.6 What frequencies are provided through the back-panel BNC?