PURPOSES AND TYPES OF SIGNAL GENERATORS [How to use signal and sweep generators (1953)]

1-1. Definition of a Signal Generator

A signal generator is a device used to generate a signal. The term "signal generator" is self-descriptive. Under this general definition, even a device which generates a signal in the normal operation of a piece of equipment is covered. However, a signal generator, as the term is used here, is normally a test instrument, used in measuring, testing, and servicing.

The signal generator generates a signal for test purposes. But what is a "signal?" A signal can be any alternating current or voltage. In its original sense, the word "signal" referred to the modulation or intelligence of any composite electrical wave. However, through loose usage, it is now generally applied to any alternating current or voltage, regard less of whether it is a carrier or its intelligence component, as long as it is used directly in the transmission of intelligence, or participates directly in modification of a transmitted or received wave.

For example, the alternating current in the power transformer or in the heaters of the tubes in a radio receiver is not a signal since it is not directly involved with the received intelligence. However, the electrical wave received from the antenna, that produced by the local oscillator, the intermediate-frequency current, and the a-f current after demodulation are all referred to as signals.

A. signal generator is thus an instrument which can provide artificially a signal which is as nearly as possible identical with, or representative of, a signal present in a circuit or device when it is in normal use.

1-2. Purposes of Signal Generators

A signal generator provides a signal in a circuit or device so that device will operate or react 'in the same way it would with a signal from its normal source. For a radio or tv receiver, the normal signal source is the antenna. A suitable signal generator is used to supply to the antenna circuit a signal which substitutes for the normally-received antenna signal.

But the signal generator is so designed that the available signal can be modified in intensity, frequency, and modulation type and percentage, and in other ways in which there is no control over the antenna signal normally received. The receiver can thus be subjected to various signal input conditions, and its performance checked. Certain signal factors, such an amplitude-modulation percentage or frequency, which are continually varying in broadcast-station reception, can be kept constant at desired values while receiver tests are made.

Signal generators are not limited in their use to radio and tv receivers.

They are suitable wherever alternating current or voltages over a desired range of frequencies are required. For example, in radio-frequency bridges which measure r-f impedance, signal generators provide a suit able source of r-f current at the radio frequency at which measurements are desired. They also can be used as a source of signal voltage in measuring the response of a transformer, or the response of a filter. Many of the hundreds of possible uses are discussed later in this book.

1-3. Test Oscillators

The term "test oscillator" has often been used interchangeably with the term "signal generator." There is on record no official or generally accepted definition which distinguishes a test oscillator from a signal generator. "Test oscillator" is the older term; that is, at one time most instruments now referred to as signal generators were called test oscillators. In present usage, there is some tendency to classify the simplest signal-generating instruments which do not include output voltage measuring or indicating devices, or attenuators, as test oscillators, and others as signal generators. However, such usage is certainly very far from standardized, and the reader is warned not to make any arbitrary classification according to these terms.

Because of the lack of any well-recognized definitions of what a test oscillator is as compared to a signal generator, in this book we shall use only the term "signal generator." Test oscillators, of whatever classification, will then fall in as special examples of signal generators, and will be treated as such.

1-4. How a Signal is Generated

There are several ways in which a radio-frequency signal can be generated, but all ordinary signal generators employ electronic oscillators. In practically all cases this is a vacuum-tube oscillator. Regular oscillator circuits are used, with the Hartley and Colpitts among the most common. In most units, several tuning bands are required to cover the desired range of frequencies. These bands are selected through a range switch which connects any of several coils, or taps on a coil. Tuning within each band is usually by means of a variable capacitor controlled by a knob with a calibrated dial on the front panel. In other cases the output frequency generated is fixed for any given switch position, each position corresponding to a specific frequency or band of frequencies, such as a television channel. Sometimes crystal control is used, either directly, or for use in calibrating a variable oscillator.

Most signal generators make provision for modulating the generated frequency. This may be either amplitude modulation, frequency modulation, or both. Frequency modulation is ordinarily provided for use in sweep analysis, in which a response curve is traced out on an oscilloscope rather than as an f-m signal for f-m receivers. Methods of modulation are discussed more fully in the next three sections, for each type of generator.

1-5. Types of Signal Generators

There are a number of ways of classifying the many types of signal generators. There are so many varieties and combinations that no classification method can be complete. However, it is common practice to designate generators by application, frequency range, and type of modulation. For a complete specification of the type of instrument, a combination of all three of these factors must be stated. For example, a-m signal generators for radio receivers ordinarily cover the range from 100 khz to about 30 mhz. But there are generators with the same type of signal out put and modulation whose frequency range may extend from 5 or 10 mhz to thousands of megacycles. The frequency range and purpose of the latter should be stated to distinguish them from simply ordinary "a-m generators." For the introductory discussion of generators in the next three sections, we have divided them, according to both application and type of signal output, into three classes:

1. A-M SIGNAL GENERATORS. Here we include all those generators designed to produce a steady r-f carrier signal, with optional amplitude modulation by a tone or other modulation signal. These can be further divided into:

a. Radio servicing type generators, which normally cover from about 100 khz to about 20 or 30 mhz.

b. Laboratory type a-m generators. These have various frequency ranges, many about the same as for radio servicing generators, but many others extending higher and lower in frequency range. Special types for vhf and uhf ranges may not include the lower frequencies at all, but start at 50 to 100 mhz or higher.

Generally laboratory types have relatively more rugged construction and tolerances as to stability, calibration, and output voltage readings, and are much more exacting than those for servicing instruments.

c. Special units, in which the amplitude modulation is by a specialized waveform or frequency. For example, for testing tv receiver linearity, an r-f carrier signal may be amplitude modulated by a sine wave or pulse wave of various frequencies up to about 400 khz.

2. MARKER GENERATORS. These are generators of unmodulated r-f signals of desired spot frequencies. The signal output is the same in nature as that of a-m generators with the modulation shut off, but the definite difference in use sets these units apart. The r-f unmodulated signal must be exactly of a known definite frequency, and is ordinarily used to mark a spot along a response pattern to indicate the frequency coverage of that pattern, or the relative location of parts of the pattern in the frequency spectrum. Since different response-pattern tests may involve frequencies from a few cycles per second to thousands of megacycles, marker generators are not limited to any particular frequency range. However, the majority of marker generators now in practical use are employed in checking tv receiver response, and have ranges in the region from 1 mhz to 220 mhz. Many marker generators are installed as part of tv sweep generators, with which they are used in tv alignment or response checking. Since, as was explained above, the marker signal is simply an unmodulated carrier, any a-m signal generator of suitable frequency range can be used to provide it, although the required accuracy of frequency may not be obtained. The principles of marking and marker generators them selves are covered more fully in Section 3.

3. SWEEP GENERATORS. These units provide a signal which is of variable frequency, the variation taking place in a rapid, periodic manner called sweep. In other words, the r-f carrier is frequency modulated.

The modulating signal is ordinarily the 60-cycle power-line frequency, so that the radio-frequency carrier changes from its lowest frequency to its highest frequency and back again 60 times each second. The same power-line voltage which is used to sweep the frequency is usually brought out through a terminal to the horizontal-deflection circuit or the oscilloscope on which a response pattern is to be observed. Sweep generators are available in a variety of potential sweep widths from a few kilocycles to many megacycles. Most common is the type designed for tv response checking and alignment, in which sweep widths up to 15 mhz are ordinarily available, with center frequencies in the range 0 to 220 mhz and above. Other units are designed only for f-m receivers, and are limited to maximum sweep widths of about 1 mhz, and may have center frequencies extending only to 108 mhz.

In this book, we shall discuss the three main types listed above. The audio signal generator, producing signals up to 20 khz or more from a low limit in the order of 15 to 20 cycles, is considered in a separate field of work entirely. It is thus not discussed in this book, except where a-f generators are part of a-m generators, where they are used to provide modulation. However, video sweep types, in which the frequency of the signal is swept from near 0 mhz up to about 10 mhz, are so closely allied to other sweep types that they are discussed in connection with them.