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January 1980 A Cardiff Publication
FREQUENCY MHz
( 5532.[][]QQQ]
AM FM RF
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Signal Generator Specifications
Part I
To an engineer or technician faced with the requirement for an RF
signal generator, a proliferation of instruments is available. The
challenge is to purchase the signal generator which has the specified
capacity to fulfill all of the applications requirements. To aid the user
in this task a review of the important signal generator specifications
and how they affect various applications is presented here.
By Rob Oeflein
Marketing Engineer
Hewlett-Packard
T o be classified as a signal generator an instru-
ment must have three basic characteristics:
1. Calibrated and variable frequency over a broad
standard signal generator characteristics which are
required to perform them.
range.
2. Calibrated and variable output level over a wide
dynamic range. Types of Signal Generators
3. One or more forms of calibrated modulation. Most oscillators only cover a single octave, but
It is important to notice that not all frequency sources many applications require greater than one octave
or synthesizers are signal generators. Sweepers, of frequency coverage. Three techniques are com-
test oscillators, and traditional frequency synthe- monly used to obtain wide frequency coverage in
sizers cannot be classified as signal generators RF signal generators - reactance band-switching,
because they usually lack a calibrated output or high frequency tuned resonators with dividers or
some form of calibrated modulation. heterodyne circuits, and synthesis.
The wide variety of applications addressed by The reactance band-switched signal generator
signal generators and some of the critical specifica- was the earliest design. Most of the older tube-type
tions associated with each are listed in Figure 1. generators used switched inductors with variable
The primary application in today's market, however, capacitors and many updated solid-state signal gen-
is that for which the signal generator was originally erators sH11 employ this technique.
designed - receiver testing. Most signal generators The advent of broadband output power amplifiers
can be easily classified, therefore, according to their and solid-state high frequency oscillators led to the
capacity to perform in-channel (sensitivity, audio development of signal generators that use a single
bandwidth, squelch threshold) or out-of-channel (ad- high frequency tuned resonator. Dividers or a hetero-
jacent channel selectivity, intermodulation distor- dyne technique can then be used to obtain the
tion, spurious attenuation) receiver tests. Figure 2 lower frequencies.
lists some of the typical receiver tests and the Division of the higher frequencies improves the
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Major Uses of Signal Generators Important Parameters
()
Spurious
1) Receiver Testing and Calibration Output Level Accuracy
Phase Noise
Modulation Distortion
2) R&D Design of Amplifiers, Antennas Frequency Range
and Filters Output Level Accuracy
Sweep Capability
3) Component Testing Frequency Accuracy
Output Level Range
Frequency Stability
4) Local Oscillator Substitution
Spectral Purity
Leakage
5) EMI/RFI Susceptibility Testing Output Level Accuracy
or Calibration of Equipment
Frequency Range
Modulation Flexibility
6) Communication System Maintenance Frequency Range
Output Level Range
Frequency Accuracy
7) Metrology Lab Standard Output Level Accuracy
Modulation Accuracy
Frequency and Output Level
8) Test Equipment Calibration
Range and Accuracy
Figure 1. The major uses of signal generators and the signal generator
parameters associated with each application.
Signal Generator
Typical Receiver Tests Characteristics Required
Usable Sensitivity Low Leakage/Accurate, Low Level Signals
Image and IF Rejection - Tests Primarily Low Spurious/Output Levels >IV for Testing
RF Selectivity Large Rejection Ratios/Coverage of Both
IF and RF Frequencies
Adjacent Channel Selectivity - Tests Primarily Low Noise at Typical Channel Spacings
IF Selectivity
Intermodulation - Tests RF Selectivity and
Good Isolation Between Two Generators
Linearity
AM Rejection (on FM Receivers) - Tests Low Incidental FM/Simultaneous AM and FM
Receivers' Immunity to AM Noise Modulation
AGC Characteristics Accurate, Wide Range Output Level
Audio Hum and Noise Low Residual AM and FM
Audio Harmonic Distortion - Tests IF
Amplitude and Phase Response Plus Low Modulation Distortion (Particularly
Discriminator Linearity Stringent for FM Broadcast, Typically <.1 %)
IF and Discriminator Alignment Wide Modulation Bandwidth/Sweep
Figure 2. Typical receiver tests and the critical signal generator
characteristics associated with each test.
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noise performance of these generators by a factor of
about 6 dB for each divide by two. The tradeoff
here is FM deviation capability which is halved with
every division of two. Division also requires addi-
tional design to eliminate spurious, harmonics and
noise caused by the dividers themselves.
Heterodyning, on the other hand, preserves the
Control Spectral
primary band FM deviations at the lower frequencies.
BCD Purity
Noise performance at the lower frequencies, how- IEEE - 488/75 Phase noise
ever, is not improved from that of the primary band. Keyboard Spurious
To insure that the signal to noise performance is Increments Harmonics
not degraded, a clean stable reference must be Residual FM & AM
provided in the heterodyne design to act as local Switching Output Level
oscillator (La) to down convert the primary oscilla- Speed Range
tor's output. Resolution
Sweep Accuracy
Depending on the design, the output of either of Control Time Leakage
these fundamental signal generators can be phase- Lock Time Reverse Power
locked to an external synchronizer or an internal Settling Time Modulation Protection
reference to improve its long term stability. Depths
Synthesizer techniques have recently been em- Deviations
ployed to yield the third type of signal generator. Distortions
Incidentals
The synthesized signal generator is a source in
which all of the output frequencies are derived
from a fixed frequency reference oscillator. In this
manner, the long and short term stability of this Figure 3. Six major categories for signal gen-
reference oscillator can be translated to the output. erator specifications.
Two techniques are employed to synthesize a
signal. Indirect synthesis uses a combination of
phase-lock loops and their associated voltage con-
trolled oscillators (VCO's) to generate the RF spec- receive frequency. In addition, the range of a funda-
trum. A direct synthesizer has one or more fixed mental generator's bands may restrict its usefulness
frequency oscillators and all of the output frequen- if they are limited or divide certain commercial or
cies are arithmetically generated by mixing, multi- military frequency groupings. In short, the versatility
plying, or dividing with suitable filtering. of a signal generator depends to a large extent on
The advantages of a synthesized signal generator its frequency range.
over a fundamental signal generator are better reso- Frequency resolution determines the minimum
lution (settability), higher long and short-term stability, frequency change which can be made. The resolution
and programming capability. The disadvantages may depend on the output frequency, providing
include the presence of spurious signals from the less settability as the frequency increases. Syn-
synthesis process and the lack of a continuous thesized RF generators available today may offer
tuning since synthesizers must increment frequency resol utions as broad as 1 kHz or as narrow as 0.1 Hz
in discrete steps. at an output frequency of 500 MHz. Fine resolutions
are important for checking narrowband filters and
testing radios which have 12.5 kHz or less channel
Signal Generator Specifications spacings. In addition, if the source is to be multiplied
up in frequency the resolution is reduced and finer
The numerous specifications needed to quantify resolution will allow better settability at the higher
signal generator performance can be grouped in six frequencies.
basic categories. Figure 3 illustrates that these cate- Repeatability of the frequency setting is deter-
gories are frequency, spectral purity, output level, mined primarily by the frequency accuracy. On mechan-
modulation, switching speed, and control. Each of ically tuned generators this is usually specified as a
these functions plays an important role in determin- percent of the frequency set or as dial accuracy.
ing the overall capability of a signal generator Synthesized or phase-locked sources which digitally
to perform in a specific application. display the generator frequency usually derive their
accuracy from the accuracy and aging rate of the
reference oscillator. These generators will have their
Frequency accuracy specified in parts per million (ppm) or as
The choice of frequency range might at first parts times ten to a minus power(x10-Y).
seem fairly straightforward. In the maintenance of The frequency accuracy is particularly important
communication systems, however, it might be desir- in applications such as component testing or when
able for the generator to cover the IF, La, and base- the signal generator is used as a meteorology lab
band frequencies as well as the RF. Image response standard. It is important to notice that the resolution
testing, spurious checks and harmonic analysis all of a generator does not determine its accuracy. If a
require RF capability greater than just the radio's synthesizer has a resolution of 100 Hz but has a time
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I Adjacent Chan-nel Selectivity I Spurious Attenuation
Measure of receiver's ability to differentiate between a
Measure of receivers ability to discriminate between a desired
desired modulation signal and modulation signals on
adjacent channels and undesired signal. IF and image frequencies should be
included in test.
Test Setup Test Setup:
Power Splitter ,........;.--~
I Generator #1 : /
16.7Q
Distortion
Analyzer
5001IReceiverl_
Under
Distortion
16.7Q Analyzer
Test
16.7Q
CD
I Generator #21
Desired
Channel
Measure input