EB38

3

RF Application Reports

Except for the position of the carrier in respect to the two

tones, displays of the signals produced by systems A, B and

C appear identical on a spectrum analyzer screen.

Sometimes, however, the suppressed carrier may remain

below the noise level of the instrument. Any spectrum

analyzer used for SSB linearity measurements must have

an IF bandwidth of less than 50 Hz to allow the two closely

spaced tones to be displayed with good resolution. Figure

1 shows a low distortion, two-tone envelope displayed on

a scope screen. On a spectrum analyzer screen the same

signal displays as two discrete frequencies separated by the

difference of the audio frequency or frequencies. See Figure

2. The display represents the rate at which peak power

occurs when the two frequencies are in phase and the

voltages add. Thus, one peak contains one-fourth (– 6 dB)

of the peak envelope power (PEP). An average reading

power meter would read the combined power of the tones,

or half the PEP, assuming the envelope distortion is

power (PEP) of each distortion product can be obtained by

deducting the number of decibels indicated by the analyzer

from the average power. This value may be useful in

determining the linearity requirements of the signal source.

While the maximum permissible distortion levels of the driver

stages in a multi-stage amplifier may be difficult to specify,

a 5- to 6-dB margin is usually considered sufficient.

Types of Distortion

The nonlinear transfer characteristics of active devices

are the main cause of amplitude distortion, which is both

device and circuit dependent. On the other hand, harmonic

and phase distortion, also present in linear amplifiers, are

predominantly circuit dependent. Even order harmonics,

particularly noticeable in broadband designs, cause the

harmonic distortion. Push-pull design will eliminate most of

the even-order-caused harmonic distortion and the driver

stages, where efficiency is of less concern, can be biased

to class A.

Phase distortion can be caused by any amplitude or

frequency sensitive components, such as ceramic capacitors

or high-Q inductors, and is usually present in multi-stage

amplifiers. This distortion may have a positive or negative

sign, resulting in occasions where the level of some of the

of the driving signal, due to canceling effects of opposite

phases. Actual levels depend on the relative magnitude of

each distortion product present.

From the above it is apparent that the distortion figures

presented by the spectrum analyzer represent a combination

of amplitude, harmonic and phase distortion.

Measurement Standards

As indicated earlier, there are two standard methods of

measuring the IM distortion:

Method 1 — In military standard (1131 A-2204B), the

distortion products are referenced to one of the two tones

of the test signal. The maximum permissible IMD is not

specified but, numbers like – 35 dB are not uncommon in

some equipment specifications. However, when this

measuring system is employed in industrial applications, the

3 shows the frequency spectrum of IM distortion products

and their relative amplitudes for a typical class AB linear

amplifier. Biasing the amplifier more toward class B will

cause the lower order distortion products to go down and

the amplitudes of the higher order products to increase.

equal resulting in 2 – 5 dB improvement in the lower order

IMD readings.

Method 2 — ln the proposed EIA standard, the amplitude

of the distortion products is referenced to the peak envelope

power, which is 6 dB higher in power than that represented

by one of the two tones. The amplifier or device indicating

a maximum distortion level of – 30 dB in Method 1 represents

– 36 dB with the EIA proposed standard. Conversely, a

– 30 dB reading with ElA’s PEP reference would be – 24 dB

when measured with the more conservative military method.

In practical measurements, the two tones can be adjusted

6 dB down from the zero dB line, and direct IMD readings

can be obtained on the calibrated scale of the analyzer.

Alternatively, the tone peaks can be set to the zero dB level

and 6 dB deducted from the actual reading.

The military standard, with the relaxed –30 dB IMD

specification, is employed by most manufacturers of high

power commercial transmitters and marine radio base

stations. *The EIA measuring method is used by the majority

of ham radio equipment and CB radio manufacturers. It is

also used to measure IMD in various mobile radio

applications operating from a 12.5 V nominal dc supply.

Because of the importance to your design, data sheets

of the newer generation Motorola devices specify linearity

tests appropriate to the expected application of the particular

device and test conditions are always indicated.

REFERENCES:

1. Pappenfus, Brueue & Schoenike, “Single-Sideband Prin-

ciples and Circuits,” McGraw-Hill.

2. William I. Orr, “Radio Handbook,” 18th Edition, Editors

and Engineers, Ltd.

3. Stoner, Goral, “Marine Single-Sideband,” Editors and En-

gineers, Ltd.

4. Hooton, “Single-Sideband, Theory and Practice,” Editors

and Engineers, Ltd.

* FCC specifications are now in effect covering maximum permissible

distortion up to the 11th order products.

Freescale Semiconductor, Inc.

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Go to: www.freescale.com