EB38 データシートの表示（PDF） - Motorola => Freescale
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EB38 Datasheet PDF : 4 Pages
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Freescale SeEmB3ic8 onductor, Inc.
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
negligible. The third order distortion products (d3), fifth order
(d5), etc., can be seen on each side of the tones. The actual
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
final IMD products (d3 or d5, or both) may be lower than that
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.
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
IMD requirement (d3) is usually relaxed to – 30 dB. Figure
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.
There is a bias point where the d3 and d5 products become
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.
Measurement Standards
As indicated earlier, there are two standard methods of
measuring the IM distortion:
* FCC specifications are now in effect covering maximum permissible
distortion up to the 11th order products.
RF Application Reports
For More Information On This Product,
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