Updated July 17,
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This page was contributed by
Matrix Test Equipment, Inc..
Matrix Test Equipment offers
some excellent technical literature that will further your understanding
on the topic of distortion, that you can download at: http://www.matrixtest.com/techlit.htm.
MTN-107 Test Setup for Measuring
X-Mod, CSB & CSO
Describes a method that uses a mean square circuit as a detector
and a fast Fourier analyzer as a readout.
MTN-108 Notes on Composite
Second & Third Order Distortion
Presents formulas for determining the number of second and third
order distortion products that can be expected in a group of carriers.
Graphs are shown depicting the distortion components versus frequency.
MTB-109 The Relationship
of Intercept Points & Composite Distortions
Presents a discussion of the relative strength of the various second
and third order distortion products.
MTN-110 Extending the Limits
of Composite Intermodulation Distortion Measurements
Methods for measuring distortion that occurs near the noise level.
MTN-111 PM to AM Conversion of Distortion Products by Slope Detection
in TV Receivers
Here's a clickable index to the
material presented on the page that you are currently looking at.
It is presented in a "frequently asked questions" format:
- What causes
- Can passive
devices cause distortion?
- What types
of distortions are important in a cable system?
- What are composite
triple beat and composite second order distortions?
- What RF
bandwidth filter should I use for distortion measurement?
- How do
I measure CTB and CSO?
- How can
the effects of coherent carrier frequencies on the consistency
of distortion measurement be minimized?
- What is
the relationship between carrier level and second and third order
- What are
the important considerations in setting signal source levels for
can I tell if distortion is coming from test equipment (e.g. spectrum
analyzer) or the DUT?
- What is carrier-to-noise
ratio (C/N) and how is it measured?
measuring C/N how do I compensate/determine test equipment noise
- How do
I measure C/N in the presence of digitally modulated carriers?
is crossmodulation measured?
I make a two-tone intermodulation distortion measurement and extrapolate
to get the CTB, CSO and crossmodulation?
- We usually
measure the amplitude intermodulation products, are phase intermodulation
products present and are they a problem?
does the carrier level drop when the carrier is modulated?
- Why is square
modulation used as a test signal when the TV signal SYNC is clearly
a much narrower pulse?
Frequently asked questions about
Distortion is the result of a nonlinear transfer function. If the
output of the device under test (DUT) is not an exact replica of
the input, distortion products are present. Although usually caused
by active devices, passive components such as couplers and connectors
can cause distortions.
Can passive devices cause distortion?
Yes, absolutely! Distortion in passive devices, also known as passive
intermodulation, is a very real problem. At some level all components
generate distortion. Significant distortions can be caused by RF
connectors because they may have dissimilar metals or dirt and oxidation
at their contacts, ferrite devices such as splitters and couplers,
relays, attenuators, PIN diodes, carbon resistors, and even coaxial
cable with an impure dielectric. The main problem is usually due
to the center conductor, not the ground. You can minimize passive
intermodulation by using the same metal for all conductors that
make mechanical contact to each other.
types of distortions are important in a cable system?
The important distortions are primarily second and third order.
Higher order distortions do occur but at normal operating level
they usually are small enough to be ignored. Second order distortions
are the result of a second order non-linearity. They are formed
by one mixing operation and result in products such as A+B, A-B,
2A, 2B. Third order distortions are the result of third order non-linearity
and are formed by two mixing operations such as (A)+(A)+(A) or 3A,
(A)+(A)+(B), 2A+B, A+B+C. Crossmodulation is a third order distortion.
The products do not all have the same amplitude, for example the
2A product in 6 dB lower than the A+B product. For more information
on this subject, see MTN-108.
What are composite triple beat and composite second order distortions?
Composite triple beat distortion (CTB), also called composite
third-order beat, is the ratio of carrier to composite third order
distortion products. Composite second-order distortion (CSO) is
the ratio of carrier to composite second-order distortion products.
Both of these distortions result from multiple carriers experiencing
second and third order non-linearity, which generate multiple distortion
products. These products are measured as a group; thus the term
composite distortion as opposed to discrete distortion.
Triple beat (CTB) products fall
close to the carriers because of the choice of equally spaced carrier
frequencies. Second order (CSO) distortion products fall +/- 1.25
MHz from each carrier. A small quantity of second order products
fall 0.75 MHz from some carriers. They are a result of the 10 MHz
instead of the usual 6 MHz spacing between Channel 4 and Channel
5. The definition of these distortions refers to spectrum analyzer
measurements. The spectrum analyzer does not read the power in CTB
or CSO with 100% accuracy, but because it is used as a standard
other methods for making the measurement are adjusted to agree with
spectrum analyzer measurements. See MTN-110
for more information.
RF bandwidth filter should I use for distortion measurement?
If CSO is being measured then the BW should be at least 2.5 MHz
in order to pass the second order products that may appear 1.25
MHz above and/or below the video carrier or expected center frequency.
For non-NTSC plans or FCC frequency offset carriers, this bandwidth
How do I measure CTB and CSO?
The basic way to measure these products is with a multiple frequency
signal generator and a spectrum analyzer. For almost all applications
a band pass filter is required at the input of the spectrum analyzer
because the spectrum analyzer, when driven with multiple carriers
will generate its own distortion products. For CTB we seek the ratio
of the carrier to the cluster of distortion products that fall in
a narrow band centered at the carrier frequencies. For CSO the measurements
are the same except the distortion products fall +/- 1.25 MHz from
each carrier. For a description of the CTB and CSO products and
how they vary across the band, see MTN-108.
For these measurements the analyzer is set as follows.
- Frequency carrier frequency
- Resolution Bandwidth 30
- Video Bandwidth 30 Hz or
lower, Video averaging is also useful
- Span 200 kHz, Nominal
Then: 1. Supply the required
number of carriers to the input of the DUT. 2. Measure the output
carrier level at the channel being measured. 3. Turn that carrier
off. 4. Measure the peak of the distortion. The ratio of carrier
to the peak of the distortion is defined as the CTB. A similar procedure
is used for CSO except that the products are +/- 1.25 MHz from the
carrier. For these CSO tests the carrier should be off.
How can the effects of coherent carrier frequencies on the consistency
of distortion measurement be minimized?
If the carriers in the multiple frequency generator are set for
example, to within 1 kHz of the correct frequency, all the composite
beats will fall within a 6 kHz band. This stresses the measurement
because the high crest factor of the distortion. One way to minimize
is to randomly offset the signal source frequencies by up to 5 kHz.
The Matrix ASX-16B and later models have the capability to remotely
adjust the carrier frequency trim. Matrix CATV Equipment control
program has the capability to automatically randomize the carrier
frequencies. The use of a square law detector eliminates the crest
factor problem and extends the lower limit of the measurement. See
is the relationship between carrier level and second and third order
When measured on a dB versus dB scale, the signal has a slope of
1, second order distortions have a slope of 2 and third order distortions
have a slope of 3. This means for every 1 dB change in carrier level
the second order distortions change by 2 dB and the third order
distortions change by 3 dB. Therefore, carrier to second order distortion
(CSO) changes 1 dB for 1 dB change in carrier level and carrier
to third order distortion (CTB) changes 2 dB for 1 dB change in
are the important considerations in setting signal source levels
for my DUT?
It is very important to correctly adjust the signal level to the
DUT, because every dB of discrepancy from the expected level may
cause a 2 dB of error in third order distortion measurements. When
leveling an amplifier for example, the target reading at the measuring
device should account for any cable or system losses, or attenuator
pads from the output of your device. In addition, if dBmV units
are being used to specify the target levels, an adjustment must
be made if the output of your device is 75 ohms and the input of
the measuring device is 50 ohms.
How can I tell if distortion is coming from test equipment (e.g.
spectrum analyzer) or the DUT?
To determine if distortion is coming from the spectrum analyzer,
increase the attenuator value at the front end of the analyzer and
observe if the distortion decreases. If it does, then the distortion
is being created at the spectrum analyzer. Make sure there is adequate
bandpass filtering between the device under test and the spectrum
analyzer. Another powerful test is to jump out the DUT and measure
the distortion. Here the distortion should be well below the required
distortion level. Typical distortion levels for Matrix equipment
is well below –100 dB.
is carrier-to-noise ratio (C/N) and how is it measured?
Carrier to noise ratio is the ratio of the carrier signal power
to the noise power in some specified channel, usually expressed
in decibels (dB). For the analog channels the noise is assumed flat
and the result of thermal and amplifier noises. Measurements are
made with a spectrum analyzer. The carrier is used as a reference
and the noise is usually measured using the “noise marker” of the
spectrum analyzer set to a quiet spot in the band. The noise marker
reading is now normalized to 4 MHz and the measured C/N is the ratio
of the carrier to the normalized noise level, measured in dB. The
4 MHz normalization frequency is for the American standard, other
standards may require a different bandwidth.
When measuring C/N how do I compensate/determine test equipment
One way is to disconnect the device under test from the test equipment
and observe if the noise floor drops. If it does not, then it means
that the noise of the device is below the noise of the test equipment
and you cannot determine the real noise of the device. An amplifier
with sufficient gain placed after the filter can bring up the noise
of the device so that it rises above the noise floor of the test
equipment. If the noise drop is at least 2 dB then a correction
may be applied to determine the correct noise floor of the device.
How do I measure C/N in the presence of digitally modulated carriers?
The distortion from a digital source manifests itself as noise.
The total of the test device noise, thermal noise and distortion
from the digital carriers is called carrier to composite noise (CCN).
If the noise level is measured without the digital carriers, the
result is the carrier to noise. The difference in the noise levels
is a measure of the intermodulation products from the digital carriers.
What is crossmodulation?
Crossmodulation or XMOD is third order distortion product which
transfers the modulation of one carrier to another carrier. Crossmodulation
is defined as the ratio of the residual modulation on a carrier
compared to the modulation of the carrier at 100% modulation. On
a TV picture crossmodulation may appear as a faint image from another
channel. This is not to be confused with an image from a distant
station at the same frequency as the desired channel.
How is crossmodulation measured?
Crossmodulation is measured by supplying one or many modulated carriers
to the Device under test along with one carrier operating CW. Crossmodulation
is the ratio of the residual modulation to 100% modulation. For
this measurement, the direct measurement of the 15.750 kHz sidebands
turns out to be unsatisfactory. There are many reasons for this,
one is that for the measurement the carrier must be present and
the carrier noise and CTB components limit the level of the measurement.
Another problem is that many amplifiers generate phase modulation
distortions, which are generated by the nonlinear reactances in
the semiconductors. Phase modulation sidebands appear on the spectrum
analyzer spectrum display but are not important for our measurement.
Actually what we are interested in is AM crossmodulation primarily
because the TV video modulation is AM. So how do you measure crossmodulation?
The answer is to first demodulate the carrier in question. AM demodulation
is immune to the PM modulation so the spectrum analyzer can operate
as an excellent demodulator with the following setup.
- Frequency: set to carrier
- Span: 0.0 Hz
- Detector mode: linear
- Resolution BW:1 MHz
- Video BW: 100 kHz
Extract the vertical output signal,
usually available on the back panel of the spectrum analyzer, and
connect it to a low frequency spectrum analyzer tuned to 15.750
kHz (note that many RF spectrum analyzers can tune to an input frequency
as low as 15 kHz and it may be tempting to use an RF spectrum analyzer
for this application but they are not satisfactory because their
performance is very poor at low frequencies.) The magnitude of the
level of the 15.750 kHz is measured first with the test carrier
modulated and again with the test carrier operating CW. The ratio
of these voltages is the crossmodulation. There is one other important
point, this method measures crossmodulation with reference to 100%
modulation. This is equivalent to measuring the ratio of the amplitude
modulation sidebands with and without modulation. Sometimes another
definition for crossmodulation is used which defines it as the ratio
of the first sideband to the carrier.
Can I make a two-tone intermodulation distortion measurement and
extrapolate to get CTB, CSO and crossmodulation?
You can certainly obtain data this way, but the question is how
well do these measurements correlate to "real" data. For
perfect conditions the measurement of two-tone second order and
third order distortions should allow the calculations of all other
MTN-109 for simple calculations. Experience has shown that extrapolation
to multiple frequency measurements results in unacceptable variations.
usually measure just amplitude intermodulation products, are phase
intermodulation products present and are they a problem?
In many amplifiers phase intermodulation exists because the semiconductor
junction capacity is a function of the applied RF voltage. The resultant
products are as real as the amplitude intermodulation products and
at exactly the same frequencies. The PM and AM intermodulation products
are usually not in phase but do contribute to the overall distortion.
The analog signals are almost immune to closely spaced pairs of
phase intermodulation products (see
MTN-111) but QAM signals can be affected. Widely spaced PM products
behave as AM products and because of phase offsets may add or subtract
Why does the carrier level drop when the carrier is modulated?
For normal AM modulation the carrier should stay constant but for
the TV industry only downward modulation is used. For 100% square
wave downward modulation the carrier decreases by 6 dB from the
CW level and the first sidebands are 3.9 dB below the new carrier.
is square modulation used as a test signal when the TV signal SYNC
is clearly a much narrower pulse?
Square modulation is easy to generate and has become the standard.
Also, all modulations are synchronous for the same reason.