For full format paper (including table, picture, graphic etc) Please open at TV TRANSMITTERS: INTERMODULATION PRODUCTS
Test Methods
he term “Intermodulation”, in television communications, normally identifies the generation
of spurious signals caused by the amplifiers’ non-linearity or, more in general, by the circuit
of the transmitter, which generates, converts and amplifies video, audio and colour carriers.
More poor is the linearity of the amplifiers, more high are the intermodulation products. There are
two kinds of intermodulation products: “in band” and “out of band”. The first kind is located
between the video and audio carriers, in the transmission channel, and the second ones fall in
the adjacent channels.
Normally among the intermodulation products “out of band”, the highest ones are placed at
multiple of the frequency difference between audio and video carriers, with respect to the same
video carrier (V.C. -5.5 MHz, V.C. +11MHz for B,G standard; V.C. -6.5 MHz, V.C. +13 MHz for
D,K standard...).
To reduce or eliminate intermodulation products ”out band”, which may cause disturbances in
adjacent channels, it can be used band pass filters and notch filters, normally placed at the output
of the transmitter.
Example of “out of band” intermodulation products in
B,G standard, with acceptable level (£ -60 dB)
© ABE Elettronica SpA 02/1996 All specifications contained in this document may be changed without prior notice.
To reduce or eliminate “in band” intermodulation products it is normally used the linearity
precorrection, that is to generate (normally at IF) an amplitude distortion equal and opposite to
that one caused by power amplifiers, which generally cause the non-linearity .
The two distortion, teorically equal and opposite, cancel themselves, removing or otherwise
reducing “in band” intermodulation products.
If “in band” intermodulation products aren’t 50 or more dB lower with respect to the video carrier
(sync peak power), the transmitted image may appear affected.
For the measure of the “in band” intermodulation products, it is largely used the “3 tone” test, that
is the insertion, in the IF of the transmitter of 3 non-modulated carriers, with amplitude (respect to
nominal sync power):
-8 dB at video carrier frequency;
-10 dB at audio carrier frequency;
-16 dB at colour carrier frequency.
The specified level (-8, -10, -16 dB), represent the medium thermal contents of carriers, so they
simulate a normal working condition for the measure of intermodulation products (“in band”).
Example of “in band” intermodulation product with 3 tone test
The limit of 3 tone test is represented by the fact that it is a static test. It is not the best solution for
the measure of intermodulation products with AB class amplifiers, which are the most common
kind of solid state amplifiers of the new generation.
In this cases it is preferred to use a test system more practical and immediate, that is to use a
video test signal made by a red bar.
TTeeTeccchhhnnniicciaacll aN Nolto eN teo t e
Red signal bar (PAL)
By this way is simulated a practical condition of working: the output power can be regulated to the
nominal value (or the desired level) and the intermodulation products are visible with a spectrum
analyser connected to a RF monitor of the output signal.
The spectrum analyser, with the purpose of displaying the most clearly possible the
intermodulation products, will be an analogic one, (not digital) and the IF filter resolution will
possibly be of 100 KHz, for the best visibility.
© ABE Elettronica SpA 02/1996 All specifications contained in this document may be changed without prior notice.
Intermodulation test with red bar
Visualization with spectrum analyzer.
By this way, it can be easily regulated the optional linearity precorrector of the transmitter which
will be regulated for the minimum amplitude of intermodulation products.
With the video carrier modulated with the red bar, in standards with 90% of modulation depth (B,
G, D, K,.......), the medium thermal contents of the carrier, is -5dB with respect to the sync peak
power; it is higher than the -8 dB used with the 3 tone test. The test with red bar modulation is
more severe than 3 tone test.
Obviously it must be considered that intermodulation products don’t represent the only problem
caused by the non-linearity of a transmitter: it can be seen effects also on differential gain,
differential phase, luminance linearity, sync amplitude, ICPM, etc., that have to be corrected with
appropriate techniques.
Test Methods
he term “Intermodulation”, in television communications, normally identifies the generation
of spurious signals caused by the amplifiers’ non-linearity or, more in general, by the circuit
of the transmitter, which generates, converts and amplifies video, audio and colour carriers.
More poor is the linearity of the amplifiers, more high are the intermodulation products. There are
two kinds of intermodulation products: “in band” and “out of band”. The first kind is located
between the video and audio carriers, in the transmission channel, and the second ones fall in
the adjacent channels.
Normally among the intermodulation products “out of band”, the highest ones are placed at
multiple of the frequency difference between audio and video carriers, with respect to the same
video carrier (V.C. -5.5 MHz, V.C. +11MHz for B,G standard; V.C. -6.5 MHz, V.C. +13 MHz for
D,K standard...).
To reduce or eliminate intermodulation products ”out band”, which may cause disturbances in
adjacent channels, it can be used band pass filters and notch filters, normally placed at the output
of the transmitter.
Example of “out of band” intermodulation products in
B,G standard, with acceptable level (£ -60 dB)
© ABE Elettronica SpA 02/1996 All specifications contained in this document may be changed without prior notice.
To reduce or eliminate “in band” intermodulation products it is normally used the linearity
precorrection, that is to generate (normally at IF) an amplitude distortion equal and opposite to
that one caused by power amplifiers, which generally cause the non-linearity .
The two distortion, teorically equal and opposite, cancel themselves, removing or otherwise
reducing “in band” intermodulation products.
If “in band” intermodulation products aren’t 50 or more dB lower with respect to the video carrier
(sync peak power), the transmitted image may appear affected.
For the measure of the “in band” intermodulation products, it is largely used the “3 tone” test, that
is the insertion, in the IF of the transmitter of 3 non-modulated carriers, with amplitude (respect to
nominal sync power):
-8 dB at video carrier frequency;
-10 dB at audio carrier frequency;
-16 dB at colour carrier frequency.
The specified level (-8, -10, -16 dB), represent the medium thermal contents of carriers, so they
simulate a normal working condition for the measure of intermodulation products (“in band”).
Example of “in band” intermodulation product with 3 tone test
The limit of 3 tone test is represented by the fact that it is a static test. It is not the best solution for
the measure of intermodulation products with AB class amplifiers, which are the most common
kind of solid state amplifiers of the new generation.
In this cases it is preferred to use a test system more practical and immediate, that is to use a
video test signal made by a red bar.
TTeeTeccchhhnnniicciaacll aN Nolto eN teo t e
Red signal bar (PAL)
By this way is simulated a practical condition of working: the output power can be regulated to the
nominal value (or the desired level) and the intermodulation products are visible with a spectrum
analyser connected to a RF monitor of the output signal.
The spectrum analyser, with the purpose of displaying the most clearly possible the
intermodulation products, will be an analogic one, (not digital) and the IF filter resolution will
possibly be of 100 KHz, for the best visibility.
© ABE Elettronica SpA 02/1996 All specifications contained in this document may be changed without prior notice.
Intermodulation test with red bar
Visualization with spectrum analyzer.
By this way, it can be easily regulated the optional linearity precorrector of the transmitter which
will be regulated for the minimum amplitude of intermodulation products.
With the video carrier modulated with the red bar, in standards with 90% of modulation depth (B,
G, D, K,.......), the medium thermal contents of the carrier, is -5dB with respect to the sync peak
power; it is higher than the -8 dB used with the 3 tone test. The test with red bar modulation is
more severe than 3 tone test.
Obviously it must be considered that intermodulation products don’t represent the only problem
caused by the non-linearity of a transmitter: it can be seen effects also on differential gain,
differential phase, luminance linearity, sync amplitude, ICPM, etc., that have to be corrected with
appropriate techniques.
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