SIST EN 60510-2-6:2002
(Main)Methods of measurement for radio equipment used in satellite earth stations -- Part 2: Measurements for sub-systems -- Section 6: Frequency demodulators
Methods of measurement for radio equipment used in satellite earth stations -- Part 2: Measurements for sub-systems -- Section 6: Frequency demodulators
Methods are given for the measurement of the electrical characteristics of frequency demodulators. Threshold and carrier-to-noise measurements are included because these are essential for satellite systems. Where possible, only measurements involving the basic demodulator are considered, excluding the equipment comprising the de-emphasis network and the networks associated with sound sub-carrier signals, pilot signals and auxiliary signals.
Meßverfahren für Funkgerät in Satelliten-Erdfunkstellen -- Teil 2: Messungen an Untersystemen -- Hauptabschnitt 6: Frequenzdemodulatoren
Méthodes de mesure pour les équipements radioélectriques utilisés dans les stations terriennes de télécommunication par satellites -- Partie 2: Mesures sur les sous-ensembles -- Section 6: Démodulateurs de fréquence
Dans cette section sont décrites des méthodes de mesure des caractéristiques électriques des démodulateurs de fréquence, y compris les mesures du seuil et du rapport signal à bruit, mesures essentielles pour les télécommunications par satellite. Dans la mesure du possible, seules les mesures relatives au démodulateur proprement dit seront évoquées, à l'exclusion des mesures spécifiques des sections comprenant le réseau de désaccentuation, les réseaux associés à la sous-porteuse de signaux sonores, aux signaux pilotes et aux signaux auxiliaires.
Methods of measurement for radio equipment used in satellite earth stations - Part 2: Measurements for sub-systems - Section 6: Frequency demodulators (IEC 60510-2-6:1992)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN 60510-2-6:2002
01-oktober-2002
Methods of measurement for radio equipment used in satellite earth stations - Part
2: Measurements for sub-systems - Section 6: Frequency demodulators (IEC 60510
-2-6:1992)
Methods of measurement for radio equipment used in satellite earth stations -- Part 2:
Measurements for sub-systems -- Section 6: Frequency demodulators
Meßverfahren für Funkgerät in Satelliten-Erdfunkstellen -- Teil 2: Messungen an
Untersystemen -- Hauptabschnitt 6: Frequenzdemodulatoren
Méthodes de mesure pour les équipements radioélectriques utilisés dans les stations
terriennes de télécommunication par satellites -- Partie 2: Mesures sur les sous-
ensembles -- Section 6: Démodulateurs de fréquence
Ta slovenski standard je istoveten z: EN 60510-2-6:1994
ICS:
33.060.30 Radiorelejni in fiksni satelitski Radio relay and fixed satellite
komunikacijski sistemi communications systems
SIST EN 60510-2-6:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN 60510-2-6:2002
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SIST EN 60510-2-6:2002
NORME CEI
INTERNATIONALE IEC
60510-2-6
INTERNATIONAL
Première édition
STAN DARD
First edition
1992-05
Méthodes de mesure pour les équipements
radioélectriques utilisés dans les stations
terriennes de télécommunication par satellites
Deuxième partie:
Mesures sur les sous-ensembles
Section six — Démodulateurs de fréquence
Methods of measurements for radio equipment
used in satellite earth stations
Part 2:
Measurements for sub
-systems
Section Six — Frequency demodulators
© IEC 1992 Droits de reproduction réservés — Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in
utilisée sous quelque forme que ce soit et par aucun any form or by any means, electronic or mechanical,
procédé, électronique ou mécanique, y compris la photo- including photocopying and microfilm, without permission in
copie et les microfilms, sans l'accord écrit de l'éditeur. writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http: //www.iec.ch
CODE PRIX
Commission Electrotechnique Internationale
S
PRICE CODE
International Electrotechnical Commission
IEC
McNvavHapogHae 311eKTpOTexHN4eCHaa HOMNCCNA
Pour prix, voir catalogue en vigueur
•
For price, see current catalogue
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SIST EN 60510-2-6:2002
510-2-6 ©IEC - 3 -
CONTENTS
Page
FOREWORD 5
SECTION SIX: FREQUENCY DEMODULATORS
Clause
1 Scope 7
2 Definition 7
3 General 7
4 I.F. input return loss 9
9
5 Baseband output impedance and return loss
11
6 Deviation sensitivity
7 Sense of demodulation 15
17
8 Differential gain/non-linearity and differential phase/group-delay
9 Baseband amplitude/frequency characteristic 21
25
10 Frequency division multiplex (f.d.m.) telephony measurements
25
11 Television measurements
12 Threshold pe rformance 25
13 Measurement of impulsive noise in telephone channels near the threshold 31
36
Figures
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INTERNATIONAL ELECTROTECHNICAL COMMISSION
METHODS OF MEASUREMENT FOR RADIO EQUIPMENT
USED IN SATELLITE EARTH STATIONS
Part 2: Measurements for sub-systems
Section six: Frequency demodulators
FOREWORD
1)
The formal decisions or agreements of the IEC on technical matters, prepared by Technical Committees on
which all the National Committees having a special interest therein are represented, express, as nearly as
possible, an international consensus of opinion on the subjects dealt with.
2) They have the form of recommendations for international use and they are accepted by the National
Committees in that sense.
3) In order to promote international unification, the IEC expresses the wish that all National Committees
should adopt the text of the IEC recommendation for their national rules in so far as national conditions will
permit. Any divergence between the IEC recommendation and the corresponding national rules should, as
far as possible, be clearly indicated in the latter.
This standard has been prepared by Sub-Committee 12E: Radio relay and fixed satellite
communications systems, of IEC Technical Committee No. 12: Radiocommunications.
The text of this standard is based on the following documents:
Six Months' Rule Report on Voting
12E(CO)119 12E(CO)130
Full information on the voting for the approval of this standard can be found in the Voting
Report indicated in the above table.
The following /EC publications are quoted in this standard:
Publications Nos. 510-1-3 (1980): Methods of measurement for radio equipment used in satellite earth
stations - Part 1: Measurements common to sub-systems and combi-
nations of sub-systems - Section three: Measurements in the i.f. range.
510-1-4 (1986): Section four: Measurements in the baseband.
510-2-5 (1992): Part 2: Measurements for sub-systems - Section five: Frequency
modulators.
510-3: Part 3: Methods of measurement for combinations of sub-systems.
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METHODS OF MEASUREMENT FOR RADIO EQUIPMENT
USED IN SATELLITE EARTH STATIONS
Part 2: Measurements for sub-systems
Section six: Frequency demodulators
1 Scope
Methods are given in this section for the measurement of the electrical characteristics of
frequency demodulators. Threshold and carrier-to-noise ratio measurements are included
because these are essential for satellite systems. Where possible, only measurements
involving the basic demodulator are considered, excluding the equipment comprising the
de-emphasis network and the networks associated with sound sub-carrier signals, pilot
signals and auxiliary signals.
Methods of measurement for frequency modulators are given in section five. Measure-
ments between the baseband terminals of modulator/demodulator assemblies are covered
by the various sections of part 3 of this publication.
2 Definition
For the purpose of this standard a frequency demodulator is a sub-system which, by
analogue means, demodulates an intermediate frequency (i.f.) carrier which has been
frequency modulated by a baseband signal. This may be a multi-channel telephony or
television signal with associated sound sub-carrier signals, pilot signals and auxiliary
signals.
Such baseband signals are normally analogue but digital signals are not excluded.
However, the methods or measurement described in this section are intended for
assessing the pe rformance of the demodulator when analogue signals are transmitted. A
demodulator sub-system usually comprises the following three main sections:
- an intermediate frequency (i.f.) section;
an i.f. to baseband section (e.g. discriminator);
- a baseband section.
3 General
A block diagram for a typical demodulator as used in satellite earth stations is shown in
figure 1.
Currently, two different types of demodulator are used, namely conventional demodulators
and threshold-extension demodulators.
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The characteristics to be measured can be divided into three principal categories:
-
non-transfer characteristics;
- i.f. to baseband characteristics;
- certain baseband-to-baseband transmission characteristics in conjunction with a
measurement modulator.
The first category of measurements applies to i.f. input measurements (see 4) and
baseband output measurements (see 5).
The second category of measurements forms the essential part of this section because of
the nature of the device under test - transfer from i.f. to baseband. In order to assess the
influence of the i.f. input level, some specified tests shall be made at nominal, minimum
and maximum specified i.f. input levels.
NOTE - Measurement of the influence of spurious amplitude modulation is not included in this Standard
since the input level is assumed to be entirely within the operating range of the limiter, the amplitude/phase
conversion of the latter being assumed to be negligible.
The third category of measurements includes those to be carried out on the complete
modulator/demodulator (modem) assembly except that the actual or system modulator is
replaced by a measurement modulator.
It is very impo rtant to know the separate contribution of a demodulator to the total
ormance characteristics because, in an operational situation,
permitted tolerance of pe rf
demodulators of one design or manufacturer may have to work with modulators of another
design or manufacturer. Compensation effects between modulator and demodulator are
therefore undesirable and each demodulator should fulfil the prescribed specification in
association with a measurement modulator. This procedure requires that the measurement
modulator has a better pe ormance than that specified for the demodulator under test.
rf
4 I.F. input return loss
See pa rt 1, section three of this publication: Measurements in the i.f. range.
Measurements at harmonics of the intermediate frequency may also be required.
5 Baseband output impedance and return loss
See part 1, section four of this publication: Measurements in the baseband.
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6 Deviation sensitivity
6.1 Definition and general considerations
The deviation sensitivity (Sd) of a demodulator for a sinusoidal signal of a given frequency
b
is expressed as the ratio of the peak value of the baseband output voltage (V ) to the
frequency deviation (fit):
V
SA - (V/MHz)
(6-1)
^f
Vb and Mare both expressed in peak or r.m.s. values.
The deviation sensitivity of the demodulator is usually a function of the baseband
frequency because of the effect of the de-emphasis network. In some cases, however, it is
possible to gain access to the baseband output point (figure 1) before the de-emphasis
network: in such cases, the measured deviation sensitivity of the discriminator is inde-
pendent of the baseband frequency used.
6.2 Methods of measurement
Two methods for obtaining the deviation sensitivity by means of a test signal of accurately
known deviation may be used, namely, the Bessel zero and the two-signal methods as
discussed below.
In the first method, the measurement is made with a well-defined modulation index of
2,404 83 at relatively low modulation frequencies, e.g. less than about 2 MHz, whilst in the
second method a low modulation index (e.g. not exceeding about 0,2) at relatively high
modulation frequencies (e.g. above 2 MHz) is used. This latter method is therefore
especially applicable to measurements at the pilot and sound sub-carrier frequencies.
6.2.1 The Bessel zero method
A suitable arrangement for measuring the deviation sensitivity of the demodulator and for
calibrating the deviation of the measurement modulator is shown in figure 2.
The method of measurement is known as the Bessel zero method and calibration of the
deviation sensitivity of the measurement modulator is based upon the fact that, in the case
of sinusoidal modulation, the carrier frequency spectral line first disappears for a modu-
lation index (mi) given by:
Af - 2,404 83
mf = (6-2)
f
where .Af is the peak frequency deviation and f
is the modulating frequency.
The "zero" or point of first disappearance of the i.f. carrier is observed on the spectrum
analyzer, but a perfect zero may not be obtained due to residual harmonic distortion of the
baseband signal generator. However, a decrease in carrier level of 30 dB or more is
regarded as adequate.
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Since there are many values of the modulation index at which a carrier-zero may be
obtained, the best way of ensuring that the first zero is used is by increasing the modu-
lating voltage smoothly from zero to the point where the carrier disappears for the first
time.
The measurement procedure is as follows:
a) the baseband generator is set to the required frequency at which the deviation
sensitivity is to be measured;
b) the output level of the generator is set to zero and then smoothly increased until the
i.f. carrier on the spectrum analyzer first disappears;
c) the r.m.s. voltage (Vb) at the baseband output of the demodulator is measured;
d) The demodulator deviation sensitivity (Sd) at modulation frequency f is then
calculated from equation 6-3:
-V 2 Vb
Sd V/MHzHz
(6-3)
2,404 83 f
NOTE - As a modulation index of 2,404 83 corresponds to an occupied i.f. bandwidth which increases
linearly with modulation frequency, the use of this method is restricted to modulation frequencies which do
not cause the modulated signal spectrum to exceed the system bandwidth. An alternative method is to
employ a calibrated measurement demodulator in place of the spectrum analyzer.
6.2.2 The two-signal method
A suitable arrangement for measuring demodulator deviation sensitivity by the two-signal
method is shown in figure 3. The method is used to calibrate the demodulator deviation
sensitivity at low modulation indices, up to about 0,2 and uses high modulating
frequencies between 2 MHz and 10 MHz; it is therefore especially applicable at the pilot
and sound sub-carrier frequencies.
An accurate frequency deviation at a specified frequency is generated by means of two i.f.
crystal oscillators having equal output levels but different frequencies – the first at the
nominal carrier frequency (e.g. 70 MHz) and the second at a frequency differing from the
carrier frequency by a known value fx.
As shown in figure 3, the output signal from crystal oscillator No. 2, suitably attenuated as
specified below, is added to the signal from crystal oscillator No. 1. The level of the
composite signal is then adjusted by attenuator No. 2 to the appropriate input level of the
demodulator under test. Due to the limiting action in the demodulator, a practically pure
angle-modulation signal is generated. In order to reduce the unwanted amplitude modu-
lation, an extra limiter has to be inserted before the demodulator under test. This limiter
shall have a low a.m/p.m. conversion in order to reduce the measurement error to an
acceptable level.
The r.m.s. frequency deviation is given by:
fx
Af=
(6-4)
2
a' 11
where a' is the voltage attenuation of attenuator No. 1.
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From this equation, the required attenuation can be calculated. For example, to produce a
frequency deviation of 140 kHz r.m.s. at a frequency fX of 8 500 kHz, the required attenu-
ation is 20 log o a' where a' is given by:
8 500
(6-5)
a' _
140 J 2
which corresponds to 32,7 dB.
It is advisable in practice to apply a high enough modulation frequency so that fX »Of
(e.g. 20 log in a' >14 dB).
Once the known frequency deviation is produced by the method described above, the
demodulator deviation sensitivity may be calculated from:
A/2 V
bb
V/MHz Sd = a' (6-6)
f
x
where Vb is the r.m.s. voltage of frequency at the demodulator output.
X
6.3 Presentation of results
The results should be given as in the following examples:
"The deviation sensitivity (Sd) was . V/MHz" or
"At an r.m.s. frequency deviation of . kHz the baseband output level was . dBm".
6.4 Details to be specified
The following items should be included as required in the detailed equipment specification:
the method of measurement (see 6.2.1 or 6.2.2);
a)
the modulation frequency of the i.f. input signal in the case of the Bessel zero
b)
fX between the two input carriers in the case of the two-signal
method or the difference
method;
c) the frequency deviation of the i.f. input signal;
d) the required deviation sensitivity or output level at the specified deviation;
the baseband connection point (i.e. before or after de-emphasis – see figure 1);
e)
f) the de-emphasis characteristic employed (if appropriate);
g) the i.f. input levels (maximum, nominal and minimum values).
7 Sense of demodulation
7.1
Definition and general considerations
The sense of demodulation of a frequency demodulator is positive if an increase in the
intermediate frequency results in a positive-going change in the output voltage. The sense
of modulation is important in television transmission, see part 3, section three of this
publication.
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7.2 Method of measurement
A simple method of checking the sense of demodulation is to modulate a measurement
modulator having a known sense of modulation with an asymmetrical waveform and to
apply this i.f. signal to the demodulator under test. If the demodulator output signal polarity
and the modulator input signal polarity are the same, then the sense of demodulation is
the same as the known sense of modulation.
An alternative method is to produce a high deviation of an i.f. carrier by a low-frequency
modulating signal and to apply this modulated carrier together with a small c.w. i.f. carrier
of known frequency to the input of the demodulator under test.
At the output of the demodulator, the beat-frequencies between the interfering carrier and
the modulated carrier will be visible on an oscilloscope display. If, when changing the inter-
fering carrier to a higher i - f -
the beat-frequency points change to a higher voltage level, the
sense of demodulation is positive.
The measurement arrangement and the oscilloscope display are shown in figure 4.
8 Differential gain/non-linearity and differential phase/group-delay
8.1 Definition and general consideration
The demodulator under test is driven by an O. carrier which has a sinusoidal test-signal
modulation of constant deviation magnitude and deviation phase, superimposed on a low-
frequency sweep signal. At the baseband output of the demodulator, the demodulated
test-signal amplitude and phase are found to be dependent upon the instantaneous value
of the swept carrier frequency. Differential gain (DG) and differential phase (DP) of the
demodulator under test are defined as functions of this instantaneous value as given in
the following equations:
DG(x) = (A(x) / A 0) — 1
(8-1)
DP(x) = (p(x) — cpo (8-2)
where
x is the instantaneous value of the input carrier frequency
DG(x) is a function representing the differential gain of the demodulator
A(x) is the baseband output test-signal amplitude as a function of
x
Ao is the baseband output test-signal amplitude at mid-band carrier frequency
DP(x) is a function representing the differential phase of the demodulator
(p(x) is the output test-signal phase as a function of x
is the output test-signal phase at mid-band carrier frequency
(po
For an ideal demodulator with no distortion, both the differential gain and the differential
phase are zero. For a practical demodulator, the above functions will show variations. A
practical demodulator is characterized either by the functions themselves or by the
differential gain and phase distortion. These are defined as the difference between the
extreme values of the above functions, usually expressed as a percentage and in degrees
respectively, as follows:
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510-2-6 © I EC
(
Amax — Aminl
(8-3)
DG distortion (%) = 100
Ao
DP distortion (degrees)
= (8-4)
'max — (Pmin
The choice of the test-signal frequency depends upon which section of the demodulator is
to be assessed and which parameter is to be measured (i.e. differential gain or non-
linearity, differential phase or group-delay). Definitions of non-linearity and group-delay,
rt 1, section
and factors governing the choice of the test-signal frequency are given in pa
four of this publication: Measurements in the baseband.
DG and non-linearity are measured by the same method but using different test
frequencies. Non-linearity is an important performance parameter of demodulators since it
represents the departure of the output voltage/input frequency characteristic from the ideal
linear response. It is measured by using relatively low test-signal frequencies within the
typical range of 50 kHz to 500 kHz.
8.2 Method of measurement
For measurement of the differential gain/non-linearity and differential phase/group-delay of
a demodulator an ideal modulator is needed. By definition, an ideal modulator, when
driven by a composite test and sweep signal, will produce a test-signal modulation of
constant deviation magnitude and phase that is independent of the instantaneous value of
the swept carrier frequency.
For this application, an ideal modulator is well approximated by the following arrangement.
Two modulators are used at frequencies much higher than the intermediate frequency and
differing in frequency by the intermediate frequency. One of them is frequency modulated
by the sweep signal and the other is frequency modulated by the test signal. A swept i.f.
signal having constant test-signal deviation magnitude and deviation phase is generated
by heterodyning these two signals down to the intermediate frequency.
A simplified arrangement for measuring the DG and DP of a demodulator is given in
figure 5. The arrangement of the ideal modulator as explained above is shown within the
rt". Within the broken line designated "receiver
broken line designated "transmitter pa
part", the test signal component is extracted by a band-pass filter tuned to the test
frequency. The amplitude and phase modulation of the output test signal are detected by
an envelope detector and a phase detector, thus supplying the DG and DP signals for
ical deflection of the display. In some cases, the sweep voltage applied to the
ve rt
oscilloscope can be obtained through separation by placing a low-pass filter at the
demodulator output. In other cases, this voltage is supplied by the sweep-signal gen-
erator. A suitable phase shifter is also required.
NOTES
1 Commercial test equipment, frequently called a "link analyzer", is available for achieving the test
arrangement within the broken lines in figure 5. Although not indicated in figure 5, this test equipment
normally contains additional facilities for calibrating both the vertical and horizontal axes of the display.
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2 When using high test-signal frequencies, the explored frequency range will not be approximated by the
sweep width but by the sweep width plus twice the test-signal frequency.
3 It is necessary to ensure that baseband amplifiers following the demodulator should not be over-driven
by the large-amplitude sweep-signal. The need to satisfy this requirement often limits the sweep width
which can be applied.
Alternatively, the baseband part of the demodulator may be excluded from the measurement, thus allowing
a sweep width high enough to explore the whole demodulator characteristic. This exclusion may also be
necessary when the lower cut-off frequency of the baseband amplifiers does not allow the sweep signal to
be transmitted.
8.3 Presentation of results
Differential gain and differential phase shall preferably be presented by photographs of the
displayed functions with both axes appropriately calibrated. Often a single photograph
showing a simultaneous display of both functions is presented. Alternatively, the differen-
tial gain distortion, differential phase distortion and sweep limits may be stated.
Details to be specified
8.4
The following items shall be included, as required, in the detailed equipment specification:
i.f. sweep range (e.g. ± 10 MHz);
a)
b) permitted DG distortion in the above range (e.g. 3 %);
permitted DP distortion in the above range (e.g. 0,8°);
c)
d) test frequency to be used;
baseband connection point (e.g. before or after the baseband amplifier);
e)
f) i.f. input levels (maximum, nominal and minimum values).
9 Baseband amplitude/frequency characteristic
9.1 Definition
The baseband amplitude/frequency characteristic of a demodulator is the curve repre-
senting the ratio, expressed in decibels, of the baseband output level to a reference level
as a function of the baseband modulation frequency for a constant deviation at the i.f.
input. The reference level is the output level at a specified baseband frequency.
9.2 General considerations
For measuring the baseband amplitude/frequency characteristic of a demodulator, a
measurement modulator is needed. By definition, a measurement modulator for measuring
this characteristic provides a nominally constant deviation of the i.f. output signal as a
function of the input baseband frequency, with constant baseband input level. A low
deviation shall be used in order to avoid higher order sidebands of significant amplitude at
the higher modulation frequencies.
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If the demodulator under test cannot be separated from the de-emphasis network, the
measurement modulator has to be used with a calibrated and corresponding pre-emphasis
network. In some cases however, the de-emphasis network may be separated from the
demodulator, so that the amplitude/frequency characteristic of the basic demodulator can
be measured. In such cases, the baseband amplitude/frequency characteristic of the
de-emphasis network shall be measured separately.
The measurement of the baseband amplitude/frequency characteristic of the demodulator
shall preferably be carried out at several specified i.f. input levels.
NOTE - At present, it is not possible to separate all the baseband frequency characteristic contributions of
the modulator/demodulator under test as the measurement demodulator/modulator has a contribution of the
same order. It is therefore customary to use for this test the system demodulator/modulator, and to specify
the overall modulator/demodulator characteristic.
9.3 Method of measurement
1, section four of this
The arrangement for the measurement is given in figure 3 of pa rt
publication: Measurements in the baseband, noting that the "equipment under test"
between the baseband terminals comprises the measurement modulator and the demodu-
lator under test, interconnected at their i.f. terminals.
Presentation of results
9.4
For sweep-frequency measurements, a photograph of the c.r.t. display or an X-Y recording
should be given. When the results of the measurement are not presented graphically, they
should be given as in the following example:
"The baseband amplitude/frequency characteristic of the demodulator (or modulator
and demodulator connected back to back) is within +0,2 dB to —0,1 dB from 300 kHz
to 8 MHz relative to the value at 1 MHz."
Point-by-point measurements may be tabulated or expressed as above.
Details to be specified
9.5
The following items should be included, as required, in the detailed equipment
specification:
reference frequency;
a)
b) baseband frequency limits;
permitted variation of the baseband amplitude/frequency characteristic;
c)
i.f. deviation at the reference frequency;
d)
pre-emphasis/de-emphasis characteristics, when required;
e)
f) i.f. input levels (maximum, nominal and minimum values).
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10 Frequency division multiplex (f.d.m.) telephony measurements
At present, it is not possible to separate the intermodulation noise contribution of the
demodulator under test, as the measurement modulator has a noise contribution of the
same order. It is therefore common practice to utilise for this test the system modulator,
and to specify only the overall modulator/demodulator noise values. In addition to the
details to be specified listed in part 3, section four of this publication: Measurements for
frequency division multiplex (f.d.m.) transmission, the i.f. input level range may also be
specified.
For measuring the basic noise of the demodulator (i.e. without noise loading), an
extremely low-noise c.w. generator such as a crystal oscillator or a synthesizer may be
used, instead of the unloaded system modulator.
11 Television measurements
At present, it is not possible to separate the waveform distortion contributions of the
demodulator under test as the measurement modulator may have a distortion contribution
of the same order. It is therefore common practice to utilise for this test the system
modulator, and to specify only the overall modulator/demodulator distortion values, using
the methods described in part 3, section three of this publication: Measurements for
monochrome and colour television.
NOTE - Most of the linear and non-linear waveform distortions are affected not by the basic
modulator/demodulator itself but by the baseband sections (including band-limiting filters, pre- and
de-emphasis networks, etc.). In cases where these sections may be separated, their performance may be
measured directly at baseband.
In addition to the measurements specified in pa 3, section three, the intermediate
rt
frequency input level range may also be specified.
For measuring the basic noise of the demodulator as described in part 3, section three, an
extremely low-noise c.w. generator such as a crystal oscillator or a synthesizer may be
used instead of the unloaded system modulator
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Questions, Comments and Discussion
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