USE OF THE DUAL CHANNEL SYNCHRONOUS DETECTOR
TYPE DSD 02 AND SIGNAL ANALYZER TYPE 2033
FOR HIGH-RESOLUTION RADIO FIELD STRENGTH
AND FREQUENCY MEASUREMENTS
1.
NOVAK
Department of Microwave Telecommunication,
Technical University, H-1521 Budapest
Received September 29, 1983
Presented by Dr. I. BOZsOKl
Summary
Co-channel sound broadcasting transmissions have the same nominal frequency.with a
frequency offset tolerance of the order of some Hz. Consequently the spectra of different
transmissions overlap and the selectivity of conventional measuring receivers is insullicient to
separate them.
In this paper the theoretical background of dual-channel radio-frequency measurement
based upon the orthogonal down conversion to about zero frequency is given together with
information on its practical implementation.
Introduction
In the field of radio monitoring it is often necessary to measure a channel
occupied by several co-channel signals. In general, however, the selectivity of
radio-frequency measuring equipment does not allow the individual measurement of signals with very close frequency separation,
A possible way to increase the resolution is to convert down the
intermediate-frequency output signal of the measuring receiver either by
envelope or by synchronous detector, followed by low-frequency spectrum
analysis for separating co-channel carrier [1]. In this way the field strengths
and - by auxiliary methods - the directions of all co-channel carriers can be
independently measured, but as during the down conversion the spectrum is
folded around the local oscillator frequency
only the absolute values of
frequency differences can be determined,
This latter problem can be overcome by using the Dual Channel
Synchronous Detector, in which the local oscillator paths are fed with signals of
identical frequency with a certain phase offset.
84
I. NovAK
The magnitude spectra of downconverted signals are the same,
nevertheless the phase spectra differ depending on whether the frequency of
incoming signal was higher or lower than the local oscillator frequency.
Consequently the true frequencies of downconverted carriers can be
obtained based on their phases in the resulting spectra.
In this paper the theoretical background of dual-channel radio-frequency
measurement is given together with information on its practical implementation to form a high-resolution monitoring system based on the Dual
Channel Synchronous Detector and Signal Analyzer type 2033.
Principle of dual-channel frequency measurement
The basic idea of the high-resolution radio-frequency field strength
measurement is to use a signal analyzer for the evaluation of downconverted
radio-frequency channel. The process of downconversion necessarily means
the fold of spectrum around the local oscillator frequency as shown in Fig. 1. In
the downconverted signal only positive frequencies exist, i.e. the original
frequency cannot be simply reconstructed from the measured baseband and
local oscillator frequencies.
The boundaries of the radio-frequency spectrum to be analyzed are set by
the bandwidth of signal analyzer.
The result is that a part of spectrum, twice wider than the bandwidth of
signal analyzer, can be evaluated at a time, at the price of obtaining absolute
values of frequencies.
If the true frequencies on both sides of local oscillator frequency are
needed, any of the single side band detection techniques - well known in the
field of radiocommunication - may be used.
All of the conventional single-sideband detection methods use, however,
some kind of analog filtering, in the form of side band filter or single or multiplepath phase shifters, to suppress the unwanted sideband. Obviously the analog
filtering cannot provide the cut-off performance which could meet the high
resolution of an FFT signal analyzer.
In the particular case of identifying carrier signais, however, it is quite
unnecessary to suppress all spectral components in one side of the local
oscillator frequency, only the sign of the frequency of downconverted signal is
important instead.
The sign of frequency can be obtained by using a dual-channel
downconverter having local oscillator signals of the same frequency with a
definite phase offset.
USE OF DETECTOR AND SIGNAL ANALYZER FOR RADIO FIELD STRENGTH MEASUREMENTS
85
radio -frequency
spectrum
2xbandwidth of Signal Analyzer
'"
RF channel
'"
baseband
spectrum
o f; f2
Fig. I. Principle of high-resolution field strength measurement. The folding of spectrum during
downconversion around the local oscillator frequency/o. /1 and/2denote the frequencies of cochannel carriers
(/'1 Il - j~ and I~ = j~ /2)
The principle is shown in Fig. 2.
The two downconverter routes contain identical mixers fed by the same
input and orthogonal local oscillator signals. The output signals also will be
orthogonal.
It is important that at the first output (<pd the phase of signal does not
depend on the sign of frequency difference (arc cos (<p)=arccos (-<p)) but at
the second output it does (arc sin (<p)= -arc sin (-<p)). As a result the sign of
phase difference between the two output signals provides the sign of freq uency
difference between the incoming and the local oscillator frequency (sign (w l
-wo) sign (<PZ-<Pl))'
Note that the result of above derivation does not depend on the initial
phases of signals. Here, for the sake of simplicity, these values were choosen to
be zero.
86
I. NOVAK
cos!w,-woJt
~, = arc coslw,-w:;it
signal input
local oscillator Input
sinw, t
output 2
.,.
0
sin Iw,-w:it
're =arc slnlw.-w·Jt
Fiy. 2. Block diagram of dual-channel synchronous detector. Signals with unity amplitude for
derivation of true frequencies are given at the main points
When using orthogonal local oscillator signals, the phase difference ((p
to be detected is + or 90 degrees, which gives the highest possible noise
margin for identification.
Finally, taking the tuning frequency of measuring receiver into account,
and provided the local oscillator frequency of synchronous detector equals the
nominal intermediate frequency, the frequency of measured signal is given by:
- qJ 1)
j . = _t't + k· Jar
where.~
tuning frequency of measuring receiver:
frequency of selected signal as measured in the downconverted
spectrum by the signal analyzer;
k is + 1 or 1 depending on the phase conditions as described in Fig. 2.
The phase difference between the output signals can be measured by the
FFT analyzer using complex spectra. To obtain both phase values either
single-channel or dual-channel signal analyzers may be used.
In the case of a single-channel signal analyzer the two outputs of the dualchannel synchronous detector should be connected to, and analyzed by, the
signal analyzer one after the other. Note that the phase shift corresponding to
the time interval between the two samples should be taken into account and the
measured signal should have negligible frequency and phase fluctuations
during the measuring time.
The use of synchronous detection offers some further important
advantages:
j~
USE OF DETECTOR AND SIGNAL ANALYZER FOR RADIO FIELD STRENGTH MEASUREMENTS
87
-
The inherently linear process eliminates all the systematic errors
(error of measured levels, presence of higher order products) which
appear when using envelope detectors.
-
The screen of the Signal Analyzer provides a true panoramic facility
for the measuring receiver even if the levels of co-channel signals cross
each other due to fading. The level variations of all signals can be
followed independently, which greatly helps manual directibn finding.
Instrumentation and measurements
Figure 3 shows the typical interconnection of Dual Channel Synchronous Detector and Signal Analyzer Type 2033 with a measuring receiver and
controller in an automated high-resolution monitoring set-up.
The Synchronous Detector receives the intermediate-frequency output
signal of measuring receiver which has been previously tuned to the channel to
be analyzed.
The receiver should be in its linear mode of operation.
The receiver must be synthetiser tuned to ensure the required stability.
Moreover the phase noise of its tuning system - that has no effect when an
envelope detector is used. but can impair the performance of synchronous
detection
should be low.
The Synchronous Detector contains band-pass filtering to reject hum
and harmonics of receiver output. The bandwidth allows the measurement of a
total radio-frequency channel in one receiver setting. The extremely linear
ext
ref. mput
1
Synthet! zer
Hlgn-Resoi u bon
Dual-Charmel output i
~
Input
receIver
lnterrT'r?dlote
1----I>-iSynchronous
output 2
irequenc y
Det ec tor
signal
Controller
Signal Ara',yzer
Type 2033
(DC verSion I
I EO bus
f
Fig. 3. Typical instrumentation set-up showing the interconnection of Dual Channel
Synchronous Detector and High-Resolution Signal Analyzer with the measuring receiver and
controller
88
I. NOV.4K
balanced mixers are fed with local oscillator signals from a crystal controlled
synthetizer.
The Dual Channel Synchronous Detector can be used with practically
any of high-performance measuring receivers, since the nominal input level and
the local oscillator frequency can be changed if required.
The outputs of Synchronous Detector are connected to the Direct and
Preamplifier Inputs of Signal Analyzer Type 2033. The switch-over of inputs
can be performed at the proper time by the controller via the IEC bus.
Spectra obtained from the Signal Analyzer are evaluated by the
Controller. In this system the use of a controller together with an appropriate
software is essential, since the phase information of spectra may be retrieved
from the Signal Analyzer through its digital outputs alone.
The software available for the Dual Channel Synchronous Detector
searches for, and identifies the stable sinusoid signals (originating from carriers
of AM broadcasting transmitters first of all in the LF and MF bands),
determines their field strengths and true frequencies.
Figures 4 and 5 show an example of measuring result hardcopies.
The tuning step of synthetizer-tuned measuring receiver - in the
frequency band of our scope
is generally not higher than 100 Hz,
consequently the frequency of spectral components to be measured by the
Signal Analyzer needs not exceed 50 Hz. The Signal Analyzer ""ith its zoom
facility provides a 4000 line resolution still improved in the subsequent
spectrum evaluation by software means by a factor of at least 10. It results in a
resolution of measured frequencies in the entire radio-frequency band to be
about 1 mHz.
The high resolution may require the use of external frequency reference
for the instruments to improve the absolute accuracy of measurements. The
Synchronous Detector has an external reference input for standard frequencies
most common in measuring receivers.
Since the frequencies to be measured by the signal Analyzer are generally
by 3 to 5 orders of magnitude lower than those appearing in the receiver and
Synchronous Detector, the accuracy of internal reference of Signal Analyzer is
adequate even for precise measurements.
Conclusions
The Dual-Channel Synchronous Detector together with its associated
software, the Briiel and Kjaer Signal Analyzer Type 2033, an appropriate
measuring receiver and IEC bus controller, offer high-resolution radio field
strength and frequency measurements as well as high-performance monitoring.
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USE OF DETECTOR AND SIGNAL ANALYZER FOR RADIO FIELD STRENGTH MEASUREMENTS
91
The system provides the true frequencies of measured signals, and an
equivalent selectivity which is far better than that obtained by conventional
methods. Owing to the good linearity of the system, individual measurements
of co-channel carriers of stable AM signals are possible.
The measurement is based upon the orthogonal downconversion of
intermediate-frequency output of measuring receiver to about zero frequency.
On the baseband signals FFT analysis is performed. In the spectra peaks
corresponding to carriers may be separated from stochastical spectra by virtue
of the narrow analysis bandwidth.
The difference of phases related to the selected peaks shows if the original
frequency was lower or higher than the local oscillator frequency of the
synchronous detector. A controller is needed to evaluate spectra, to have access
to the necessary phase information and to control the operation of equipment.
Owing to its automatic operation, high frequency resolution and
dynamic range, the system may be well used at monitoring and measuring
stations.
References
I. NOY,.\K. I.: The envelope analysis method
a new technique for measuring co-channt:!
interference in the LF and MF bands, EBU Review-Technical, No, 188. August 1981.
2. KOPITZ, D,: Analysis of the Geneva LF/MF Plan of 1975, and comparison of its outcome with
the present situation in the European Broadcasting Area. EBU Review. No. 160.
December 1976. pp. 258--267.
Or. Istvan
NOVAK
H-1521 Budapest