Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se
Reexamination Certificate
2000-03-16
2002-05-21
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
C324S076220, C324S076270
Reexamination Certificate
active
06392397
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
Applicant claims priority of PCT application Ser. No. PCT/GB98/01868; filed May 28, 1998.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a spectrum analyser.
2. Description of the Related Art
An ideal spectrum analyser is illustrated in
FIG. 1
of the accompanying drawings. A control processor
1
tunes the bandpass filter
2
such that it passes only a selected range of the frequencies present in input signal
3
. The power of this signal is detected by power detector
4
. The basic output of the power detector
4
is then processed by processor
5
, where the signal may be averaged to reduce noise, peak detected, and converted to a logarithmic representation, before being displayed on a display device
6
. Usually, the control processor
1
will organise the display such that a graph is drawn of power versus frequency.
An RF spectrum analyser of this simple form is impractical, certainly if intended as a general purpose test instrument. The problem lies in the implementation of the tuneable filter
2
. It is impractical to make a filter that has a bandwidth which is selectably wide or narrow (e.g. 3 MHz to 10 Hz) that will tune over a large RF frequency range (e.g. 10 kHz to 10 GHz). An interesting contrast to this is the optical spectrum analyser, where a cavity resonator and/or a diffraction grating can be tuned over the full range of interest, and the heterodyne techniques described to follow are not practical.
A solution to the implementation problem is found with a frequency converting front end to the spectrum analyser.
FIG. 2
of the accompanying drawings shows the use of such a device. Instead of tuning a bandpass filter on the input, the control processor
1
sets the frequency of a frequency synthesiser
7
, which provides a frequency reference to a frequency converter
8
. The function of the frequency converter
8
is to take a block of frequencies, which frequencies are related to the reference frequency, and to convert them, maintaining their relative power, into a block of frequencies with the same range, but a much lower centre frequency. This block of frequencies is passed to a fixed bandpass filter
9
, where one narrow range is selected and passed on for power detection by the power detector
4
.
The frequency conversion stage
8
, combined with the reference synthesiser
7
and the fixed bandpass filter
9
emulates the bandpass filter
2
of
FIG. 1
, but frequency shifted up in frequency by an amount related to the frequency reference. As it is implemented at a fixed and low frequency, the bandpass filter
9
of
FIG. 2
is possible to realise.
An ideal frequency conversion stage for use in this type of spectrum analyser has one important property, which will be called the 1:1 property: whenever signal power emerges from the frequency converter, there is always exactly one input signal at a specific frequency difference and power difference on the input signal that has caused it. The specific power difference allows the controller
1
to correctly estimate the input power that it has detected. The specific frequency difference allows the controller
1
to correctly estimate the frequency of the signal that it has detected.
It is not easy to build an ideal frequency converter. Referring to
FIG. 3
of the accompanying drawings, the simplest type uses a fundamental mixer
10
. Consider an input signal of 1 GHz, and a reference signal of 1.01 GHz. The frequency mixer
10
will output two signals of frequencies 2.01 GHz, and 10 MHz. The bandpass filter
9
following the mixer
10
will reject the 2.01 GHz signal, and pass the 10 MHz signal. Unfortunately, there are other input signals that can cause an output of 10 MHz. The first and most obvious is an input signal of 1.02 GHz, which being different from the reference by 10 MHz, will cause a 10 MHz output signal. In addition, non-idealities in the mixer
10
will cause input signals such as 3.02 GHz, which is 10 MHz different from the third harmonic of the reference to cause an output of 10 MHz. Thus, it can be seen that the fundamental mixer violates the 1:1 property, of only one input signal causing an output. The result of this violation would be that the spectrum analyser display would indicate the presence of input power where there was none, causing ‘images’ and ‘spurious signals’ on the display.
There are instruments that can be built using a fundamental mixer. A modulation analyser is one such instrument. Here the assumption is made that in normal operation of the instrument, the only input signal is the modulation under test. Though the simple RF front end is capable of creating images and spurious signals, these are known to be absent under normal use, and so are ignored.
A spectrum analyser cannot make the assumption of a single input signal. A measurement that is often made with a spectrum analyser is a search for signals. Here any signal seen on the display must be able to be interpreted as a genuine input signal, and not ignored as an artefact of the frequency converter.
Referring to
FIG. 4
of the accompanying drawings, a more ideal frequency converter can be built with multiple mixers and filters. A low pass filter
11
passes only signals below some cut-off frequency, for instance 2 GHz. Therefore signals only in the range DC to 2 GHz are present on the mixer input. The reference synthesiser
7
generates frequencies higher than this range, for instance 3 GHz to 5 GHz, and the bandpass filter
12
is tuned to 3 GHz. This ensures that when the reference synthesiser
7
is set to a frequency of 4 GHz, only input signals with a frequency of 1 GHz will appear in the bandpass filter
12
. The input lowpass filter
11
ensures that there will not be images caused by a 7 GHz input mixing with the 4 GHz reference to create a mixer output frequency also of 3 GHz. This describes the input stage of the classic ‘upconversion’ heterodyne receiver. While a power detector could be placed at the output of the bandpass filter
12
, it is very difficult to make a narrow bandwidth filter at such a high frequency, and so one or more further stages of mixing and filtering are employed to get the final signal down to a reasonable frequency.
A second reference source
13
is used together with a second mixer
21
. If the reference is chosen to be 3.01 GHz, then the output frequency in the final bandpass filter
9
will be 10 MHz. However, there are practical problems with these particular frequencies. If bandpass filter
12
contains a signal at 3.02 GHz, it too will mix down to an output of 10 MHz. It is not easy to make the bandpass filter
12
have a passband centred on 3 GHz, and also to provide adequate rejection of signals at 3.02 GHz. Without adequate rejection of these image frequencies, the complete frequency converter will violate the important 1:1 property, even though the first mixing stage does not. The best that can be routinely obtained from a typical filter at 3 GHz is to reject signals 300 MHz away from the passband. The second reference
13
must be offset more from 3 GHz, perhaps to 3160 MHz, such that the following bandpass filter
9
is centred on 160 MHz.
This final output frequency may still be too high to build narrow resolution filters. However, it is now low enough to build a filter which will discriminate against images at 10 MHz away. A further stage of mixing and filtering can now be employed to bring the final output frequency down to 10 MHz, while retaining the 1:1 property.
In the classical analogue spectrum analyser (FIG.
2
), filter
9
is switch selectable between several resolution bandwidths, perhaps 100 Hz to 1 MHz, and defines the resolution bandwidth of the instrument. The power in this filter is detected by power detector
4
, and represents the amount of power present at the tuned frequency. The frequency reference
7
to the frequency converter
8
is stepped or swept across a range of frequencies in order to buil
Hamdan Wasseem H.
IFR Limited
Metjahic Safet
Reising, Ethington, Barnes, Kisselle, Learman and McCulloch, P.C
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