Spectrum analyzer design and use
- the block diagram and operation of a spectrum analyzer
A spectrum analyzer is a very useful test or measuring
instrument for the radio design or maintenance engineer. It enables the
spectrum of a signal to be shown, revealing the presence of signals that may
not be seen if other test equipment or measuring instruments are used. To
enable the most effective use to be made of a spectrum analyzer it is
necessary to have a basic understanding of the way in which it works. This
will enable many of the pitfalls, including false readings, using an
analyzer to be avoided.
Basic principle
The spectrum analyser uses the superhet principle used in many radio
receivers as the underlying principle on which its operation depends. The
superhet principle uses a mixer and a second locally generated or local
oscillator to translate the frequency.
The mixing principle used in the spectrum analyzer
operates in exactly the same manner as it does for a superhet radio. (See
the article under the Radio receivers section of this website for further
explanation). The signal entering the front end is translated to another
frequency, typically lower in frequency. Using a fixed frequency filter in
the intermediate frequency section of the equipment enables high performance
filters to be used, and the analyzer or receiver input frequency can be
changed by altering the frequency of the local oscillator signal entering
the mixer.
Although the basic concept of the spectrum analyzer is exactly the same as
the superhet radio, the particular implementation differs slightly to enable
it to perform is function as a spectrum analyzer.
The frequency of the local oscillator governs the
frequency of the signal that will pass through the intermediate frequency
filter. This is swept in frequency so that it covers the required band. The
sweep voltage used to control the frequency of the local oscillator also
controls the sweep of the scan on the display. In this way the position of
the scanned point on the screen relates to the position or frequency of the
local oscillator and hence the frequency of the incoming signal. Also any
signals passing through the filter are further amplified, detected and often
compressed to create an output on a logarithmic scale and then passed to the
display Y axis.
Although the basic concept of the analyser is fairly
straightforward a few of the circuit block may need a little further
explanation.
RF attenuator
The first element a signal reaches on entering the spectrum analyzer is an
RF attenuator. Its purpose is to adjust the level of the signal entering the
mixer to its optimum level. If the signal level is too high, not only may
the reading fall outside the display, but also the mixer performance may not
be optimum. It is possible that the mixer may run outside is specified
operating region and additional mix products may be visible and false
signals may be seen on the display.
In fact when false signals are suspected, the input
attenuator can be adjusted to give additional attenuation, e.g. +10 dB. If
the signal level falls by more than this amount then it is likely to be an
unwanted mix product and insufficient RF attenuation was included for the
input signal level.
The input RF attenuator also serves to provide some
protection to very large signals. It is quite possible for very large
signals to damage the mixer. As these mixers are very high performance
components, they are not cheap to replace. A further element of protection
is added. Often the input RF attenuator includes a capacitor and this
protects the mixer from any DC that may be present on the line being
measured.
Low pass filter and pre-selector
This circuit follows the attenuator and is included to remove out of band
signals which it prevents from mixing with the local oscillator and
generating unwanted responses at the IF. These would appear as signals on
the display and as such must be removed.
Microwave spectrum analyzers often replace the low pass
filter with a more comprehensive pre-selector. This allows through a band of
frequencies, and its response is obviously tailored to the band of interest.
Mixer
The mixer is naturally key to the success of the analyser. As such the
mixers are high performance items and are generally very expensive. They
must be able to operate over a very wide range of signals and offer very low
levels of spurious responses. Any spurious signals that are generated may
mix with incoming signals and result in spurious signals being seen on the
display.
IF gain
Despite the fact that attenuation is provided at the RF stage, there is also
a necessity to be able to alter the gain at the intermediate frequency
stages. This is often used and ensures that the IF stages provide the
required level of gain. It ahs to be used in conjunction with the RF gain
control. Too high a level of IF gain will increase the front end noise level
which may result in low level signals being masked. Accordingly the RF gain
control should generally be kept as high as possible without overloading the
mixer. In this way the noise performance of the overall unit is optimised.
Local oscillator
The local oscillator within the spectrum analyzer is naturally a key element
in the whole operation of the unit. Its performance governs many of the
overall performance parameters of the whole analyser. It must be capable of
being tuned over a very wide range of frequencies to enable the analyzer to
scan over the required range. It must also have a very good phase noise
performance (more details may be found under the Frequency Synthesizer
portion of the Radio Receivers section of this website). If the oscillator
has a poor phase noise performance then it will not only result in the unit
not being able to make narrow band measurements as the close in phase noise
on the local oscillator will translate onto the measurements of the signal
under test, but it will also prevent it making any meaningful measurements
of phase noise itself - a measurement being made increasingly these days.
Summary
Spectrum analyzers are widely used to make RF measurements. They offer very
high levels of performance, especially when compared to what was available a
few years ago. Typically today they make widespread use of digital signal
processing techniques, the signals being converted into a digital format
after the IF stage. This enables very flexible filtering to be offered along
with a host of other useful facilities that would not be possible if only
analogue techniques were employed.
With the development of electronics technology that is
taking place, the performance of the analyzers will no doubt improve, while
the cost for the facilities offered will fall.
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