Sweep Spectrum Analyzer Tutorial

- the sweep spectrum analyser, or as it is also known the swept or superheterodyne test instrument a summary or tutorial about the sweep or superheterodyne format of spectrum analyser that sweeps the required span with block diagram and operational details.

Of the two types of RF spectrum analyzer that are available, namely the swept or superheterodyne spectrum analyzer and the Fast Fourier Transform, FFT spectrum analyzer, it is the swept or sweep spectrum analyzer that is the most widely used.

The swept spectrum analyser is the general workhorse RF test equipment of the spectrum analyzer family. It is a widely used item of RF test equipment that is capable of looking at signals in the frequency domain. In this way this form of spectrum analyser is able to reveal signals that are not visible when using other items of test equipment.

To enable the most effective use to be made of a sweep 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.

Advantages & disadvantages of a sweep spectrum analyzer

The sweep or swept spectrum analyzer has a number of advantages and disadvantages when compared to the main other type of analyzer known as the FFT spectrum analyzer. When choosing which type will be suitable, it is necessary to understand the differences between them and their relative merits.

    Advantages of superheterodyne spectrum analyser technology

  • Able to operate over wide frequency range:   Using the superheterodyne principle, this type of spectrum analyzer is able to operate up to very high frequencies - many extend their coverage to many GHz.
  • Wide bandwidth:   Again as a result of the superheterodyne principle this type of spectrum analyzer is able to have very wide scan spans. These may extend to several GHz in one scan.
  • Not as expensive as other spectrum analyzer technologies:   Although spectrum analyzers of all types are expensive, the FFT style ones are more expensive for a similar level of performance as a result of the high performance ADCs in the front end. This means that for the same level of base performance, the superheterodyne or sweep spectrum analyzer is less expensive.

    Disadvantages of superheterodyne spectrum analyzer technology

  • Cannot measure phase:   The superheterodyne or sweep spectrum analyzer is a scalar instrument and unable to measure phase - it can only measure the amplitude of signals on given frequencies.
  • Cannot measure transient events:   FFT analyzer technology is able to sample over a short time and then process this to give the required display. In this way it is able to capture transient events. As the superheterodyne analyzer sweeps the bandwidth required, this takes longer and as a result it is unable to capture transient events effectively.

Balancing the advantages and disadvantages of the swept or superheterodyne spectrum analyzer, it offers excellent performance for the majority of RF test equipment applications. Combining the two technologies in one item of test equipment can enable the advantages of both technologies to be utilised.

Sweep spectrum analyser basics

The swept spectrum analyser uses the same superheterodyne principle used in many radio receivers as the underlying principle on which its operation depends. The superheterodyne principle uses a mixer and a second locally generated local oscillator signal to translate the frequency.

The mixing principle used in the analyzer operates in exactly the same manner as it does for a superheterodyne radio. 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 superheterodyne radio, the particular implementation differs slightly to enable it to perform is function.

Diagram of a superheterodyne based spectrum analyzer showing the various circuit blocks
Superheterodyne or swept frequency spectrum analyzer block diagram

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.

Elements of a sweep spectrum analyzer

Although the basic concept of the sweep spectrum analyser is fairly straightforward a few of the circuit blocks may need a little further explanation.

  • RF attenuator:   The first element a signal reaches on entering the test instrument 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. Thus the dynamic range performance of the mixer is of crucial importance to the analyser as a whole.

    Great care must be taken when using a sweep spectrum analyzer not to feed excessive power directly into the mixer otherwise damage can easily occur. This can happen when testing radio transmitters where power levels can be high and accidentally turning the attenuator to a low value setting so that higher power levels reach the mixer. As a result it is often good practice to use an external fixed attenuator that is capable of handling the power. If damage occurs to the mixer it will disable the spectrum analyzer and repairs can be costly in view of the high performance levels of the mixer.
  • IF amplifier:   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 has 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 test instrument is optimised.
  • IF filter:   The IF filters restrict the bandwidth that is viewed, effectively increasing the frequency resolution. However this is at the cost of a slower scan rate. Narrowing the IF bandwidth reduces the noise floor and enables lower level spurious signals to be viewed.
  • 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. 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.
  • Ramp generator:   The ramp generator drives the sweep of the local oscillator and also the display. In this way the horizontal axis of the display is directly linked to the frequency. In other words the ramp generator is controlled by the sweep rate adjustment on the spectrum analyser.
  • Envelope or level detector:   The envelope detector converts the signal from the IF filter into a signal voltage that can be passed to the display. As the level detector has to accommodate very large signal differences, linearity and wide dynamic range are essential.

    The type of detector may also have a bearing on the measurement made. Whether the detector is an average level detector or whether it provides an RMS value.

    An RMS detector calculates the power for each pixel of the displayed trace from samples allocated to the pixel, i.e. for the bandwidth that the pixel represents. The voltage for each sample is squared, summed and the result divided by the number of samples. The square root is then taken to give the RMS value.

    For an average value, the samples are summed, and the result is divided by the number of samples.
  • Display:   In many respects the display is the heart of the test instrument as this is where the signal spectra are viewed. The overall display section of the spectrum analyser contains a significant amount of processing to enable the signals to be viewed in a fashion that is easy comprehend. Items such as markers for minimum signal, maximum peak, auto peak, highlighting and many more elements are controlled by the signal processing in this area. These features and many more come as the result of significant increases in the amount of processing provided.

    As for the display screens themselves, cathode ray tubes were originally used, but the most common form of display nowadays are forms of liquid crystal displays. The use of liquid crystal displays does have some limitations, but overall with the level of development in this technology they enable the required flexibility to be provided.

The superheterodyne spectrum analyser, or as it is also called the sweep spectrum analyser is still widely used although with the development of processing technology, other forms of analyser such as the FFT spectrum analyser are becoming increasingly widely used. However the superheterodyne analyser is able to provide a particularly useful function within the analyser marketplace.

By Ian Poole

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