Superheterodyne, Sweep Spectrum Analyzer

- a summary or tutorial about the swept or superheterodyne format of spectrum analyser that sweeps the required span with block diagram and operational details.

This spectrum analyzer tutorial is split into several pages each of which address different aspects of spectrum analyser operation and technology:

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 and disadvantages of a swept or 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 the 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 the 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.

Swept or 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 spectrum 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 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.

Elements of a superheterodyne 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.

1. RF attenuator:   The first element a signal reaches on entering the spectrum analyser 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.


2. 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


3. 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.
   
   Great care must be taken when using a superheterodyne 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.


4. 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 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.


5. 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.


6. 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.


7. 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.


8. Level detector:   The level detector converts the signal from the IF filter into a signal voltage that can be passed to the display. Normally a logarithmic output is required for the display, although occasionally linear displays may be required. Any conditioning and switching for this will be contained within the level detector and associated display circuitry.


9. Display:   In many respects the display is the heart of the test instrument as this is where the spectra are viewed. Originally cathode ray tubes were used, but a variety of more modern types of display are used these days. Additionally significant amounts of signal processing are used in spectrum analysers these days, and this enables far higher degrees of functionality to be introduced into these test instruments.

Summary

Superheterodyne spectrum analyzer technology also referred to as sweep or swept frequency spectrum analyzer technology is widely used in its own right within analyzers to make RF measurements. Superheterodyne or sweep analyzers 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. By combining the superheterodyne analyzer techniques with FFT analyzer technology, particularly high performance analyzers can be made.

Further pages from this tutorial
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