Sweeping Spectrum Analyzer Basics: Advantages of Superheterodyne Spectrum Analyzer Technology

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Sweeping Spectrum Analyzer Basics: Advantages of Superheterodyne Spectrum Analyzer Technology

Posted Date: 2024-01-16

The superheterodyne spectrum analyzer, also known as a swept or swept spectrum analyzer, was the first form of spectrum analyzer used.

Essentially, a superheterodyne spectrum analyzer or swept/swept spectrum analyzer is a radio receiver with a display at the output that indicates the output level. Tune or scan the receiver over the desired range and select filters to accept the desired signal bandwidth.

Spectrum analyzers use the superheterodyne principle used in many radio receivers as the basic principle on which their operation depends. The superheterodyne principle uses a mixer, in addition to a locally generated or local oscillator, to convert the frequency.

Advantages and Disadvantages of Sweep/Sweep Spectrum Analyzers

Swept or swept spectrum analyzers have many advantages and disadvantages compared to other types of spectrum analyzers. When choosing the right type, it's necessary to understand their differences and their relative merits.

The other major type of spectrum analyzer uses digital technology and relies on the Fast Fourier Transform, and is therefore often referred to as an FFT analyzer.

Advantages of Superheterodyne Spectral Analyzer Technology

Broadband: Due to the superheterodyne principle, this type of spectrum analyzer is capable of a very wide scanning range. The scan span can be extended to several GHz.

Capable of operating over a wide frequency range: Using the superheterodyne principle, this type of spectrum analyzer is capable of operating at very high frequencies - many spectrum analyzers extend their coverage to many GHz, as the range from very high to The frequency is converted to the processing frequency band.

Disadvantages of Superheterodyne Spectrum Analyzer Technology

Cannot measure phase: A superheterodyne or swept spectrum analyzer is a scalar instrument and cannot measure phase - it can only measure the signal amplitude at a given frequency.

Transient events cannot be measured: FFT analyzer technology is able to take samples over a short period of time and then process them to provide the desired display. In this way, it is able to capture transient events. Due to the bandwidth required for superheterodyne analyzer scanning, this takes longer and therefore cannot capture transient events efficiently.

In the past, FFT spectrum analyzers were more expensive than more traditional swept or superheterodyne spectrum analyzers. However, technology has moved forward and now in many cases there is little cost difference. Typically, most modern spectrum analyzers will use digital processing, although they may still use the superheterodyne principle to provide the required frequency range.

Sweep Spectrum Analyzer Basics

Swept spectrum analyzers use the same superheterodyne principle as many radio receivers as the basic principle on which their operation depends. The superheterodyne principle uses a mixer and a locally generated local oscillator signal to convert frequency.

The mixing principle used in the analyzer is exactly the same as that used in superheterodyne radio.

The signal entering the front end is converted to another frequency, usually a lower frequency. Using a fixed-frequency filter in the IF section of the device allows the use of high-performance filters and allows the input frequency of the analyzer or receiver to be changed by changing the frequency of the local oscillator signal entering the mixer.

Although the basic concept of a spectrum analyzer is exactly the same as a superheterodyne radio, the specific implementation that enables it to perform its functions is slightly different.

Superheterodyne or swept spectrum analyzer block diagram

The frequency of the local oscillator determines the frequency of the signal passing through the IF filter. This is swept in frequency (frequency increases linearly) in order to cover the desired frequency band. The sweep voltage used to control the local oscillator frequency also controls the sweep of the sweep on the display. In this way, the position of the scan point on the screen is related to the position or frequency of the local oscillator and therefore to the frequency of the input signal. Additionally, due to the wide range of input levels received, any signal that passes through the filter is further amplified, detected and usually converted to a logarithmic scale before being passed to the display Y-axis.

Scanning Spectrum Analyzer Components

While the basic concept of a swept spectrum analyzer is fairly simple, some circuit blocks may require further explanation.

RF Attenuator: The first component a signal reaches when entering a test instrument is the RF attenuator. Early models used manually switched attenuators, but modern analyzers are usually controlled by a processor in the test instrument. Its purpose is to adjust the level of the signal entering the mixer to an optimal level. If the signal level is too high, not only may the reading fall off the display, but the mixer performance may not be optimal. The mixer may operate outside of the designated operating area and may see other mixed products and error signals may be seen on the display.

In fact, when a false signal is suspected, the input attenuator can be adjusted to provide additional attenuation, such as +10 dB. If the signal at the display level drops by more than this amount, it is likely that there are unwanted mixing products and insufficient RF attenuation of the input signal level.

Input RF attenuators are also used to provide some protection for very large signals. Very large signals are likely to damage the mixer. Because these mixers are very high-performance components, they are not cheap to replace and are relatively easy to damage. Also adds an element of protection. Typically, the input RF attenuator includes a capacitor, which protects the mixer from any DC current that may be present on the line under test.

Low Pass Filter and Preselector: This circuit is followed by an attenuator to eliminate out-of-band signals. This filter in the spectrum analyzer prevents unwanted signals from mixing with the local oscillator and producing an unwanted response at the intermediate frequencies. These will appear on the display as signals and must therefore be removed.

Microwave spectrum analyzers often replace low-pass filters with more comprehensive preselectors. This allows a frequency band to be passed through, and its response is clearly tailored to the band of interest.

Mixer: The mixer is naturally the key to the success of the analyzer. Therefore, blenders are high-performance products that are often very expensive. They must be able to operate over a very wide signal range and provide very low levels of spurious response. Any spurious signals generated may cause spurious responses that will be shown on the display along with the actual signal. Therefore, the dynamic range performance of the mixer is critical to the entire analyzer.

Great care must be taken when using a swept spectrum analyzer not to feed too much power directly into the mixer, otherwise damage can easily occur. This can happen when testing a radio transmitter that may have very high power, and accidentally turn the attenuator to a low setting so that the higher power level reaches the mixer. Therefore, it's usually best to use an external fixed attenuator that can handle the power. A damaged mixer will render the spectrum analyzer unusable and require repair, which is often expensive.

IF Amplifier: The signal leaving the mixer is usually low level and needs to be amplified. The gain of the stage is adjustable, usually in 10dB stages. Changing the gain here changes the position of the signal on the analyzer's vertical scale. IF gain must be used in conjunction with RF gain control, and in modern analyzers the two are often interconnected and adjusted to provide the best overall performance. Excessively high IF gain levels increase front-end noise levels, which can cause low-level signals to be blocked. Therefore, the RF gain control should generally be kept as high as possible without overloading the mixer. In this way, the noise performance of the entire test instrument is optimized.

IF Filter: The IF filter limits the bandwidth viewed, effectively increasing frequency resolution. However, this comes at the cost of a slower scan rate. Narrowing the IF bandwidth reduces the noise floor and enables viewing of lower level spurious signals.

Local Oscillator: The local oscillator within a spectrum analyzer is naturally a critical component in the overall operation of the device. Its performance determines many of the overall performance parameters of the entire analyzer. It must be able to tune over a very wide frequency range to enable the analyzer to scan over the required range. It must also have very good phase noise performance. If the oscillator has poor phase noise performance, not only will it render the device incapable of making narrowband measurements, since the near-phase noise on the local oscillator will be translated into a measurement of the signal being measured, but it will also prevent it from making any useful measurements of the phase noise itself. Measurement of meaning - This type of measurement is increasingly carried out today.

Ramp Generator: The ramp generator drives the local oscillator and the scan of the display. This way, the horizontal axis of the display is directly connected to frequency. In other words, the ramp generator is controlled by the sweep rate adjustment on the spectrum analyzer.

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. Because level detectors must accommodate very large signal differences, linearity and wide dynamic range are critical.

The type of detector may also have an impact on the measurements taken. Whether the detector is an average level detector or provides RMS values.

The RMS detector calculates the power of each pixel in the display trace based on the samples assigned to that pixel, which is the bandwidth represented by that pixel. Add the squared voltages for each sample and divide the result by the number of samples. Then taking the square root gives the RMS value.

For the mean, add the samples and divide the result by the number of samples.

Display: In many ways, the display is the heart of test instrumentation because this is where the signal spectrum is viewed. The entire display portion of a spectrum analyzer contains extensive processing capabilities that allow signals to be viewed in an easy-to-understand manner. Items such as marking of minimum signal, maximum peak, autopeak, highlighting and many more elements are controlled by the signal processing in this area. These features and more are the result of a significant increase in the amount of processing offered.

As for the display itself, cathode ray tubes were originally used, but now the most common form of display is in the form of an LCD. The use of LCD monitors does have some limitations, but overall, with the state of the art in this technology, they can provide the flexibility needed.

Superheterodyne spectrum analyzers, or also called swept spectrum analyzers, are still in use although they have been superseded by spectrum analyzers that use digital FFT technology. However, it helps illustrate the principles of a spectrum analyzer, and often older equipment may be available in some laboratories.

Review Editor: Huang Fei

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