One of the biggest challenges that radio frequency (RF) and microwave spectrum analyzers present to users is to calculate the measurement accuracy of these two spectrum analyzers.
RF and microwave spectrum analyzers are not simple even in principle. Calling these two types of spectrum analyzers corrected superheterodyne receivers can only reflect what they have and how to implement them. If they are called frequency domain oscilloscopes, they will reflect less. In addition, if you just take a look at this technology, you will come to the wrong conclusion: RF and microwave spectrum analyzers have not changed much in the past decade. However, in the rapidly developing field of wireless technology, which can display frequencies as high as 3 GHz or more-often up to 7 GHz, and sometimes up to 20 GHz-the signal spectrum analyzer is actually undergoing major changes, and its importance is also greatly increasing.
To make matters worse, choosing the most suitable analyzer for a task can be a big problem. When your boss does not understand why this selection process is not just about the price and some parameters on one or two manufacturer's product instructions This is especially true for comparison. For the following situations, there are not many engineers who understand, and even fewer managers who understand: the spectrum analyzer manual often hides the data that it does not give, and the most important information on the product manual may be that the spectrum analyzer manufacturer does not The key words spoken. For example, as a worst-case scenario, some manufacturers only provide typical parameter values ​​given by their competitors.
In addition, the test conditions have a great influence on the technical specifications, but the manufacturer hopes that you think that the technical specifications are not related to these test conditions. The attached article "1dB Gain Compression Point of RF Spectrum Analyzer" explains such a strange technical specification. Gain compression can quantify the effect of an interfering signal whose amplitude and frequency you do not want to measure on the measured amplitude of a signal you are studying.
Most suppliers understand that if you have not used any of the spectrum analyzers of the same model series that you plan to buy, your evaluation of the spectrum analyzer will take at least two weeks, and may be as long as one month. Ideally, you would place two top-level alternative product demonstration equipment in the lab for a full month. This way, you can compare the two not only under the same conditions, but also in relation to your application. In practice, it sometimes takes a month to obtain the necessary equipment, and then conduct such an evaluation. Modern spectrum analyzer design simplifies the evaluation and use of spectrum analyzers, and some instruments have an automatic setting function that can be used to carry out tests applicable to various wireless communication protocols specified by various standardization organizations.
After-sales service is important
You will often find that evaluating and using a spectrum analyzer requires some accessories, such as splitters, directional couplers, and so on. If you do not have these necessary accessories, or if the manufacturer and distributor cannot provide them in time, the field engineer of the instrument manufacturer in your area will lease these accessories to you.
You may think that the cost of serving such customers is too high. This service method should have ceased to exist ten years ago, or only for the largest companies. However, manufacturers of spectrum analyzers believe that good after-sales service will bring good business, and they have never stopped this service. Manufacturers have included service fees in the product price framework. Since a company is likely to purchase more than one spectrum analyzer, but only the probability of supporting its first one is high, manufacturers will have many opportunities to recover technical support costs or provide attractive repurchase customers. discount. Although the price of portable spectrum analyzers is often lower than 10,000 US dollars, and some portable spectrum analyzers are much cheaper, the price of high-performance RF spectrum analyzers (that is, broadband low-noise spectrum analyzers) is often Quite expensive. Many high-end spectrum analyzers are priced above US $ 30,000.
It is difficult for users to evaluate a device, which is beneficial to the manufacturer selected by the user. Complicated products, coupled with repeated sales to customers, may benefit both customers and manufacturers. After spending so much time evaluating products, figuring out how to use the unique features of the product, and learning how to deal with product failures, a customer who is satisfied with the service will probably buy more products from the selected manufacturer. When customers change manufacturers, they have to make a large investment and take a long time to understand the advantages of another complex product.
mixing
The working principle of the spectrum analyzer is called mixing, heterodyne or frequency conversion. Fundamentally speaking, mixing is frequency multiplication, which is an inherently nonlinear processing method. The spectrum analyzer mixes the input signal (the frequency of which is fIN-short for any frequency in the range of fIN1 to fIN2) and the signal of variable frequency LO (local oscillator) of frequency fLO to produce an IF of frequency fIF (Intermediate frequency) signal. Usually, fLO = fIN + fIF, that is, the frequency of the local oscillator is higher than the input signal frequency, but the design using fLO = fIN-fIF is also possible. A classic spectrum analyzer scans fLO, so in a short period of time, the mixed output signal represents the input signal in the fIN1 ~ fIN2 frequency band. Then, the spectrum analyzer detects the envelope of the output signal of the mixer (removes the fIF component in the output signal) and displays it as an envelope of a function of time to generate a curve in which the amplitude of the input signal is a function of frequency.
The architecture of the spectrum analyzer is very complicated, which is mostly caused by an inevitable feature of mixing. That is, the mixed output signal with frequency fIF not only represents the input signal in the fLO-fIN frequency band, but also represents the fLO + fIN input signal. If you try to convert a frequency band, such as the 30MHz to 3GHz frequency band, to a common 10.7MHz intermediate frequency in one step, the scan range of the local oscillator may be 40.7 to 3010.7MHz (the scan range of your local oscillator may also be For 19.3 ~ 2989.3MHz). If you choose the first option, the mixed output signal not only represents the input signal in the desired frequency band, but also represents the input signal in the 51.4 ～ 3021.4MHz frequency band. At any time, the mixed output signal represents the sum of two input signals separated by 21.4MHz (fIF · 2)-the required frequency + the unwanted frequency called the mapping (in this case, the higher frequency).
In further discussion, the mixer output signal never represents an input signal with only one (or only two) frequencies; obtaining such an output signal means that the intermediate frequency bandwidth is zero. The intermediate frequency bandwidth, which is the RBW (resolution bandwidth) of the spectrum analyzer, is always greater than zero. The minimum RBW you can choose is the figure of merit of the spectrum analyzer. However, if you deliberately choose RBW to be zero, then all swept spectrum analysis requires infinite time, because a zero-bandwidth bandpass filter requires infinite time to react to changes in its input signal.
One way to solve the problem of image frequency is to use multiple frequency conversions. Spectrum analyzers usually have 3 intermediate frequency levels. A spectrum analyzer with a frequency coverage ranging from 30MHz to 3GHz may first convert the input signal frequency to greater than 3GHz, which makes all unnecessary reflection frequencies higher than the spectrum analyzer's input frequency range, so a fixed cut-off frequency low pass The filter can filter them out. If the first intermediate frequency is 3.4 GHz, for example, the scan range of the local oscillator is 3.43 to 6.4 GHz, and the coverage of the image frequency band is 6.83 to 9.8 GHz. Another benefit of this is that the ratio of the highest fLO to the lowest fLO is reduced. In this example, this ratio is reduced from more than 6 octaves (40.7 to 3010.7 GHz) to less than 1 octave (3.43 to 6.4 GHz).
Transform frequency domain to time domain
At the input end of the swept frequency analyzer, signals of various frequencies within the entire input frequency range of the instrument may exist at the same time, however, at the output end of the mixer, this frequency range is greatly reduced, because when sweeping the sweep frequency When the instrument is tuned in the relevant frequency band, these signals—transformed to near fIF frequencies—do not exist at the same time, but appear in chronological order. Therefore, after the first mixer, the swept frequency analyzer does not need a very wide bandwidth, which greatly simplifies the design of most circuits of the swept frequency analyzer. On the other hand, the existence time of modern communication signals is extremely short, and the duty ratio is very small, which requires the frequency sweep analyzer to be able to continuously scan a wide frequency band more or less continuously.
There are already such instruments. Manufacturers have given them various names, including signal analyzers, vector signal analyzers, and wireless communication analyzers. All of these instruments use DSP technology in large quantities, and so are more and more spectrum analyzers. However, in general, spectrum analyzers and signal analyzers using DSP technology are quite different in specifications and expected applications. The signal analyzer can capture data at a faster rate, can store long digital time domain data records, can process vectors (phase and amplitude), and can digitally modulate in formats such as 60QAM (64-level quadrature amplitude modulation) Complex analysis. Spectrum analyzers are usually smaller and cheaper than signal analyzers, but they have a much larger dynamic range.
The block diagrams of most spectrum analyzers based on DSP technology are at least superficially similar to those of spectrum analyzers using traditional analog signal processing techniques. Like the spectrum analyzer using the traditional architecture, the DSP-based spectrum analyzer also uses a large number of analog intermediate frequency signals. However, after the last mixer, you will find that there are no analog filters except for high-speed high-resolution ADCs and DSPs that function as digital filters. The advantage of using a DSP spectrum analyzer is that the selectivity is improved (RBW is narrower), and when you reduce the RBW, the scanning speed is reduced less. However, Agilent recently announced that its PSA series of DSP-based spectrum analyzers have vector modulation analysis capabilities. These functions are a major change for spectrum analyzers that previously had only scalar (size) measurement capabilities.
The days of major reforms in this common architecture may not be too far away. The spectrum analyzer can use an analog mixer instead of an analog mixer. An ADC with a sampling rate of 64Ms / s (sampling accuracy of 14 bits) has been released. If a suitable T / H (track-and-hold) amplifier is installed in front of such an ADC, you can deliberately reduce the sampling rate according to the required sampling accuracy and sample communication signals that are much higher than the 32MHz Nyquist frequency of the ADC . Suppose you sample the modulated 200MHz carrier in this way. If the sidebands on both sides of the carrier frequency do not exceed half the sampling frequency, and no other signals are mixed into the baseband, then you can get an accurate digital copy of this modulation.
Of course, most new digital communication systems have a carrier frequency range of 2.35 to 5.8 GHz, so a signal analyzer that includes a T / H amplifier that amplifies a modulated 200 MHz carrier still has to use mixing technology. However, today you don't need to buy a T / H amplifier with a bandwidth greater than 200MHz and an accuracy comparable to a 14-bit resolution ADC. Although such a T / H amplifier has not yet been put into production, it is said to be feasible now. If such devices are put on the market, they will revolutionize the architecture so that one or several frequency conversion stages can be removed from the signal analyzer and spectrum analyzer using DSP.
Signal Analyzer Modular
A modular 2.7GHz signal analyzer recently announced by National Instruments (NI) has broken many rules for such instruments. PXI-5660 (Figure 1) is not a high-end instrument, but two 3U height PXI modules-a frequency converter with 3 slots and a slot with 16M words (32M bytes) digital conversion memory 14-bit 64Ms / s digitizer. The SFDR (dynamic range without spurious signals) is 80 dB, but you can sacrifice measurement speed to achieve a larger dynamic range. NI claims that the measurement speed of SFDR of 80 dB is 200 times that of SFDR equivalent instrument-level signal analyzer.
The sum of these two modules is still much smaller than an instrument-level signal analyzer with the same function, but it cannot constitute a complete analyzer. The two modules can be inserted into a PXI plug-in box that can house a CPU module, including the necessary power supply and other additional modules.
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