How to make a cheap spectrum analyzer expensive. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

 Comments on the article

If it becomes necessary to evaluate the bandwidth of the emitted signal, the instability of the operating frequency, the suppression of out-of-band and spurious emissions, the distortion of the baseband signal of the radio transmitter, what do we do? That's right, take your spectrum analyzer (AC) HP 8560 series E and measure everything you need! But let me tell you, I do not have HP, I have the most ordinary analyzer of the most domestic production in the world! In this case, you will agree with me that the sensitivity of a spectrum analyzer is never too high! Sensitivity, frankly, is always not enough, because. dealing with very small signals. The second thing you will surely agree with is that there is always little dynamic range, you always want more! Large dynamic range is needed when looking at the spectrum of a signal in the presence of very strong interference or other signal. Most often, such a problem arises when assessing the level of the second or third harmonic of the transmitter signal.

Studying the brochures of eminent manufacturers of measuring equipment, sometimes it becomes a shame for your own analyzer. So, in order to have something to answer the "imperialists", we will share with you a few tips and recommendations on how to achieve the sensitivity and dynamic range necessary to solve problems that only expensive imported devices can do.

Dynamic range

The dynamic range of any active receiving device is estimated by some predetermined parameter that characterizes the various distortions that occur in this device when an RF signal passes through it. In other words, this is the difference between the maximum and minimum signal levels at which no distortion is yet observed. The reason for these distortions is the nonlinearity of the amplifying path of the device in question. There are different types of non-linearity, so different characteristics are used to estimate the dynamic range. The most important characteristics are the linear dynamic range and the 3rd order IMD dynamic range at the IP3 point (Fig. 1). When considering both, one cannot do without the use of such a concept as an amplitude characteristic, by which one can judge the degree of non-linear distortion.

Ris.1.

The generalized amplitude characteristic (ACH) of the device under consideration is presented on a double logarithmic scale in Fig. 1 (curve 1). The minimum detectable signal is considered to be 3 dB higher than the device's own noise. Therefore, the beginning of the linear section of the characteristic from below is considered to be a point on the AX, corresponding to an excess of 3 dB of its own noise at the output, and the corresponding minimum input P_in.min and output R_{output min} power.

The upper limit of the linear section of the AX is the point at which the actual characteristic deviates from the ideal (linear) by 1 dB. This point corresponds to the input P_1dBv and output R_1dBout saturation power (compression point). The difference (in decibels) between the saturation input power and the minimum input signal power determines the linear dynamic range.

As is known, the effect of any changing signal on a non-linear element is the enrichment of its spectrum - harmonics and combination frequency components appear. When studying the spectrum of signals, a lot of troubles are caused by combination frequencies of odd orders that fall directly into the band of the signal under study. The most dangerous combinational components of the third order, namely the components at frequencies 2f1-f2 and 2f2-f1, where f1 and f2 are the two most significant spectral components of the input signal (for example, carrier and side, first and second harmonics, signal and strong interference and etc.). Let us consider the harmful effect of the combinational components of the third order on a typical, in relation to the problem under consideration, example - measuring the level of side oscillations of the transmitter. On fig. 2 shows the combinational distortions of the signal spectrum at the output of the transmitter.

Ris.2.

In the case when the ratio of the level of the second and higher harmonics to the first is small enough, there is a danger of going beyond the limit of the linear section of the amplitude characteristic of the analyzer's amplifying path, since trying to see weak signals of higher harmonics, we excessively (in relation to a strong first harmonic) increase the gain of the device. Then, as a result of the influence of a polyharmonic (containing two or more spectral components) signal on the nonlinear path, combinational spectral components arise, two of which (in the simplest case, taking into account only the combinational components from the first and second harmonics, and neglecting the rest) at frequencies 2f1- f2 and 2f2-f1 fall directly into the operating band of the signal under study. It should be noted here that third-order combination components do not arise with any kind of nonlinearity (they do not arise with quadratic nonlinearity). On fig. 2, these combination frequencies are highlighted in bold. It can be seen that the component 2f2-f1 falls on the frequency of the third harmonic and distorts its true value. As a result, the observer makes erroneous conclusions about the spectrum of the signal!

It is convenient to determine the value of the dynamic range from third-order combinational distortions using curve 2 in Fig. 1, which displays the dependence of the data level of the combinational components on the level of the input signal. The extensions of the linear parts of the third-order pitch and combination frequency characteristics intersect at a point called the characteristic power point (or compression point) of third-order distortion IP3. It corresponds to the input (P_IP3in) and output (Р_IP3out.) characteristic third-order distortion powers.

The dynamic range for combinational distortions of the third order (by point IP3) is defined as the difference between the input power corresponding to the absence of distortion and the power of the minimum input signal. The higher the IP3 point, the higher the dynamic range, respectively.

It follows from the above that the dynamic range can be determined according to different criteria. In practice, this is exactly what is done, and then, according to the results, the worst value is taken as the value of the dynamic range.

Give sensitivity!

In order to increase the sensitivity of the speakers, i.e. to provide the ability to process low-level signals without getting inside the device, it is enough to put a preamplifier in front of its input. A number of questions immediately arise. The first question is which amplifier to use, what should be its main parameters: gain (hereinafter referred to as simply gain), noise figure and dynamic range. The second, no less important question is how the inclusion of a preamplifier at the AC input affects the operation of the entire circuit. We will try to answer these questions so that you can choose the right amplifier for your application.

When using preamplifiers, always remember that the maximum signal level at the input of the preamplifier should not exceed the maximum allowable signal level at the input of the spectrum analyzer, minus the gain of the preamplifier.

For simplicity of explanation, we will use a specific example. Assume our spectrum analyzer has a noise figure of -30dB and an IP3 combinational distortion point of +10dBm. Let's find out how different types of preamplifiers affect the characteristics of the measurement circuit. Figure 3 shows the connection diagram of the preamplifier to the analyzer.

Ris.3.

Let's say the preamp gain is 20dB, the noise figure is 6dB, and the IP3 point is +15dBm. It is necessary to determine the noise figure and dynamic range of the circuit shown in Figure 3. To calculate the noise figure of the circuit in Fig. 3, we use the formula for cascading devices:

Ш = Ш1+(Ш2-1)/К1 +(ШЗ-1)/К1К2, (1)

where:

• W - noise figure (in times);
• W1 - noise figure (in times) of the first device in the cascade connection;
• W2 - noise factor (in times) of the second device in the cascade connection;
• K1 - gain (in times) of the first device in the cascade connection;
• K2 - gain (in times) of the second device in the cascade connection.

The noise figure (in times) is related to the noise figure in decibels as follows:

N = 10log(f)

Noise figure (in times) for the circuit in Fig. 3, calculated by formula (1). equals 13,99.

Really:

W = 4+ 1000 -1/100 = 13,99 Let's express this noise figure in decibels: 10log(13.99) = 11,5 dB.

Thus, connecting a preamplifier allowed us to reduce the noise figure of the spectrum analyzer by 18,5 dB, which, in fact, was what we were trying to achieve.

Now let's see how the preamp will affect the IP3 point. Table 1 shows the relationship between the IP3 point of the preamplifier and the reduction of the IP3 point value for the circuit in Fig. 3. The data in Table 1 correspond to the worst case, when the level of the combinational components of the analyzer itself is maximum. The left column of the table indicates the excess of the IP3 point of the preamplifier over the IP3 point of the analyzer.

Table 1

In our example: Preamplifier IP3 +15dBm and Spectrum analyzer IP3 -+10dBm, the difference is 5dB. The closest values ​​of the difference in the table. 1-6 dB and 3 dB. The IP3 reduction is 3,5dB and 4,6dB respectively. In our case, the IP3 drop calculated by linear interpolation between these values ​​is 3,9 dB. That is, the IP3 point of the circuit in Fig. 3 will correspond to +6,1 dBm.

This means that at the preamp input, the IP3 point will be 20 dB lower, which corresponds to -13,9 dBm.

So by adding a preamplifier, we have improved the spectrum analyzer's ability to process low-level signals and degraded its performance in the large-signal region. This is not surprising, since with the connection of the preamplifier, another non-linear device with a far from infinite dynamic range was added to the measuring circuit. Table 1 shows that the greater the excess of the preamplifier's IP3 over the analyzer's IP3, the less the IP3 of the entire circuit drops. For example, for a difference value of 20 dB, the drop in IP3 is only 0,8 dB. Thus, the use of a preamplifier with a dynamic range that is much larger than the dynamic range of the spectrum analyzer is most preferable, since it allows almost completely avoiding a decrease in the dynamic range of the entire measuring circuit.

In some cases, in order to achieve good gain, it becomes necessary to connect several preamplifiers in series. Consider what happens when you cascade two preamplifiers before a spectrum analyzer. Let's analyze the scheme shown in Fig.4.

Ris.4.

Both preamplifiers have the same characteristics shown in Fig. 4. The total gain of the preamps is 40dB (10000 times). The total noise figure is:
W \u4d 4 + (1-100) / 1000 + (1 - 10000) / 4.13 \uXNUMXd XNUMX;
In decibels, it is 10log(4,13) = 6,17 dB. Thus, the noise figure is reduced by
30 dB - 6,17 dB = 23,8 dB.

Now let's calculate the decrease in IP3. Both amplifiers have the same IP3 value of +30 dBm. According to Table. 1, with a difference of 0 dB, the reduction in IP3 at the output of preamplifier 2 is 6 dB. Thus, IP3 at the output of preamplifier 2 is equal to
30 dBm + (-6 dB) = +24 dBm.

This is 14 dB more than the IP3 value of the spectrum analyzer. Again, look at the table. 1 and obtain by interpolation between the nearest values: -2,4 dB for 10 dB and -1,4 dB for 15 dB, the value of -1,6 dB. Calculating the IP3 value for the analyzer
+10 dBm + (-1,6 dB) = +8,4 dBm.

Conclusions. Thus, the sensitivity of the analyzer when using a preamplifier improves, and the dynamic range generally deteriorates, and the less the dynamic range of the preamplifier exceeds the dynamic range of the analyzer itself, the stronger it is. Preamplifiers can be used to analyze weak signals. The use of preamplifiers should be avoided when analyzing strong signals, as well as when analyzing weak signals in the presence of strong noise.

Give dynamic range!

As mentioned above, the danger of going beyond the dynamic range is greatest when assessing the level of the second or third harmonic of the transmitter signal, i.e. when the first harmonic is a strong interference, leading to the appearance of combination components with the harmonic under study. Let's consider how this unpleasant phenomenon can be eliminated and the harmonic level can be measured.

This problem can be solved by using a notch filter at the input of the spectrum analyzer, which suppresses the carrier while the second or third harmonic enters the passband. In reality, the analyzer's dynamic range is not expanded, but rather the difference between the observed input signals is reduced.

It is important to remember that the specified maximum input signal level for the spectrum analyzer must not be exceeded. The stated maximum input level should not be confused with the 1dB compression point or the IP3 point. The maximum allowable input signal level is the level at which the input attenuator or mixer remains within acceptable operating limits. The IP3 point is typically 10 to 15 dB higher than the 1 dB compression point.

Consider the circuit in Fig.5.

Ris.5.

The attenuator is used to limit the output of the transmitter to a level that is safe for the analyzer to operate. Assume that the analyzer's maximum input level is +30 dBm, the 1 dB compression point is 0 dBm, and the transmitter output power is 100 W (50 dBm). If the attenuation in the attenuator installed between the transmitter and the spectrum analyzer is 20 dB, then the signal level at the analyzer input is equal to the maximum allowable. It is better to use a 30 dB attenuator, which will give us 10 dB of headroom.

Assume that the dynamic range of the spectrum analyzer is 70 dB. This means that we can measure the levels of two signals if the difference between them does not exceed 70 dB. Also, the level of the larger signal should be a few decibels below the 1 dB compression point or the analyzer's IP3 point.

Let's consider an example when we need to measure the levels of the second and higher harmonics of the signal under study with respect to the carrier. Assume that the second harmonic level is 80 dB below the carrier level. The dynamic range of the analyzer is 70 dB, therefore, the harmonics of the studied signal will be distorted by combination components of odd orders.

To get around this difficulty, we install a filter between the attenuator and the analyzer in order to lower the carrier level and introduce minimal losses into the second harmonic. In order for our measurements to be accurate, we need to know the losses caused by the notch filter at the second harmonic frequency. It can be a resonator or LC filter. The latter is quite small and convenient compared to conventional resonator filters. As a rule, 20...30 dB of carrier suppression is sufficient, so making and setting up a compact LC filter is not difficult.

First, we determine the losses in the filter, for this the signal generator and spectrum analyzer are tuned to the carrier frequency. Then, according to the analyzer readings, the filter is adjusted to the maximum carrier suppression. Next, the signal generator is tuned to the second harmonic frequency and the signal level is set to 0 dBm. According to the analyzer readings, we determine the losses in the filter. For example, if the analyzer is -3 dBm, then the filter loss is 3 dB.

Now we determine the value of the second harmonic. Let's assemble the installation shown in Fig.6.

Ris.6.

We put a notch filter and set it to maximum carrier suppression. Now, by increasing the sensitivity of the spectrum analyzer, by increasing the gain of the input amplifier, we determine the level of the second harmonic of the signal. Assume that the second harmonic level is -60 dBm and the filter loss at this frequency is 3 dB. Therefore, the true second harmonic level is -60 dBm - (-3 dBm) = -57 dBm. Since the carrier level is +20 dBm, the second harmonic level is 77 dB below the carrier level.

The accuracy of such measurements depends on many factors, for example, on losses in connecting cables, etc. At high powers, part of the power may leak. Therefore, we recommend using well-shielded connecting cables for measurements and positioning the transmitter away from the analyzer. Using this approach, very accurate measurement results can be achieved.

Conclusions. The use of notch filters makes it possible to investigate the spectra of signals that do not fit into the dynamic range of the spectrum analyzer or signals in the presence of strong interference, causing the appearance of combination components in the band of the signal under study. In this case, the accuracy of measurements, to a large extent, is determined by the parameters of these filters.

Author: G. Melnikov, Moscow; Publication: radioradar.net

See other articles Section Measuring technology.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Machine for thinning flowers in gardens 02.05.2024

In modern agriculture, technological progress is developing aimed at increasing the efficiency of plant care processes. The innovative Florix flower thinning machine was presented in Italy, designed to optimize the harvesting stage. This tool is equipped with mobile arms, allowing it to be easily adapted to the needs of the garden. The operator can adjust the speed of the thin wires by controlling them from the tractor cab using a joystick. This approach significantly increases the efficiency of the flower thinning process, providing the possibility of individual adjustment to the specific conditions of the garden, as well as the variety and type of fruit grown in it. After testing the Florix machine for two years on various types of fruit, the results were very encouraging. Farmers such as Filiberto Montanari, who has used a Florix machine for several years, have reported a significant reduction in the time and labor required to thin flowers. ... >>

Advanced Infrared Microscope 02.05.2024

Microscopes play an important role in scientific research, allowing scientists to delve into structures and processes invisible to the eye. However, various microscopy methods have their limitations, and among them was the limitation of resolution when using the infrared range. But the latest achievements of Japanese researchers from the University of Tokyo open up new prospects for studying the microworld. Scientists from the University of Tokyo have unveiled a new microscope that will revolutionize the capabilities of infrared microscopy. This advanced instrument allows you to see the internal structures of living bacteria with amazing clarity on the nanometer scale. Typically, mid-infrared microscopes are limited by low resolution, but the latest development from Japanese researchers overcomes these limitations. According to scientists, the developed microscope allows creating images with a resolution of up to 120 nanometers, which is 30 times higher than the resolution of traditional microscopes. ... >>

Air trap for insects 01.05.2024

Agriculture is one of the key sectors of the economy, and pest control is an integral part of this process. A team of scientists from the Indian Council of Agricultural Research-Central Potato Research Institute (ICAR-CPRI), Shimla, has come up with an innovative solution to this problem - a wind-powered insect air trap. This device addresses the shortcomings of traditional pest control methods by providing real-time insect population data. The trap is powered entirely by wind energy, making it an environmentally friendly solution that requires no power. Its unique design allows monitoring of both harmful and beneficial insects, providing a complete overview of the population in any agricultural area. “By assessing target pests at the right time, we can take necessary measures to control both pests and diseases,” says Kapil ... >>

Random news from the Archive Pay in person for lunch 24.10.2021 In North Ayrshire, Scotland, nine schools will begin using facial recognition to pay for lunch. As arguments for making such a decision, they called the fact that payment is made in a contactless way, and this will increase the safety of students against the backdrop of the ongoing Covid pandemic, as well as the fact that transactions will be significantly accelerated. "In a secondary school, we have about 25 minutes to serve potentially 1000 students. So we need high throughput at the point of sale," David Swanston, managing director of CRB Cunninghams, who installed recognition systems, told the Financial Times in an interview. on the face. According to him, the system reduces the average transaction time when paying for lunch to about five seconds per student. This is far from the first time schools in the UK have used biometric systems (some are already using fingerprint technology), but North Ayrshire will be the first to use facial recognition, which is seen as potentially a bigger privacy threat. However, Swanston noted that technology will be used that is not "live" recognition, as it does not rely on an online database. Instead, the system checks faces against an encrypted database of face templates stored on servers at schools. The vast majority of parents and students (97%) agreed to run the recognition system, although some parents expressed concern that students were not provided with enough information to make an informed decision. They also suspect that peer pressure may have played a role for some of the students.

Other interesting news:

▪ Secrets of home walking cats

▪ solar phone

▪ Banana skin flour

▪ Different types of trees affect the climate in different ways.

▪ Transparent adapter for cameras

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Standard instructions for labor protection (TOI). Selection of articles

▪ article Pneumatic tires. History of invention and production

▪ article What is the Milky Way? Detailed answer

▪ article Liquidation of enterprises in case of violation of labor protection

▪ article Reflex radio receiver for local reception. Encyclopedia of radio electronics and electrical engineering

▪ article How to improve the selectivity of the receiver. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024