Design scheme of spectrum analyzer in full analog mode based on superheterodyne receiving component

The spectrum analyzer is known as the “microwave engineer’s multimeter”. It can perform intuitive measurement and analysis of the wave component, parasitics, intermodulation, and noise sidebands of the signal. It is one of the indispensable measuring instruments in microwave measurement. . For a long time, due to the high price of traditional desktop spectrum analyzers, and the domestic application of microwaves is mainly concentrated in the fields of radar, Electronic countermeasures, space technology, satellite ground stations, EMC testing, etc., the popularity of spectrum analyzers is not high.

introduction

The spectrum analyzer is known as the “microwave engineer’s multimeter”. It can perform intuitive measurement and analysis of the wave component, parasitics, intermodulation, and noise sidebands of the signal. It is one of the indispensable measuring instruments in microwave measurement. . For a long time, due to the high price of traditional desktop spectrum analyzers, and the domestic application of microwaves is mainly concentrated in the fields of radar, electronic countermeasures, space technology, satellite ground stations, EMC testing, etc., the popularity of spectrum analyzers is not high. In recent years, with the rapid development of communication technology, especially the advent of the 3G era, more and more field operations need the support of spectrum analyzers. In this situation, the huge body and expensive price of traditional spectrum analyzers increasingly restrict it. With the expansion of its application, foreign instrument manufacturers such as Agilent, R&S, Anritsu, etc. have launched their own portable spectrum analyzer products, but domestic portable spectrum analyzer products are still very rare. The original design concept of the handheld spectrum analyzer was put forward based on the above-mentioned demand situation.

1 Introduction to the full analog spectrum analyzer solution

Most modern broadband spectrum analyzers adopt the sweep frequency superheterodyne receiving scheme, and the simplified block diagram of the full analog superheterodyne spectrum analyzer is shown in Figure 1.

Design scheme of spectrum analyzer in full analog mode based on superheterodyne receiving component

The spectrum analyzer shown in Figure 1 works in a frequency sweep mode: the IF filter bank is composed of multiple filters with the same center frequency and different bandwidths. By constantly changing the frequency of the sweep local oscillator signal, the mixer can make different frequencies The input signal sequentially falls within the passband of the intermediate frequency filter, thereby completing the spectrum analysis of the entire frequency band signal.

2 Scheme design of a handheld spectrum analyzer

2.1 Enlightenment of the full analog spectrum analyzer program

The full analog mode spectrum analyzer program explains the basic composition of the spectrum analyzer well, as shown in Figure 2.

Design scheme of spectrum analyzer in full analog mode based on superheterodyne receiving component

(1) The superheterodyne receiving component composed of the swept local oscillator and the mixer converts the wideband input signal into an intermediate frequency narrowband signal, which is convenient for further analysis and processing of the subsequent circuit.
(2) The IF filter bank is composed of a series of band-pass (or low-pass filters). Different filter bandwidths determine different frequency resolutions. It is used to realize the resolution bandwidth of the spectrum analyzer, which is indispensable for the spectrum analyzer. One of the few core components.
(3) The detector is used to detect the signal energy within the bandwidth of the resolver, and is one of the essential core components of the spectrum analyzer.
(4) The video filter is composed of a series of low-pass filters, which are used to filter out the high-frequency components of the spectrum information after detection, and can improve the visual effect of the spectrum Display. It is one of the important components of the spectrum analyzer.
(5) The spectrum Display device is the platform for the spectrum analyzer to display the results of the spectrum analysis, and it is an indispensable and important functional component of the spectrum analyzer.

2.2 A scheme design of a handheld spectrum analyzer

The first thing that needs to be solved in the design of a handheld spectrum analyzer is the volume problem. The fundamental way to solve this problem is to make the functional components as digital as possible.

The frequency range of the handheld spectrum analyzer is 250 kHz~2.7 GHz. In the core composition block diagram of the frequency submersible instrument shown in Fig. 2, limited by the A/D sampler and sampling bandwidth, the superheterodyne receiving component still needs to be simulated (Microwave) circuit design.
The signal after superheterodyne reception is an intermediate frequency signal that is already a narrowband signal (bandwidth less than 20 MHz). Through a reasonable selection of the center frequency and bandwidth of the signal, A/D sampling can be performed at the current device level. In the design of the handheld spectrum analyzer, the center frequency of the intermediate frequency signal is selected as 21. 4 MHz, and the bandwidth is selected as 3 MHz.

In this way, resolution bandwidth filtering, video filtering, etc. can be realized by digital signal processing. Since the handheld spectrum analyzer has 11 resolution bandwidths and 10 video bandwidths, it needs to design 21 filters (some of which are Design is very difficult), after adopting digital signal processing, only one FPGA chip is needed, which greatly saves the volume. A design scheme of a handheld spectrum analyzer is shown in Figure 3.

Design scheme of spectrum analyzer in full analog mode based on superheterodyne receiving component

The following introduces the core part-the design of the superheterodyne receiving component.

2.3 Design of superheterodyne receiving component

The superheterodyne receiving component is the core component of the handheld spectrum analyzer, and its performance directly determines the phase noise, spurious, background noise and other performance indicators of the spectrum analyzer. The composition block diagram of the superheterodyne receiving component is shown in Figure 4.

Design scheme of spectrum analyzer in full analog mode based on superheterodyne receiving component

The components are implemented in a three-stage mixing mode. The first intermediate frequency is 4 021. 4 MHz, the second intermediate frequency is 821.4 MHz, and the third intermediate frequency is 21.4 MHz. The purpose of this selection is that the 800 MHz local oscillator required for the third mixing can be The 3.2 GHz local oscillator required for the second mixing frequency is generated by 4, thereby reducing the number of local oscillators and reducing the volume.

The input attenuator is a 0~60 dB high-power RF attenuator with steps of 10 dB to realize the measurement range of a handheld spectrum analyzer (-120~+30 dBm).

3 Main performance indicators

At present, the prototype of the handheld spectrum analyzer designed in the above scheme has been successfully developed and has been applied in a project of the General Staff. The core part has a volume of only 190 mm×100 mm×60 mm and a power consumption of about 12 W. Its main performance indicators are as follows:

(1) Frequency range: 250 kHz~2.7 GHz;
(2) Resolution bandwidth: 30 Hz~1 MHz (1, 3 steps);
(3) Video bandwidth: 10 Hz~1 MHz (1, 3 steps);
(4) Span setting: zero span, 100 Hz~2.7 GHz;
(5) Average display noise level (DANL, RBW: 30 Hz, VBW: 10 Hz);
250 kHz~1 MHz: “-100 dBm, 1 MHz~2.7 GHz: “-120 dBm;
(6) Measuring range: DANL~+30dBm;
(7) Single sideband phase noise: -80 dBc/[email protected]

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