CN104702279A - Frequency synthesizer of phase-locked loop - Google Patents

Abstract

The invention discloses a phase-locked loop frequency synthesizer, which belongs to the technical field of radio. The phase-locked loop frequency synthesizer includes a phase-locked loop composed of frequency and phase detectors, loop filters, voltage-controlled oscillation components, and frequency dividers connected end to end in sequence; the voltage-controlled oscillation components include a set of center frequency VCOs that are different from each other but have the same VCO gain, and the tuning ranges of VCOs with adjacent center frequencies overlap each other; the input terminals of each VCO are connected to the loop filter through a switch The switch is used to select one of the voltage-controlled oscillators in the voltage-controlled oscillation components to communicate with the loop filter; the output terminals of each voltage-controlled oscillator are connected to the input terminals of the frequency divider. Compared with the prior art, the invention has good phase noise characteristics in a wide tuning range, has a simple structure, and is convenient for miniaturization and integration.

Description

A Phase Locked Loop Frequency Synthesizer

technical field

The invention relates to a frequency synthesizer, in particular to a phase-locked loop frequency synthesizer, and belongs to the field of radio technology.

Background technique

In modern electronic products, frequency synthesis technology, as a key technology, has been widely used in various fields. A frequency synthesizer is a device that uses one or more standard signals to generate a large number of discrete frequency signals through various technical approaches.

There are three implementation methods of frequency synthesizer: direct analog frequency synthesis, indirect frequency synthesis and direct digital frequency synthesis.

Direct analog frequency synthesis techniques. Using one or more different crystal oscillators as a reference signal source to directly generate output signals of many discrete frequencies through frequency multiplication, frequency division, and frequency mixing is called direct frequency synthesis. The signal obtained by this method has the characteristics of high long-term and short-term frequency stability and fast frequency conversion, but it is difficult to debug and spurious suppression.

Phase-locked frequency synthesis technology. Phase-locked frequency synthesizers, also known as indirect synthesizers, appeared in the 1950s. It uses one or several reference frequency sources to generate a large number of harmonics or combined frequencies through harmonic generator mixing and frequency division, and then uses a phase-locked loop (Phase-Locked Loop, PLL) to convert the voltage-controlled oscillator Frequency locked to a certain harmonic or combination of frequencies. The desired frequency output is indirectly generated by a voltage controlled oscillator. The advantage of this method is that since the phase-locked loop is equivalent to a narrow-band tracking filter, it can well select the signal of the required frequency, suppress spurious components, avoid a large number of filters, and is conducive to integration and miniaturization.

Direct digital frequency synthesis technology. Since the 1970s, with the development of digital integrated circuits and microelectronics technology, a new synthesis method - direct digital frequency synthesis (DDS) technology has emerged. It conducts frequency synthesis from the concept of phase, adopts digital sampling storage technology, and has conflicting advantages such as precise phase and frequency resolution, and fast conversion time.

Among them, the phase-locked loop frequency synthesis technology, especially the DDS-PLL combined frequency synthesizer combined with DDS technology is widely used at present. The core of the phase-locked loop frequency synthesizer is the phase-locked loop, and its basic structure is shown in Figure 1. (Voltage Controlled Oscillator, VCO), frequency divider and other components. The frequency tuning range is one of the main indicators of the VCO, which is related to the topological structure of the resonator and the circuit. Generally, the larger the tuning range, the smaller the Q value of the resonator. The Q value of the resonator is also related to the phase noise of the oscillator. The smaller the Q value, the worse the phase noise performance. Therefore, in the wide tuning range PLL frequency synthesis application, a compromise point should be taken between the bandwidth tuning range and the phase noise, and the indicators of the two should be taken into account. In the prior art, a voltage-controlled oscillator is used to cover all frequency ranges, and it is difficult to achieve good phase noise characteristics in the entire bandwidth tuning range.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a phase-locked loop frequency synthesizer with good phase noise characteristics in a wide tuning range.

A phase-locked loop frequency synthesizer, comprising a phase-locked loop composed of a frequency discriminator, a loop filter, a voltage-controlled oscillation component, and a frequency divider connected end to end in sequence; the voltage-controlled oscillation component includes a set of Voltage-controlled oscillators with different center frequencies but the same voltage-controlled gain, and the tuning ranges of voltage-controlled oscillators with adjacent center frequencies overlap each other; the input terminals of each voltage-controlled oscillator are connected to the loop through a switch The filter is connected, and the switch is used to select a certain voltage-controlled oscillator in the voltage-controlled oscillation component to communicate with the loop filter; the output ends of each voltage-controlled oscillator are connected to the input end of the frequency divider.

Preferably, the voltage-controlled oscillator is an LC voltage-controlled oscillator.

Preferably, it also includes a control unit connected to the switching switch and the frequency divider respectively.

Preferably, the frequency divider is a sigma-delta modulator.

Compared with the prior art, the present invention has the following beneficial effects:

The present invention utilizes a plurality of switchable voltage-controlled oscillators with different center frequencies to form a voltage-controlled oscillation component, which can be switched to the best voltage-controlled oscillator according to the working frequency band, thereby ensuring a very wide tuning range while ensuring Excellent phase noise characteristics can be obtained in the whole tuning range;

The invention has a simple structure, avoids problems such as cost increase caused by using a plurality of phase-locked loop frequency synthesis chips to synthesize corresponding frequencies and difficulty of PCB layout, and is beneficial to miniaturization and integration of equipment.

Description of drawings

Fig. 1 is the basic structure schematic diagram of existing PLL frequency synthesizer;

FIG. 2 is a schematic diagram of the basic structure of the phase-locked loop frequency synthesizer of the present invention.

Detailed ways

The technical scheme of the present invention is described in detail below in conjunction with accompanying drawing:

The present invention aims at the problem that a single voltage-controlled oscillator in the prior art cannot obtain better phase noise characteristics in a wide tuning range, and uses a group of parallel-connected voltage-controlled oscillators with different center frequencies to replace the existing technology. A single voltage-controlled oscillator is used, and the best voltage-controlled oscillator is selected according to the operating frequency of the frequency synthesizer to be connected to the phase-locked loop, so as to obtain better phase noise characteristics; when the operating frequency changes, the corresponding ground to switch the connected voltage controlled oscillator. In this way, the entire frequency synthesizer can obtain better phase noise characteristics in the entire tuning range while obtaining an extremely wide tuning range.

A preferred solution of the phase-locked loop frequency synthesizer of the present invention is shown in Figure 1, and its phase-locked loop part includes a frequency and phase detector, a loop filter, a voltage-controlled oscillation component, and a frequency divider, wherein the voltage-controlled oscillation component includes multiple VCOs with different center frequencies, and the tuning ranges of VCOs with adjacent center frequencies overlap each other; each VCO is connected to the loop filter through a switch, and the switch is used for Selecting a certain voltage-controlled oscillator in the voltage-controlled oscillation unit to communicate with the loop filter. Since a plurality of voltage-controlled oscillators will share the frequency and phase detector and the loop filter in the loop, each voltage-controlled oscillator in the voltage-controlled oscillation part should have the same voltage-controlled gain Kv value, like this, each voltage-controlled oscillator Both controlled oscillators work in conjunction with other components in the phase-locked loop. The frequency divider in this embodiment is a fractional frequency divider, which is realized by a Σ-Δ modulator. In order to realize automatic control, as shown in FIG. 2 , the phase-locked loop frequency synthesizer in this embodiment also includes a control unit composed of a microcontroller, which is respectively connected with a switch and a frequency divider. The control unit can select a corresponding voltage-controlled oscillator from the voltage-controlled oscillator to connect to the loop according to the preset corresponding relationship according to the actual working frequency.

Existing voltage-controlled oscillators can be divided into two types based on their circuit structure: loop oscillators and LC oscillators. The ring oscillator has a large tuning range, and the available volume is relatively small, and it is easy to make an integrated circuit, but its phase noise performance is not as good as that of the LC oscillator. After the LC oscillator adopts a MOS varactor with a large ratio Cmax/Cmin, its tuning range can become relatively large and can be used in a wide frequency range; at the same time, its Q value is also relatively high, and its phase noise is also good. Therefore, the present invention preferably uses an LC oscillator.

In order to facilitate the public to understand the technical solution of the present invention more clearly, an application example is used below to illustrate the superiority of the multi-VCO design of the present invention. The wide tuning range designed in this example is 1800MHz to 3600MHz, and the frequency synthesizer is HMC703LP4E. In order to improve its performance, the frequency range is divided into four sections, and four voltage-controlled oscillators are selected respectively, and they are respectively operated in:

(1) 1800MHz to 2050MHz; VCO uses DCRO170230-10

(2) 2040MHz to 2500MHz; VCO uses ROS-2490C+

(3) 2450MHz to 2980MHz; VCO uses ROS-2960-119+

(4) From 2962MHz to 3388MHz, the VCO uses ROS-3600-619+

The selection of voltage-controlled oscillators and the use of their respective frequency bands must consider the working conditions of the entire phase-locked loop. Which frequency band is needed is switched to the corresponding VCO through the control unit, so that the voltage-controlled oscillator and the phase detector work together. It is necessary for each voltage-controlled oscillator to be suitable for the phase-locked loop where it is located, which is mainly reflected in their control sensitivity. When selecting devices, try to choose a voltage-controlled oscillator with Kv close to it, and the frequency band in which they need to work The corresponding voltage should be as same as possible, probably between 0 and 16v.

Comparing the phase noise of the above scheme with that of DCYS160360-5 (oscillating chip from 1800MHz to 3600MHz) alone, it can be seen that when DCYS160360-5 is used alone, the phase noise for different center frequencies at 1000Hz is about -60dBc /Hz. However, with the solution of the present invention, the phase noise of the same offset frequency of 1000 Hz is approximately -80 dBc/Hz. It can be seen that the structure of using multiple VCOs can effectively improve the performance of phase noise in a wide frequency range.

Claims (4)

1. a phase-locked loop frequency synthesizer, comprises the phase-locked loop be made up of end to end phase frequency detector, loop filter, VCO parts, frequency divider successively; It is characterized in that, it is different but have the voltage controlled oscillator of identical voltage controlled gain that described VCO parts comprise a group switching centre frequency, and the tuning range mutual of the adjacent voltage controlled oscillator of centre frequency is overlapping; The input of each voltage controlled oscillator is connected with loop filter by a diverter switch, and described diverter switch is communicated with loop filter for selecting the some voltage controlled oscillators in voltage-controlled oscillating component; The output of each voltage controlled oscillator is all connected with the input of described frequency divider.

2. phase-locked loop frequency synthesizer as claimed in claim 1, it is characterized in that, described voltage controlled oscillator is LC voltage controlled oscillator.

3. phase-locked loop frequency synthesizer as claimed in claim 1, is characterized in that, also comprise the control unit be connected with described diverter switch and frequency divider respectively.

4. phase-locked loop frequency synthesizer as claimed in claim 1, it is characterized in that, described frequency divider is sigma-delta modulator.