Table of contents

• Introduction
• Scientific innovation. Goals and objectives
• Review of associated research
  • In the university
  • In Ukraine
  • In the world
• Body
  • Panoramic radio frequency receivers
  • PLL frequency synthesizers
  • Design and calculation of a synthesizer model
  • On PLL synthesizer modelling
• Conclusion

Introduction

At the present time, the radio is one of the dominating communication media. Such situation is caused by a number of factors, among them are the high bandwidth, flexibility, cheap maintenance costs and relative simplicity of radio channel routing. However, there is also a problem of high radio frequency range commitment, and, as a result, significant interference issues of a quite unpredictable kind. A possibility of switching to other, 'clean' channels, could be the solution to this problem.

Furthermore, there are other purpose radio systems, that utilize the same frequency range as the communication ones. Providing solutions for their mutual functioning is one of the most important tasks for today's communications engineering. There is also a problem of detecting any spurious radiation that can cause noise and interference, and in that case, it is necessary to have an estimation of spectra in a wide frequency range.

The requirements for radio transmission quality become more and more strict every day. In such conditions it is necessary to provide the lowest noise level possible, without degradation of other features.

Low-noise frequency synthesizers are used in main and zone radio relay broadband lines (TV, multichannel telephony), panoramic receivers and spectrum analyzers, measurement devices, as well as any other systems that require fast and accurate switching in a wide frequency range. [2, p.191]

The cardinal problem of frequency synthesizer design and construction is the inevitable output noise rise when widening the frequency range. Existing solutions include increasing Q of the VCO, reference oscillator, precise PLL calculation, or using multiple VCO's for different frequency sub-ranges.[4].

Scientific innovation

• design of high precision frequency synthesizer based on general purpose elements
• research of remnant noise issues and developing the ways to eliminate them
• developing of optimal computer modelling methods

Intended practical result: working prototype of programmable PLL frequency synthesizer

Goals and objectives

The goal of the research is widening the PLL synthesizers frequency range, that will make possible to employ the sequential mode of functioning for radio communications, diagnostics and control systems.

The objectives are:

• Research the existing solutions and analyze them for the possibility of improvement
• Design an optimal PLL frequency synthesizer structure
• Calculate the synthesizer element parameters
• Build a computer model based on the designed structure and its parameters. Run the simulation and evaluate the results.
• Desing the synthesizer electrical circuit and PCB
• Build the working prototype and proceed with experimental research

Review of associated research

In the university

The research on PLL systems, as well as the other fundamental communications issues is led by professor Alexander G. Vorontsov

Searching DonNTU masters site yielded only one reasonable result:
"Development of signal spectral evaluation methods for telecommunication systems" by Oleg V. Khmelevoy.

In Ukraine

Most of the research concerning PLL synthesizers is concentrated around the FM radio and GSM frequency ranges. There is no apparent interest in or any articles on UHF range.

In the world

A number of well-known researchers work in this area, among them is Michael Perrott of MIT. Moreover, there are numerous publications concerning the topic in question, but most of them are of commercial/advertising kind, and provide little to no useful scientific information.

• Enhance CMOS Charge Pumps And Phase-Frequency Detectors — several ways of improving the phase-frequency detector are proposed in this article.
• 7 GHz Prescaler — an example of how to program a PLL synthesizer chip to make a UHF measuring prescaler

Body

Panoramic receivers.

Panoramic devices (such as receivers and spectrum analysers) allow to receive all the radiation in a specific frequency range, or to analyse the spectrum of any radiation. Like the panoramic receiving, spectrum analysis is based on identification of difference between carrier frequency and spectral components frequencies of received signal.

Devices designed for monitoring a large number of radiation sources in a wide frequency range, are called panoramic receivers. Devices for analysing the spectra of radiation or any other processes, are called spectrum analysers.

There are two modes of frequency analysis: parallel and sequential. There are also a number of their combinations that unite the advantages of both while partially evading their drawbacks.

The parallel frequency analysis is based on simultaneous detection of all the spectral components in the specified frequency range called swath. It is performed using a large number of filters with biased frequency bands. All the filters are sumultaneously influenced by the received signal or signals.

Fig.1. Parallel panoramic receiver structure

The means of operation for parallel panoramic receiver are the following: at the input, the signal passes the broadband preselector (bandwidth of the preselector is equal to swath). This part of the circuit is called broadband section. It amplifies the analysed signal to the level required by the following analyser elements. In the mixer, the spectrum is down converted to an intermediate frequency without losing component amplitude ratios and frequency distribution. Thus, the shape of analysed spectrum remains the same.

Fig.2. Spectrum down conversion

Here it is necessary to take into consideration that mixer properties and mode of operation should not cause interference within analysed frequency band.

A set of narrowband filters is used to analyse the downconverted spectrum. Their resonant frequencies are evenly distributed within analysed frequency range. Every filter responds to the frequency component that is within its band. A conclusion can be made on the analysed spectrum structure by inspecting the number and distribution of excited filters. In this case, the precision is limited by the bandwidth of every filter. Therefore, to increase precision given the same frequency range, it is necessary to increase the quantity of filters, which becomes the main drawback for the parallel mode.

Another type of panoramic receiver is sequential. It is based on sequential detection of frequency components in a specified swath. This type of panoramic devices became quite widespread due to its simplicity. It is only necessary to have one narrowband filter to build such device.

There are sequential receivers with resonator tuning:

Fig.3. Structure of panoramic receiver with resonator tuning

…and with spectrum shifting:

Fig.4. Structure of panoramic receiver with spectrum shifting.

Sequential analysers with resonator tuning are usually quite difficult to implement in circuit. While simple resonators do not provide the required resolution, more precise and complicated systems are difficult to tune within wide range, because their gain frequency characteristic becomes uneven. That's why resonator tuning is used only when it is impossible to implement spectrum shifting, but frequency resolution requirements can be lowered.

Therefore, panoramic devices with moderate bandwidth usually employ shifted spectrum frequency analysis.

To shift the spectrum along the frequency axis, the output from the broadband section (which is, in fact, the analysed spectrum) and the voltage from linear frequency-modulated oscillator are fed to a mixer. Thus, the frequency components of the analysed spectrum, are sequentially transformed into the resonant frequency of the filter, and the spectrum shifts relatively to this frequency, according to the law of oscillator frequency variation. It is important that the filter gain and frequency response are matched to this law.

Oscillator frequency can be changed manually or automatically. Manual tuning is used for thorough inspection of radiation characteristics in relatively narrow swath.

Sequential panoramic devices suffer from transients caused by switching of the filter or variation of frequency. These transients cause limitations to switching and, as a result, analysis speed. Also, sequential analysis of rapidly variating or very brief processes is possible only provided their periodicity is proportionate (or even higher) to the periodicity of sweep scanning.

Let's examine shifted spectrum receiver structure more thoroughly. The sweep generator and frequency-modulated oscillator together are, in fact, a chirp generator. The idea of using chirp signal is based on the following. Multichannel processing is needed to detect a signal with unknown frequency bias. This means that every channel should have an optimal detector in it (a matched filter etc.), that detects specific bias. On the other hand, using chirp signal allows for only one channel, since there will always be a response on the output of matched filter, with some time delay and amplitude loss.

A chirp signal is a radio pulse with frequency linearily increasing or decreasing from its beginning to the end. The momentary frequency of such frequency is calculated using the following expression: ω(t)=ω0+kt, where ω0 is the starting signal frequency, and k — linear time variation quotient.

The field of application for chirp signals is rather wide and includes:

1. Target detection and evaluation

Today's radars widely use chirp signals for target detection, since using them do not require great number of Doppler channels. This is due to the comb-shaped signal ambiguity function, that greatly mitigates even significant Doppler distortion influence on output signal amplitude.

2. Space object recognition

3. Radioaltimeters

4. Physical measurements

5. Communications

Due to the great variety of requirements to chirp generator parameters and precision, a number of different devices are used for their implementation, like passive ones with dispersive delay lines, controlled autogenerators, or high precision synthesizers with multiple control systems.

PLL frequency synthesizers

PLL frequencey synthesizers are widely used in all areas of today's radio technics. Typical PLL synthesizers can generate a sine signal with several GHz frequency, sampling rate over 10MHz, synchronous to the reference frequency, and with equal phase stability. Morever, PLL synthesizers have a high spectral purity that is required for high resolution radar equipment. The main drawbacks of such synthesizers are the limited switching speed, limited frequency range, and the complexity of construction and tuning.

The VCO, is a determinant element of PLL synthesizer. It should work in the whole frequency range, while producing low noise and spurs. To keep the loop stable, the VCO must have a low frequency drift. Designing such an oscillator is rather complex, particularly for frequencies over 100 MHz, even with specialized VCO IC's available. These oscillators also require thorough and precise tuning.

Another pitfall in PLL synthesizer design is the controversy of requirements to reference frequency value. Lowering this value can help increase the output sample rate, while increasing settling time. With the development of electronics this problem, along with many others, was successfuly solved, and today PLL synthesizers are the most widespread synthesizer type, used in television, radio and computers. However, the complexity of calculation and tuning still significantly hampers the evolution of such systems.

These requirements are essential when designing a PLL synthesizer:

1. Frequency range fmax−fmin
2. Frequency resolution Δf
3. Output power Pout
4. Relative output frequency instability Δf/f
5. Output noise level relative to noise fluctuation frequency Fш: Where Pш(Fш) is the noise power in 1 Hz bandwith with frequency offset Fn
6. Reference spur power
7. Switching time τ

Design and calculation of a synthesizer model

Model structure and purpose

An important stage of synthesizer design is modelling its inner processes. In fact, it is an early experiment stage, and its results can significantly influence further research and design. In this work, Elanix SystemVue software is used for modelling.

Fig.5. Synthesizer structure

Calculation of loop filter element parameters

In this work, loop filter element parameters are calculated according to National Semiconductor Application Note 1001 recommendations.

Table 1. Calculation of loop filter element parameters

Model set up using the calculated values:

Fig. 6. Charge pump with calculated element parameters

Fig. 7. LPF with calculated element parameters

On PLL synthesizer modelling

Most of the simulation software have the possibility of visually building the model using the basic blocks (oscillators, filters, etc.) However, these blocks are usually assumed ideal, thus making it impossible to simulate some particular device. Moreover, calculus of approximations usage, as well as time window limitation, are also the sources of inaccuracy.

The main problem in output spectrum analysis is so called spectrum leakage [6]. It is caused by an assumption, when calculating FFT, that the sample sequence is periodically repeating back and forth in time. Additionally, quick frequency alterations caused by VCO switching transient, significantly influence the output spectrum.

Typically, a weighting function is used to justify these inaccuracies.

A moving average operator can be used to additionally mitigate the influence of aperiodical components.

Another way of avoiding spurious components in output signal is using a limited time window to minimize transient influence.

Fig. 8 illustrates the improvements that can be achieved using these methods

Fig. 8. Output spectrum using various modelling methods.

Conclusion

Having researched the basic features of PLL synthesizer, a conclusion can be made that this type of synthesizer mostly meets the requirements to chirp generators. Today's integrated PLL solutions have the possibility of program control. Thus, by setting the required parameters and varying them according to the needed law, it is possible to get an output signal with a corresponding law of variation.

The design of loop filter is by far the most important stage of development, since it determines the speed-to-noise ratio of the system.

A number of requirements have to be met to obtain the correct results with computer modelling. They include using an optimal time window, exclusion of aperiodical component influence, averaging the results. However, it is prudent to take the features of particular model, such as transient duration and loop filter bandwidth, into consideration.