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Frequency synthesiser

From Academic Kids

A frequency synthesiser is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc.

Prior to widespread use of synthesizers, radio and television receivers relied on manual tuning of a local oscillator. Some might remember the classic turret tuner commonly used in television receivers prior to the 1980s. Variations in temperature and aging of components caused frequency drift. Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary. Since transmitter frequencies are well known and very stable, an accurate means of generating fixed, stable frequencies would solve the problem.

A simple and effective solutions employs the use of many stable resonators or oscillators for each tuning frequency. Quartz crystals offer good stability and are often used for this purpose. This "brute force" technique is practical when only a handful of frequencies are required, but quickly becomes costly and impractical in many applications. For example, the FM radio band in many contries supports 100 individual frequencies from about 88 MHz to 108 MHz. Cable television can support even more frequencies or channels over a much wider band. A large number of crystals increases cost and requires greater space.

Many coherent and incoherent techniques have been devised over the years. Some approaches include phase locked loops, double mix, triple mix, harmonic, double mix divide, and direct digital synthesis (DDS). The choice of approach depends on several factors, such as cost, complexity, frequency step size, switching rate, phase noise, and spurious output.

Coherent techniques generate frequencies derived from a single, stable master oscillator. In most applications, crystal oscillator are common, but other resonators and frequency sources can be used. Incoherent techniques derive frequencies from a set of several stable oscillators, typically through frequency multiplication, division, and summing/differencing (mixing). The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost.

Synthesizers used in commercial radio receivers are largely based on simple division phase-locked loops or PLLs. Many types of frequency synthesiser are available as integrated circuits, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination.

Contents

Principle of PLL synthesizers

A phase locked loop does for frequency what the operational amplifier does for voltage. It compares the frequencies of two signals and produces an error signal which is proportional to the difference between the input frequencies. The error signal is used to drive a voltage-controlled oscillator (VCO) which creates an output frequency. The output frequency is fed through a frequency divider back to the input of the system, producing a negative feedback loop. If the output frequency drifts, the error signal will increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input. This input is called the reference and is derived from a crystal oscillator, which is very stable in frequency. The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesiser.

image:Freq_Synth.jpg

The key to the ability of a frequency synthesiser to generate multiple frequencies is the divider placed between the output and the feedback input. This is usually in the form of a digital counter, with the output signal acting as a clock signal. The counter is preset to some initial count value, and counts down at each cycle of the clock signal. When it reaches zero, the counter output changes state and the count value is reloaded. This circuit is straightforward to implement using J-K or D-type flip-flops, and because it is digital in nature, is very easy to interface to other digital components or a microprocessor. This allows the frequency output by the synthesiser to be easily controlled by a digital system.

Example

Suppose the reference signal is 100 kHz, and the divider can be preset to any value between 1 and 100. The error signal produced by the comparator will only be zero when the output of the divider is also 100 kHz. For this to be the case, the VCO must run at a frequency which is 100 kHz x the divider count value. Thus it will produce an output of 100 kHz for a count of 1, 200 kHz for a count of 2, 1 MHz for a count of 10 and so on. Note that only whole multiples of the reference frequency can be obtained.

Practical considerations

In practice this type of frequency synthesiser cannot operate over a very wide range of frequencies, because the comparator will have a limited bandwidth and may suffer from aliasing problems. This would lead to false locking situations, or an inability to lock at all. In addition, it is hard to make a high frequency VCO that operates over a very wide range. This is due to several factors, but the primary restriction is the limited capacitance range of varactor diodes. However, in most systems where a synthesiser is used, we are not after a huge range, but rather a finite number over some defined range, such as a number of radio channels in a specific band.

For much radio use, the frequencies desired may be quite high - higher than may be directly input to the digital counter in many cases. To overcome this, the counter could be contructed using high-speed logic such as ECL, or more commonly, using a fast initial division stage called a prescaler which reduces the frequency to a manageable level. Since the prescaler is part of the overall division ratio, a fixed prescaler can cause problems designing a system with narrow channel spacings - typically encountered in radio applications. This can be overcome using a dual-modulus prescaler.

Further practical aspects concern the amount of time the system can switch from channel to channel, time to lock when first switched on, and how much noise there is in the output. All of these are a funtion of the loop filter of the system, which is a low-pass filter placed between the output of the frequency comparator and the input of the VCO. Usually the output of a frequency comparator is in the form of short error pulses, but the input of the VCO must be a smooth noise-free DC voltage. (Any noise on this signal naturally causes frequency modulation of the VCO.). Heavy filtering will make the VCO slow to respond to changes, causing drift and slow response time, but light filtering will produce noise and other problems with harmonics. Thus the design of the filter is critical to the performance of the system and in fact the main area that a designer will concentrate on when building a synthesiser system.

See also

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