Dual-modulus prescaler

Last updated

A dual modulus prescaler is an electronic circuit used in high-frequency synthesizer designs to overcome the problem of generating narrowly spaced frequencies that are nevertheless too high to be passed directly through the feedback loop of the system. The modulus of a prescaler is its frequency divisor. A dual-modulus prescaler has two separate frequency divisors, usually M and M+1.

Contents

The problem

A frequency synthesizer produces an output frequency, fo, which, divided by the modulus, N, is the reference frequency, fr:

The modulus is generally restricted to integer values, as the comparator will match when the waveform is in phase. Typically, the possible frequency multiples will be the channels for which the radio equipment is designed, so fr will usually be equal to the channel spacing. For example, on narrow-band radiotelephones, a channel spacing of 12.5 kHz is typical.

Suppose that the programmable divider, using N, is only able to operate at a maximum clock frequency of 10 MHz, but the output fo is required to be in the hundreds of MHz range. Interposing a fixed prescaler that can operate at this frequency range with a division ratio M of, say, 40 drops the output frequency into the operating range of the programmable divider. However, a factor of 40 has been introduced into the equation, so the output frequency is now:

If fr remains at 12.5 kHz, only every 40th channel can be obtained. Alternatively, if fr is reduced by a factor of 40 to compensate; it becomes 312.5 Hz, which is much too low to give good filtering and lock performance characteristics. It also means that programming the divider becomes more complex, as the modulus needs to be verified so that only those that give true channels are used, not every 1/40th of a channel that is available.

The solution

The solution is the dual modulus prescaler. The main divider is split into two parts, the main part N and an additional divider A, which is strictly less than N. Both dividers are clocked from the output of the dual-modulus prescaler, but only the output of the N divider is fed back to the comparator. Initially, the prescaler is set to divide by M + 1. Both N and A count down until A reaches zero, at which point the prescaler is switched to a division ratio of M. At this point; the divider N has completed A counts. Counting continues until N reaches zero, which is an additional N - A counts. At this point, the cycle repeats.

So while we still have a factor of M being multiplied by N, we can add an additional count, A, which effectively gives us a divider with a fractional part. Only the prescaler needs to be constructed from high-speed parts, and the reference frequency can remain equal to the desired output frequency spacing.

The diagram below shows the elements and arrangement of a frequency synthesizer with a dual-modulus prescaler. (Compare with the diagram on the main synthesizer page).

Dual modulus prescaler frequency synthesiser.svg

One can compute A and N from the formulae:

where V is the combined division ratio V = MN+A. For this to work properly, A must be strictly less than M, as well as less than or equal to N. These restrictions on values of A imply that you can't get every division ratio V. If V falls below M(M - 1), some channels will be missing.

Example

Dual modulus prescaler waveform with a 10 microsecond scale. DualModulusPrescalerWaveform.png
Dual modulus prescaler waveform with a 10 microsecond scale.
Dual modulus prescaler waveform with a 200 nanosecond scale. DualModulusPrescalerWaveform-zoom.png
Dual modulus prescaler waveform with a 200 nanosecond scale.

Today, most dual-modulus prescalers exist inside PLL chips, making it impossible to probe actual signals during operation. The first dual-modulus prescalers were discrete ECL devices, separate from the PLL chips. Here is an example of a dual-modulus prescaler in use. This circuit happens to use a Motorola MC145158 with a Fujitsu MB-501 dual-modulus prescaler operating in the 128/129 mode. The PLL is locked at 917.94 MHz (fo) with a channel spacing frequency of 30 kHz (fr). The total integer count, therefore, is 30,598. Dividing this by 128 (M) yields a quotient of 239 with a remainder of 6, N, and A, respectively. The result of this frequency choice is that the prescaler spends most of its time counting at 128 and just a brief period at 129.

This is shown by the upper purple trace, the modulus control, A, counter output. These two screen captures differ only in the horizontal scale. The lower, yellow trace is the N counter output whose frequency corresponds to the channel spacing frequency of 30 kHz. The green trace is the output from the dual-modulus prescaler, which happens to correspond to 7.1714 MHz in the case that the prescaler is at 128 and 7.1158 when it is at 129. It is plainly obvious that the modulus control is low for precisely 6 cycles of the prescaler output. What is not obvious is the fact that the frequency changes by less than one percent between the two states of the modulus control. There will be cases where A = 0, resulting in the dual-modulus prescaler counting only by 128. This would happen at 906.24, 910.08, 913.92, 917.76, 921.60 MHz, and so on.

See also

Related Research Articles

<span class="mw-page-title-main">Bandwidth (signal processing)</span> Range of usable frequencies

Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in unit of hertz.

<span class="mw-page-title-main">Frequency modulation</span> Encoding of information in a carrier wave by varying the instantaneous frequency of the wave

Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing.

<span class="mw-page-title-main">PAL</span> Colour encoding system for analogue television

Phase Alternating Line (PAL) is a colour encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields per second, and associated with CCIR analogue broadcast television systems B, D, G, H, I or K. The articles on analog broadcast television systems further describe frame rates, image resolution, and audio modulation.

<span class="mw-page-title-main">Analog-to-digital converter</span> System that converts an analog signal into a digital signal

In electronics, an analog-to-digital converter is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide an isolated measurement such as an electronic device that converts an analog input voltage or current to a digital number representing the magnitude of the voltage or current. Typically the digital output is a two's complement binary number that is proportional to the input, but there are other possibilities.

Noise figure (NF) and noise factor (F) are figures of merit that indicate degradation of the signal-to-noise ratio (SNR) that is caused by components in a signal chain. These figures of merit are used to evaluate the performance of an amplifier or a radio receiver, with lower values indicating better performance.

A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and output frequencies the same, thus a phase-locked loop can also track an input frequency. And by incorporating a frequency divider, a PLL can generate a stable frequency that is a multiple of the input frequency.

<span class="mw-page-title-main">Speed of sound</span> Speed of sound wave through elastic medium

The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. More simply, the speed of sound is how fast vibrations travel. At 20 °C (68 °F), the speed of sound in air is about 343 m/s, or 1 km in 2.91 s or one mile in 4.69 s. It depends strongly on temperature as well as the medium through which a sound wave is propagating.

<span class="mw-page-title-main">Low-noise block downconverter</span> Receiving device on satellite dishes

A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver inside the building. Also called a low-noise block, low-noise converter (LNC), or even low-noise downconverter (LND), the device is sometimes inaccurately called a low-noise amplifier (LNA).

A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned over some range. It is a necessary component in any tunable radio transmitter and in receivers that work by the superheterodyne principle. The oscillator controls the frequency to which the apparatus is tuned.

<span class="mw-page-title-main">555 timer IC</span> Integrated circuit used for timer applications

The 555 timer IC is an integrated circuit used in a variety of timer, delay, pulse generation, and oscillator applications. It is one of the most popular timing ICs due to its flexibility and price. Derivatives provide two or four timing circuits in one package. The design was first marketed in 1972 by Signetics and used bipolar junction transistors. Since then, numerous companies have made the original timers and later similar low-power CMOS timers. In 2017, it was said that over a billion 555 timers are produced annually by some estimates, and that the design was "probably the most popular integrated circuit ever made".

<span class="mw-page-title-main">Voltage-controlled oscillator</span> Oscillator with frequency controlled by a voltage input

A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop. VCOs are used in synthesizers to generate a waveform whose pitch can be adjusted by a voltage determined by a musical keyboard or other input.

In electronic instrumentation and signal processing, a time-to-digital converter (TDC) is a device for recognizing events and providing a digital representation of the time they occurred. For example, a TDC might output the time of arrival for each incoming pulse. Some applications wish to measure the time interval between two events rather than some notion of an absolute time.

<span class="mw-page-title-main">Direct digital synthesis</span> Method for creating waveforms

Direct digital synthesis (DDS) is a method employed by frequency synthesizers used for creating arbitrary waveforms from a single, fixed-frequency reference clock. DDS is used in applications such as signal generation, local oscillators in communication systems, function generators, mixers, modulators, sound synthesizers and as part of a digital phase-locked loop.

A prescaler is an electronic counting circuit used to reduce a high frequency electrical signal to a lower frequency by integer division. The prescaler takes the basic timer clock frequency and divides it by some value before feeding it to the timer, according to how the prescaler register(s) are configured. The prescaler values, referred to as prescales, that may be configured might be limited to a few fixed values, or they may be any integer value from 1 to 2^P, where P is the number of prescaler bits.

A frequency divider, also called a clock divider or scaler or prescaler, is a circuit that takes an input signal of a frequency, , and generates an output signal of a frequency:

MUSE, commercially known as Hi-Vision was a Japanese analog high-definition television system, with design efforts going back to 1979.

In GSM cellular networks, an absolute radio-frequency channel number (ARFCN) is a code that specifies a pair of physical radio carriers used for transmission and reception in a land mobile radio system, one for the uplink signal and one for the downlink signal. ARFCNs for GSM are defined in Specification 45.005 Section 2. There are also other variants of the ARFCN numbering scheme that are in use for other systems that are not GSM. One such example is the TETRA system that has 25 kHz channel spacing and uses different base frequencies for numbering.

A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in devices such as radio receivers, televisions, mobile telephones, radiotelephones, walkie-talkies, CB radios, cable television converter boxes, satellite receivers, and GPS systems. A frequency synthesizer may use the techniques of frequency multiplication, frequency division, direct digital synthesis, frequency mixing, and phase-locked loops to generate its frequencies. The stability and accuracy of the frequency synthesizer's output are related to the stability and accuracy of its reference frequency input. Consequently, synthesizers use stable and accurate reference frequencies, such as those provided by a crystal oscillator.

A PLL multibit or multibit PLL is a phase-locked loop (PLL) which achieves improved performance compared to a unibit PLL by using more bits. Unibit PLLs use only the most significant bit (MSB) of each counter's output bus to measure the phase, while multibit PLLs use more bits. PLLs are an essential component in telecommunications.

<span class="mw-page-title-main">Charge-pump phase-locked loop</span>

Charge-pump phase-locked loop (CP-PLL) is a modification of phase-locked loops with phase-frequency detectors and square waveform signals. A CP-PLL allows for a quick lock of the phase of the incoming signal, achieving low steady state phase error.