GB2338128A

GB2338128A - Voltage controlled oscillators

Info

Publication number: GB2338128A
Application number: GB9811952A
Authority: GB
Inventors: Simon Charles Jones
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1998-06-03
Filing date: 1998-06-03
Publication date: 1999-12-08
Anticipated expiration: 2018-06-03
Also published as: GB2338128B; GB9811952D0

Abstract

A voltage controlled oscillator with a flat modulation characteristic i.e frequency deviation versus output frequency characteristic across a wide frequency band is obtained by incorporating in the control circuit of the oscillator including a varactor D3 receiving a steering or control signal Vsteer and a varactor D2 receiving a fixed dc bias and a modulation signal a further varactor D1 in parallel with D2 and receiving a variable bias voltage and a modulation signal which may differ in amplitude to that applied to varactor D2. Varactor D1 with a variable applied bias has a characteristic which is complementary to that of varactor D2 with a fixed dc bias and the combination results in a flat modulation characteristic. The voltage controlled oscillator may form part of a transmitter in a communications system e.g. a mobile radio device.

Description

2338128 MCUIT ARRANGEMENT

Field of the Invention

This invention generally relates to a circuit arrangement for modulating a control voltage for a voltage controlled oscillator (VCO) which results in a substantially flat modulation characteristic.

Background of the Invention

A voltage controlled oscillator (VCO) is a circuit which is designed to produce an output signal at a frequency determined by the level of a voltage which is input to the oscillator. The output frequency of the oscillator can be varied by varying the magnitude of the voltage which is input to the oscillator. Typically, a voltage controlled oscillator may be used to provide a signal for transmission by the transmitter of a communications system. Such a transmitter may be part of, for example, a portable or a mobile radio.

Prior art designs of VCO's often meet requirements for phase noise performance and transmitter modulation linearity only with great difficulty and expense, often requiring relatively high control voltages to do this.

A typical WO design is described in the background of the same applicant's related GB patent application number 9810524.0, filed May 16, 1998, (CM00584P). The conventional method of deviating a synthesised transmitter is to modulate the control voltage on a varactor controlling the W0. The main problem with this method is that the capacitance to voltage (CJV) ratio of the diode varies with control voltage. The C[V ratio is greater at lower control voltages. This means that as an operating frequency increases along with the control voltage, the frequency deviation caused by modulation of the control voltage will become progressively less. This variation can be compensated by using a variable attenuator in series with the modulating audio. However, the case with the attenuator may be that the minimum to maximum attenuation ratio is only approximately 5:1 and sometimes this is not sufficient especially on a wide bandwidth radio. This can make it extremely difficult to ensure that deviation can be adjusted at all frequencies across the operating band. One solution to this problem is to have a first 'modulation' varactor with a constant control voltage applied to it. Thus, a second varactor being unmodulated and used to control the carrier frequency can be loosely coupled into the resonant circuit (tank) of the ocillator. However, even though the C/V ratio is kept constant for the 'modulation' varactor, the frequency deviation using this method also varies across the band. This is because at high frequencies the 'modulation' varactor is adding a far greater proportion of the total tank capacitance than at lower frequencies and thus causes far greater deviation at the top of the band than at the bottom. In fact, the change can be almost identical to the first deviation method but inverse to it.

FIG. 1 depicts a known circuit configuration used in existing radios today. Referring to FIG. 1, varactor diode D1 is driven from a charge pump and controls the output frequency. Modulation is applied to varactor diode D2 which has a fixed dc bias. However, the voltage applied to varactor DI increases as the output frequency increases, with the result that the percentage contribution (of capacitance) from D2 increases with frequency. As a result, the peak frequency deviation increases with frequency, shown in FIG. 2.

In order to obtain a flat modulation characteristic, the applied modulation voltage is routed through a variable attenuator under control of the microprocessor and adjusted accordingly. The appropriate values for the attenuator are determined at the tuning stage during manufacture and stored in a look-up table in the radio's memory. These values are used by the radio's microprocessor, sometimes with iteration between stored values, depending on the frequency of the selected channel. However, this method is time consuming and expensive since it requires factory time and test systems for tuning of the radio.

Thus, there is still a desire to develop a circuit that automatically provides a substantially flat modulation output to stabilise frequency deviation across a frequency band for a W0.

is Summary of the Invention

According to the present invention, a circuit arrangement is provided for a radio communications system comprising a first varactor having a variable bias and a modulating signal input and a second varactor having a fixed bias and a modulating signal input connected in parallel to the first varactor resulting in a substantially flat modulation output for the circuit arrangement.

According to an embodiment of the present invention a circuit arrangement is provided for a radio communications system comprising a first varactor having a variable bias and a modulating signal input; and a second varactor having a fixed bias and a modulating signal input connected in parallel to the first varactor so that the output of the first varactor is substantially compensated for by the second varactor.

Brief Descrij2tion of Drawings FIG. 1 is a block diagram of an embodiment of the prior art for the present invention.

FIG. 2 is a peak deviation chart for the prior art of FIG. 1.

FIG. 3 is a voltagelcapacitance chart for a typical varactor.

FIG. 4 is a block diagram of a circuit arrangement for the present invention.

FIG. 5 shows a resultant frequency deviation chart for the circuit of FIG. 4.

Detailed Description of the Drawings

FIG. 4 shows a circuit arrangement for a radio communications system having a first varactor D1 having a variable bias and a modulating signal input and a second varactor D2 having a fixed bias and a modulating signal input connected in parallel to the first varactor D1 resulting in a substantially flat modulation output for the circuit arrangement.

In FIG. 4, varactor D1 has the inherent non-linear voltage/capacitance characteristic of any typical varactor, as illustrated in FIG. 3. Consequently it may be seen that, for a dc bias voltage of Vx, a modulation signal, amplitude Vm, applied to D1 only, would cause a varation in capacitance of delta x. For a dc bias voltage of Vy (Vy > Vx), the same modulation signal, amplitude Vm, will cause a variation in capacitance of delta y (delta y < delta x).

The peak frequency deviation resulting from the modulation signal will decrease with output frequency, in a manner illustrated in FIG. 5 curve b.

The dc bias voltage applied to D1 is under control of a microprocessor and is chosen such that the voltage applied to varactor D3, under control of the synthesiser loop, remains approximately constant.

Thus, the first varactor D1 having a variable bias and a modulating signal input connected in parallel to the second varactor D2 having a fixed bias and a modulating signal results in a circuit arrangement with an output frequency deviation curve c as shown in FIG. 5 whereby the output of the first varactor DI is substantially compensated for by the second varactor D2.

The circuit arrangement as described in FIG. 4 includes the modulating signal inputs to the first and second varactors having different amplitudes.

The present invention is based around the principle of applying the modulation signal to both diodes D1 and D2. By adjusting the relative amplitudes of the signals applied to each diode and the fixed bias applied to D2, a reasonably flat deviation response can be obtained, independent of the output frequency, as shown in FIG. 5 curve c.

A significant advantage of the present invention is that the need for tuning modulation in the factory is drastically reduced or eliminated.

The present invention includes modulating DI and D2 as shown in FIG. 4. The present invention combines two methods of frequency deviation. If both varactors D1 and D2 are modulated and balanced to give an equal and opposite effect across the band the net result is a much flatter response. For example, a deviation variation across a 20Mhz band can come down from approximately 3:1 to 1:1 for a constant channel bandwidth, i.e., only a single value of deviation is required. If the channel spacing varies, for example 12.5 kHz and 25 kHz, the variation in deviation reduces from 6:1 to 2A. This advantageously puts the variation well within the dynamic range of various attenuators used in radio communication systems rather than right on their limits.

Thus, the present invention provides a circuit arrangement where the output is a substantially flat modulation output effectively reducing tuning efforts in a radio communications system.

Claims

Claims:

1. A circuit arrangement for a radio communications system comprising:

a first varactor having a variable bias and a modulating signal input; and a second varactor having a fixed bias and a modulating signal input connected in parallel to the first varactor resulting in a substantially flat modulation output for the circuit arrangement.

2. A circuit arrangement for a radio communications system comprising:

is a first varactor having a variable bias and a modulating signal input; and a second varactor having a fixed bias and a modulating signal input connected in parallel to the first varactor so that the output of the first varactor is substantially compensated for by the second 20 varactor.

3. The circuit arrangement of any of the preceding claims wherein the modulating signal inputs to the first and second varactors have different amplitudes.