GB2095491A

GB2095491A - Constant amplitude sawtooth generator

Info

Publication number: GB2095491A
Application number: GB8106464A
Authority: GB
Original assignee: CROMA RESEARCH Ltd
Current assignee: CROMA RESEARCH Ltd
Priority date: 1981-03-02
Filing date: 1981-03-02
Publication date: 1982-09-29

Abstract

The present invention relates to a constant amplitude generator for generating a field of vertical scanning current in a deflection yoke. The sawtooth generator comprises a variable frequency oscillator (10), a counter (12) acting as a fixed ratio divider, and a phase comparator (14) connected to form a phase locked loop operative to lock the frequency of the variable oscillator (10) onto a multiple of the frequency of a received synchronisation signal, and a circuit (16) Fig. 1 for producing a sawtooth voltage signal, the gradient of the ramp of which is dependent upon the frequency of the variable oscillator (10) and the frequency of which is dependent upon the frequency of the divider output signal. In the embodiment of Fig. 2, a capacitor (24) is charged stepwise (18, 22) at the oscillator frequency, and discharged (20, 26) under the control of counter (12). <IMAGE>

Description

SPECIFICATION Constant amplitude sawtooth generator The present invention relates to a constant amplitude generator for generating a field or vertical scanning current in a deflection yoke.

In a television display or pick-up device a sawtooth generator is often required to generate the vertical scanning current in the deflection yoke. A common way of achieving this is to charge a capacitor through a constant current source, discharging it at the time when the field fly-back is required. This gives a linear sawtooth, but if a different field frequency is required the sawtooth amplitude produced by the generator will also be different.

At present different countries of the world fall under two standards, namely those using 50 Hz field rate and those using 60 Hz field rate. Any equipment which is designed to work on either standard and uses a conventional sawtooth generator will require a switch to alter the resulting sawtooth amplitude depending on which standard is selected. The present invention seeks to provide a sawtooth generator which may be used at either standard without the need for switching.

In accordance with a first aspect of the present invention, there is provided a sawtooth generator for use in a raster scanning circuit comprising a variable frequency oscillator, a fixed ratio divider and a phase comparator connected to form a phase locked loop operative to lock the frequency of the variable oscillator onto a multiple of the frequency of a received synchronisation signal, and a circuit for producing a sawtooth voltage signal, the gradient of the ramp of which is dependent upon the frequency of the variable oscillator and the frequency of which is dependent upon the frequency of the divider output signal.

Conveniently, the fixed ratio divider is a counter and the circuit for producing a sawtooth voltage is a digital analogue converter connected to the output of the counter.

In an alternative embodiment of the invention, the output signals of the variable frequency oscillator are connected to gate predetermined charges to a charge storage device which is periodically discharged by the output of the fixed ratio divider, According to a second aspect of the invention, a sawtooth generator for producing a sawtooth wave form having the same frequency as an input synchronisation signal and a constant amplitude includes a charge storage device connected to be charged at a rate determined by the frequency of the input synchronisation signal and to be periodically discharged at the frequency of the input synchronisation signal.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a block circuit diagram of the first embodiment of the invention and, Figure 2 shows a block circuit diagram of a second embodiment of the invention.

In Fig. 1, a variable oscillator 10, a binary counter 1 2 and a phase comparator 14 are connected to form a phase locked loop locking the frequency of the variable oscillator 10 on to a multiple of the field synchronisation or locking pulses applied to one of the two inputs of the phase comparator 14. The binary outputs from the counter 1 2 are fed to a digital analogue converer 1 6 which produces a sawtooth output. The amplitude of the output is related only to the maximum binary count of the binary counter 1 2 and is therefore independent of the frequency of which the system is running, the frequency being determined by the field locking pulses.

The resulting sawtooth output from the generator will have high frequency transients present which are due to small propagation delay differencies in the counter and due to any inaccuracy in the digitalanalogue conversion, subsequent filtering is employed to remove these transients In the circuit shown in Fig. 2, variable oscillator 10, counter 1 2 and phase comparator 1 4 once again form a phase locked loop in which the variable oscillator 10 is locked onto a multiple of the field synchronisation pulse frequency. The output pulses of the variable oscillator 10 are applied to a monostable circuit 1 8 which in turn serves to control a gated current source 22 to apply discrete charges to a storage capacitor 24.When the counter 1 2 achieves a count of zero, a decoder 20 switches on a transistor 26 connected in parallel with the capacitor 24 to discharge the capacitor 24. The output signal is derived from the capacitor 24 by way of a buffer amplifier 28 In this circuit, regardless of the frequency of the variable oscillator 10, a predetermined number of discrete charges are applied to the capacitor 24 during each ramp and therefore the amplitude of the sawtooth wave form is once again independent of the frequency of the field synchronisation pulses. The resulting output will be a sawtooth made up of a number of small steps and again filtering is required in order to smooth out these small steps.

1. A sawtooth generator for use in a raster scanning circuit comprising a variable frequency oscillator, a fixed ratio divider and a phase comparator connected to form a phase locked loop operative to lock the frequency of the variable oscillator onto a multiple of the frequency of a received synchronisation sig- nal, and a circuit for producing a sawtooth

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Constant amplitude sawtooth generator The present invention relates to a constant amplitude generator for generating a field or vertical scanning current in a deflection yoke. In a television display or pick-up device a sawtooth generator is often required to generate the vertical scanning current in the deflection yoke. A common way of achieving this is to charge a capacitor through a constant current source, discharging it at the time when the field fly-back is required. This gives a linear sawtooth, but if a different field frequency is required the sawtooth amplitude produced by the generator will also be different. At present different countries of the world fall under two standards, namely those using 50 Hz field rate and those using 60 Hz field rate. Any equipment which is designed to work on either standard and uses a conventional sawtooth generator will require a switch to alter the resulting sawtooth amplitude depending on which standard is selected. The present invention seeks to provide a sawtooth generator which may be used at either standard without the need for switching. In accordance with a first aspect of the present invention, there is provided a sawtooth generator for use in a raster scanning circuit comprising a variable frequency oscillator, a fixed ratio divider and a phase comparator connected to form a phase locked loop operative to lock the frequency of the variable oscillator onto a multiple of the frequency of a received synchronisation signal, and a circuit for producing a sawtooth voltage signal, the gradient of the ramp of which is dependent upon the frequency of the variable oscillator and the frequency of which is dependent upon the frequency of the divider output signal. Conveniently, the fixed ratio divider is a counter and the circuit for producing a sawtooth voltage is a digital analogue converter connected to the output of the counter. In an alternative embodiment of the invention, the output signals of the variable frequency oscillator are connected to gate predetermined charges to a charge storage device which is periodically discharged by the output of the fixed ratio divider, According to a second aspect of the invention, a sawtooth generator for producing a sawtooth wave form having the same frequency as an input synchronisation signal and a constant amplitude includes a charge storage device connected to be charged at a rate determined by the frequency of the input synchronisation signal and to be periodically discharged at the frequency of the input synchronisation signal. The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a block circuit diagram of the first embodiment of the invention and, Figure 2 shows a block circuit diagram of a second embodiment of the invention. In Fig. 1, a variable oscillator 10, a binary counter 1 2 and a phase comparator 14 are connected to form a phase locked loop locking the frequency of the variable oscillator 10 on to a multiple of the field synchronisation or locking pulses applied to one of the two inputs of the phase comparator 14. The binary outputs from the counter 1 2 are fed to a digital analogue converer 1 6 which produces a sawtooth output. The amplitude of the output is related only to the maximum binary count of the binary counter 1 2 and is therefore independent of the frequency of which the system is running, the frequency being determined by the field locking pulses. The resulting sawtooth output from the generator will have high frequency transients present which are due to small propagation delay differencies in the counter and due to any inaccuracy in the digitalanalogue conversion, subsequent filtering is employed to remove these transients In the circuit shown in Fig. 2, variable oscillator 10, counter 1 2 and phase comparator 1 4 once again form a phase locked loop in which the variable oscillator 10 is locked onto a multiple of the field synchronisation pulse frequency. The output pulses of the variable oscillator 10 are applied to a monostable circuit 1 8 which in turn serves to control a gated current source 22 to apply discrete charges to a storage capacitor 24.When the counter 1 2 achieves a count of zero, a decoder 20 switches on a transistor 26 connected in parallel with the capacitor 24 to discharge the capacitor 24. The output signal is derived from the capacitor 24 by way of a buffer amplifier 28 In this circuit, regardless of the frequency of the variable oscillator 10, a predetermined number of discrete charges are applied to the capacitor 24 during each ramp and therefore the amplitude of the sawtooth wave form is once again independent of the frequency of the field synchronisation pulses. The resulting output will be a sawtooth made up of a number of small steps and again filtering is required in order to smooth out these small steps. CLAIMS

1. A sawtooth generator for use in a raster scanning circuit comprising a variable frequency oscillator, a fixed ratio divider and a phase comparator connected to form a phase locked loop operative to lock the frequency of the variable oscillator onto a multiple of the frequency of a received synchronisation sig- nal, and a circuit for producing a sawtooth voltage signal, the gradient of the ramp of which is dependent upon the frequency of the variable oscillator and the frequency of which is dependent upon the frequency of the divider output signal.

2. A sawtooth generator as claimed in claim 1, wherein the fixed ratio divider is a counter and the circuit for producing a sawtooth voltage is a digital/analog converter connected to the output of the counter.

3. A sawtooth generator as claimed in claim 1, wherein the output signals of the variable frequency oscillator are connected to gate pre-determined charges to a charge storage device which is periodically discharged by the output of the fixed ratio divider.

4. A sawtooth generator as claimed in claim 3, wherein the fixed ratio divider is a counter and the charge storage device is a capacitor, output from the counter being coupled to a decoder output from which triggers discharge of the capacitor.

5. A sawtooth generator as claimed in claim 4, wherein the output signals of the variable frequency oscillator is applied to a monostable circuit which controls a gated current source so as to apply discrete charges to the capacitor.

6. A sawtooth generator as claimed in claim 4 or 5, wherein an output signal is derived from the capacitor by way of a buffer amplifier.

7. A sawtooth generator for producing a sawtooth waveform having the same frequency as an input synchronisation signal and a constant amplitude includes a charge storage device connected to be charged at a rate determined by the frequency of the input synchronisation signal and to be periodically discharged at the frequency of the input synchronisation signal.

8. A sawtooth generator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.