DE2203323A1 - Filter circuit - Google Patents

Description

Hewlett-Packard Company

1501 Page Mill Road

Palo Alto

California 94304

Case 629

January 18, 1972 FILTER SWITCH

The invention relates to an electronic circuit arrangement with a filter for attenuating a signal when the frequency of the signal is greater than a variable cut-off frequency.

The conventional phase locked loops require a search method to find out the correct frequency for the voltage controlled oscillator to find the tuned state to be achieved if the bandwidth of the loop is smaller than the desired tuning range. The to one Such a search method required circuit results in a complicated structure of the phase-locked loop.

The invention is based in particular on the object of creating a filter circuit in which in many cases Such a search method can be dispensed with and is based on the knowledge that the beat frequency between the voltage controlled oscillator and the input

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The phase-locked loop signal is used to control the oscillator frequency until the tuned state is achieved.

In the case of an electronic circuit arrangement of the type mentioned at the outset, this object is achieved according to the invention solved that the filter has a variable impedance network, which by a detector according to the frequency of the signal is regulated and the impedance of the changeable impedance network and thus the cutoff frequency of the filter will be changed.

According to a preferred embodiment of the invention, the output of a phase detector can be at the input of a low-pass filter with a variable frequency and also to the input of a control amplifier, which has a detector circuit includes. The output of the control amplifier can in turn be fed to the control input of the low-pass filter. The output signal from the low-pass filter is fed to a voltage-controlled oscillator. The oscillator output will in turn be the Phase detector fed and compared with an input signal for the phase-locked loop that the other input of the Phase detector is supplied.

If the phase-locked loop is in the regulated state, the output of the phase detector contains-

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de signal has a DC component and AC components. The DC component is the desired phase error signal, and the AC components result from the penetration of the signal frequency and from background noise on both phase detector inputs here. The purpose of the variable bandwidth low pass filter is to remove all AC components above a given frequency must be removed to ensure that the voltage controlled oscillator is not through these unwanted components are modulated.

When a phase loop is established with a much larger drive area than the loop bandwidth is to be, then the frequency of the oscillator must be brought into the capture range bandwidth of the input signal frequency will. The capture range bandwidth is always smaller than the drive range and is approximately equal to the loop bandwidth. In the conventional circuits, a wobble voltage, usually a sawtooth voltage, was applied to the oscillator fed, which was triggered when the phase alignment was lost, and which was turned off when the Phasing has been restored. Such systems have a major disadvantage when the loop bandwidth is very narrow, since the highest permissible wobble speed is then determined by the loop bandwidth. Therefore the fishing time depends on the wobble speed and

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the frequency difference between the voltage controlled oscillator and the input signal.

In contrast, the loop bandwidth according to the invention as a function of the frequency difference between the The oscillator and the input signal are changed in such a way that the capture range is very close to the entrainment range of the loop comes. The time required to catch or effect phasing is a function of the time that it takes is to set the loop bandwidth to a value greater than the difference between the frequencies of the oscillator and the input signal. With narrow loop bandwidths, the response time can be several orders of magnitude can be reduced compared to the conventional circuit arrangements.

The use of a bandwidth-changeable low-pass filter enables a search operation with a simpler circuit than was previously known. Such a filter also allows the loop to be very has a low bandwidth and thus lowers high-frequency noise in the output signal of the phase-locked loop, while maintaining a large capture area.

The following are preferred embodiments of the invention explained with reference to the drawings; it represent

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1 shows a phase-locked loop and

Figs. 2 and 3 show other embodiments of the invention.

The phase locked loop according to FIG. 1 has a phase detector or some other mixing or superimposing circuit 10, a bandwidth-changeable low-pass filter 12, a high-pass filter 13, a detector 14, a Amplifier 16 and a voltage controlled oscillator 18. The input 20 to the phase detector 10 is also the input for the phase locked loop. The phase detector input is connected to the output of the oscillator 18; therefore a signal at the output 24 of the phase detector 10 contains the Sum and difference frequencies of the signals at the inputs 20 and 22 / if the phase-locked loop is not entrained state. If this is the case, however, the output signal from phase detector 10 contains a direct voltage component, which is a function of the phase difference between the signals at inputs 20 and 22. About it contains the output of the phase detector in this case also unwanted AC voltage components, for example hum from the Oscillator interference voltages and other modulation products appearing at one or both of the inputs 20 and 22. The filter 12 regulates the capture range of the control circuit and the noise bandwidth by setting its cutoff frequency changes in accordance with a control voltage at input 26. This control voltage is derived from the difference frequency at the output

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output of the phase detector 10 derived when the phase-locked loop is not in the driving state. These Control voltage at input 26 is derived in the following way:

When the phase-locked loop is matched to the signal at input 20, the cutoff frequency of the low-pass filter with variable bandwidth assumes its lowest value F _T and the control saving at output 26 also assumes its lowest value V. If, on the other hand, the phase control circuit is not in the driving range, the difference frequency F passes through the high-pass filter 13 and is rectified by the detector 14.

The high-pass filter 13 has a lower limit frequency below F_; therefore, if F is above the lower cut-off frequency of the filter 13, a voltage occurs at the output of the detector 14, which voltage is amplified by an amplifier 16 and used to increase the cut-off frequency of the low-pass filter 14 with variable bandwidth. This allows the beat frequency F _{D to} pass through the filter 12 and change the frequency of the voltage controlled oscillator 18 until the frequencies of the two signals 20 and 22 are close enough to allow the phase control circuit to be entrained or tuned. When this is achieved, the beat frequency F goes back to zero, that is to say to a direct voltage, and causes the output of the de-

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tector 14 assumes the value zero. The control voltage at the output 26 therefore assumes the lowest value V_ and the limit frequency of the filter 12 is lowered _{from a higher value F "to the lowest value F T.} The time constant

π Li

the change in the cutoff frequency in the filter 12 is determined by the response time of the detector 14 and the amplifier 16. Therefore, only DC voltage signals and low-frequency signals can reach the input 28 of the oscillator.

Another embodiment of a phase control circuit in which this invention could be used is explained in German patent application P 2 061 227, which on the same applicant goes back.

Fig. 2 shows a preferred embodiment of the detector 14, of amplifier 16 and filter 12. Detector 14 is shown as a peak detector and includes diodes 30 and 32 and Capacitors 34 and 36, which form a voltage doubler circuit form. The detector input 35 is connected to the output 24 and the output of the detector 14 is connected to the Input 40 of the amplifier 16 connected. A resistor 38 is between the positive terminal 51 of a voltage source and connected to the amplifier input 40. The amplifier is shown as a single transistor 42, its emitter is connected to the diode 32, the capacitor 36 and the negative voltage source 50.

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The filter 12 has a field effect transistor 46 which is connected in parallel with a resistor R 1 and is connected in series with a capacitor C 1 and a resistor 47. When the phase-locked loop is in the driving state, the AC voltage signal at the output 24 is very small and does not generate sufficient current to overcome the base current delivered by the resistor 38 to the transistor 42. The transistor 42 therefore goes into saturation, the field effect transistor 46 has a high resistance and the cutoff frequency F ₁ . of the low-pass filter follows the equation

L 2 7Γ R ₁ C ₁

If, on the other hand, the phase-locked loop is not in the driving state, a large AC voltage signal occurs at output 24 and causes the detector output current to approach the value determined by resistor 38 and the positive terminal 51 is set, so that the transistor 42 is switched off. When switching off the transistor 42 increases the voltage at the gate of the field effect transistor, so that the resistance between the output 24 and the input 28 is reduced. As a result, the cutoff frequency of the low-pass filter with variable bandwidth is increased.

According to Fig. 3 can have additional changeable impedance elements to be added to the filter 12 to increase the slope

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- Q _

at the frequency F to increase. A field effect transistor and a parallel resistor 49 are connected in series with the resistor 47. A transistor 43 goes in parallel to a transistor 42 and connected to the gate of the field effect transistor 41 and a collector resistor 45. The resistance value of the field effect transistor 41 becomes

decreased in the same way as the resistance of the field effect transistor 46 when the phase-locked loop falls out of step, i.e. the entrainment status is interrupted.

The filter 12, the detector 14 and the amplifier 16 can be configured in other ways. Just a changeable one Impedance element has been shown in filter 12, however there could be several such elements including varactor diodes, photoresistors and bipolar transistors as well like field effect transistors. The detector 14 could be a one-way or two-way peak or mean value detector, or be designed in the manner shown. The amplifier 16 is shown simply as a single bipolar transistor stage, but it could also be a multi-stage amplifier or an operational amplifier or other types of active elements are provided.

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Claims (3)

Hewlett-Packard Company ' λλ 1501 Page Mill Road Palo Alto California 94304 United States Case 629 January 18, 1972 Claims ( 1.! Electronic circuit arrangement with a filter for attenuating a signal when the frequency of the signal is greater than a variable cut-off frequency, characterized in that the filter has a variable impedance network (12) which is determined by a detector (14) in accordance with the Frequency of the signal is regulated and the impedance of the changeable impedance network and thus the cutoff frequency of the filter is changed. 2. Circuit arrangement according to claim 1, characterized in that the detector (14) has an output control signal a first value for input signal frequencies above a threshold frequency and a second value for Emits input signal frequencies below the threshold frequency, the changeable cut-off frequency assumes a first value, when the output control signal reaches the first value and assumes a second lower value when the output control signal reaches the second value and the threshold value 209836/1046 frequency essentially equal to the second value of the variable Cutoff frequency is. 3. Circuit arrangement according to claim 1, characterized / that the variable in frequency Oscillator (18) is controlled by a signal from the filter, a phase detector (10) the signal of the oscillator and a compares another signal and outputs the resulting signal to the filter circuit and to the detector (14) and that the detector emits a first output signal at a signal from the phase detector whose frequency is greater than one Threshold frequency and emits a second output signal, in the case of a signal from the phase detector, its frequency is equal to or less than the threshold frequency, and the filter circuit has an upper limit frequency that is greater than the threshold frequency corresponding to the first output signal and which is equal to or less than the threshold frequency is corresponding to the second output signal. 209836 / 104Ö Blank page