CA1166705A - Phase detector circuit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A reference pulse signal is applied to both a detector multiplier and a gate circuit by utilizing the same trans-mission path, i.e., same circuit. More specifically, the reference pulse signal is supplied to the detector multiplier and gate circuit through a transistor and a resistor. Therefore, the reference pulse signal received at the base of the transistor constituting the detector multiplier is substantially identical in level and pulse width to the reference signal received a-t the base of the transistor constituting the gate circuit.
Owing to the use of the same transmission path described above, the timing of pulse detection coincides very accurately with the timing of gate control. Therefore, a signal can be detected accurately and the efficiency of phase detection is improved.

Description

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"PHASE DETECTOR CIRCUIT"
This invention relates to phase detector circuits.
This sort of the circuits are used for, fo~ example, color signal processing circuits in video tape recorders and the like.
In the recording of NTSC video signals on magnetic tape with a video tape recorder, the luminance signal and chrominance signal are separated from the video signal to be recorded. The luminance signal is frequency modulated ~ a frequency modulator, while the chrominance signal is converted into a signal with a low frequency subcarrier in a color signal processing circuit. The frequency modulated luminance signal and the converted chrominance signal are combined in a~
mixing circuit, and the resultant signal is recorded on a magnetic tape by a magnetic head. In the color signal processlng clrcuit, the subcarrier frequency of the chrominance signal is converted from fsc = 3.58 MHz to (44 - 4)fH (fH denotes the frequency of the horizontal~sync signal). The color signal processing circult lncludes a converting~ signal generator for converting the original chrominance into the converted chrominance slgnal. The converting signal has a frequenc~y of the sum of the original subcarrier frequency (3.58 MHz) and the;converted subcarrier frequency (688 kHz), and is synchronized to the S ~/i1 C
` ~ horizontal ~i~ signal. The converting signal is , ~, ,., -~
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also controlled such that it can be used as a hurst siynal.
An automatic frequency control circuit (abbreviated as AFC circuit) synchronizes the converting signal frequency (3.58 MHz -~ 688 kHz = fsc + 4 fH) horizontal s~nc signal frequency. The automatic frequency control ~ircuit includes a voltage-controlled oscillator (abbreviated as VCO), a frequency divider for dividing the frequen~y oscillated from the VCO output, an integrating circuit for integrating the frequency divider output and generating a sawtooth wave signal, and an AFC detector to which the sawtooth wave signal (which is a compared pulse signal) and the horizontal sync signal (which is a reference pulse signal) are coupled. The ~FC detector provides an output which corresponds in voltage to the phase difference between the sawtooth wave signal and the horizontal sync signal.
This output is applied to a frequency control terminal of the VCO. The output of the VCO is thus synchronized in frequency to the horizontal sync signal. The circuit described above is termed an AFC loop.
In the AFC loop described above, it is important that the sawtooth wave signal generated from the integrating circuit has steady slant portions, that is, the slant portions of the sawtooth wave signal have good linearity. However, the characteristics of resistors ; and capacitors that are employed in the i~tegrating circuit are susceptible to tempexature changes, which -~7 . ~,,~ .

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s causes the slan-t portions of the sawtooth wave signal to fluctuate.
The sawtooth wave signal output from the integrating circuit is given as a function Ao which is t Ao = Eo~l - eR~), S Eo: the amplitude of the pulse input to the integrating circuit, t : time in second, RC: the time constant of the integrating circuit.
The slope n of the sawtooth wave signal is thus n = ~C eRC (V/sec) where 0 ~ t < T ( T being the pulse width of pulse input to the integrating circuit). It will be seen from the above equation that the slope of the output sawtooth wave signal is inEluenced by changes of the time constant with temperature change. This means that the sensitivity of the AFC detector circuit is also changed with temperature change. Under the constant sensitivity of the AFC detector~circuit, an output voltage from the AFC detector circuit~ is in proportion to a phase difference between the sawtooth wave signal and.the horizontal sync signal always changes linearly in accordance with the change of the phase difference.
Where the sensitivity of the AFC detector circuit varies with temperature change in the above described :
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conventi.onal AFC detector cir~uit which employs an integrating circuit~ an accurate output siynal can no longer be obtained. Besides, the slant portions of the output of the conventional integrating ~ircuit employing the time constant ~ircuit is non-linear, so that the draw-in frequency and the hold frequency of the AFC loop with respect to the ~enter frequency cannot be equalized.
Further, in case of fabricating the AFC loop employing the conventional integrating circuit on an IC chip, the capacitor for the time constant circuit cannot but be constructed in an external circuit to the IC chip, so that the large number of connection pins cannot be reduced.
An object of the invention is to provide a phase detector circuit, which alleviates the adverse effects of temperature changes and has steady and stable sensi-tivity of phase difference detection when detecting -phase difference among a plurality of signals and also reduces the number of required terminal pins in case of implementing on an IC circuit.
For achieving the above object, the phase detector circuit according to the present invention comprises:
a phase detector including a first differential amplifier and a second differential amplifier, a reference pulse signal and a comparison pulse signal being applied to f irst in.put terminals of said respective first and second differenti.al amplifiers, :;
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, ;7~35 preset bias voltages of first and second DC blas voltage sources being applied to second input terminals of said respective first and second differential amplifiers, said second differential amplifier providing from an output terminal thereof a phase detection pulse of a pulse width proportional to the phase difference between the reference pulse and comparison pulse, a buffer circuit connec-ted to the output terminal of said second differential amplifier, for extracting said phase detection pulse output, a gate circuit connected to an output terminal of said buffer circuit, a current path of said buffer circuit between a phase detection pulse input terminal and an output terminal being on-off controlled by said reference pulse signal, said buffer circuit providing an output pulse while said current path is in the "on"
state, the pulse duty of said output pulse varying according to the pulse width of said phase detection pulse, and a filter for receiving the output of said gate circuit.
This invention can be more fully understood from ~the following detailed description when ta~en in conjunction with the accompanying drawings, in which:
Fig. l is a block diagram showing a color signal processing circuit to which the present invention is applied;

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Fi~ . 2 is a circuit diagram showing ~n embodiment of the present invention; and Fig. 3 is a waveform diayram showiny siynal wave-forms at various parts of -the circuit of Fig. 2 with the letters designating the waveforms indicating the part in Fig. 2.
Fig. 1 shows a color signal processing circuit for a video tape recoxder (VTR), particularly a circuit for recording operation. The color signal (with subcarrier having frequency of 3.58 MHz) separated from the video sigrlal is applied through an input terminal 11 to a first input terminal of a first balanced modulator 12 and then converted to a low frequency converted color signal des-cribed later in detail. The converted color signal is then filter~d out through a filter 13 to an output terminal 14.
The original color signal is also applied to an automatic phase control detector (abbreviated as APC
detector) 15 which constitutes part of an automatic phase control loop (APC loop). In the APC detector 15 the burst signal included in the original color signal is compared in phase with the output (at ~.58 MHz) from a first VCO 18 among the gating period according to the burst pulse. The resultant voltage which corres-ponds to the phase difference between the burst signal and the output of the first VCO 18 is sampled and held by a follo~ing sample-and-hold circuit 16 under the control of the burst gating pulse. The output voltage of` the sample-and-hold circuit 16 is ~,...

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J~s coupled throuyh a low~pass filter 17 to an oscillation frequency control terminal of the first VCO 1~. Thus, the output of the first VCo 18 is locked in phase to the burst signal.
The output of the VCO is also fed to a first input terminal of a second balanced modulator 19. To a second input terminal of the second balanced modulator 19 is fed an output (688 kHz) of a frequency divider 29, which divides the frequency (2.7 MHz) of the output of second VCO 27 to 1/4.
The output of the second balanced modulator lg thus has a frequency of 3.58 MHz ~ 688 kHz or roughly 4.2 MHz. This output (4.2 MHz) is applied through a filter 20 to a second input terminal of the first balanced modulator 12 as a converting signal. The original color signal is modulated at the first balanced modulator 12 with the output or the converting signal from the filter 20. As a result, the low frequency converted color signal is obtained at the output terminal of the first balanced modulator 12.
The second VCO 27 is automa~ically frequency controlled such that its output frequency ~(175 x fH =~27 MHz) is synchronized to the horizontal sync signal frequency (fH).
~ In the color signal processing system for a video tape recorder, the frequency (688 kHz) of the low frequency converted color siynal has to be synchronized : -7~S

to the frequency (fH) of the horizontal sync sign.al included in the video sign.al. Accordinyly, this is because of the :Eact thak the horizontal sync signal variations have effects upon the color signal hase variations. The circuit that is involved is designed to maintain a constant relation of the low frequency converted color signal frequency and horizontal sync signal frequency to each other.
The video signal is ~ed through an input terminal 10 21 to a sync separator 22. The sync separator 22 separates the horizontal sync signal from the video signal. The horizontal sync signal is applied to a first input terminal of a phase detector multiplier 23 and also to a gating pulse input terminal of a gate circuit 25 both being included in an AFC loop. To a second input terminal of the phase detector multiplier 23 is applied the output (5fH) of a frequency divider 28, which divides the output frequency (175 x fH = 2.7 MHz) of the second VCO 27 to 1/35.
Phase detector multiplier 23 detects the phase difference between the horizontal sync signal and the frequency divider output, and provides a phase detection pulse output which corresponds to the deviation from synchronization between the horizontal sync signal (at fH = 15.73 kHz) and the frequency divider output (at 5fH). The phase detector output pulse is applied through a bu~fer 24 to the gate circuit 25~ In the gate ~"~

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t~3s circuit 25, the path between i-ts phase detection pulse input terminal and output terminal is enabled, i.e. ren-dered conductive, during the horizontal sync signal period, thus producing an output pulse of a pulse dut~ proportional to the pulse width of the phase detection pulse.
The invention can be effectively applied to the circuit composed of the phase detector 23, buffer 24 and gate circuit 25 shown in Fig. 1. 6 Fig. 2 shows a specific circuit construction of an embodiment of the invention. Referring to the Figure, labeled el is a reference pulse signal (for instance the horizontal sync signal) from a first signal source such as in the Fig. 1 embodiment the signal from sync separation circuit 22, and labeled e2 is a compared pulse signal (for instance a frequency divider output such as in the Fig. 1 embodiment the signal from frequency divider 28) from a second signal source (for instance a VCO). Labeled El is a first DC bias voltage source, and labeled E2 is a second DC

bias voltage source. The second DC bias voltaqe E2 is applied to the base of a transistor Ql The transistor Ql has its collector connected to the first DC bias voltage source El and its emitter connected in series with resistors Rl and R2 to a reference potential terminal. The emitter of the transistor Ql is also connected in series with resistors R3 and R4 to the collector of a transistor Q2. The transistor Q2 has its emitter connected to the reference pote~tial terminal and upon its base the aforemention~d 3~

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, 1?~ 5 reference pulse siynal el P
The compared pulse signal e2 is applied to the base of a transistor Q3. The first DC bias voltage source El is connected throuyh resistors R5 and R6 to the collector of the transistor Q3, and this collector is connected through a resistor R7 to the base of the transistor Q4. The transistor Q4 has its emitter connected to the reference potential terminal and its collector to h~ the DC bias El -i{.~ d through resistors R8 and Rg. The collector of the transistor Q4 is also connected through a resistor Rlo and a diode Dl to the reference potential terminal. The diode D
is a temperature compensation diode.
The juncture between the resistors R1 and R2 is connected to the bases of transistors Q6 and Q13' and the juncture between the resistors R8 and Rg is connected to the base of a transistor Q8. In other words, the potential at the junc-ture between the resistors Rl and R2 is set as the base bias for the transistors Q6 and Q13~ and the potential at the juncture between the resistors R8 and Rg is set as the base bias for the transistor Q~. Transistors Q5 and Q6 form a first differential amplifier A, transistors Q7 and Q8 form a second differential amplifier B, and transistors Q12 and Q13 form a third differential : amplifier _.
~he reerence pulse el is applied through the ~ ' ~ ' '' .

transistor Q2 and resistor R4 to the base of the b transistor Q5 of the first differential amplifier A
and also to the base of the transistor Q12 of the second differential amplifier C. The compared pulse signal e2 is applied through the transistor Q3 and resistor R6 to the base of the transistor Q7 of the second differential amplifier B. The transistors Qs and . Q6 have their emitters commonly connected to the collector of a transistor Qg which forms a current source. The emitter of the transistor Qg is connected through a resistor R12 to the reference potential terminal. The first DC bias voltage source El is connected to the collector of the transistor Qs, and the transistors Q7 and Q8 have their emitters common~y connected to the collector of the transistor Q6. The first DC bias voltage source is directly connected to the collector of the transistor Q7, and it is also connected through a resistor Rll to the collector of the transistor Q8. The collector of the transistor Q8 is also connected to the base of a transistor Qlo which is a component el.ement of the buffer 24.
The first differential amplifier A is constituted by the transistors Q5 and Q6 and the second differential amplifier B is constituted by the transistors Q7 and Q~ form the phase detector 23.
The transistor Qlo of the buffer 24 has its collector to which the first DG bias voltage source - :
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El is connected and its emitter connected to the collector of a transistor Qll The transistor Q11 forms a constant current source, and its emitter i5 connected to the reference potential terminal. The emitter of the 5~ transistor Qlo is connected to the output terminal of the buffer 24, and it is connected through a resistor Rl4 to the base of a transistor Ql4-The transistors Q14' Ql2 and Ql3 form the g circuit 25. The collector of the transistor Q12 is connected to the base of the transistor Ql4~ which has its emitter connected to the collector of the transistor Q13 The first DC bias voltage source El is connected to the collector of the transistor Ql4 The transistors Ql2 and Q13 have their emitters commonly connected to lS the collector of a transistor Ql5 which forms a constant current source. The emitter of the transistor Ql5 is connected through a resistor Rl5 to the reference potential source. The emitter of the transistor Ql4 is connected to the filter 26.
A series circuit including a resistor Rl6, a diode D2 and a resistor R17 sets the base bias for the constant current source transistors Qg~ Q11 and Q15-The diode D2 serves the roles of bias setting and temperature compensation~ The current through the collector-emitter path of the transistor Qg is set equal to the çurrent through the diode D2. In other words, the current path formed by the transistor Qg .

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~ 13 -and resistor Rl2 and that by -the resistor Rl6, c1iode D2 and resistor Rl7 form a current mirror circuit.
The operation of the phase detector 23, buffer 24 and gate circuit 25, having the above construction, will now be described. It is now assumed that the reference pulse signal el and compared pulse signal e2 are related to each other as shown in (a) and (b) in Fig. 3. When the reference pulse signal el goes to a high level, the potential of the collector on the -tran-sistor Q2 is reduced. In consequence, the signal -to the base of the transistors Q5 and Ql2 goes to a low level as shown in (c) in Fig. 3. When the compared pulse signal e2 goes to a high level, the collector potential on the transistor Q3 is reduced. In consequence, the signal to the base of the transistor Q7 goes to a low level as shown in (d) in Fig. 3. The transistors Q6 and Q8 are both on state during a period t shown in (e) in Fig. 3. During this period t, a pulse as shown in (e) in Fig. 3 prevails at the base of the transistor Qlo. The current through the transistor Qlo is controlled by the pulse shown in (e) in Fig. 3.
When the reference pulse el goes to a high level, the transistor Ql2 is cut off, while the transistor Ql3 is tr1ggered. (While the reference pulse el is at the low level, the transistor Ql2 is "on" and the transistor Ql3 if "off".). Thus, the base potential o~ the transistar Ql4 is increased when the transistor ', ' ' ~, ~ . ': .
., ., , ~ ,, ' ~, ~ 14 -Q12 is cut off and is reduced when the current through the transistor Qlo is subsequently controlled. The signal to the base of the Q14 thus has a waveform as shown in (f) in Fig. 3, and a signal as shown in (g) in Fig. 3 appears from the emitter of the transistor Q14 In other words, in the gate circuit 25, the current path between the phase detection pulse input terminal and output terminal is conductive while the reference pulse el is at the high level, and during this period the pulse duty of the output pulse is set. When the reference pulse and compared pulse go to high level at the same time a phase detection pulse can be obtained, a pulse width of said phase detection pulse proportion between the reference pulse and compared pulse, to the collector of transistor Q8 Thus, the output (as shown in (h) in Fig. 3) of the filter 26 can be utilized as a voltage which corresponds to the phase difference between the reference pulse and compared pulse.
The current I through the emitter of the transistor Q9 in the automatic frequency control phase detector 26 is given as I = R +R R ---; (1) VF : the forward voltage across the diode D2, R12: the resistance of the resistor R~2, ~16: the resistance of the resistor R16, R17: the resistance of the resistor R17.

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This means that the current path constituted by the resistor Rl6, diode D2 and resistor R17 and that constituted by the transistor Qg and resistor R12 form a current mirror circuit.
The sensitivity ~ of the phase detector 26 is given as Rl6+Rl7 Rl2' ~ /sec) ---- (2) under the assumption that T 1 _ T2 where T l is the pulse width of the reference pulse el and T 2 is the pulse width of the compared pulse e2.
As is seen from the above equation, the sensitlvity of the phase detector is determined by the resistance R17 Rll Since the resistOrS Rll~ 12 16 R17, etc. can be realized as distrlbuted resistance within a semiconductor integrated circuit, the error of the aforementioned resistance ratio R +R R - can be held within 3%.
Thus, sufficiently high precision of the sensitivity can be obtained. Also, satisfactory linearity of the sensitivity can be obtained as is seen from equation (2).
In equation (2), only VF is dependent upon~the temperature. If the circuit constants are selected such that El ~ VF and Rl6 >> Rl7, ~ can be appro~imated as R17 Rll El - R16 R12 Tl ~., ~. .

: ~' In this case, the temperature changes have practically no influence upon the sensitivity.

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Claims (3)

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. A phase detector circuit comprising:
a phase detector including a first differential amplifier and a second differential amplifier, a reference pulse signal and a comparison pulse signal being applied to first input terminals of said respective first and second differential amplifiers, preset bias voltages of first and second DC bias voltage sources being applied to second input terminals of said respective first and second differential amplifiers, said second differential amplifier providing from an output terminal thereof a phase detection pulse of a pulse width proportional to the phase difference between the reference pulse and comparison pulse, a buffer circuit connected to the output terminal of said second differential amplifier, for extracting said phase detection pulse output, a gate circuit connected to an output terminal of said buffer circuit, a current path of said buffer circuit between a phase detection pulse input terminal and an output terminal being controlled to on or off by said reference pulse signal, said buffer circuit providing an output pulse while said current path is in the "on" state, the pulse duty of said output pulse varying according to the pulse width of said phase detection pulse, and a filter for receiving the output of said gate circuit.

2. A phase detector circuit according to claim 1, which comprises:
a first transistor having the base connected through a first resistor to said first DC bias voltage source and also connected through a diode and a second resistor to a reference potential terminal and the emitter connected through a third resistor to said reference potential terminal;
a second transistor having the emitter connected to the collector of said first transistor, the base connected to a signal line to which said reference pulse signal is applied and the collector connected to said first DC bias voltage source;
a third transistor having the emitter connected to the emitter of said second transistor and the base connected to a first biasing means, said second and third transistors constituting said first differential amplifier;
a fourth transistor having the emitter connected to the collector of said third transistor, the base connected to a signal line to which said comparison pulse signal is applied and the collector connected to said first DC bias voltage source; and a fifth transistor having the emitter connected to the emitter of said fourth transistor, the base connected to a second biasing means, and the collector connected through a fourth resistor to said first DC bias voltage source, said fourth and fifth transistors constituting said second differential amplifier.

3. A phase detector circuit for providing an output signal indicative of a phase difference between an input signal and a reference signal, comprising:
a multiplying circuit for multiplying said input and reference signals and providing a multiplied signal indicative thereof;
a signal transmission path for transmitting said multiplied signal to an output circuit; and means for interrupting said signal transmission path in synchronism with said reference signal, thereby obtaining at said output circuit said output signal in accordance with the phase difference between said input and reference signals.