JPH1038933A - Apparatus for converting cycle-voltage of cyclic signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect and output a voltage corresponding to a cycle of cyclic signals at high speed and highly accurately even when the cycle of the cycle signals detected continuously or intermittently changes in a short time. SOLUTION: An input signal S2 is shaped to a rectangular wave by a rectangular wave-shaping circuit 6. A pulse signal S4 of a predetermined pulse width is output by a one-shot circuit 7 synchronously with a rise or fall of a rectangular wave signal S3. The pulse signal S4 is delayed at a delay circuit 8 and output as a switch signal S5 to an RC-charging discharging circuit 9. The charging discharging circuit 9 charges a capacitor 9aa while the switch signal S5 is input, and discharges through a resistor 9ab thereafter. A level shift signal S6 due to the discharging is sample-held at a sample hold circuit 10 with the pulse signal S4 used as a synchronous signal, so that a voltage corresponding to a cycle is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects a voltage corresponding to a period with high speed and high accuracy even if the period of a periodic signal received continuously or intermittently fluctuates within a short time. The present invention relates to a periodic-to-voltage conversion device for a periodic signal that can be output as a signal.

[0002]

2. Description of the Related Art Conventionally, in a velocity measuring device using ultrasonic waves, a moving object is irradiated with ultrasonic waves of a known frequency, a reflected wave from the moving object is received, and a frequency change of the reflected wave is used. The speed of a moving object is determined (for example,
See Japanese Patent Application No. 1-296288).

[0003] In this specification, a case where a moving object approaches an observer will be described for convenience of explanation.
It will be added just in case that it can be explained in the same way when moving away.

When an ultrasonic wave is applied to a moving object, the frequency of the reflected wave changes according to the moving speed of the moving object due to the Doppler effect. Therefore, the speed of the moving object is obtained from the change in the frequency.

In this case, it is necessary to know the frequency (reciprocal of the period) of the received reflected wave. FIG. 5 shows such a speed measuring device.

[0006] For the sake of convenience in the following description, a velocity measuring device is provided with a transmitting system for emitting ultrasonic waves, a receiving system for receiving Doppler frequency by receiving reflection from a moving object, and a moving system for detecting the Doppler frequency based on the Doppler frequency. The description will be roughly divided into a processing system for obtaining and displaying the speed.

The transmission system includes a reference oscillator 1 for generating a reference signal S11 having a reference frequency Fout, a driver 2, and a transmitter 3 for emitting ultrasonic waves toward a moving object based on signals from the driver 2. doing.

[0008] Further, the receiving system is provided with a frequency Fi from a moving object.
receiver 4 for receiving the n reflected waves, preamplifier 5 for amplifying received signal S12 from receiver 4, and received signal S
12 and a reference signal S11, and has a mixer 100 that outputs a Doppler signal S13 that is a difference frequency Fd of these frequencies.

Then, the processing system shapes the Doppler signal S13 from the mixer 100 into a rectangular wave, and forms a rectangular wave signal S14.
Square wave shaping circuit 101 that outputs a signal, Doppler signal S13
Signal detector 10 which receives and outputs a detection signal S15
2. A one-shot circuit 103 which receives the detection signal S15 and outputs a count control signal S16 having a predetermined pulse width, performs a logical product of the rectangular wave signal S14 and the count control signal S16, and outputs the result as a count pulse signal S17. An AND gate 104, a counter 105 for counting the count pulse signal S17, and a display 106 for displaying the count result as speed.

The operation of the speed measuring device having the above configuration will be described with reference to a timing chart shown in FIG. The reference signal S11 output from the reference oscillator 1 is input to the transmitter 3 via the driver 2, and the transmitter 3 outputs the reference signal S1.
The ultrasonic wave is emitted toward the moving object based on 1.

The ultrasonic wave applied to the moving object is reflected by the moving object and received by the receiver 4, converted into an electric signal, amplified by the preamplifier 5 to a predetermined level, and received by the receiving signal S12. Becomes

The reference signal S11 is output from the mixer 100.
Reference frequency Fout and frequency Fin of received signal S12
Is output as a Doppler signal S13 (FIG. 6A).

At this time, since the Doppler frequency Fd satisfies the relationship of Fin = Fout + Fd (1) with the reference frequency Fout and the frequency Fin, the frequency of the Doppler signal S13 output from the mixer 100 becomes the Doppler frequency Fd.

The Doppler signal S obtained as described above
13 is input to the square wave shaping circuit 101 and the signal detector 102, and the Doppler signal S1
3 is shaped into a rectangular wave signal S14 having a waveform as shown in FIG.

On the other hand, in the signal detector 102, in synchronization with the Doppler signal S13, the detection signal S15 (see FIG. 6 (c)) which is at "H level" while the burst-like Doppler signal S13 is being input is one-shot. Output to the circuit 103.

In the one-shot circuit 103, the detection signal S
The count control signal S 16 having the pulse width T is output to the AND gate 104 in synchronization with the rise of the signal 15.

The AND gate 104 performs a logical operation on the rectangular wave signal S14 and the count control signal S16, and
A count pulse signal S17 as shown in FIG. The counter 105 counts the count pulse signal S17 and outputs a count signal to the display 106.

The AND gate 104 outputs a rectangular wave signal S14.
An "H level" signal is output only when both the signal and the count control signal S16 are at "H level".

Incidentally, the count control signal S16 is a pulse signal having a predetermined pulse width T in synchronization with the input of the Doppler signal S13, and the rectangular wave signal S14 is a signal having the same frequency as the Doppler frequency Fd.

Therefore, the count pulse signal S17 output from the AND gate 104 is the same as the Doppler signal S13 during a predetermined time T after the input of the Doppler signal S13.
Means how many cycles have been input.

Therefore, the above-mentioned Japanese Patent Application Laid-Open No. 61-29628 is also applicable.
In Japanese Unexamined Patent Publication No. 8 (1993) -1990, the speed of a moving object is obtained by multiplying the number of cycles of the Doppler signal S13 input during a predetermined time T, that is, the count number from the counter 105 by the time T.

[0022]

However, since the Doppler frequency and the moving speed of the moving object have a function dependency of Equation 2 described later, the count number is multiplied by a predetermined time T to reduce the speed of the moving object. There was a problem that the error would increase if found.

That is, the Doppler frequency F when the ultrasonic wave emitted at the reference frequency Fout is reflected by the moving object
d is given by Fd = (2 · V · Fout) / (C−V) (2) where V is the speed of the moving object and C is the sound speed.

Therefore, the frequency of the reflected wave that has undergone such a Doppler shift, that is, the frequency Fin of the ultrasonic wave received by the receiver 4, is given by the following equations (1) and (2): Fin = Fout + Fd = Fout · (C + V) / (CV) (3)

As can be understood from Equation 3, the Doppler frequency Fd is not linearly proportional to the moving speed V of the moving object. FIG. 7 shows this. FIG. 7 shows a case where the reference frequency Fout is 40 kHz and the sound speed is 346.5 m.
3 is a graph in which Equation 3 is graphed as / sec, where the horizontal axis is the velocity V of the moving object, and the vertical axis is the Doppler frequency Fd.

In the figure, the solid line is the Doppler frequency Fd according to equation 1.
The dotted line indicates that the speed V of the moving object is 5 km / h and 200
It is a straight line connecting two points on the Doppler frequency Fd at km / h. The difference between the straight line and the curve is an error. FIG. 8 is a graph of such an error, and it can be understood that an error of about 9 km / h occurs at the maximum.

Such an error is further increased if the frequency Fin of the reflected wave fluctuates during measurement or is temporarily interrupted.

Therefore, even when the present invention is used in a speed measuring device or the like, even if the period of a periodic signal received continuously or intermittently fluctuates within a short period of time, the present invention provides a high speed and An object of the present invention is to provide a period-to-voltage converter for a period signal that enables high-speed and high-precision speed measurement by detecting and outputting a voltage corresponding to a period with high accuracy.

[0029]

According to a first aspect of the present invention, an input periodic signal is shaped into a rectangular wave signal, and a pulse is synchronized with a rising or falling edge of the rectangular wave signal. A periodic pulse generating means for outputting a signal, and after a lapse of a predetermined time from the input of the pulse signal,
A displacement signal generating means for outputting a level displacement signal whose signal level changes with a predetermined time constant based on a predetermined level value; and sampling the level displacement signal in synchronism with the pulse signal, for each cycle of the periodic signal. And a periodic voltage detecting means for outputting a voltage corresponding to the above.

That is, a periodic signal such as a sine wave is shaped into a rectangular wave signal by a periodic pulse generating means, and a pulse signal having a predetermined pulse width is output in synchronization with the rise or fall of the rectangular wave signal. I do. The pulse signal is delayed by a predetermined time by the displacement signal generating means, and the pulse signal is delayed by a predetermined time or a signal delayed by a predetermined time from the pulse signal, and the level is changed by a predetermined time constant from the predetermined level by the signal. Output a level displacement signal. Therefore, the level displacement signal is sampled and held, and a voltage corresponding to the cycle of the periodic signal is output, using the pulse signal output for each cycle of the periodic signal by the periodic voltage detection means as a synchronization signal. .

According to a second aspect of the present invention, the periodic signal is a signal of a reflected wave reflected by a moving object and subjected to a Doppler shift, and the displacement signal generating means includes a capacitor and a resistor. The values of the capacitor and the resistor are set to approximate the relationship between the moving speed of the moving object and the period of the reflected wave, and the capacitor is set for a predetermined time after a predetermined time has elapsed since the input of the pulse signal. Charging, accumulating a certain amount of electric charge, and thereafter discharging the electric charge stored in the capacitor with a time constant determined by the capacitor and the resistor via the resistor, and outputting a level displacement signal. .

That is, a displacement signal generating means having a capacitor and a resistor is provided. Then, when outputting a voltage corresponding to the period of a periodic signal formed by receiving a reflected wave whose frequency has been changed by receiving a Doppler shift due to the Doppler effect,
The relationship between the moving speed of the moving object and the period of the reflected wave is similar to the discharge time of the level displacement signal attenuated by the time constant determined by the values of the capacitor and the resistor, and the discharge voltage at that time.
It is characterized in that the capacitance and the resistance value are set.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a speed measuring device provided with a periodic signal period / voltage converter according to the present embodiment.

The velocity measuring device is adapted to correspond to a transmitting system for emitting ultrasonic waves, a receiving system for receiving reflection from a moving object and outputting a period of an input signal which is a periodic signal, and a period of the input signal. And a processing system (including a period-to-voltage converter) that outputs a voltage to be applied.

The transmission system includes a reference oscillator for generating a reference signal S1 having a reference frequency Fout, a driver for power-amplifying the reference signal S1, and a transmitter for emitting the power-amplified reference signal S1 as ultrasonic waves. 3. The moving object is irradiated with ultrasonic waves.

The receiving system receives the ultrasonic wave of the frequency Fin reflected by the moving object by Doppler shift and converts the received ultrasonic wave into an electric signal, and amplifies the signal from the receiving unit 4. It has a preamplifier 5 for outputting as an input signal S2.

The processing system also includes a rectangular wave shaping circuit 6 for shaping the input signal S2 from the preamplifier 5 into a rectangular wave and outputting it as a rectangular wave signal S3, and a pulse having a minute pulse width in synchronization with the rise of the rectangular wave signal S3. A one-shot circuit 7 for outputting the signal S4, a delay circuit 8 for delaying the pulse signal S4 by the pulse width of the pulse signal S4 and outputting it as a switch signal S5, and charging is started by the input of the switch signal S5 to start the charging and level displacement. An RC charge / discharge circuit 9 for outputting a signal S6, a sample / hold circuit 10 for sampling and holding the level displacement signal S6 and outputting a periodic voltage signal S7, and a subtraction voltage source (Vsub) 12 are provided. A subtractor 11 that outputs a subtraction signal S8 from which the offset has been removed, amplifies the subtraction signal S8 by a predetermined gain, and outputs an output voltage S9. An amplifier 13 for power.

The RC charge / discharge circuit 9 includes a capacitor (C) 9
A differentiating circuit 9a comprising aa and a resistor (R) 9ab, and a switch (SW) 9c which operates when a switch signal S5 is input and connects the differentiating circuit 9a to a reference voltage (Vref) 9b.
And

The periodic pulse generating means comprises a rectangular wave shaping circuit 6 and a one-shot circuit 7, the displacement signal generating means comprises a delay circuit 8 and an RC charging / discharging circuit 9, and the periodic voltage detecting means comprises , One-shot circuit 7 and sample / hold circuit 10.

With the above configuration, the operation will be described according to the timing chart shown in FIG.

The reference signal S1 oscillated by the reference oscillator 1
Is amplified to a predetermined level by the driver 2 and
Sent to Thereby, the reference frequency Fo is transmitted from the transmitter 3.
out ultrasonic waves are emitted.

The ultrasonic wave emitted from the transmitter 3 is reflected by the moving object, received by the receiver 4 and converted into an electric signal. At this time, since the ultrasonic wave is subjected to the Doppler effect by the moving object, the frequency is shifted by Doppler and the frequency F
in. Then, the signal is amplified to a predetermined level by the preamplifier 5 and becomes a sinusoidal input signal S2 as shown in FIG.

The input signal S2 is shaped into a rectangular pulse wave as shown in FIG. 2B by the rectangular wave shaping circuit 6, and is output to the one-shot circuit 7 as the square wave signal S3.

In the one-shot circuit 7, the rectangular wave signal S3
A pulse signal S4 having a very small pulse width is generated in synchronization with the rising edge of the pulse signal, and is output to the delay circuit 8 and the sample / hold circuit 10.

The delay circuit 8 delays the pulse signal S4 by a time corresponding to the pulse width of the pulse signal S4 and outputs it to the RC charge / discharge circuit 9 as a switch signal S5.

Switch 9c in RC charge / discharge circuit 9
Operates when the switch signal S5 is input to connect the differentiating circuit 9a to the reference voltage 9b, whereby the capacitor 9aa of the differentiating circuit 9a starts charging.

Since the pulse width of the switch signal S5 is the time during which the capacitor 9aa is completely charged, a constant charge is always stored in the capacitor 9aa at the end of charging.

Thereafter, when the switch signal S5 ends,
The differentiating circuit 9a is disconnected from the reference voltage 9b, and the electric charge accumulated in the capacitor 9aa is discharged via the resistor 9ab.

FIG. 2 (e) shows the discharge voltage, which is attenuated according to a time constant determined by the capacitor 9aa and the resistor 9ab. The discharge voltage is the level displacement signal S
6 and input to the sample / hold circuit 10.

On the other hand, the sample / hold circuit 10
The pulse signal S4 from the one-shot circuit 7 is input, the level displacement signal S6 is sampled and held by the pulse signal S4, and is output as the periodic voltage signal S7.

Since the pulse signal S4 is input to the RC charge / discharge circuit 9 after being delayed by the delay circuit 8 for a predetermined time, the level displacement signal S6 output from the RC charge / discharge circuit 9 is output.
Are also delayed by the same time.

Accordingly, the periodic voltage signal S7 sampled and held and output by the sample / hold circuit 10 is the data immediately before the switch 9c is operated, that is, the level displacement signal S7 immediately before the rise of the level displacement signal S6.
6.

The periodic voltage signal S7 is supplied to the subtractor 1
1 and the subtractor 11 subtracts the subtraction voltage (Vsub) 12 from the periodic voltage signal S7 by the subtracter 11 to obtain the signal shown in FIG.
, Is input to the amplifier 13, amplified by a predetermined gain, and output as an output voltage (Vout) S9.

The frequency Fin of the reflected wave reflected by the moving object undergoes a Doppler shift due to the Doppler effect.
As the Doppler frequency Fd increases, the period of the reflected wave decreases (frequency increases), and the pulse signal S
The interval of 3 becomes smaller. Conversely, if the Doppler frequency Fd decreases, the period of the reflected wave increases (the frequency decreases), and accordingly, the interval between the pulse signals S3 increases.

Therefore, if the decay time of the level displacement signal S6 that attenuates with a time constant determined by the values of the capacitor 9aa and the resistor 9ab corresponds to the period of the reflected wave, the decay voltage at that time corresponds to the period of the reflected wave. The voltage can be determined.

Since the cycle and the frequency of the input signal S2 have a reciprocal relationship, the frequency Fin can be easily obtained from the cycle, and the Doppler frequency Fd can be obtained from Equation 1 using the frequency Fin and the reference frequency Fout.

Therefore, the output voltage S9 is subjected to voltage / frequency conversion by a voltage-frequency converter (not shown) to determine the frequency Fin of the reflected wave.
And a reference frequency Fout. The difference between the frequency and the reference frequency Fout is obtained by a frequency comparison device (not shown), and is output as a Doppler frequency Fd.

As a result, the speed V of the moving object can be known from V = Fd · C / (2 · Fout + Fd) obtained by modifying the equation (2).

In the above description, the level displacement signal S6 is sampled in synchronization with the pulse signal S4 output every cycle of the input signal, and the sampled discharge voltage at that time is output as the periodic voltage signal S7.

In this case, since the level displacement signal S6 is a signal that attenuates monotonously, if the interval between the pulse signals S4 at the time of sampling is different, that is, if the period is different, different discharge voltages are sampled, and the period and the discharge are different. It has a linear relationship with voltage.

However, simply satisfying the linearity,
The error between the frequency obtained based on the level displacement signal S6 and the frequency Fin of the reflected wave increases.

In order to reduce this error, the capacitor 9aa in the differentiating circuit 9a is set so that the level displacement signal S6 has a function dependency represented by (C + V) / (C−V).
And the value of the resistor 9ab must be properly selected. The values of the capacitor 9aa and the resistor 9ab are appropriately set for the frequency band (or the range of the cycle) handled by the speed measuring device and for the required measurement accuracy.

In the above description, the case where the pulse signal S4 is output by the delay circuit 8 as the switch signal S5 with a delay of the pulse width of the pulse signal S4 has been described. Or a new switch signal S5 having the same pulse width as the pulse signal S4 may be generated and output after a predetermined time elapses.

The pulse width of the switch signal S5 does not need to be the same as the pulse width of the pulse signal S4. This is because the pulse width of the switch signal S5 is
This is because it suffices if it is time to completely charge a. On the other hand, the pulse width of the pulse signal S4 may be any time as long as it is received as a signal in the delay circuit 8 and the sample hold circuit 10.

Although the case where the input signal S2 is shaped into a rectangular wave by the square wave shaping circuit 6 has been described, the input signal S2 is not limited to a rectangular wave and may be a triangular wave or the like. That is, since the rectangular wave shaping circuit 6 and the one-shot circuit 7 jointly output a signal for identifying each cycle of the input signal S2, if it is possible to output a signal corresponding to this signal, It is possible to achieve the object of the invention.

The amplifier 13 can be omitted by giving the subtractor 11 a gain.

Furthermore, as a method for obtaining the level displacement signal S6, the RC differentiating circuit 9a using the capacitor 9aa and the resistor 9ab is used. However, the present invention is not limited to this, and a known circuit using a coil or the like is used. It may be.

As described above, an output voltage corresponding to each cycle of the input signal can be obtained. Therefore, even when the frequency of the input signal fluctuates in a short time or when the input signal is input discontinuously, high speed and high accuracy can be obtained. It is possible to determine the period.

Since the values of the capacitor and the resistor can be set as appropriate, when a speed measuring device is constructed using such a period-to-voltage converter, the time dependence of the voltage in the level displacement signal can be reduced by using the moving object of the Doppler frequency. Speed dependency can be sufficiently approximated, and the reliability of the speed measuring device can be improved.

[0070]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a speed measuring apparatus using a period / voltage converter according to the present invention will be described with reference to FIG. Note that the measurement range of the speed measurement device is 5 to 200 km / h.

FIG. 3 shows the reference frequency Fou of the reference oscillator 1.
t is 40 kHz, the capacity of the capacitor 9aa in the RC charge / discharge circuit 9 is 6800 PF, and the value of the resistor 9ab is 3.4.
The level displacement signal S6 when kΩ is represented by time on the horizontal axis,
The vertical axis shows the voltage.

In the figure, time t0 indicates the discharge start time, time t1 is the time when 18.1 μsec has elapsed since the start of discharge,
Time t2 indicates a time when 24.8 μsec has elapsed from the start of discharge.

Then, the frequency Fin when the output voltage S9 based on the level displacement signal S6 at the time t1 is frequency-converted is 55.276 kHz, and the speed V of the moving object at that time is 200 km / h from the equation (1). The output voltage S based on the level displacement signal S6 at time t2
9 is 40.32.
2 kHz, and the speed V of the moving object at that time is 5 km
/ H.

FIG. 4 is a diagram showing a difference (error) between the speed of the moving object obtained using the level displacement signal S6 of FIG. 3 and the speed of the moving object obtained based on the equation (1). As can be understood from this figure, by setting the capacitor 9aa and the resistor 9ab to the above values, the speed of the moving object is 5 km / km.
In the range of h to 200 km / h, the speed error is 0.1
It is within km / h, which enables very high-precision measurement.

[0075]

According to the present invention, the period of the periodic signal is obtained for each period of the periodic signal. Therefore, even if the periodic signal is received continuously or intermittently, the period of the periodic signal can be obtained. It is possible to obtain the cycle at high speed and with high accuracy even if the value fluctuates in a short time.

When a sound wave or the like is reflected by a moving object, a voltage corresponding to the period of a periodic signal obtained by receiving a reflected wave whose frequency has changed due to a Doppler shift due to the Doppler effect is output. , So that the relationship between the moving speed of the moving object and the period of the reflected wave approximates the discharge time of the level displacement signal attenuated by the time constant determined by the values of the capacitor and the resistor, and the discharge voltage at that time. Since the resistance and the resistance value were set, it became possible to measure the speed of a moving object with high accuracy when applied to a speed measurement device.

[Brief description of the drawings]

FIG. 1 is a block diagram of a speed measuring device provided with a period / voltage conversion device according to the present invention.

FIGS. 2A and 2B are timing charts of output waveforms output from respective blocks in FIG. 1, wherein FIG. 2A is an input signal, FIG. 2B is a rectangular wave signal, FIG. 2C is a pulse signal, FIG.
(E) is a level displacement signal, (f) is a periodic voltage signal,
(G) is a subtraction signal, and (h) is an output voltage.

FIG. 3 is a diagram showing a level displacement signal of an RC charge / discharge circuit.

FIG. 4 is an example showing an error between the speed of a moving object obtained by the present invention and the speed of a moving object according to a theoretical formula.

FIG. 5 is a block diagram of a speed measuring device provided with a period / voltage converter applied to the description of the conventional technique.

FIGS. 6A and 6B are timing charts of output waveforms output from each block of FIG. 5, wherein FIG. 6A is a Doppler frequency, FIG. 6B is a rectangular wave signal, FIG. 6C is a detection signal, FIG.
(E) is a count pulse signal.

FIG. 7 is a diagram illustrating a relationship between a speed of a moving object and a Doppler frequency.

FIG. 8 is a diagram illustrating the relationship between the speed of a moving object and the Doppler frequency error.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 reference oscillator 2 driver 3 transmitter 4 receiver 5 preamplifier 6 rectangular wave shaping circuit 7 one-shot circuit 8 delay circuit 9 RC charging / discharging circuit 9a differentiating circuit 9aa capacitor 9ab resistance 10 sample / hold circuit 11 subtractor 13 amplifier

Claims (2)

[Claims] 1. A periodic pulse generating means for shaping an input periodic signal into a rectangular wave signal and outputting a pulse signal in synchronization with the rise or fall of the rectangular wave signal; After a lapse of time, a displacement signal generating means for outputting a level displacement signal whose signal level changes with a predetermined time constant with reference to a predetermined level value, and sampling the level displacement signal in synchronization with the pulse signal; A period-voltage converter for outputting a voltage corresponding to each period of the signal. 2. The method according to claim 1, wherein the periodic signal is a signal of a reflected wave reflected by a moving object and subjected to a Doppler shift, and
The displacement signal generating means includes a capacitor and a resistor, and the values of the capacitor and the resistor are set so as to approximate the relationship between the moving speed of the moving object and the period of the reflected wave. After a lapse of a predetermined time from the input of the signal, the capacitor is charged for a predetermined time to accumulate a certain amount of electric charge, and then the electric charge stored in the capacitor via the resistor with a time constant determined by the capacitor and the resistance. A periodic-to-voltage converter for a periodic signal, which discharges and outputs a level displacement signal.