DE2330357C3 - Bandpass filter circuit - Google Patents

Description

Usually, a band-pass filter circuit has a band-pass filter adapted to pass only the signals in the desired band and a limiter circuit. If the input signal frequency to the center frequency of Δ / ₀ fluctuates / the band pass filter, band-pass filter in such a corresponding change of Δ θ in the output stage of the bandpass filter is present because of its phase characteristic.

In addition, there is a possibility that the center frequency / 0 of the bandpass filter may differ due to Temperature changes or aging can change, although the input signal frequency is thus constant In this case too, an output phase difference occurs. In addition, there is a likelihood that such output phase differences have adverse effects when used in circuits that employ a require very precise phase synchronization, e.g. B. Carrier regeneration circuits of multiphase PCM systems.

An AM demodulator is known in which a * 5 Phase shift between the input signal and the output signal by a phase detector is compensated (VHF reports, vol. 11, September 1971, p. 144). The phase shifter generates eim independent of the frequency of the input signal! Phase shift, which is why a stationary phase error of the negative feedback loop occurs

Also known is a band-pass filter circuit with a phase detector which determines the phase difference between see the input and output signals to and in front of a bandpass filter, with a voltage ge controlled oscillator, which has a low-pass filter through dai Output signal from the phase detector is controlled, with first and second frequency converters, the upstream or downstream of the bandpass filter and their conversion ratio through the output signal of the voltage-controlled oscillator is determined by isl (DT-OS 21 39 039). In this known arrangement in which an AFC circuit is provided to keep the input frequency constant, the stationary phase error reduced, but not reduced to zero.

The invention is based on the object of providing a bandpass filter circuit of the type mentioned above to create, which is designed so that stationary phase errors can be reduced to zero. Solved this task is achieved by the Merkn.ale specified in the characterizing part of claim 1. Refinements of the invention are specified in the subclaims.

In addition to reducing the stationary phase error to zero, the invention also enables a Phase difference, which is based on factors outside the circuit, suppressed or reduced to zero will.

The invention is described by way of example with reference to the drawing in which is

F i g. 1 is a block diagram showing the principle of the bandpass filter circuit the invention,

F i g. 2 oscillation and phase frequency characteristics of the bandpass filter,

3 (a) and 3 (b) are exemplary circuit diagrams of the circuit according to FIG. 1,

4 shows a frequency characteristic of the constant phase difference, in which the delay time of the delay circuit DL is used as a parameter,

F i g. Figure 5 is a block diagram of an embodiment of the invention in which factors are out of the loop are included,

F i g. 6 is a diagram for explaining the compensation of a phase delay due to factors outside the loop and

F i g. 7 a circuit diagram of a delay circuit DL used

F i g. 1 is a block diagram illustrating an embodiment of the band pass filter circuit of the invention. The input signal frequency is subjected to a frequency conversion in the first frequency converter FCKl in accordance with the output signal of the voltage-controlled oscillator VCO and after passing through the bandpass filter BPF a frequency conversion is carried out again in order to obtain an input signal frequency at the second frequency converter, which ultimately becomes the output signal. The phases of the input and output signals of the bandpass filter BPF are determined by the phase detector. The output signal obtained in accordance with the input and output phase difference is applied to the voltage controlled oscillator VCO through a low pass filter LF.

In this way, the frequency converters FCV1 and FCK2 mentioned above are to be read

Oscillation frequency is controlled according to the output signal of the voltage controlled oscillator When there is a difference between the input and output phases, the oscillation frequency of the voltage controlled oscillator VCO becomes so controlled that the phase difference is kept constant. However, since the loop gain with this species a circuit arrangement of the band-pass filter is not unlimited, necessarily becomes a constant one Phase errors generated,

This can be said to be a disadvantage of a band pass filter of this type.

In order to reduce the constant phase error to zero, according to the invention, the output oscillation of the voltage-controlled oscillator is applied to the first frequency converter FCV \ via the delay circuit DL

The principle of the invention is described in detail below.

Assuming that the input carrier frequency ωό, the center frequency of the bandpass filter ω / and the center frequency of the voltage controlled oscillator VCOw are vs, the following equation is obtained:
ω / = (U ₀ - cot). (1)

If the input carrier frequency is changed from ωο by Δ ω, the result is ω _ο + Δω.

The oscillation frequency of the voltage controlled oscillator VCO changes by Δ ω ' and becomes ω ν + Δ ω' because it is controlled in accordance with the output of the phase detector PD to keep the input frequency of the bandpass filter constant.

As a result, the input signal frequency of the band pass filter BPF becomes as follows:

(D ₀ + Αω - (ων + Λω ') = ω / + (1 ω - Λ ω).

(2)
However, there the loop gain

35

G =

Λ ω 'Λ ω - .1 ιο

is, we get \ Δω \> \ Δω '\.

The bandpass filter BPF 'is a device having an oscillation characteristic a and a phase characteristic b as shown in Fig. 2. When the frequency of the input signal to the bandpass filter BPF differs from the center frequency ω / of the bandpass filter by Δω , the output of the bandpass filter becomes BPFB.

This phase difference between the input and output signals of the bandpass filter is determined by the phase detector PD which controls the voltage-controlled oscillator VCO

Since (Δω-Δ ω ') in equation (2) is proportional to the change Δ ω in the oscillation frequency of the voltage controlled oscillator VCO , the following applies

= K ■ Λ υ / = α · Λ ω.

(3)

Assuming that the delay time τ of the delay circuit becomes DL

= K ■ K ',

(5)

the constant phase difference θ can be reduced to zero.

Assuming that the initial phase difference in the input signal! according to FIG. 1 becomes zero, the signals at points A through Fin can be expressed as follows:

The signal at point A or the input signal to the band pass filter circuit becomes

A ; cos (<u ₀

(6)

The signal at point B or the signal obtained by passing the output oscillation of the voltage controlled oscillator VCO through the delay line DL becomes

B ; cos {(ων + Λ (u ') t - τ · Δω')

(7)

The signal at point C or the output signal from the first frequency converter FCVl is obtained via a low-pass filter which recovers the low-frequency component from the product of the multiplication of the signals A and B again

Here, the low frequency component has been restored in place of the high frequency component to carry out suppression and operate the band pass filter BPF at low frequency.

It applies

(i) ') t - τ / J ω'}
[COS {((U ₀ + ων + Λ ω + Δω ') ί - ι · Δω'} ί

COS (o) ₀ + Au)) t ■ COS {((uü
-γ

Here, K and a are constants of proportionality that are positive and greater than zero.

From the phase characteristic in Fig . 2 it follows that the phase differences θ and (Δω-Δ ω ') are proportional in a range near the center frequency ω / of the bandpass filter, which can be expressed in the following way: (> 5

θ = -K '(Δω- Δα,') = -K '■ K ■ Λω'. (4)

In this, K ' is not a constant of proportionality> 0.

+ COS {(θ) ₀ - ων + A (UA ω ') ί + T · Λ <//}].

Therefore, the signal C represents the difference component (low frequency component) expressed as follows will

C; COS {((U ₀ - ων + Δω - Αω ') ΐ + τ- Δω'} = COS {(«</ + KA ω ') ί + τ- Δω'}. (8)

A signal at point D or the output signal of the bandpass filter BPF has the following form, since the constant phase error of this bandpass filter BPF is given by θ:

D ; cos {(«j / + KA (o ') t + Θ + τ ■ A ω'}. (9)

A signal at point E or the output signal of the voltage controlled oscillator VCO is

F ,; cos (ω ν + Δ ω ') t.

A signal at point F or the output signal of the second frequency converter FCV2 is obtained by multiplying the signals D and E and reproducing the high-frequency component via a high-pass filter.

Because of the following relationship, the signal at point F is the sum component (high frequency component).

From (1) and (3) is obtained

F ; COsftoj / + ων + K ■ Λ ω + A ω) ί + θ + τ ■ , 1 ω}

= cos («j ₀ + Δ ω) ί + Θ + τ · / 1 ω) (H)

α = COs (OJ ₀ + Αω) ί + Θ + τ- -JT- Δ ω.

Here, θ and τ ■ Δω ', as obtained from (4) and (5), θ K- K'- Δω' = - τ ■ Δω '.

Therefore, the equation becomes the signal at point F or the output of the bandpass filter 11 to represent:

Δω) ί.

(12)

This expression is the same as (6) which equation gives the input waveform. As a result, the constant phase error can be reduced to zero by setting the delay time τ of the delay circuit DL as in equation (5).

F i g. 3 (a) and 3 (b) show in detail an example of the band-pass filter circuit of the present invention as shown in FIG Fig. 1.

The FCVi section of the frequency converter shown in FIG. 3 (a) is formed from three sets of amplifiers in emitter circuit, which consist of transistors TR 1 to TR 3, capacitors Cl to C 7, resistors R 1 to R 13, a double balanced mixer from transformers 7Ί and 7 * 2, diodes Di to D 4 and a low-pass filter with inductors CX and C2 and a capacitor 8 exist.

The signal from the input terminal A is amplified by the transistor amplifier TRi and mixed in the double balanced mixer with the signal, which is the output signal of the voltage controlled oscillator VCO , and received via a delay line.

This signal from the delay line is supplied from the terminal B through the amplifier TR 2

The signal subjected to suppression by the double balanced mixer passes through the amplifier of the transistor TR 3, and unnecessary noise is filtered out by the low-pass filter. The output from this low-pass filter is given to the band-pass filter fl /> F.

In the bandpass filter, transistors TR 4 and 77? 5, capacitors C9 to CIl and resistors /? 14 to R 23 form two sets of amplifiers in a common emitter circuit and diodes DS and D 6 form an amplitude limiter, the bandpass filter BPF being a helical filter with a center frequency / b— // (input frequency) - fVCO (oscillation frequency of the voltage controlled oscillator VCO)

The helical filter used is a distributed constant filter that has a high <? ensures

The signal from FCVi of the first frequency converter is in the transistor 77? 4 amplified and fed to the bandpass filter BPFi The signal, after it has been freed of noise in the bandpass filter SFFl, is fed to the amplitude limiter.An output signal of the amplitude limiter is divided into two signals and is then sent to the second frequency converter FCV2 (connection D in Fig. 3 ( a) and the phase detector PD supplied

In the phase detector PD , transistors TRi and TR 7, capacitors C12 to C15 and resistors R 24 to R 3t form two emitter circuit amplifiers and transformers 7 * 3 and 7 * 4 and diodes Dl bii Z? 10 form a phase detector in the manner of a double balanced mixer, while DL 1 acts as a delay line in order to generate a phase shift of till radians for the center frequency f ₀ zi.

ίο A phase comparison between the input signal! of the helical filter (this is obtained, unlike in FIG. 1, via the amplifier which is formed from the transistor 77-2 in the bandpass filter BPF) of the bandpass filter BPF and the output signal of the amplitude limiter is carried out in the phase detector PD . The signals are each amplified by the transistors TR 6 and TR 7 and then fed to the doubly balanced phase detector DL i is provided in order to set a phase bias in the doubly balanced phase detector.

An output signal from the double-balanced phase detector in the phase detector part PD is compared to the one shown in FIG. 3 (b) shown low-pass filter LF is supplied via the terminal G.

Form in the low-pass filter part LF as shown in Fig. 3 (b). Resistors / 32 and 33 and a capacitor C16 form a low-pass filter, while a differential amplifier / Cl, the capacitor C16 and resistors / 34 and R 35 form a direct current amplifier. The output from the phase detector PD [Fig. 3 (a)] goes through the low-pass filter via connection G [Fig. 3 (b)] and is amplified by the direct current amplifier (differential amplifier / C1) and finally the voltage controlled oscillator

VCO supplied

In the voltage controlled oscillator VCO , a transistor 77? 8, capacitors C17 to C19, resistors R 36 to Ä38, an inductance L 3 and a variable capacitance diode DIl a voltage

controlled Hartley-type oscillator while transistor 77? 9, capacitors C20 and C21 and resistors R 39 to Ä41 form an emitter circuit amplifier. The output signal of the low-pass filter is sent to the voltage-controlled

Oscillator VCO applied

Then the output signal of this voltage controlled oscillator KCO is amplified and converted into two signals in transistor 77? 9 split
One of the two signals obtained in this way

becomes directly to the second frequency converter FCV2

given as logic signal E. The other signal H

is applied to the delay circuit DL , which is a

represents an essential feature of the invention

In the delay circuit DL , a transistor / R10, a capacitor C22 and resistors R 42 to /? 46 form an emitter circuit amplifier, and DL2 is a hetical filter with a signal delay effect. The signal (at terminal H) from the voltage controlled oscillator VCO runs over the hetical filter

6c DL 2 and after the necessary phase shift by the transistor amplifier 71? Finally, the signal is sent to the first frequency converter FCVi (connection Binder figure). Form FCV2 in the second frequency converter

Transistors 77? II to TR 14, resistors λ47 to i 64 and capacitors C23 to C30 four sets of common-emitter amplifiers. Transformers T6 and 77 and diodes D12 to D15 make one double

Balanced mixer and a transistor TRiS, a resistor R 65 and a capacitor C30 form an amplifier in a collector circuit, while capacitors C31 and C32 and an inductance L 4 form a high-pass filter. The output from the band pass filter BPF [ Fig. 3 (a)] is passed through a terminal D , mixed with the output signal from the voltage controlled oscillator VCO in the double balanced mixer and finally subjected to suppression.

The output from the double balanced mixer is amplified by the transistor amplifier TR 13 and then unnecessary noise is cut off in the high pass filter. Ultimately, the output signal from the high-pass filter to the output terminal F via the transistor amplifiers TR 14 and 77? 15 transferred.

F i g. 4 shows the normal phase error frequency characteristic when the delay time τ of the delay line DL is used as a parameter. In this case, the origin is _{assumed to be the input carrier frequency of ω 0} with the horizontal axis as the frequency fluctuation Δ ω and the vertical axis as the phase error Θ.

Due to the above-mentioned equation (5), ie τ = Κ ■ K ', the normal phase difference Θ is reduced to zero. If τ> K ■ K 'or τ <Κ ■ K', the characteristics a and b are changed.

When the phase delay element, e.g. B. the cable CB, is outside the loop, as in FIG. 5, a constant phase error is generated due to the delay time of τ 'of the cable CB. Assuming that the phase delay characteristic of the cable is as shown in Fig.6 (line FI) , and if the phase delay characteristic between points A and Fin Fig.5 is set as shown by the Line A-F is shown, the characteristic of the phase difference between the points A and / becomes as shown by the line AI.

Therefore, a constant phase difference including the phase delay elements can be outside of the loop can be compensated.

ίο If phases can be changed as a result of changing the frequency of the delay circuit DL according to the example above, this will suffice as a function of the delay line DL. As in Fig. 7 is shown, a filter circuit consisting of

■ 5 capacitors Cl and C2 and an inductance L is formed, or a delay line cable can be used.

If a delay line DL can be implemented with a delay time -τ , this delay line can also be connected between the frequency converter on the output side, ie the second frequency converter FCV 2, and the voltage-controlled oscillator VCO .
As explained above, not only can the invention reduce the constant phase difference due to factors in the loop, but, if necessary, the phase difference due to factors outside the loop can be reduced to zero by controlling the phase of the signal sent from the voltage-controlled oscillator VCO is fed to the frequency converter on the input side, ie the first frequency converter FCKl, with the aid of a phase control circuit such as a delay line.

In addition 5 sheets of drawings

709603/276

Claims (4)

Patent claims: 1. Bandpass filter circuit with a phase detector that determines the phase difference between the input s and output signals to and from a bandpass filter, with a voltage-controlled oscillator that is controlled by the output signal from the phase detector via a low-pass filter, with first and second frequency converters, which the Bandpass filters are connected upstream or downstream and their conversion ratio is determined by the output signal of the voltage-controlled oscillator, characterized in that a phase control circuit (DL) between the first frequency converter (FCV 1) on the input side of the bandpass filter and the voltage-controlled oscillator (VCO) is switched, the effective phase shift of which depends on the instantaneous frequency of the voltage-controlled oscillator, so that the input signal and the output signal of the bandpass filter always have a constant phase shift or the phase shift zero. 2. band-pass filter circuit according to claim 1, characterized in that the phase control circuit is a delay line 3. band-pass filter circuit according to claim 1, characterized in that the phase control circuit is a filter circuit. 4. Bandpass filter circuit according to claim 1, characterized in that the first frequency converter (FCVX) from the multiplication of the input signal and the output signal of the voltage-controlled oscillator (VCO) generates and outputs only the difference component and that the second frequency converter (FCV2) from the multiplication of the input signal of the frequency converter and the output signal of the voltage-controlled oscillator only generates and emits the sum component.