Abstract:
The subject of this invention is a filter circuit, designed especially for the separation of two channels, with a low-pass filter ( 10, 14 ) and an additional all-pass filter ( 12 ).

Description:
[0001]    This invention refers to an electronic filter circuit designed especially for the separation of two channels with a low-pass filter that is designed preferably as a Chebyshev filter.  
         BACKGROUND OF THE ART  
         [0002]    Filter circuits of this type are well known. They are used in stereo transmitters for the transmission of spatial sound signals. Stereo transmitters transmit a mono-signal, which contains a complete sound impression—so that a mono-signal can be reproduced in a monophonic form—and a second signal, which, as the so-called auxiliary signal, together with the mono-signal, makes possible a spatial impression of the sound signal.  
           [0003]    The auxiliary signal is shifted by 38 kHz in relation to the mono-signal by modulating it upon a subcarrier frequency voltage of a frequency of 38 kHz with a resulting amplitude-modulated signal. Before the transmission of the auxiliary signal, the subcarrier frequency voltage is suppressed. In this connection we wish to make a remark that an additional pilot voltage is transmitted for the purpose of recovering the subcarrier frequency voltage at the receiver&#39;s side. The pilot voltage frequency equals half of the subcarrier voltage frequency, i.e., the frequency of the pilot voltage is 19 kHz.  
           [0004]    Furthermore, there is a stereo-coder, in which a multiplication with the 38 kHz auxiliary signal is performed.  
           [0005]    The stereo-coder can include e.g. a rectifier modulator with diodes connected in ring for the suppression of the carrier voltage of the amplitude-modulated signal. The diodes are switched by the subcarrier frequency voltage. The generated amplitude-modulated signal is, among other things, transmitted to an output terminal through the switched-through diodes. At the exit of the rectifier modulator is generated an auxiliary signal, which contains only the two side oscillations. For the subcarrier frequency voltage to be able to switch the diodes, the subcarrier frequency voltage must be significantly higher than the voltage of the auxiliary signal. Therefore, the stereo transmitter works with a high operational voltage. However, since the diodes are not linear above the range of modulation, undesired non-linear distortion arises.  
           [0006]    As an alternative, analog multipliers—designed especially as integrated circuits—can be used, which often include differential amplifiers consisting of six transistors (the so-called Gilbert cells) with small modulations. However, the disadvantage of this solution includes temperature drift, non-linear distortion and insufficient signal-to-noise spacing due to the very small modulations of the semiconductor characteristics.  
           [0007]    Finally, stereo-coders also exist that scan the signal to be transmitted, the signal containing a mono signal, a pilot signal and an auxiliary signal. The scanning of the signal to be transmitted is performed by an electronic switch that switches periodically. In the switch position “ON”, a capacitor is charged to the instantaneous value of the signal. In the switch position “OFF”, the capacitor holds the previously scanned signal voltage and transfers it to an amplifier with a high-impedance input. If the scanned frequency is smaller than the double of the frequency of the highest partial oscillation of the signal to be transmitted, a signal of a lower frequency (alias frequency signal) arises, and the signal to be transmitted cannot be faithfully recovered. In order to prevent an undesired signal distortion, the stereo-coder includes a low-pass filter (anti-alias filter), which cuts off the signal band. Depending on the particular design, a stereo-coder with a low-pass filter must be balanced/gauged, is dependent on temperature, and is expensive to construct. Therefore, such a switch-coder performs the multiplication with a 38 kHz square-wave signal and not with a sinusoidal signal. The signal amplitudes can represent not only a few milli-volts as with a rectifier modulator or an analog multiplier, but advantageously also several volts, which allows achieving excellent signal-to-noise spacing. Another advantage is that there basically arise no non-linear distortions because no non-linear components are used since the electronic switches are “ideal” to such degree.  
           [0008]    The switched 38 kHz signal consists of dominant waves and harmonic waves. However, the harmonic waves are a disturbance since they reduce the channel separation of the transmitted stereo signals during the demodulation by a receiver. Therefore, a filter circuit is required with a pass-band width, within which the transmitted signal passes as much frequency-independent and undistorted as possible, while the disturbing harmonic oscillation is suppressed as much as possible.  
           [0009]    The known filter circuits according to Chebyshev and the known filter circuits according to Butterworth show a strong frequency-dependent group retardation in the pass-band range. The known filter circuits according to Bessel show a very uneven attenuation in the pass-band range.  
           [0010]    An object of this invention is to design a filter circuit especially for a stereo-coder that is especially suitable for the transmission of sound signals, and in which the disadvantages of the known circuit arrangements, especially a frequency-dependent group retardation and an uneven attenuation in the pass-band range, are prevented.  
         SUMMARY OF THE INVENTION  
         [0011]    According to this invention, this task relating to the filter circuit of the previously described type is resolved in such a manner that an all-pass filter is added.  
           [0012]    Compared to the current state of technology, the filter circuit designed according to this invention can achieve, in a relevant pass-band range, a highly even attenuation and group retardation, i.e., an approximately linear amplitude characteristic and an approximately linear phase characteristic. Using the filter circuit designed according to this invention, stereo-coder can be constructed on the switching principle, with an extraordinary large signal-to-noise spacing with a low distortion and without any balancing. Therefore, the filter or filter circuit designed according to this invention is suitable for the attenuation of harmonic waves that arise in the switch of the stereo-coder due to the switching operations. The filter according to this invention is especially suitable to effectively suppress harmonic waves of the third and fifth order outside the pass-band range, namely in the stop range of the filter, and to guarantee a high-frequency band width of the signal that complies with the strict requirements of the regulating agencies such as the European Telecommunication Standard Institute (ETSI).  
           [0013]    The all-pass filter can be connected to the output of the low-pass filter. However, it is also possible that the low-pass filter include a filter order low-pass, typically passive, which is connected between the input of the filter circuit and the all-pass filter, and possibly also an additional low-pass of the first or higher order, which is connected to the output of the all-pass filter. Such a circuit design could be advantageous to the extent as the (normally negligible) noise of the all-pass filter is eliminated by the subsequent low-pass.  
           [0014]    One embodiment teaches the connection of a passive RC low-pass to the input of the invention-related filter circuit in order to reduce the band width and the rise speed of the slopes of the input signal, and the all-pass filter is connected on the output side of the low-pass filter. This design version of the filter circuit prevents rise distortions, which would normally develop if the signal is generated by an analogue switch with steep signal slopes and transferred to an operational amplifier.  
           [0015]    In one of the design versions, at least two pole points of the filter circuit are in the stop range, preferably at the frequencies to be attenuated, especially with harmonic waves of the scanned frequency and/or the frequencies of the sidebands of the scanned frequency.  
           [0016]    In some embodiments, the low-pass filter will consist of a first low-pass filter of the first order and a subsequently connected low-pass filter preferably of the fifth order. Such a low-pass filter shows a relatively low portion of standing waves so that the signal amplitude in the pass range is especially even. The filter circuit includes a filter terminal impedance connected between the output of the low-pass filter and the ground.  
           [0017]    In many embodiments, the low-pass filter is an active RC low-pass filter. Furthermore, the filter terminal impedance is possibly designed as a capacitor. This design version of the invention minimizes the number of inductive components. Inductive components such as coils are relatively more costly than ohmic resistors and capacitors. Since many embodiments of the invention will minimize the number of inductances, the cost of such a circuit is especially low.  
           [0018]    In many embodiments of the invention, the low-pass filter comprises in-series connected resistors and two frequency-dependent negative resistors (FDNR), which are connected between the connection points of the resistors and the ground.  
           [0019]    If the all-pass filter is designed as an all-pass filter of the first order, the design version is especially simple. The all-pass filter is preferably designed as an active RC all-pass filter. The active RC all-pass filter can be economically manufactured without any inductive components, especially without any coils.  
           [0020]    In many embodiments, a first or a second trap amplifier is incorporated between the input of the invention-related filter circuit and the input of the low-pass filter as well as possibly between the output of the low-pass filter and the output of the filter circuit, particularly between the output of the low-pass filter and the input of the all-pass filter. Due to the first trap amplifier, the components of the input low-pass can be of high impedance and, therefore, can be kept small. The second trap amplifier makes a low-resistance dimensioning of the all-pass circuit possible, so that the resistors of the all-pass circuit emit only a low noise, and the capacitor&#39;s capacitance can be dimensioned relatively big and, therefore, simply and relatively precisely. In this design version, the all-pass filter circuit guarantees a flat frequency characteristic.  
           [0021]    Additional embodiments of this invention are characterized in the related claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The following text describes in more detail design examples of this invention based on drawings, in which:  
         [0023]    [0023]FIG. 1 shows a circuit diagram illustrating the design principle of the filter according to this invention; and  
         [0024]    [0024]FIG. 2 shows a design version of the filter according to this invention with frequency-dependent negative resistors.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The filter shown in FIGS. 1 and 2 comprises a first circuit section  10  that is designed as a low-pass filter of the fifth order, as well as a second circuit section  12  that is designed as an all-pass filter of the first order. The all-pass filter  12  is connected at the output side of the fifth-order low-pass filter  10 . A first-order low-pass filter  14  is connected on the input side of the fifth-order low-pass filter  10 . Trap amplifiers  16 , 18  are connected between the output of the first-order low-pass filter  14  and the input of the fifth-order low-pass filter  10 , and between the output of the fifth-order low-pass filter  10  and the input of the all-pass filter  12 , respectively.  
         [0026]    The low-pass filter  10  of the fifth order is designed as an in-series circuit of three coils L 1 , L 2 , L 3 . The connection points  20 ,  22  between the coils L 1  and L 2  as well as L 2  and L 3  are connected with the ground GND through a series oscillatory circuit, which comprises a coil L 4  and a coil L 5  as well as a capacitors C 1  and C 2 . The output  24  of the low-pass filter  10  is terminated in the ground GND by means of a resistor R 1 .  
         [0027]    The all-pass filter  12  comprises three resistors R 2 , R 3 , R 4 , a capacitor C 3  as well as an operational amplifier Op 1 . Both the resistor R 2  and the resistor R 3  are connected at the input  42  of the all-pass filter  12 . The resistor R 2  is connected with the inverting input  26  and the resistor R 3  is connected with the non-inverting input  28  of the operational amplifier Op 1 . The resistor R 4  forms a feedback between the output  30  of the operational amplifier Op 1  and its inverting input  26 , while the non-inverting input  28  of the operational amplifier Op 1  is, in addition, connected to the ground GND through the capacitor C 3 . The third resistor R 4  as well as the output  30  of the operational amplifier are connected to the output  32  of the all-pass filter  12 .  
         [0028]    The low-pass filter  14  of the first order comprises a resistor R 5 , which is located between the input  34  of the circuit and a capacitor C 4  connected to the ground GND. In this way, the input of the low-pass filter  14  simultaneously forms the input  34  of the circuit, and the connection points of the resistor R 5  and the capacitor C 4  form the output  36  of the low-pass filter  14 .  
         [0029]    The trap amplifier  16  comprises an operational amplifier Op 2 , and is connected between the output  36  of the low-pass filter  14  and the input  38  of the low-pass filter  10  in such a manner that the non-inverting input of the operational amplifier Op 2  is connected to the output  36  of the low-pass filter  14 , and the output of the operational amplifier Op 2  is coupled as a feedback to its inverting input. Furthermore, the output of the operational amplifier Op 2  is connected to the input  38  of the low- pass filter  10 .  
         [0030]    The second trap amplifier  18  comprises an operational amplifier Op 3.  The non-inverting input of the operational amplifier Op 3  is connected to the output  24  of the low-pass filter  10 , while the output of this operational amplifier Op 3  is coupled as a feedback to its inverting input.  
         [0031]    Furthermore, the output of the operational amplifier Op 3  is connected to the input  42  of the all- pass filter  12 .  
         [0032]    The low-pass filter  14  reduces the rise speed of the slope especially with impulse signals and thus prevents rise distortions that would normally arise when driving an amplifier with such signals. The first trap amplifier  16  provides, independent from the frequency, an output resistance of zero Ohm. The first trap amplifier  16  allows the second low-pass filter  14  to be dimensioned with high-impedance. Due to the second trap amplifier  18 , which is connected at the output side of the first low-pass filter  10  and has the same design as the first trap amplifier  16 , the all-pass filter  12 , which is connected at the output side of the second trap amplifier  18 , can be dimensioned with low resistance, which minimizes the disadvantageous effects of resistor noise and capacitor tolerances.  
         [0033]    The circuit arrangement according to this invention illustrated in FIG. 2 differs from the circuit arrangement illustrated in FIG. 1 in that the low-pass filter  10  is designed as an active low-pass filter that comprises a combination of operational amplifiers Op 4  to Op 7 , resistors R 6  to R 8 , R 9  to R 12  and R 13  to R 16 , and capacitors C 5  to C 8 . The resistors R 6  to R 7  and R 8  are connected in-series between the input  38  and the output  24  of the first low-pass filter  10 . Frequency-dependent negative resistors  46  and  48  are connected to the connection points  43  and  44  between resistors R 6  and R 7  as well as R 7  and R 8  through a resistor R 9  and R 13 , accordingly. Both frequency-dependent resistors  46  and  48  are connected against ground. Each of the two frequency-dependent negative resistors  46  and  48  comprises a series connection consisting of a first capacitor C 5  and C 7 , a first resistor R 10  and R 14 , a second capacitor C 6  and C 8 , a second resistor R 11  and R 15 , and a third resistor R 12  and R 16 , as well as of two operational amplifiers Op 5  and Op 6  and Op 7  and Op 8 .  
         [0034]    The inverting input  50  of the first operational amplifier Op 5  is connected to the connecting point  62  between resistor R 9  and the first capacitor C 5 . The non-inverting input  52  of the first operational amplifier Op 5  is connected to the connecting point  70  between the second resistor R 11  and the third resistor R 12 . The output  64  of the operational amplifier Op 5  is connected to the connecting point  64  between the first capacitor C 5  and the first resistor R 10 . The inverting input  72  of the second operational amplifier Op 6  is connected to the connecting point  66  between resistor R 10  and the second capacitor C 6  The non-inverting input  74  of the operational amplifier Op 6  is connected to the connecting point  70  between the second resistor R 11  and the third resistor R 12 . The output  76  of the operational amplifier Op 6  is connected to the connecting point  68  between the second capacitor C 6  and the second resistor R 11 .  
         [0035]    The frequency-dependent negative resistor  48  has an analog circuit design.  
         [0036]    According to the described design example, the low-pass filter  14  and low-pass filter  10  together form a low-pass of the sixth order. The low-pass filter  14  of the first order, which is connected after the input  34 , is designed to reduce or eliminate slewing distortions, and the low-pass filter  10  performs the additional five orders, e.g. by means of frequency-dependent negative resistors as shown in the circuit arrangement illustrated in FIG. 2.  
         [0037]    In an especially preferred design version as shown in the circuit design illustrated in FIG. 2, the circuit elements have the following values:  
                                                     Index   R/Ω   C/pF                                1   —   —       2   2210   —       3   2370   1000       4   2210   47       5   22186   1000       6   1850   1000       7   2870   1000       8   402   1000       9   1050   1000       10   2320   —       11   2260   —       12   4750   —       13   432   —       14   2050   —       15   2210   —       16   5110   —                  
 
         [0038]    The dimensioning of the circuit elements in the circuit according to this invention is not restricted to the aforementioned example. On the contrary, additional advantage of this invention can be achieved by combining the circuit elements and their values.