Method and apparatus for supplying a system for processing electrical signals

In a circuit supplying a digital system, the frequency at which the switching device therein is controlled is synchronized to be the same as one or several times half the sampling frequency. In this way, the original frequency spectrum can be kept free from interfering signals.

FIELD OF THE INVENTION 
The present invention relates to method and apparatus for supplying power 
at a predetermined level to a system for processing electrical signals 
obtained from a main signal by sampling at a particular frequency and 
including a switching device for producing stable frequency electrical 
pulses. 
BACKGROUND OF THE INVENTION 
Switching networks operating with a constant switching frequency and using 
pulse width modulation are per se known. Such known systems are used for 
supplying electronic systems with direct current or direct voltage. These 
prior art switching networks are operated at the frequency of their own 
internal oscillator. 
The aforementioned systems include inter alia digitally operating audio 
systems, in which the information signal is sampled at a frequency fs and 
in which the resulting pulse-like signals are then processed. Since the 
oscillator frequency fo in the supplying network is different from the 
sampling frequency fs, it is not possible to avoid undesired signals, such 
as e.g. interference in such systems which have a strong spectral 
component at the sampling frequency fs and a strong component at the 
oscillator frequency fo. Such signals can occur as digital signals or as 
signals sampled with the sampling frequency fs. 
More particularly, such interference often occurs in analog - digital 
converters. The new signals resulting from the interference can be in the 
audible range of the human ear and the frequency of such interference 
signals within the audible frequency range can change. More particularly, 
this occurs if the oscillator frequency fo is not absolutely constant in 
the switching network. For example, such frequency changes occur in the 
case of varying loads on the oscillator in the power supply. 
SUMMARY OF THE INVENTION 
The problem to which the present invention is directed is to provide an 
apparatus for supplying digitally operating systems so that the spectrum 
of the original signal is free from the aforementioned interfering 
signals. 
The invention is based on synchronization between the sampling frequency 
and the switching frequency of the switching device to thereby avoid 
interference between them. Hitherto, interference was possible between a 
signal frequency, the sampling frequency, the corresponding Nyquist 
frequency and the switching frequency. Due to the fact that the switching 
frequency coincides with the Nyquist frequency or the sampling frequency, 
certain interference formation possibilities are eliminated. Thus, in the 
case of digital audio systems, it is possible using the invention to avoid 
fixed interfering frequencies in the audible range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an apparatus for supplying a system 2, which is used for 
processing signals resulting from the sampling of an input or information 
signal with a given frequency fs. System 2 can e.g. be digital audio 
systems for recording or reproducing sound signals or individual devices 
for such systems, in which sampling values or signals are to be processed. 
System 2 can have its own integrated sampling frequency generator or the 
sampling frequency can be supplied to system 2 from an external source. 
For example, the sampling frequency fs can emanate from a sampling 
frequency generator 3, which can synchronize system 2 and the present 
apparatus with half the sampling frequency fs/2 or with a multiple of one 
or more times half the sampling frequency fs/2. 
The circuit 1 shown in FIG. 1 contains a transformer 4. A rectifier bridge 
5 and a switching device 6 are connected to the secondary winding of 
transformer 4. In the represented embodiment, the switching device 6 
comprises a transistor 60. However, several transistors can also be used 
in such a switching device. All of the various means able to perform the 
same function are covered by the term "switching device". 
A line 7 connects the rectifier bridge 5 to the switching device 6, while 
another line 8 connects the switching device 6 to system 2. A third line 9 
connects the second pole of the rectifier bridge 5 to system 2. Switching 
device 6 has a control input 10, which is located on the base of 
transistor 60 and is connected to a pulse width regulator 11. Lines 12 and 
13 are used for supplyihg the pulse width regulator 11. A line 14 is used 
to indicate the output voltage-to-current ratios in line 8 to the pulse 
width regulator 11. 
An oscillator 17 is connected between lines 7 and 9 by lines 15 and 16 
respectively. The output terminal of oscillator 17 is connected to one of 
the poles of a reversing switch 18, which is connected to the pulse width 
regulator 11. The other pole of the reversing switch 18 is connected to a 
line 19, which connects the sampling frequency generator 3 to system 2. 
A voltmeter 20 is connected to lines 8 and 9 by lines 21 and 22 
respectively. A further line 23 connects the latter with the reversing 
switch 18 and is used for the transmission of switching instructions. 
"Switch" 18 can be a relay with the signals on line 23 operating the coil 
thereof. This function could also be performed using a device such as a 
solid state controlled switching device, such as an FET switch, and for 
this reason line 23 is shown dashed. All such means are intended to be 
covered by the terms "switch", "switching device" and the like as used 
herein. 
FIG. 2 shows the possible frequency response of a system 2 connected to the 
apparatus. In FIG. 2 the amplitude is given in dB on the vertical axis 24 
and the frequency is on the horizontal axis 25. The operating range of 
system 2 is below half the sampling frequency fs/2. Signals with higher 
frequencies than fs/2 are filtered out by filters not shown in a per se 
known manner. A transition region between permitted signals and undesired 
signals is indicated in simplified form by line 26. According to the 
Shannon sampling theorum, the original spectrum 27 appears as a 
convolution 28, 29 and 30 around the sampling frequency fs. 
A main function of the circuit 1 of the present invention is to supply 
system 2 with a given power level at a specific and as far as possible 
constant voltage across lines 8 and 9. Generally, this power is taken from 
an alternating current network on the primary coil of transformer 4. The 
secondary winding of transformer 4 takes this power from the network and 
carries it into the rectifier bridge 5 which supplies direct current to 
lines 7 and 9. In order to be able to adapt the supplied power to the 
characteristics of system 2, the switching device 6 interrupts the 
connection between lines 7 and 8 in a periodic manner. The current pulses 
occurring in line 8 are smoothed in a per se known manner by means not 
shown. Although said current pulses always have the same voltage, they do 
not have the same pulse duration and/or width. In per se known manner, the 
pulse width is modified by pulse width generator 11. Such pulses then 
control the switching device 6 correspondingly. The frequency at which the 
switching device 6 is controlled is constant and is normally determined by 
the oscillator 17, when the reversing switch 18 is in the corresponding 
position, opposite that shown in FIG. 1. 
In sensitive systems 2, in part dominant signals appear, which on the one 
hand have the sampling frequency fs and which on the other hand have the 
oscillator frequency fo. These signals interfere with one another, so that 
in certain circumstances further signals occur at frequencies fi, whose 
frequency can be relatively low. The formula for the resultant 
interference frequencies is: 
EQU fi=((m.multidot.fs/2)+(n.multidot.fo/2)) modulo fs/2 
in which m and n can be random positive or negative integers. The modulo 
fs/2 operation indicates that the positive or negative frequencies 
resulting from the use of m and n are convoluted back into the base band 
fs/2, as is known in sampling systems. If fs=48 kHz and fo=20 kHz, then 
e.g. a frequency fi=4 kHz is obtained with m=+1 and n=-2 in the formula. 
If system 2 is e.g. an audio system, then signals with frequency fi can be 
easily heard. 
When reversing switch 18 is connected to the sampling frequency generator 3 
in the manner shown in FIG. 1, then the pulse frequency of pulse width 
regulator 11 and consequently also the switching device 6 is determined by 
the sampling frequency fs. Sampling frequency fs reaches the pulse width 
regulator 11 either from sampling frequency generator 3 or from system 2 
if no separate sampling frequency generator 3 is provided. 
Voltmeter 20 monitors the voltage in lines 8 and 9. If the voltage between 
lines 8 and 9 exceeds the desired value, then voltmeter 20 supplies a 
switching instruction to the reversing switch 18 on line 23. As a result 
of this switching instruction, switching device 6 is again connected to 
oscillator 17 with frequency fo. This occurs mainly during switching 
processes on and off, as well as in the event of a fault in the power 
supply. 
Where several circuits 1 are provided in one apparatus for one or more 
systems 2 as shown in FIG. 6, it is advantageous to synchronize their 
switching devices 6 to the same sampling frequency. This can be done by 
providing a common sampling frequency generator 3 for all of the circuits 
1, in such a "ganged" system, as shown in FIG. 6. FIG. 6 shows, for 
example, a common voltage source 31 connected through lines 62 to each one 
of three circuits 1. Three systems 2 are connected to the circuits 1 
through lines 8 and 9. A common sampling frequency generator 3, shown here 
as a separate element, may also be part of one of the systems 2. The 
sampling frequency generator 3 is connected through lines 63 to all 
systems 2 for synchronizing them to the sampling frequency, but it is also 
connected through a line 64 to the circuits 1 in order to transmit the 
sampling frequency to each of those circuits 1. The sampling frequency 
generator 3 may also be connected through lines 65 to a power supply such 
as for example one of the circuits 1. 
It can be advantageous for the different circuits 1 to operate against one 
another with a phase displacement. In the case of a number of circuits 1, 
the phase displacement between the individual circuits can be 2pi/n, where 
n indicates the number of circuits 1 involved. This is shown in FIG. 7 
where pulses 69 of signals 66, 67 and 68 each corresponding to signals 
transmitted by line 8 of a circuit 1 are phase displaced by a factor 
amounting to 2pi/3. The voltage of the subsequently smoothed signal is 
indicated by a line 70. This has an advantage that the primary side 
loading of transformer 4 is more uniform. 
For supplying a system 2 with direct current the present apparatus can also 
be constructed in such a way that the frequency of the alternating current 
from which direct current is obtained is one or several times half of the 
sampling frequency fs. A corresponding circuit arrangement is shown is 
FIG. 3. 
The circuit 1a shown in FIG. 3 is supplied with power from an a.c. voltage, 
e.g. mains voltage source 31. Parts in FIG. 3 the same as or similar to 
parts shown in FIG. 1 are indicated by the same reference numeral or the 
same reference numeral followed by "a", respectively. The voltage is 
supplied via lines 32, 33 to a per se known converter 34, which converts 
the voltage supplied into an a.c. voltage with a frequency which is a 
multiple of one or several times half of the sampling frequency fs. Lines 
35, 36 connect the output of converter 34 to a transformer 37, which is 
series-connected to a controllable rectifier 40 which acts like the 
aforementioned switching device 6 across lines 38, 39. Rectifier 40 is 
preferably constructed as a FET-bridge. 
The embodiment of FIG. 3 operates on fs, or fs/2 or an integer multiple of 
it. The converter 34 and the rectifier 40 also operate on said frequency. 
Otherwise, interferences between the frequency on which the converter 34 
and the rectifier 40 operate and the frequency of the system 2 (that is 
the sampling frequency) would be possible and the resulting signals could 
disturb the sampled signals within the system 2. 
Rectifier 40 is connected across further lines 41, 42 to system 2 and to 
the sampling frequency generator 3. An inductor 43 is connected in line 41 
which, together with a capacitor 44 which is connected across a line 45 
between lines 41 and 42, forms a means for smoothing the current pulses. 
The sampling frequency generator 3 is connected via lines 46, 47 and a 
transmitter 48 is connected to converter 34. The voltmeter 20 is placed 
across lines 41 and 42/45. A control unit 49 is connected by means of 
lines 50, 51 and 52 to rectifier 40, sampling frequency generator 3 and 
voltmeter 20. Line 19 connects the sampling frequency generator 3 to 
system 2, as in FIG. 1. 
The transmitter 47 and the transformer 37 also serve to separate or 
electrically isolate the signal handling parts of the circuit from the 
source voltage 31. 
It is assumed that the voltage source 31 supplies a sinusoidal voltage 
(FIG. 4), e.g. a mains voltage, which is converted in converter 34 to a 
higher frequency sinusoidal voltage 53. Voltage 53 is supplied across 
transformer 37 to the controllable rectifier 40, which produces from said 
voltage 53 a rectified voltage 53, 54. This takes place at times indicated 
by arrows 55. In this case and at such time, system 2 is supplied with 
maximum power. 
For the control of circuit 1a, on the one hand converter 34 receives a 
timing signal from sampling frequency generator 3 via lines 46, 47 and 
transmitter 48, and on the other hand rectifier 40 receives the same 
timing signal or a corresponding frequency-divided (with integral parts) 
timing signal on line 50 from control 49. For this purpose, control unit 
49 is controlled with the timing signal via line 51. 
However, if system 2 receives too much power in this way, then this is 
detected by voltmeter 20 in the form of an excessively high voltage on 
lines 41, 42 and/or 45 and said voltmeter supplies a control signal to 
control unit 49 via line 52. Control unit 49 converts the control signal 
for rectifier 40 which had been supplied at regular intervals in 
accordance with arrows 55, in such a way that new control signals appear 
at times indicated by arrows 56 and 57. The distance between two adjacent 
arrows 56 or two adjacent arrows 57 corresponds to the time spacing of one 
period or cycle , as with arrows 55. However, now the period is displaced 
by one spacing a and is subdivided into spacings a plus b plus a. 
This means that rectifier 40, instead of switching over at times 
corresponding to arrows 55, now switches over at times corresponding to 
arrows 56 and then earlier at the times corresponding to arrows 57. This 
gives a voltage gradient 58 as shown in FIG. 5. The sum of the regions 
between voltage gradient 58 and the time axis t gives the new power. 
Particularly due to the negative power fractions indicated by regions 59, 
60, said power is smaller than that corresponding to those regions defined 
by time axis t and voltage curves 53, 54 in FIG. 4. 
If the power level is to be further reduced the spacing a must be 
increased. This is determined in control unit 49, which e.g. contains a 
read-only memory, which stores switchover times for rectifier 40 at given 
voltages which then act as memory addresses. 
The constant voltage to be kept on the lines 8 and 9 corresponds to a 
precise value for the spacing a. If the voltage measured by the voltmeter 
20 is slightly greater, then the control unit 49 will cause the spacing a 
to increase. This also means that the power delivered to the system 2 
increases. If the voltage measured by the voltmeter 20 is slightly smaller 
than the value to be kept, then the control unit will cause the spacing a 
to decrease. Accordingly, the power will then increase. Therefore, the 
spacing b will decrease or increase accordingly. 
It should be clear that also in the case of the embodiment of FIG. 3 of the 
present apparatus, the sampling frequency generator 3 can be part of the 
system or can be omitted because the actual system supplies the sampling 
frequencies. 
In the embodiments shown and described, the systems 2 need a constant 
voltage, e.g. 5 V. The system 2 needs some power, but this need may not be 
constant. As the power equals the product of voltage and current, to adapt 
the power in the system 2 the invention operates by adapting the current. 
This adaptation of the current to the power needed is done by the switch 
6. The switch 6 releases bursts of direct current (as pulses 69, FIG. 7, 
or pulses 58, FIG. 5) whose length within the period 2pi is calculated 
from the sampling frequency fs, as is well known, 1/fs. Therefore, the 
leading edge of such a pulse is produced at the sampling frequency and is 
synchronized with it. The trailing edge defines the length of the pulse 
and is therefore produced at varying time intervals following the leading 
edge in the case of the embodiment according to FIG. 1. With respect to 
the embodiment of FIGS. 3 to 5, the switching times are as indicated by 
the arrows 55, 56 and 57 and are tied to the sampling frequency and 
therefore no interferences will be possible, i.e., switching according to 
the arrows 55 is produced at the sampling frequency (FIG. 4). 
Voltage has nothing to do with dB in the invention. There is no relation 
between FIG. 5 and dB. dB's are only mentioned in FIG. 2 when discussing 
the spectrum of the frequencies occurring in a system 2, when system 2 is 
a digital audio system. Then the output of such a system is measured in 
dB, but this has no direct relation with the power supply of such a system 
except that the power supply of a system 2 having an enormous output in dB 
will have a power supply capable of supplying enough power to make such an 
output possible. 
As to the operation of switch 18 and oscillator 17, under normal conditions 
a precise voltage should be kept in the lines 8 and 9. This is arranged by 
means of controlling the switch 6 as explained by means of the pulse width 
regulator 11 which constantly measures the voltage in the lines 8 and 9 as 
indicated by the lines 14 and 13. If this voltage cannot be kept simply by 
adjusting the length of the pulses, then the voltage will increase or 
decrease and the departure from the required voltage is sensed by the 
voltmeter 20 and it will send out a signal through line 23 to the unit 18. 
The switch 18 will connect the pulse width regulator 11 to the oscillator 
17. This is essentially the case when the system 2 is started or shut down 
or when it does not operate correctly. 
In case of the embodiment according to FIG. 3 the voltage only is sensed by 
the voltmeter 20. Signals transmitted from the voltmeter 20 to unit 49 are 
used to adjust the switching times as indicated by the arrows 55 to 57. If 
this procedure does not succeed in keeping the voltage constant, then 
departure from the right voltage is transmitted to unit 49. Of course this 
embodiment could as well be equipped with switching means 18 and an 
oscillator 17 as shown in FIG. 1. 
While the invention has been described in detail above, it is to be 
understood that this detailed description is by way of example only, and 
the protection granted is to be limited only within the spirit of the 
invention and the scope of the following claims.