AN INTEGRATED DYNAMIC AMPLITUDE LIMITER INDEPENDENT OF THE SUPPLY VOLTAGE

Abstract: An integrated dynamic amplitude limiter is disclosed for lim­ iting the deviation of a transmitter operating at audio fre­ quencies. In an amplitude limiter according to the invention, a dynamic AGC amplifier block (1), a level clipper block (3) and a block (2) which forms the higher and lower limiting levels are implemented using switched capacitors (SC), and they are formed into an integrated amplitude limiter cir­ cuitry, and the voltages which define the higher and lower limiting levels are formed from the voltage of the band-gap voltage reference (Vbgref), which voltage is formed in rela­ tion to the virtual ground voltage of the amplifier circuitry, in which case the limiting levels are independent of the sup­ ply voltage (Vdd).

Description: The present invention relates to an amplitude limiter operating at audio frequencies, and it is particularly intended for a deviation limiter in a mobile telephone or a base station. In the transmitter of a radio telephone, the amplitude of the input voltage of the FM modulator has to be limited in order to limit the frequency deviation of an FM-modulated signal. Signals of an excessively high level are clipped by a level clipper which limits the amplitudes of the signals to a level between a higher level and a lower level. A normal way of implementing the level clipper is to form limiting levels on the voltage divider directly from the supply voltage or to use a reference voltage formed in relation to the supply voltage. This known method has the disadvantage that variations and disturbances in the supply voltage affect the reference voltages and thereby also directly the limiter output signal. In order to eliminate these disadvantages the disturbance components within a wide frequency range must be filtered out from the supply voltage, and efforts must be made to keep the absolute value of the supply voltage as stable as possible. This for its part will lead to the use of expensive precision components. Such specialized components include precision capacitors, precision resistors and a regulator, which have been used in prior-art mobile telephones and base stations implemented using, for example, an NE 571 circuit. However, it has been difficult to achieve sufficiently precise limiting levels and sufficiently precise build-up and decay times. US-A- 4 802 236 describes a modulation limiter comprising an amplifier and, a clipper which limits the amplitude of the signals to a level between a higher level and a lower level. Parts of the limiter are implemented in switched capacitor form. The object of the present invention is to decrease the deficiencies of prior-art clippers and to provide an integrated dynamic amplitude limiter which is independent of the voltage and limits the amplitude of the input signal to a predetermined level so that variation and disturbances in the supply voltage will not be directly visible in the output signal. The amplitude limiter according to the invention is characterized in that the voltages defining the higher and lower limiting levels are formed from the voltage of a band-gap voltage reference (Vbgref), which voltage is formed in relation to the virtual ground voltage of the amplifier circuitry, in which case the limiting level is independent of the supply voltage (Vdd). The virtual ground is common to the whole circuitry, and it is formed, for example, by dividing resistors from the supply voltage. The precision of the virtual ground does not affect the functioning of the circuitry. The amplitude limiter according to the invention comprises a dynamic AGC (automatic gain control) amplifier and a level clipper subsequent to it. The circuitry is implemented by using switched capacitors (SC). These enable very precise limiting levels to be achieved in an integrated implementation. The basic circuitries of the AGC amplifier and the limiter amplifier, implemented by the SC technique, are known as separate circuits, but the integrated circuitry according to the invention is novel. Since the limiting levels are formed from a reference voltage which is formed in relation to the virtual ground voltage, it follows from this that the limiting circuitry functions independently of the voltage, and variations and disturbances in the supply voltage are not directly visible in the output signal. The circuitry according to one embodiment of the amplitude limiter according to the invention is depicted in Figure 1. The amplitude limiter comprises three clearly distinguishable blocks: automatic gain control (AGC) 1; a reference voltage forming block 2; and a limiter block 3. In the blocks, A, B and C indicate switched capacitors which are controlled by clock pulse forms a, b, c. The description of the operation begins from block 2. The band-gap reference circuit forms from the supply voltage Vdd and the virtual ground (Vdd/2) a voltage Vbgref, which is below the virtual ground by the extent of the band-gap reference voltage. It can be seen that the voltage difference between Vbgref and the virtual ground Vdd/2 is not dependent on the absolute value of the supply voltage Vdd as long as the supply voltage is sufficient for the functioning of the band-gap reference. Of course, the absolute value of Vbgref will change as the supply voltage Vdd changes, but this does not have significance, since according to the invention all the voltage samples in the capacitors are formed in relation to the virtual ground. From the voltage Vbgref the higher and lower reference voltages for the window comparator 6, 7 on the SC amplifier pair 4, 5 are formed so that the reversing amplifier stage 4 will form the reference voltage above the virtual ground and the direct amplifier stage 5 will form the reference voltage below the virtual ground. The window comparator 6, 7 compares the value of the output voltage of the AGC amplifier 9 with the higher and lower reference voltages obtained from the amplifiers 4, 5 of the reference voltage forming block 2 and controls the functioning of the up/down counter of block 8 so that it will increase or decrease the damping of the AGC amplifier if the level of the AGC output signal is outside or respectively inside the limits of the window. The rise and decay time constants of the AGC are determined directly on the basis of the clock frequencies of the up/down counter, and therefore they are very precise. They can also be made selectable, in which case the time constants can be changed according to the application. The functioning of the up/down counter is synchronized with the clocks of the AGC amplifier so that amplification will be adjusted only at the beginning of the offset-reset sequence, since adjustment during the amplifier operating sequence would cause strong disturbances to appear in the output of the AGC amplifier. The level clipper block 3 takes a sample of the output voltage of the AGC amplifier and compares it with the reference levels by using the window comparator 10, 11. If the voltage is between the reference levels, it is applied directly to the output of the amplitude limiter. If the voltage is outside the reference limits, the corresponding reference voltage is applied directly to the output. The decision and the control of the switches S1, S2, S3 are carried out in the decision making and capacitor selection logic 12. By the solution according to the invention, supply voltage variation and disturbances in the output signal of the amplitude limiter are eliminated. The level of the signal depends on the supply voltage only to the extent of the PSR (power supply rejection) property and temperature dependence of the band-gap voltage reference. Typically the PSR is in the order of -20...-40 dB and the temperature dependence is in the order of 100-200 ppm/°C. Thus the supply voltage may vary, for example within the range 4-10 V, without affecting the limiting properties of the proposed circuitry. If it is desired that the operating level of the clipper block 3 is different from that of the control circuit block 1 of the amplification, separate reference voltages must be formed for the clipper block, but if the same operating levels are used, it is possible to use common reference voltages, as described above. The circuitry example described above can also be implemented in other ways within the scope of protection of the patent claims. Thus the control loop of the AGC amplifier can be implemented by using, for example, a combination of integrator + rectification + comparator + up/down counter, or in some other way evident for an expert in the art. The amplitude limiter according to the invention can well be integrated on a CMOS circuit, for example, together with filtering functions, since the SC technique is used in both. Since reference voltage of the circuit is formed in relation to the virtual ground level of the circuitry, an inexpensive regulator circuit can be used for forming the circuit supply voltages. The circuitry described can be used in all applications in which it is desired to make a CMOS-integrated dynamic amplitude limiter within an audio frequency range. A typical use is the deviation limiter in a mobile telephone or a base station.

Claims: An amplitude limiter circuitry for limiting the amplitude of the input signal to a predetermined level, which amplitude limiter circuitry comprises a dynamic AGC amplifier, a level clipper, and a limiting level forming circuit, characterized in that the dynamic AGC amplifier block (1), the level clipper block (3), and the block (2) which forms the higher and lower limiting levels are implemented using switched capacitors (SC), and they (1, 2, 3) are formed into an integrated amplitude limiter circuitry, and that the voltages defining the higher and lower limiting levels are formed from the voltage of a band-gap voltage reference (Vbgref), which voltage is formed in relation to the virtual ground voltage of the circuitry, in which case the limiting levels are independent of the supply voltage (Vdd). A circuitry according to Claim 1, characterized in that the AGC amplifier block (1) has a window comparator (6, 7), which compares the output voltage of the AGC amplifier (9) with the voltages defining the higher and lower limiting levels, the window comparator (6, 7) controlling the up/down counter to increase or decrease the damping of the AGC amplifier (9) so that the said output voltage will remain between the higher and lower limiting levels. A circuitry according to Claim 1, characterized in that the level clipper block (3) has a window comparator (10, 11) which compares the output voltage of the AGC amplifier with the voltages defining the higher and lower limiting levels, the window comparator (10, 11) controlling the decision making and capacitor selection circuit (12) to direct to the amplitude limiter circuitry output the output voltage of the AGC amplifier if its value is between the voltages defining the higher and lower limiting levels, and the higher reference voltage if the value of the output voltage of the AGC amplifier is higher than it, and the lower reference voltage if the value of the output voltage of the AGC amplifier is lower than it. A circuitry according to Claim 1, characterized in that the voltages defining the higher and lower reference levels are formed separately for the AGC amplifier block (1) and for the level clipper block (3).

Inventors: KORHONEN, KALLE JUKKA TAPIO, KORHONEN, PERTTI JUHANI
Applicants: NOKIA MOBILE PHONES LTD.
Application year: 1990
Main classification: H03G11
IPC codes: H03G3, H03G11