Scaling circuit

Abstract: A scaling circuit in which a signal, which contains an ambient offset portion and a desired signal portion, in current mode, is scaled using a resistance means. The offset portion is substantially eliminated using constant flow rate means in the form of a current mirror, thus enabling the entire scaling dynamic of the circuit to be utilized.

Description: The present invention relates to improvements in a scaling circuit for use in the production of an output signal characteristic of an input signal, there being a scaling factor between the input and output signals. In particular, the present invention relates to cellular radio telephone communications systems wherein the output signal is a received signal strength indicator (RSSI) indicative of a signal received in an intermediate frequency (IF) circuit. In conventional scaling circuits the output signal, which is normally in current mode, is scaled via a resistor such that as the magnitude of the resistor used increases the magnitude of the scaling factor produced increases. However, with such a system the output signal generally includes an offset portion, which is present as a function of the circuit and is independent of the magnitude of the signal. Therefore, if the magnitude of the resistor is large the offset signal will be large and if the magnitude of the resistor is small the offset signal will be small but the scaling factor will also be small. Therefore due to the offset signal it is not possible with conventional scaling circuits to utilize the full scaling dynamic of the circuit. One external resistance has so far been generally employed in the output of RSSI signal circuits and the range of the output voltage provided by the circuit for IF circuit has been narrow. In radio telephones the RSSI output voltage is converted by an A/D converter for further processing by the processor. If the range of the RSSI output voltage is narrow and the A/D converter comprises a small number of bits, the resolution of the level indication remains poor after the A/D convertion. The A/D converters generally use 8 bits and the reading range of the converter is 0 to 5 volts. In radio telephones the output current of the RSSI signals of IF circuits typically range between offset currents of about 5 to 15µA and the maximum output current is about 50 to 80µA. The maximum voltage level of the output can be about 2 to 4 volts depending on the internal adjustment. A different known approach to reducing voltage offsets is disclosed in US 4,928,059, which describes a scaling circuit having a feedback circuit comprising an integrator which responds to the scaled output voltage produced by an operational amplifier to provide a time averaged offset bias signal to the inputs of the op amp to prevent the op amp from saturating thus enabling the op amp to operate in its linear range. Known configurations for amplifiers suitable for adjusting voltages are disclosed in DE 2,446,103, which refers to a stabilised transistor amplifier which has a circuit with M diodes and a resistor connecting the transistor emitter to a reference potential, and DE 3,213,838, which refers to a voltage to current converter for an integrated circuit which has a transistor and a reference resistor in combination to compensate for the spread in width of the resistor. According to the present invention there is provided a scaling circuit including means for receiving a signal, and means for providing an output signal indicative of the input signal, wherein the output signal may include an offset component characterized by open loop adjustment means coupled to the signal output means for providing a substantially constant signal level independent of the output signal whereby the offset component is substantially eliminated. In one embodiment the output signal is in voltage mode and the adjustment means provides a substantially constant voltage drop predetermined to substantially eliminate the offset component. In another embodiment the output is in current mode and the adjustment means are coupled to the signal output means for maintaining a substantially constant predetermined current flow to substantially eliminate the offset component. An advantage of the present invention is that by eliminating the offset portion of the RSSI output signal regardless of the magnitude of the scaling resistor used it enables a large scaling resistor to be used, thus utilizing the entire scaling dynamic of the RSSI circuit. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a conventional scaling circuit; Figure 2 is a graphical illustration of the scaling factor of the circuit of Figure 1, for a range of input values, when a large magnitude scaling resistor is used; Figure 3 is a graphical illustration of the scaling factor of the circuit of Figure 1, for a range of input values when a small magnitude scaling resistor is used; Figure 4 is a scaling circuit in accordance with the present invention; Figure 5 is a graphical circuit of the scaling factor of the circuit of Figure 4 for a range of input values; Figure 6 is a modified scaling circuit in accordance with the present invention; and Figure 7 is a graphical illustration of the scaling factor of the circuit of Figure 6 for a range of input values. Figures 1 to 3 illustrate how an RSSI output signal can be scaled by a conventional scaling circuit into a desired signal (current/voltage transformer; V=R*I) by resistance means 2. The output voltage thus obtained is a linear function of the logarithmic level variation of the IF input signal. Generally the current output always includes a small offset current causing an offset portion in the output voltage. If external resistance 2 is high, the offset voltage thus formed is also high. Thus a high input level of an IF-input signal respectively gives a high RSSI output voltage. This is generally limited in its maximum to the internal supply voltage of the circuit. Fig. 2 represents the RSSI output voltage 3 as a function of the IF-signal when the external resistance 2 is high. If the external scaling resistance 2 is low, the offset voltage thus formed is also very low and the available maximum RSSI output voltage remains lower than that when a high resistance is used. Fig. 3 represents RSSI output voltage 4 as a function of the IF-signal when the external resistance is low. The dashed line 3 represents the RSSI output voltage when the external resistance is high. Figure 4 illustrates a scaling circuit wherein the offset portion of the RSSI signal is substantially eliminated and a sufficiently large scaling resistor 2 can be utilized to facilitate use of the full scaling dynamic of the circuit. The circuit includes a means of draining the current corresponding to the offset portion of the signal. Said means is in the form of a current mirror 5. The current mirror 5 is a circuit construction wherein a current generator can be guided by a reference current. The ratios of the reference current and the output current can be modified. The most simple construction of the current mirror consists of a diode 6 and a transistor 7. Current mirror 5 serves as a constant flow rate generator. The current mirror 5 is available integrated in the same casing (for example Philips BCV-61, casing SOT-142). Because the diode 6 and the transistor 7 are both integrated in the same microcircuit, they provide good thermal connection and thus fluctuations in temperature do not change the current ratios of the current mirror 5. The open end of the current mirror 5 is coupled to the RSSI output and the reference current side of the current mirror 5 is connected to the supply voltage +Vcc via resistance 8. The value of resistance 8 is so chosen that the current mirror 5 eliminates the offset current of the RSSI output. The value of resistance 2 is so chosen that the maximum current of the RSSI level indicator provides the highest available output voltage which, according to the above description, is determined in accordance with internal voltage +Vreg of the circuit. Fig. 5 represents the RSSI output voltage 9 produced by the circuit in Fig. 4 as a function of the IF signal input. The output voltage 9 has been optimised by the elimination of the offset portion of the RSSI signal and the choice of an optimum magnitude resistor 2 so as to utilize the whole of the scaling dynamic of the circuit, from a voltage of substantially zero to Vreg. Fig. 6 represents an embodiment of the present invention suitable for use with IF circuits with a low supply voltage. Output A of the RSSI level indication of IF circuit 1 is coupled to the reference current side of current mirror 10 and its open side is coupled to the supply voltage +Vcc by resistance 11. Current mirror 10 consists of diode 12 and transistor 13. In addition, current mirror 10 is coupled to the ground potential by two compensating resistances 14 an 15, which can be used to change the ratios of the output current and the reference current. When current scaling is not needed and the matching of current mirror 10 to the circuit is in order, the matching resistances 14 an 15 can be omitted. Open output B of current mirror 10 provides the RSSI output voltage. The effect of the offset current of the output can be attenuated by second current mirror 16 by coupling its open end to output A and the reference current side to supply voltage +Vcc via resistance 17. Current mirror 16 consists of diode 18 and transistor 19. The value of resistance 17 is so chosen that current mirror 16 eliminates the offset current of the RSSI output. Thus the maximum output signal strength is Vcc instead of Vreg. Fig. 7 represents RSSI output voltage 20 as a function of the IF signal on the IF circuits for the circuit of Fig. 6 in which the level indication function is reversed, i.e., low input levels of the IF signal provide a high output voltage and high levels provide a low output voltage. With the aid of the circuit, the whole usable dynamics of the RSSI level indicator and the desired voltage range of the output can be utilized in this IF circuit, despite the low internal voltage Vreg. This application is very useful in IF circuits with a low supply voltage, such as +3V or under. In addition, the resolution after the A/D convertion used in radio telephones is also essentially better with the aid of the invention. In view of the foregoing description it will be evident to a person skilled in the art that modifications and improvements to the present invention may be incorporated without departing from the scope of the claims.

Claims: A scaling circuit including means for receiving a signal (IF-input), and means for providing an output signal (RSSI-out) indicative of the input signal (IF-input), wherein the output signal may include an offset component (offset); characterized by open loop adjustment means (5,6,7,8) coupled to the signal output means for providing a substantially constant signal level independent of the output signal (RSSI-out) whereby the offset component (offset) is substantially eliminated. A scaling circuit as claimed in claim 1 wherein the output signal (RSSI-out) is in voltage mode, and the open loop adjustment means provides a substantially constant voltage drop predetermined to substantially eliminate the offset component (offset). A scaling circuit as claimed in claim 1, wherein the output signal (RSSI-out) is in current mode and the open loop adjustment means are coupled to the signal output means for maintaining a substantially constant predetermined current flow to substantially eliminate the offset component (offset). A scaling circuit as claimed in any of the preceding claims, further including current mirror (10) coupled to the signal output means and to the open loop adjustment means for extending the range of output signal levels (RSSI-out) available from the circuit. A scaling circuit as claimed in any of the preceding claims, wherein the open loop adjustment means comprises a current mirror (5,16). A scaling circuit as claimed in claim 5, wherein the current mirror (5,16) comprises a resistance means (8,17), a diode (6,18) and a transistor (7,19) arranged such that the collector terminal of the transistor (7,19) is coupled to the means for providing an output signal; and the resistance means (8,17) is coupled between a reference voltage and the base terminal of the transistor (7,19), such that the value of the resistance (8,17) magnitude will determine the base current and thus determine the voltage drop across the collector, emitter path of the transistor (7,19). A scaling circuit as claimed in any of claims 4 to 6, wherein the resistance means comprise a resistor (2,14, 8,17). A scaling circuit as claimed in any of claims 4 to 6, wherein the resistance means is a current mirror (10). A scaling circuit as claimed in claim 8, wherein the ratio of reference current to output current in the current mirror, is dependent on the magnitudes of additional resistances (14,15) connected between the diode (12) and earth and the emitter terminal of the transistor (13) and earth respectively. A scaling circuit as claimed in claim 5, wherein the magnitude of the current flow in the current mirror (5,16) is determined by the magnitude of resistance means utilized (8,17).

Inventors: NYQVIST JOUNI, NYQVIST, JOUNI
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1992
Main classification: G05F3
IPC codes: H04B17, G05F3