Circuit for forming low power levels in a transmitter of a radio telephone

Abstract: The present invention concerns a circuit for forming low power levels in a transmitter of a radio telephone. By a power amplifier (1) of the transmitter is formed the maximum power level and a plurality of lower power levels. As taught by the invention, very low power levels can be produced by controling the power of the RF signal leaking through the power amplifier (1) when said signal is not amplified by the power amplifier. For controlling the leakage power, a PIN diode attenuator (4) connected before the power ampli­ fier (1) is used.

Description: The present invention concerns a circuit by which in a transmitter consisting of a controlled amplifier of one or several stages the lowest power levels can be formed. In a transmitter of a radio telephone, C - class amplifiers are generally used, which are characterized by a good efficiency, 60 to 80 per cent, on high power levels. Because of the great non-linearity of the amplifier, difficulties arise in realizing controlled power control on low power levels, and in addition, when low power levels are used, the efficiency of the amplifier reduces. This causes no harm when a system (or a locality of use) is in question in which the smallest transmission rates need not be used. The situation will be different, for instance in the digital gsm system covering all Europe and to be started at the beginning of the decade to come, in which very low power levels will be in use. If a telephone is going to be used in an aeroplane, it is necessary to use, in order to prevent potential interferences to be caused in the aviational electronics, probably the lowest power levels, -17 dBM of order of magnitude. A typical principle block diagram of a transmitter for a gsm radio telephone is presented in Fig. 1. The block diagram only shows the blocks required for understanding the operation. A signal to be transmitted enters e.g. the input RFin of a three-step C class power amplifier. The gain of the amplifier 1 is controlled by a reference amplifier 3, the output of which is filtered before being input in the power amplifier 1. The input signals of the reference amplifier 3 are the voltage derived from a power pick-up 2, said voltage being proportional to the the output voltage RFout of the power amplifier 1, and the control voltage TXC1 derived from the logic section of the telephone. Said blocks 1,2 and 3 constitute a control loop, which tends to be controlled in a state in which the voltage to be derived from the power pick-up 2 and the control voltage TXC1 derived from the logic sections of the radio telephone are of equal magnitude. With the procedure of prior art, such power levels are obtainable which at the most remain about 25 dB below the maximum power level. If a wider power control range is wished, the following difficulties arise: the narrowness and non-linearity of the dynamic range of the power pick-up and the poor controllability of the power amplifier of the C class make the control of low power levels difficult. On small power levels the efficiency of the amplifier is extremely poor. According to the specifications of the GSM system the power increase will on low power levels conform to the COS² graph and this is difficult with a C class amplifier. The object of the present invention is to produce a circuit with which, when using a C class amplifier, small power levels may, nevertheless, be generated controllably and with a good efficiency. The circuit of the invention for producing low power levels is characterized in that a controllable attenuator is connected in series together with the power amplifier, said attenuator not controlling a radio frequency signal (RF signal) when said signal is being amplified in the power amplifier but with which the RF signal can be attenuated or is not attenuated when said signal is not amplified in the power amplifier (TXC1=0). The operation of the circuit of the invention is based on the phenomenon that although no control voltage (TXC1=0) enters the power amplifier, the RF power entering the input of the amplifier is, however, shown in the output RFout of the transmitter in the form of leakage power. The amount of said leakage power is in general of the same order of magnitude as the lowest power levels to be used in the gsm system. The quantity of said leakage power can be controlled with an attenuator and in this manner, the low power levels of the required order can be provided. The invention is described below more in detail, referring to the accompanying figures, in which Fig. 1 presents a principle block diagram of a typical transmitter of a gsm radio telephone, Fig. 2 presents a principle block diagram of the circuit of the invention. Fig. 1 presents a circuit of the state of art, the functioning whereof being already described above. The circuit of the invention shown in Fig. 2 deviates from the circuit of Fig. 1 in that in series with the power amplifier 1 has been coupled a controllable attenuator 4. In the present example it is connected before the power amplifier 1. The attenuator may be implemented in the form of a PIN diode attenuator controlled by a control voltage TXC2 derived from the logic section of the radio telephone. The circuit of the invention operates as follows. When a transmitter of a radio telephone is desired to operate on high power levels, the bias currents of the diodes of the PIN diode attenuator 4 are so controlled with the voltage TXC2 that the attenuation is minimal, whereby the RF signal is allowed to proceed without any obstruction to the power amplifier 1, and, controlled by the voltage TXC1 derived from the control logic, a transmission signal is obtained from the output RFout of the transmitter on the desired power level. If very low transmission signal power levels are required, the gain of the power amplifier is controlled to be zero by setting the control voltage TXC1 to be TXC1=0. Hereby, the radio frequency signal entering the input RFin enters the PIN diode attenuator 4, where it may be attenuated, as desired, determined by the control signal voltage TXC2. From the attenuator the RF signal proceeds on to the power amplifier stage 1, where no control voltage enters. The signal proceeds, however, as leakage of the RF power to the output RFout of the radio telephone transmitter. This leakage power may therefore be controlled by the PIN diode attenuator 4. The greatest transmission power level in the output RFout derived in this state is because of losses to some extent lower than the power of the entering RF signal in the transmitter input RFin. In comparison with the transmitter of the gsm radio telephone known in the art, by which power levels of only about 25 dB below the maximum power level can be produced, the circuit of the invention offers a simple and inexpensive solution in forming very low power levels. Since the circuit is based on a RF signal leaking through a RF power amplifier and on the attenuation thereof, only two lowest power levels can therefore be established in practice, their order of magnitude being about -17 dBm. The circuit of the invention for producing low power levels consumes little current, since on low power levels the power amplifier is not given control and therefore consumes no current. The dynamics of the transmitter power pick-up need not cover the entire power range because on low power levels the pick-up is not used at all. In principle the attenuator may be placed also after the power amplifier. In practice, this is however more difficult and less worth while because in that case the attenuator should tolerate the maximum transmission power derived from the power amplifier. In addition to the PIN diode attenuator, also a voltage or current controlled amplifier stage may be used as an attenuator.

Claims: A circuit for forming low transmission power levels in a transmitter of a radio telephone transmitter, comprising a controllable, C class power amplifier of one or several stages, wherewith the maximum transmission power level and a plurality of lower transmission power levels are produced, characterized in that in series with the power amplifier (1) is coupled a controllable attenuator (4), which does not attenuate a radio frequency signal (RF signal) when said signal is being amplified in the power amplifier (1), but with which a RF signal may be attenuated or not be attenuated when said signal is not being amplified (TXC1=0) in the power amplifier (1). A circuit according to claim 1, characterized in that the attenuator (4) is connected before the power amplifier (1). A circuit according to claim 1, characterized in that the attenuator (4) is a PIN diode attenuator, which is controlled by a control voltage (TXC2) derived from the control logic of the radio telephone. A circuit according to claim 1, characterized in that the attenuator (4) is a voltage or current controlled amplifier which is controlled by the control voltage (TXC2) derived from the control logic of the radio telephone. A circuit according to claim 1, characterized in that the gain of the power amplifier (1) is controlled by a reference amplifier (3), to a first input of which the control voltage (TXC1) derived from the control logic of the radio telephone is applied, and to a second input of which a voltage proportional to the power of the output of the power amplifier (1), derived from the power pick-up (2) is applied, whereby the power amplifier (1), the reference amplifier (3) and the power pick-up (2) constitute a control loop. A circuit according to any one of the preceding claims, characterized in that it is used in a transmitter of a digital radio telephone, e.g. in the gsm system.

Inventors: PEKKARINEN VESA, SARASMO JUKKA, VAEISAENEN RISTO, PEKKARINEN, VESA, SARASMO, JUKKA, VAEISAENEN, RISTO, Väisänen, Risto
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1990
Main classification: H04B1
IPC codes: H03G3, H04B1