CIRCUIT ARRANGEMENT COMPRISING A FEEDBACK LOOP

Abstract: A description is given of a circuit arrangement comprising a feedback loop, which arrangement has a forward path for transmitting a result signal, a feedback path for deriving a feedback signal from the output signal of the forward path and a first recombining stage for recombining an input signal with the feedback signal to generate the result signal. A possibility of measuring the loop gain of the open feedback loop is then obtained by a measuring circuit for producing a measuring signal, which signal is derived from the result signal and the feedback signal by linear combination.

Description: Circuit arrangement comprising a feedback loop. The invention relates to a circuit arrangement comprising a feedback loop, which arrangement has a forward path for transmitting a result signal, a feedback path for deriving a feedback signal from the output signal of the forward path and a first recombining stage for re-combining an input signal with the feedback signal to generate the result signal. Such a circuit arrangement is known from European Patent Application 598 585. More particularly, the feedback loop described therein has the configuration of a Cartesian loop. Such Cartesian loops - but also feedback loops in more general terms - are to be used for a loop gain for the open feedback loop of which the gain value may at most have slight deviations from the set value for which the loop gain is designed. This is necessary for not affecting the stability of the loop and its bandwidth. In the known circuit arrangement, however, a measurement of the loop gain with open or interrupted feedback is not possible. It is an object of the invention to provide a circuit arrangement of the type defined in the opening paragraph, so that the loop gain of the open loop can be measured and monitored without the transmission properties of the loop being affected. According to the invention, this object is achieved with a circuit arrangement of this type by a measuring circuit for the production of a measuring signal, which signal is derived from the result signal and the feedback signal by linear combination. An advantageous embodiment of the invention is characterized in that the measuring circuit comprises a first measuring signal path to which the result signal is applied and by which it is transmitted with a predefinable first transmission factor, a second measuring path to which the feedback signal is applied and by which the signal is transmitted with a predefinable second transmission factor, and also a second re-combining stage for additively combining the output signals of the measuring paths to form the measuring signal. Preferably, the feedback loop is then airanged as a Cartesian loop. Advantageously, the circuit arrangement according to the invention is used within an amplifier arrangement of a telecommunication apparatus. The drawing in which like elements are designated by like reference characters shows in: Fig. l a block circuit diagram of a circuit arrangement according to the invention, Fig. 2 a detailed embodiment of a measuring circuit for the circuit arrangement shown in Fig. 1, and Fig. 3 shows an illustrative embodiment of the invention for a Cartesian loop. The circuit arrangement shown in Fig. 1 comprises a forward path 1 shown as an amplifier stage having the gain factor G and whose input is connected to an output of a first re-combining stage 2 for the supply of a result signal. The first re-combining stage 2 is arranged as a subtracter, for example, a differential circuit (differential amplifier); in this first re-combining stage 2, a feedback signal applied through a second input 4 is subtract-ed from an input signal applied through a first input 3 i.e. additively re-combined in accordance with its sign. The feedback signal is generated by a feedback path 5 whose output is connected to the second input 4 of the first re-combining stage 2 and whose input is connected to the output of the forward path 1. An attenuator 6, in principle belonging to the feedback path 5, is insetted in Fig. 1 into the link between the output of the forward path 1 and the input of the feedback path 5. For simplicity, however, there will be assumed in the following that the attenuator 6 forms part of the feedback path 5 and that particularly its attenuation factor is incorporated in the transmission factor i.e. the gain of the feedback path 5 which is referenced H in Fig. 1. Therefore, if a signal is available as a result signal on the output of the first re-combining stage 2, a signal eGH will appear on the output of the feedback path 5 and thus on the second input 4 of the first re-combining stage 2. Fig. 1 furthermore includes a measuring circuit for providing a measuring signal on a measuring signal output 7. The measuring circuit shown in Fig. 1 forms the measuring signal by linearly combining the result signal on the output of the first recombining stage 2 and the feedback signal on the output of the feedback path 5. For this purpose, the measuring circuit shown in Fig. 1 comprises a first measuring signal path 8, which is supplied with the result signal from the output of the first re-combining stage 2, and by which this result signal is transmitted with a predefinable first transmit factor k2. The measuring circuit further includes a second measuring path 9 whose input is connected to the output of the feedback path 5 and by which the feedback signal is transmitted with the predefinable second transmission factor kl. The signals provided on the output of the first measuring signal path 8, or of the second measuring signal path 9 (which signal path 9 may also be referenced second measuring signal path), are additively (in accordance with their sign) re-combined in a second re-combining stage i.e. subtracted from each other in the present example. The signal forming the difference between the signals from the first measuring signal path and the second measuring signal path is provided as a measuring signal of the measuring circuit on the measuring signal output 7. The second re-combining stage in Fig. 1 is referenced 10. In this circuit arrangement, the measuring signal on measuring signal output 7 represents a loop gain error signal E. If the same signal level is applied to the first input 3 of the first re-combining stage 2 for measuring purposes, a loop error signal e will be formed as a result signal on the output of the first re-combining stage 2. This loop error signal causes an output signal of magnitude e.G.H to occur on the output of the feedback path 5 or on the second input 4 of the first re-combining stage 2. This output signal forms the origin of a loop gain error signal E of magnitude e.k2-e.G.H.kl. If the transmission factors kl and k2 are designed so that k2 is equal to the product kl.(G.H)nom, where (G.H)nom is the set value of the loop gain, the value of the loop gain error signal E will become exactly equal to zero if G.H corresponds to (G.H)nom. The value necessary for the loop gain may thus be set by setting the gain factor G or H until the state E = 0 is reached. In general, the aforesaid leads to the fact that the loop gain error signal corresponds to the expression e.kl.(G.H)nom-G.H). Thus, by appropriately selecting the transmission factor kl or k2, the circuit arrangement may be designed so that there is a desired degree of sensitivity to the loop gain error. Fig. 2 shows an embodiment of the measuring circuit 8, 9, 10 of Fig. 1. This embodiment comprises an operational amplifier 11 whose first input 12 is connected via a second resistor R2 to the output of the first re-combining stage 2 for the result signal. A first resistor Rl bridges the output 14 of the operational amplifier 1 1 and the first input 12. The output of the feedback path 5 is connected, via a third resistor R3 to a second input 13 of the operation amplifier 11, which second input is furthermore connected to ground 15 via a fourth resistor R4. The output 14 of the operational amplifier 11 forms the measuring signal output 7. In this circuit arrangement, the transmission factors kl and k2 are determined by the following equations: kl = (Rl +R2)/R2.R4/(R3+R4) and k2 = R1/R2. Fig. 3 shows a further modification of the circuit arrangement according to the invention, comprising a Cartesian loop known per se as it is also described in EP 0 598 585 A2 and is therefore represented in Fig. 3 only by way of indication. The output of the forward path 1 is then connected via a directional coupler 16 to an antenna 17 and also to the feedback loop 5. The coupling factor of the directional coupler 16 is then incorporated in the gain H (set value) or H' (actual value) of the feedback path 5. Accordingly, the set value G or actual value G' of the gain factor in the forward path 1 are calculated from the gain of the power amplifier and the transmission factors of the mixers in the Cartesian amplifier. The measuring signal E on measuring signal output 7 then presents itself as the expression K.e.(G' .H'-G.H), while the expression in brackets represents the gain error. The measuring signal E formed in this manner is applied to a controller 18 which then sets the gain factor G' via the gain of the power gain factor in the forward path 1. The described loop gain control may be used both when a Cartesian loop is formed or when it is adjusted for the first time, and for the running or periodic monitoring of the Cartesian loop. For example, a very large input signal on input 3 may lead to the forward path becoming saturated and thus the signal amplitude being reduced. The measuring circuit according to the invention detects this reduction as a reduction of the loop gain and thus provides the possibility of recognizing distortions caused by overload.

Claims: CLAIMS: 1. A circuit arrangement comprising a feedback loop, which arrangement has a forward path for transmitting a result signal, a feedback path for deriving a feedback signal from the output signal of the forward path and a first re-combining stage for re-combining an input signal with the feedback signal to generate the result signal, characterized by a measuring circuit for the production of a measuring signal, which signal is derived from the result signal and the feedback signal by linear combination. 2. A circuit arrangement as claimed in Claim 1 , characterized in that the measuring circuit comprises a first measuring signal path to which the result signal is applied and by which it is transmitted with a predefinable first transmission factor, a second measuring path to which the feedback signal is applied and by which the signal is with a predefinable second transmission factor, and also a second re-combining stage for additively combining the output signals of the measuring paths to form the measuring signal. 3. A circuit arrangement as claimed in Claim 1 or 2, characterized in that the feedback loop is a Cartesian loop. 4. An amplifier arrangement for amplifying a signal, characterized by a circuit arrangement as claimed in claim 1 , 2 or 3. 5. A telecommunication apparatus, characterized by an amplifier arrangement as claimed in claim 4.

Inventors: SPARKS GEOFFREY SIDNEY, SPARKS, GEOFFREY, SIDNEY
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS PATENTVERWALTUNG, SPARKS GEOFFREY SIDNEY, PHILIPS ELECTRONICS N.V., PHILIPS PATENTVERWALTUNG GMBH, SPARKS, GEOFFREY, SIDNEY
Application year: 1997
Main classification: H03F1
IPC codes: H03F1, H03G3