WO2013152795A1

WO2013152795A1 - Rssi system and bias method for amplifier stages in rssi systems

Info

Publication number: WO2013152795A1
Application number: PCT/EP2012/056692
Authority: WO
Inventors: Peter VAN DER CAMMEN
Original assignee: Epcos Ag
Priority date: 2012-04-12
Filing date: 2012-04-12
Publication date: 2013-10-17

Abstract

RSSI systems providing an improved accuracy and bias methods for driving amplifiers in such systems are provided. For that, an RSSI system comprises a main cascade and a reference cascade of amplifier stages, an attenuator and an additional amplifier, where the amplifier stages are controlled via a feedback loop. On top of that a method for measuring the large signal behavior of an amplifier stage in a RSSI system is given that can be used to increase the accuracy as well.

Description

RSSI system and bias method for amplifier stages in RSSI sys^¬ tems

The present invention refers to RSSI systems, and bias meth^¬ ods for such systems.

An RSSI (RSSI = Radio Signal Strength Indicator) can be used to generate information about the amplitude or power level of an electrical signal propagating in a signal line. An RSSI can be utilized in mobile communication devices to monitor the power level of an RF signal being emitted or received. Such RSSI systems should be able to monitor the power level of a large dynamic range.

An RSSI system can comprise a chain of amplifiers, e.g. lim- iter amplifiers, where the output port of one amplifier is connected to the input port of the next amplifier. It was found that RSSI systems providing an accurate level signal even at high frequencies is not a triviality.

It is therefore an object of the present invention to provide an RSSI system that provides an accurate level signal even at high frequencies. It is a further object to provide a method implementing such a system, especially a bias method for such a system.

For that, an RSSI system and a method for biasing amplifier stages in RSSI systems according to the independent claims are provided. Dependent claims provide preferred embodiments of the invention. An RSSI system comprises a main cascade of amplifier stages where the main cascade provides a main RSSI sum. The RSSI system further comprises a reference cascade of amplifier stages where the reference cascade provides a reference RSSI sum. Further, the RSSI system comprises an attenuator being serially connected to and in front of the reference cascade, and an additional amplifier stage serially connected to and in front of the reference cascade. The additional amplifier stage is provided for working in a well-defined amplification range. Further, the RSSI system comprises a feedback loop where the feedback loop controls the RSSI sums of the main cascade and of the reference cascade.

Each amplifier stage of the main cascade and of the reference cascade provides a contribution to the total RSSI sum of the respective stage. This contribution is a level signal indi^¬ cating the mode of operation of the respective amplifier stage. Amplifiers can work in a linear region where the out^¬ put of the amplifier is mainly proportional to the input voltage or current of the amplifier. Further, an amplifier can work in an overdriven mode when the input voltage or current of the amplifier exceeds the amplifier's linear working region . It is preferred that the first amplifier stage of the main cascade, i.e. the amplifier stage obtaining the smallest sig^¬ nal power, is provided to operate fully in the linear region. Further, it is preferred that the last amplifier stage of the main cascade, i.e. the amplifier stage at the output of the cascade, is provided for working in a fully overdriven mode. The level signal that every amplifier stage provides and that is a contribution to the respective RSSI sum comprises infor^¬ mation about the working mode of the respective amplifier stage. The level signal can be a voltage or a current. Then, an amplifier stage working in a fully overdriven mode provides a different voltage or current than an amplifier stage working in the linear mode. The dynamic range in which the amplifier for an RSSI system works with a satisfying accuracy is the RSSI range which is defined by the signal level of an input signal in such a way that the first amplifier stage works in a linear mode and the last amplifier stage works in a fully overdriven mode.

In this context, the quantity "small signal gain" can be in^¬ troduced: the small signal gain is the gain, voltage to volt^¬ age, current to current, voltage to current, or current to voltage, for small signals in a certain bias condition. By altering the bias condition the gain can be changed. Further, the small signal gain is a number having the effect that if the input level is increased, i.e. multiplicated, by a factor of the small signal gain, then the RSSI sum being the sum of all contributions of the amplifier stages of one cascade, in- creases with an amount proportional to VS_CLIP ^_ VS o - Here,

VSc_Lip is the level signal of an amplifier stage working in a fully overdriven mode, while VS o is the level signal of an amplifier stage obtaining no input signal at all. Thus, in other words, the small signal gain is an amplification factor for the input RF signal that has the same effect as an addi^¬ tional amplifier stage in the cascade.

The meaning of the small signal gain becomes clear when its dependence on the RSSI sum directly encoding the level of the propagating RF signal is regarded: within the RSSI range of the RSSI system, the RSSI sum mainly depends linearly on a logarithm of the signal level of the RF signal being fed into the first amplifier stage of the main cascade. The slope of the RSSI curve (compare FIG. 3), thus, depends on the small signal gain and the quantity that is proportional to VS CLI P ^_ VS ₀ . Knowing the small signal gain and the value for VS CLI P ^~ VSo with a higher precision results in obtaining a more accu- rate RSSI signal.

Thus, the feedback loop can control the RSSI-sums of the main cascade and the reference cascade by controlling the small signal gain of the respective amplifier stages.

In conventional systems, the small signal gain is frequency and to some extent temperature dependent for each RSSI ampli^¬ fier stage. By providing the reference cascade preceded by an accurate attenuator and an additional amplifier stage, iden- tical to the amplifier stages in the main cascade and the reference cascade, the small signal gain is kept constant and the frequency and/or temperature dependency is eliminated or at least reduced. The attenuator and the additional amplifier stage are seri^¬ ally connected in an RF signal path in front of the reference cascade. The feedback loop controls the RSSI sums of the main cascade and of the reference cascade to be identical. Fur^¬ ther, the additional amplifier stage - which is provided for working in a well-defined amplification range, e.g. the linear range - eliminates the effect of the attenuator. The at^¬ tenuator can attenuate RF signals independent of their fre^¬ quency and independent of an environmental temperature. Thus, by introducing the attenuator, a frequency and/or temperature drift is eliminated. The attenuator and the additional ampli^¬ fier are only provided in the signal path of the reference cascade. Thus, the signal gain of the additional amplifier stage must exactly compensate the signal attenuation of the attenuator. The additional amplifier stage obtains the same control signal via the feedback loop as the other amplifier stages. Thus, the existence of the attenuator and the addi^¬ tional amplifier stage enables to provide all amplifier stages with a control signal enabling to set the small signal gain independent of the deviations, resulting in an RSSI less sensitive for temperature and frequency deviations.

In one embodiment, the main cascade's amplifier stages, the reference cascade's amplifier stages and the additional am^¬ plifier stage comprise a control port. The feedback loop is connected to the respective control ports of the amplifier stages . The contribution to the RSSI sum of the respective cascade, i.e. the level signal provided by each amplifier stage, can depend on a control signal being sent to the respective am^¬ plifier stage via the feedback loop. Thus, by evaluating the RSSI sum of the main cascade and the RSSI sum of the refer- ence cascade, the respective contributions to the total RSSI sum can be accurately controlled resulting in a more accurate RSSI sum.

In one embodiment, the feedback loop comprises a circuit for determining the difference between the main RSSI sum and the reference RSSI sum.

The difference between the RSSI sum of the main cascade and the RSSI sum of the reference cascade can be fed into the feedback loop to control the gain of all amplifier stages via their control ports. For that, the feedback loop can comprise a loop amplifier amplifying the difference of the RSSI sums. In one embodiment, the main cascade's amplifier stages and the reference cascade's amplifier stages comprise a level port for providing signal level information. Further, the main cascade comprises a summing circuit and an RSSI integra- tor for providing the main cascade's RSSI sum and the reference cascade comprises a summing circuit and an RSSI integra^¬ tor for providing the reference cascade's RSSI sum. The inte^¬ grators are connected to the feedback loop. Thus, the level ports of the respective amplifier stages are utilized to provide the respective contributions to the RSSI sums to the RSSI integrators of the respective cascades. The integrators integrate the individual contributions of the level signals of the amplifier stages, e.g. by adding. It is essential that the level signals are added or integrated and, then, filtered, or vice versa. It is possible that a resis^¬ tive network adds the signals and divides the sum by the num^¬ ber of contributing stages. A capacitor filters the signal. It is possible to obtain the average signal by adding the single signals and dividing. The result is independent of the chronological order.

In one embodiment, the attenuator provides a frequency-inde- pendent and/or a temperature independent and/or a power inde^¬ pendent attenuation factor.

In one embodiment, the signal attenuator comprises a parallel connection of impedance elements such as resistive and/or ca- pacitance elements. However, inductance elements can be used in the attenuator as well. The signal attenuator can comprise a voltage divider which can be realized to have a high accu^¬ racy. Accordingly, attenuation elements with very precisely known attenuation factors can be obtained. Accordingly, an RSSI system with very precisely defined small signal gains which are determined by the very precisely controlled control signals is obtained.

In a preferred embodiment, the additional amplifier stage is provided for working in a linear amplification range.

Thus, the additional amplifier stage and the first amplifier stage of the reference cascade are provided for working in a linear amplification range. For that, further attenuation elements in the reference cascade and/or the main cascade can be provided. In one embodiment the amplifier stages in the main cascade, the amplifier stages in the reference cascade and the addi^¬ tional amplifier stage are identical, e.g. of the same type.

In one variant of the invention, an RSSI system comprises a cascade of amplifier stages, providing a main RSSI sum, and further comprising an additional amplifier stage connected behind the cascade. The additional amplifier stage is pro^¬ vided for working in a well-defined amplification range. This amplification range can be a range in which the amplifier stage works in a fully overdriven mode. Such a mode is also a well defined mode. Then, the level signal of this amplifier stage may be measured and used to increase the accuracy in determining the level of the input signal from the RSSI sys^¬ tem.

As already stated and with reference to FIG. 3, the accuracy of the RSSI signal is determined by the small signal gain and by the contribution VS CLI P ^_ VS Q. The first variant of the in- vention refers to the situation where an extra reference cas^¬ cade with extra amplifier (s) , attenuator and a feedback loop helps to provide increasing the accuracy of the small signal gain. In this other variant of the invention, an additional amplifier stage helps to increase the knowledge about the difference VSC_LIP ^_ VSo- The slope of the curve in FIG. 3 needed for determining the signal strength of the input sig^¬ nal V_M depends on the knowledge of both the small signal gain and the large signal behavior, specified by VS_CLIP ^_VSO.

The additional amplifier stage being serially connected to the cascade helps to accurately know the "large signal behav^¬ ior" . In one embodiment of this variant, the RSSI system further comprises a second additional amplifier stage being connected behind the cascade and the additional amplifier stage and provided for working in a well defined amplification range or in a overdriven mode. The RSSI system further comprises a circuit for determining the average value of the signal level of the additional amplifier stage and the second additional amplifier stage. In one embodiment, the additional amplifier stage is provided for working in a fully overdriven amplification range.

The additional amplifier stage and the second additional am^¬ plifier stage do not influence the feedback loop controlling the respective other amplifier stages. Further, the two addi^¬ tional amplifier stages can work in the fully overdriven am- plification range and thereby provide information about the large signal behavior. By combining two or three or four or five or more additional amplifier stages, the accuracy of the large signal behavior is statistically improved. A method for biasing amplifier stages of RSSI systems com^¬ prises the steps:

- providing an RSSI system, e.g. one of the above RSSI sys- terns ,

- controlling the amplification gains of the amplifier stages via a feedback loop,

- determining the small signal behavior by accurately setting an attenuation factor of the attenuator.

In one embodiment the method comprises the step

- measure the level signal of one ore more fully overdriven amplifier stages for determining the large signal behavior in addition to or instead of the above mentioned claims.

This information can be used to obtain a more accurate read^¬ ing of the input signal level.

Examples of RSSI systems together with their working princi^¬ ples are schematically shown in the figures.

Short description of the figures:

FIG. 1 shows an RSSI system comprising a main cascade and a reference cascade,

FIG. 2 shows an embodiment of an amplifier stage,

FIG. 3 shows the dependence between the RSSI sum and the level of the signal to be detected,

FIG. 4 shows how the contributions to the RSSI-sum can

obtained . shows an amplifier cascade m a serial connection with two additional amplifier stages to obtain an accurately known large signal behavior, shows an embodiment of an amplifier stage, shows an embodiment of an amplifier stage.

Detailed description

FIG. 1 shows an embodiment of an RSSI system A comprising a main cascade MC and a reference cascade RC . The main cascade MC comprises amplifier stages ASi, AS₂, AS_n. The reference cascade RC comprises amplifier stages AS_Ri, AS_R2,..., AS_Rn. The amplifier stages of the main cascade MC and the reference cascade RC comprise a control port PC_TRL and a signal level port PS_L- Via the control port PC_TRL, the respective amplifier stages are connected to a feedback loop FL . Via the feedback loop FL, the amplifier stages' gain can be adjusted. Via their signal level port P_SL, the respective amplifier stages of the main cascade MC are connected to an integrator INT and with their signal level ports P_SL/ the amplifier stages of the reference cascade RC are connected to a second integrator INT. An integrator can be a summing circuit comprising a filter, such as a low pass filter. Each amplifier stage provides a signal level via their signal level port P_SL to the respec^¬ tive integrator which gives a contribution to the total RSSI sum. This contribution may either be a voltage or a current that is transmitted to the integrator INT. The signal level transmitted to the integrator INT depends on the operation mode of the respective amplifier stage. As the respective first amplifier stage may be provided to work in a linear re- gion, this first amplifier stage's contribution may be smaller compared to the last amplifier stage's contribution, where the last amplifier stage may work in a fully overdriven operating mode. The higher the power level at the input port P_IN, the more amplifier stages work in a fully overdriven mode and the higher is the RSSI sum. The RSSI sum is provided at the output port POU_T ·

A loop amplifier LA amplifies the difference and controls the amplifier stages via the feedback loop FL .

In front of the reference cascade RC, an attenuator AT and an additional amplifier stage are provided. The additional am^¬ plifier stage AAS does not contribute to the RSSI sum of the reference cascade RC . However, the additional amplifier stage is also controlled via its control port by the feedback loop FL . The feedback loop FL ensures the equality of the RSSI sums of the main cascade and of the reference cascade. The attenuator attenuates the RF signal feed into the input port P_IN and further feed into the reference cascade RC . Thus, in order to keep the RSSI sums equal, the additional amplifier stage AAS must exactly compensate the attenuation of attenu^¬ ator AT. Preferably, the attenuation factor β of attenuator AT is frequency and/or temperature and/or power independent. Then, by definition of the feedback loop, the RSSI sums are frequency and temperature independent, too. As a result, the small signal gain which is one factor to accurately determine the level of the signal fed into the input port P_IN, i s accu rately known.

FIG. 2 shows an embodiment of an amplifier stage AS. The am^¬ plifier stage AS comprises a first transistor Tl and a second transistor T2 and provides operating with balanced RF signals which may be fed into the amplifier stage's input port Ρ_ΪΝ and which may be amplified at the amplifier stage's output port P_OUT- Further, the amplifier stage AS comprises a supply port PSUP, a bias port PBIAS, a control port PCTRL, and a signal level port P_SL- The signal level port P_SL provides a voltage determining the mode of operation of the amplifier stage AS. The control port P_CTRL can be used to control the amplifica^¬ tion behavior of the amplifier stage AS. Via the bias port P_BIAS, the gates of the first transistor Tl and the second transistor T2 can be biased. Therefore, a first resistance element R_G, i is connected between the bias port P_BIAS and the gate of the first transistor. A second resistance element R_G,₂ is connected between the bias port P_BIAS and the gate of the second transistor T2. Further, another resistance element R_D, i is connected between the supply port P_SUP and the drain of the first transistor and another resistance element R_D,₂ is con^¬ nected between the supply port P_SUP and the drain of the sec^¬ ond transistor T2. The control port P_CTRL is connected to the gate of the third transistor and the signal level port P_SL is connected to the drain of the third transistor whose source is connected to ground.

FIG. 3 shows the dependence of the RSSI sum and the to be de^¬ termined signal level of RF signals. The ordinate shows the RSSI sum and the abscissa shows the logarithm of the signal level. The curve has a mainly linear power range defining the RSSI range of the RSSI system. VS_CLIP defines the signal level of a fully overdriven, i.e. clipping, amplifier stage. VSo is the signal level of an amplifier stage with no input signal. In FIG. 3, a certain length of the abscissa corresponds - due to the logarithmic scale - to a certain factor in power level. Here, A_v or log (A_v) respectively is the small signal gain. The slope of the curve may equal k* (VS_CLIP ^_VSO) /log (A_v) , where k = 1/n and n is the number of amplifier stages con^¬ tributing to the RSSI sum. Thus, by knowing the small signal gain with a higher accuracy, the level of the RF signal can be obtained with a higher accuracy. The range RSSIR denotes the RSSI range of the system.

FIG. 4 shows an integrator INT comprising, as an example, four resistive elements RE to be connected to the signal level ports of four amplifier stages. Each voltage drop over a resistive element is a contribution to the RSSI-sum. However, the total RSSI-sum is

1 n

RSSI-sum = - ^ Vi with n=4 due to the parallel connection of n

the resistive elements RE. Thus, the RSSI-sum is the average over the respective contributions.

FIG. 5 shows schematically a variant of the invention where an additional amplifier stage AAS and a second additional am^¬ plifier stage AAS₂ are electrically connected behind a cas- cade C comprising further amplifier stages AS contributing to the RSSI sum by means of an integrator INT. Usually, the last amplifier stage of the cascade C works in a fully overdriven operating mode. Thus, the additional amplifier stage AAS and the second additional amplifier stage AAS₂ work in a fully overdriven mode, too. Thus, they provide the large signal behavior VSC_LIP ^_ VSo- By adding the signal level of the large signal behavior of several additional amplifier stages and by dividing the resulting sum by the number of additional amplifier stages, an average large amplifier behavior can be ob- tained with statistically higher accuracy. Further, a low pass filter LFP flattens a signal being proportional to the large signal behavior. FIG. 6A shows another embodiment of an amplifier stage AS be^¬ ing able to process unbalanced signals and comprising two transistors T, a capacitance element CE and a resistance ele^¬ ment RE .

FIG. 6B shows another embodiment of an amplifier stage for unbalanced RF signals, also comprising two transistors T, a capacitance element CE and a resistance element RE. It is possible to combine two such amplifier stages to obtain an amplifier stage for differential RF signals.

The respective amplifier stages can comprise limiter amplifi^¬ ers, the output signals of which are limited. Thus, they clip the output signal when the output signal's power level ex^¬ ceeds a certain power level.

As described above, the invention's variants utilize cascades of amplifier stages and additional amplifier stages working in a well defined mode of operation to provide RSSI systems having a higher accuracy.

An RSSI system or methods for biasing amplifier stages of RSSI systems are not limited to the embodiments described in this specification or shown in the figures. RSSI systems comprising further amplifier stages or circuit elements and methods comprising further steps are also comprised by the invention . Further, it is possible to design a system where the level signals of fully overdriven amplifier stages are smaller than the level signals of amplifiers operating in the linear mode. List of reference signs

A: RSSI system

AAS : additional amplifier stage

AAS2 : second additional amplifier stage AS : amplifier stage

AS1, AS2 , ASn: amplifier stages of the main cascade ASR1, ASR2, ASRn: amplifier stages of the reference cas^¬ cade

AT : attenuator

C: cascade

CE: capacitance element

FL: feedback loop

GND: ground

INT : integrator

LA: loop amplifier

log (AV) : small signal gain

LPF: low pass filter

MC: main cascade

PBIAS : bias port

PCTRL: control port

PIN: input port

POUT : output port

PSL: signal level port

PSUP: supply port

RC: reference cascade

RD,1, RD,2,

RG,1, RG,2: resistance element

RE : resistance element

RSSIR: RSSI range

sumVSI : RSSI sum

T : transistor

Tl, T2, T3: transistor VM: power of an RF signal propagating in a signal path

VSO: signal level of an amplifier stage

receiving no input signal

VSCLIP: signal level of an amplifier stage in a fully overdriven mode

β: attenuation factor

Claims

1. An RSSI system (A), comprising

- a main cascade (MC) of amplifier stages (AS), providing a main RSSI-sum,

- a reference cascade (RC) of amplifier stages (AS), providing a reference RSSI-sum,

- an attenuator (AT) serially connected to and in front of the reference cascade (RC) ,

- an additional amplifier stage (AAS) serially connected to and in front of the reference cascade (RC) and

provided for working in a well defined amplification range,

- a feedback loop (FL), where

- the feedback loop (FL) controls the RSSI-sums of the main cascade (MC) and of the reference cascade (RC) .

2. The RSSI system (A) of the previous claim, where

- the main cascade's (MC) amplifier stages (AS), the reference cascade's (RC) amplifier stages (AS) and the additional amplifier stage (AAS) comprise a control port (PCTRL) , and

- the feedback loop (FL) is connected to the control ports (PCTRL) ·

3. The RSSI system (A) of one of the previous claims, where

- the feedback loop (FL) determines the difference between the main RSSI-sum and the reference RSSI-sum.

4. The RSSI system (A) of one of the previous claims, where - the main cascade's (MC) amplifier stages (AS) and the reference cascade's (RC) amplifier stages (AS) comprise a level port (PSL) for providing signal level information, - the main cascade (MC) comprises a RSSI-integrator (INT) for providing the main cascade's (MC) RSSI-sum,

- the reference cascade (RC) comprises a RSSI-integrator (INT) for providing the reference cascade's (RC) RSSI- sum,

- the integrators (INT) are connected to the feedback loop (FL) .

5. The RSSI system (A) of one of the previous claims, where

- the attenuator (AT) provides a frequency independent attenuation factor.

6. The RSSI system (A) of one of the previous claims, where

- the signal attenuator (AT) comprises a parallel

connection of resistive and/or capacitance elements.

7. The RSSI system (A) of one of the previous claims, where

- the additional amplifier stage (AAS) is provided for working in a linear amplification range.

8. The RSSI system (A) of one of the previous claims, where the amplifier stages (AS) in the main cascade (MC) , the amplifier stages (AS) in the reference cascade (RC) and the additional amplifier stage (AAS) are identical.

9. An RSSI system (A), comprising

- a cascade (C) of amplifier stages, providing a main RSSI-sum,

- an additional amplifier (AAS) stage connected behind the cascade and provided for working in a well defined amplification range or in a overdriven mode.

10. The RSSI system (A) of the previous claim, further comprising

- a second additional amplifier stage (AAS2) serially connected behind the cascade (C) and provided for working in a well defined amplification range or in a overdriven mode,

- a circuit for determining the average value of the signal level of the additional amplifier stage and the second additional amplifier stage.

11. The RSSI system (A) of one of the two previous claims, where

- the additional amplifier stage (AAS) is provided for working a fully overdriven amplification range.

12. A method for biasing amplifier stages (AS) of an RSSI system (A) , comprising the steps

- providing one of the above RSSI systems (A) ,

- controlling the amplification gains of the amplifier stages (AS) via a feedback loop (FL),

- determining the small signal behavior by accurately setting an attenuation factor of the attenuator (AT) .

13. The method of the previous claim, comprising the step

- measure the level signal of one ore more fully

overdriven amplifier stages for determining the large signal behavior.