What is Envelope Tracking

What is Envelope Tracking- Envelope Tracking, ET is becoming a key technique in many RF amplifier designs where it enables much higher levels of efficiency to be achieved, thereby saving battery or other power consumption as well as bring other advantages.Envelope tracking is a technology associated with Radio Frequency, RF amplifier design.When using envelope tracking technology the power supply voltage applied to the power amplifier is constantly adjusted to ensure that the amplifier is operating at peak efficiency for the given instantaneous output power requirements.This brings many advantages to RF amplifier applications in many industry sectors and as a result it has been adopted in a wide variety of areas.While the concept behind envelope tracking technology has been understand for many years, it has not been easy to achieve in a workable form until recently because of the difficulties of implementing it in a satisfactory format.RF amplifier efficiency improvement techniquesOver the years there have been several techniques, apart from envelope tracking that have been used to improve the efficiency performance of RF amplifiers. Each of these techniques has its advantages and disadvantages.Ope rate PA i n co mpr essi on This scheme is only applicable for constant amplitude schemes such as FM and GMSK where there is no amplitude element in the modulation. Can use digital pre-distortion to provide linearity where some amplitude components are available.• Easy operation where modulation and performance requirements permit.• Provides high efficiency while in compression• Does not provide high efficiency for high peak to average power levels signals as amplifier will not run in compression all thetime.DC Trac king The PA voltage is adjusted in line with the modulation level,often using protocol commanded power supply voltage levelchanges• Relatively straightforward to implement.• Does not requireparticularly advanced techniques.• Deals only with average power.• Does not address broadbandissues.• Does not address instantaneous peaks and troughs.Doh erty Amp lifier Amplifier consists of two elements: main amplifier and a peaking amplifier that comes in to assist with peak power levels.The peaking amplifier is biased off for most of the time, onlycoming into operation for peaks.• Gives useful increase in efficiency• Requires combiner for combining output of two amplifier sections.• Gain kinks can occur• Gain and phase vary with output power.Env elop e Tr acki ng Envelope tracking, has the PA supply voltage that tracks the RF envelope. This enables the amplifier to be run at a voltage that gives the optimum efficiency or other performance level at any instantaneous power level.• Provides best performance at all power levels.• Permits broadband operation.• Provides additional advantages in terms of operation into mismatched loads, etc. .• Envelope tracking requires very fast - high bandwidth - powersupply.• Requires accurate envelope signal for power supply.Each technique has its own advantages and can be used to provide significant benefits under certain scenarios. Envelope tracking is able to offer some significant benefits in terms of efficiency, and general operation for RF power amplifiers used in many applications.Envelope tracking backgroundBefore looking further into the technology in this envelope tracking tutorial, it is worth looking at the background and need for the technique.In most RF amplifier applications, the efficiency of the amplifier has an impact on the design, operation and efficiency of the overall system. Power supply requirements, RF amplifier capabilities and heat-sinks, battery life and many more elements are dependent upon the RF amplifier, especially when it is one of the main power users within a system.For any RF amplifier power is supplied to the circuit, and a signal is produced. The output will always be less than the DC input power, the ratio of output to DC input being the efficiency.The efficiency of an amplifier depends upon the shape of the waveform and the mode in which it is operating.When operating in a linear mode, the output device must always be in conduction, with the output voltage rising and falling between the two limits.When operating in this mode, often called Class A, the maximum theoretical efficiency that can be achieved is 50%. However in a real system the achieved levels are always below this.Linear operation of an amplifierTo achieve better efficiency levels, it is possible to drive the amplifier into compression. Much greater levels of efficiency can be achieved, and if a steady waveform, like FM is used, the only degradation of the signal is that additional harmonics of the fundamental carrier are generated and these can be filtered out using RF filters.Unfortunately when modulation with an amplitude component is applied to a carrier this is distorted if it is passed through an amplifier that is run in compression.For data transmission systems that are used today like UMTS, HSPA, and 4G LTE, etc, the RF waveforms that are used incorporate an amplitude component in addition to the phase elements and therefore they require a linear amplifier.The situation becomes worse as if the peak to average ratio is high, i.e. the waveform has higher peak levels when compared to the average because the amplifier has to be able to accommodate the peaks while still only running at a low average power level.Waveform with high peak to average ratioDuring the peaks, the amplifier requires the full voltage to be able to deliver the required power without running into compression, but during the periods of lower signal, this voltage is not required and means that power is dissipated in the device. The amplifier only requires a smaller voltage to deliver the lower levels of power and therefore running with the higher voltage all the time, unnecessarily wastes power.High levels of power are dissipated if the full rail voltage is maintainedIt can be seen that the power dissipated is proportional to the aea between the top of the RF envelope and the rail voltage. For low peak to average power ratio signals, this can be high.In addition to the modulation, many requirements placed on RF amplifiers used in modern applications such as cellular telecommunications still further reduce the efficiency levels - many amplifiers are required to operate over wide bands, or multiple frequency bands for example.Envelope tracking: basic conceptIn order to improve the efficiency levels of RF amplifiers, one of the approaches that can be used is to employ envelope trackingtechnology.As the name indicates, envelope tracking employs a system whereby the amplitude envelope of the signal is tracked and utilised by the amplifier.Using envelope tracking the power supply voltage applied to the power amplifier is constantly adjusted to ensure that the amplifier is operating at peak efficiency for the given instantaneous output power requirements.Envelope tracking conceptIt can be seen that using envelope tracking technology the voltage applied to the amplifier is sufficient to enable it to handle the signal without entering compression, but it also does not dissipate unnecessary power.Envelope Tracking Systems & Block Diagramthe Envelope Tracking, ET systems requires a number of circuit blocks providing modulation information to the power supply to provide an effective operating system.In order to implement envelope tracking, several circuits and circuit blocks are needed.It is necessary to be able to control the power supply voltage to the amplifier so that it only receives the voltage, and hence the power required to deliver the signal in a linear fashion.The envelope tracking block diagram and circuits or envelope tracking system requires a system that is fast and able to respond to the quickly changing envelope levels while still being able to provide the current levels needed all the time.Traditional system block diagramIn order to develop an envelope tracking amplifier, it is necessary to modify the existing system block diagram to accommodate the additional components.A traditional RF amplifier block diagram would have the baseband generation system where the signal is created with its modulation and up-converted to the final frequency. It would then be amplified and applied to the final RF power amplifier. This would traditionally be supplied by a DC-DC converter providing a constant voltage.Traditional amplifier system block diagramIt is possible to look a little deeper into what is contained within these different areas.Most signals these days are in a digital format and they arrive at the modulator as I or In-phase signals and Q or Quadrature signals. The I and Q signals are applied to the modulator to give the overall signal.Traditional amplifier system signal pathIn the diagram it can be seen that the I and Q signals in their digital format are separately applied to a digital to analogue converter to transform them into an analogue format. This signal is passed through a low pass filter to remove the unwanted alias and higher frequency products.Envelope tracking system block diagram Envelope tracking system signal pathEnvelope tracking system signal pathThis obviously places some critical design requirements onto the envelope tracking modulator / supply, but it also place requirementsEnvelope Tracking Shaping Characteristics- Envelope tracking systems can utilise a number of different characteristics or profiles to define the envelope shape. These can be optimised to provide different results.The shaping characteristics or profiles of the envelope tracking signal applied to the envelope tracking power supply / modulator can provide different results dependent upon the requirements for the system.By selecting the approach required, it is possible to attain different types of performance within the RF power amplifier.Effect of modulating the supply voltageVarying the voltage on the RF power amplifier enables the optimum performance point to be selected for any given level of RF power output.It is found that for any given supply voltage, there is an optimum level of efficiency that can be obtained. The aim of an envelope tracking system is generally to operate at the most efficient point on the curve for any given power output.It can be seen by the graph below that the for any given supply voltage, i.e. drain voltage or collector voltage dependent upon the type of device, the power efficiency rises as the power output starts to rise, then reaches a peak before falling away.If many plots are made for different supply voltages, it can be seen that there is curve for the maximum efficiency level. If the supply voltage follows this curve, then the RF amplifier can operate at its maximum efficiency level for any given power level.Power efficiency for different supply voltagesEnvelope tracking shapingThe obvious aim for an envelope tracking system is to optimise the efficiency of the system. Under these circumstances the envelope tracking signal should be such that the points of maximum efficiency are used for each voltage.However it is also possible to adopt other envelope tracking profiles. One in particular follows a constant gain and is known as the Isogain contour and provides the maximum linearity.Envelope shaping - maximum efficiencyIn order to obtain the maximum efficiency a supply voltage shape profile that follows the points of maximum efficiency for the various output power levels is required.This can be plotted on the various curves for gain and power output for the different supply voltage levels.Maximum efficiency envelope tracking shape / contourIn order to obtain the required voltage, it is necessary to develop a shape look-up table. This ensures that the required voltage is provided to the amplifier for a given envelope amplitude. This can be developed from the curves for the amplifier.Typical shape lookup curve for maximum efficiencyEnvelope shaping - linear gainUnder some circumstances it may be necessary to optimise the envelope tracking system to provide linear gain rather than maximum efficiency. Under these circumstances it is necessary to alter the operating conditions slightly and this results in a different curve for the look-up table.Linear gain envelope tracking shape / contourThe figures from these contours are taken and added into the shape loo-up table to provide the required envelope tracking contour for linear gain.Typical shape lookup curve for linear gainThe figures in the look-up tables are those used in the envelope shaping look-up memory. They enable the required contour to be provided to match the required supply voltage for the given instantaneous amplitude of the RF amplifier.Envelope tracking system block diagramEnvelope tracking control signal block diagramPre-envelope gain: This stage of the control signal generation is required because the output power of the overall transmitterEnvelope tracking system signal pathIn this diagram, it can be seen that the I and Q signals enter a block called delay where the delay matching can be applied and theyare also split off along their different paths.The RF path passed into DACs, one for the I and another for the Q signal and these outputs are then filtered and mixed up to the required frequency where they are summed and then amplified.The envelope control signal path incorporates very different signal processing techniques and therefore it has a different level of delay.The envelope control signal path incorporates a number of different baseband blocks to ensure the contour of the control signal is correct: this involves aspects including gain control and a RAM look up table. The signal is converted from digital into an analogue format and then filtered before being applied to the envelope tracking supply.One of the major factors in the different delays between the two signal paths is the low pass filters used after the DACs. Low pass filters inherently have a delay and as their characteristics are very different, they have different levels of delay.As seen on the left most block in the block diagram, the delay balanced is added as the IQ signals are split along the two paths and in this way the delay can easily be balanced at the outset.Envelope tracking synchronisation examplesIn order to illustrate the issue of synchronisation it is useful to see some examples.There are images taken from real laboratory envelope tracking systems showing good and poor synchronisation levels.Good envelope tracking synchronisationIf the delay envelope tracking balancing is not well matched, both signals fall out of synchronism and there is a mismatch between the two signals.As a rough rule of thumb the alignment must be within about half a nanosecond for 20 MHz bandwidth LTE signal.Poor envelope tracking synchronisationEven though the apparent discrepancy appears small, there are areas where there is insufficient voltage applied to the RF amplifier to cater for the signal, and other areas where there is too much voltage which will result in excess dissipation.。