Envelope Tracking Control Signal Generation

The key process in RF amplifier envelope tracking is the generation of the control signal for the ET supply or DC modulator.

Envelope Tracking primer includes:
What is envelope tracking Block diagram PSU / DC modulator Envelope shaping Control signal Delay balancing

In order to be able to modulate the DC supply for the final amplifier it is necessary to generate a control signal that follows the envelope of the modulation.

This control signal is generated from the early stages of the transmitter where the modulation is applied to the RF signal.

Most modern transmitters that are used with envelope tracking technology carry data and this is modulated into the signal as the I- in-phase and Q - the quadrature signals.

The I and Q signals are then used to generate the modulation evnvelope by passing them through several circuit blocks to convert the signal into exactly the right format with the correct contour to provide the required performance.

Control signal basics

The basic signal used to control the envelope tracking power supply or modulator is derived from the I and Q signals present at the baseband generator.

Envelope tracking basic transmitter block diagram / concept — Basic envelope tracking block diagram / system concept

The actual generation of the envelope tracking control signal is quite sophisticated and required several blocks of circuitry as we will see below..

Envelope tracking control signal generator

The block diagram for the circuitry required for the envelope tracking control signal generation includes a number of blocks that condition the inputs to provide the required output.

Circuit blocks required for envelope tracking PSU control signal generation — Envelope tracking control signal generation

It can be seen that there are several blocks each serving separate functions.

These circuit blocks include the following:

Magnitude calculator: The magnitude calculator takes the two signals, I and Q, from the baseband generator. It squares the signals, sums them and then takes the square root to create the overall signal magnitude.

$Envelope = \sqrt{I^{2} + Q^{2}}$
Pre-envelope gain: This stage of the control signal generation is required because the output power of the overall transmitter is controlled within applications like mobile phones. The base station backs off the transmit power of the mobile to conserve battery power and to prevent the base station receiver being overloaded by nearby signals. This control is normally applied to the pre-amplifier stages and the creation of the envelope control signal is modified to take account of this.
Envelope shaping: This shaping is normally contained within a semiconductor memory. It takes the incoming envelope signal and then adds the shaping required to provide the correct response from the amplifier. This stage typically uses a look up-RAM that has the required contour programmed into it. Typically it requires a 14 bit input to give the required control.
Post envelope gain & offset: This stage is used to provide gain and offset, so that the correct control voltage is applied to the control input of the envelope tracking supply. It is required for calibration of the gain and offset errors in the DAC, filter and envelope tracking modulator.
Digital to analogue converter : As the signal has up to this point been held in a digital format, it requires converting into an analogue form to be applied to the control terminal of the envelope tracking supply
Filter: The filter is required to remove the high frequency components generated by the digital to analogue conversion.

The generation of the control signal for the envelope tracking supply modulation is a complicated process. Once designed, the control signal generator requires careful optimisation to ensure that the system operates in exactly the manner required. Normally once the system has been optimised the circuits are quite repeatable and only small adjustments may be needed in production.