Envelope Tracking Delay Balancing & Synchronisation

Delay balancing is required within envelope tracking to ensure that the peaks and troughs of the RF amplitude synchronise the voltage waveform applied to the RF power amplifier.

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

It is necessary to match the delays that occur within the arms or circuit paths of the envelope tracking system, otherwise there will be a mismatch in the delivery of the required voltage to the power amplifier.

Impact of poor envelope tracking synchronisation

It is particularly important to have good synchronisation between the RF envelope and the control signal otherwise the RF amplifier performance will be degraded:

Increased power dissipation: If there is poor synchronisation between the waveforms then power dissipation will be increased because the voltage peaks will not occur at the same time as the RF envelope peaks, resulting in increased power dissipation
Reduced efficiency: The increased power dissipation reflects into the overall operation of the amplifier as a reduction in efficiency.
Increased distortion: Poor synchronisation will also mean that there will be insufficient voltage supplied to meet the envelope peaks and this will result in the signal driving the amplifier into compression, thereby distorting the signal

Delay mismatches

In order to understand the reason for the envelope tracking delay balancing it is necessary to look at the reasons why delay mismatches occur.

The reasons for the mismatch in the RF envelope and the envelope tracking signal arise from the different delay periods that occur in the paths taken by the two signals.

Looking at the block diagram, it can be seen that there are two distinct signal paths within the envelope tracking signal: one for the RF signal and another for the envelope control signal..

Detailed block diagram of a envelope tracking transmitter showing I & Q inputs, DACs, mixers, summation, pre-amplifier and final amplifier — Envelope tracking signal path

In 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 they are 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 examples

In 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.

An oscilloscope image of a well synchronised envelope tracking system where the control signal and envelope tracking are well synchronised and the delay balanced — Good envelope tracking synchronisation

If 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.

An oscilloscope image of a poorly synchronised envelope tracking system where the control signal and envelope tracking are slightly out of synchronism and the delay in each path is not accurately balanced — Poor envelope tracking synchronisation

Even 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.