RF envelope tracking requires a number of additional circuit blocks to be incorporated into the overall transmitter circuit.
The envelope tracking systems requires an input from the earliest stages of the modulator, right through to the final amplifier.
A number of additional circuit blocks are required over that of the traditional transmitter circuit to enable envelope tracking to be introduced into the system.
Of all the circuit blocks in the envelope tracking system, the most exacting circuit block is that of the envelope tracking supply which provides the varying voltage for the final RF amplifier.
Traditional transmitter block diagram
Before looking at the elements needed to introduce envelope tracking into a transmitter system, it is first necessary to look at the basic block diagram of a traditional system. It is then easy to see the differences.
A traditional RF transmitter system and 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 typically be supplied by a DC-DC converter providing a constant voltage.
To understand a little more about the traditional transmitter, it is necessary to take a more detailed look at the block diagram, breaking it down into further blocks.
For applications where envelope tracking is applicable, most signals 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.
In 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.
The signals are then passed into a mixer where they are mixed with the local oscillator signal to bring them to the required frequency, after which they are summed to create the final signal, and then passed to the amplifier chain for amplification to the required level.
The RF power amplifier is connected to a power supply, and this is decoupled to earth to prevent any unwanted signals and noise appearing on the voltage line, as these would amplitude modulate the signal, creating unwanted noise.
Envelope tracking system
An envelope tracking system requires a little more to be added. The envelope tracking block diagram shows that the DC-DC converter has been replaced by an envelope tracking supply. This takes in information about the envelope to ensure that it provides the correct voltage output to the RF power amplifier so that it runs in its linear portion but also dissipating the minimum amount of heat.
One of the key elements of the system is being able to generate the envelope signal. Although some processing is done, the basic envelope information is derived from the I and Q baseband signals using the following calculation.
Like the traditional RF transmitter system, it is possible to look more closely at the overall signal path required for an envelope tracking system.
It can be seen from the diagram below, that the envelope tracking system incorporates a number of new circuit blocks.
There are several elements to the overall envelope tracking system. These can be summarised below:
- Main RF path: The RF path itself remains broadly the same. The I and Q signals are used to create the composite RF signal that is passed to the RF amplifiers.
- Envelope shaping signal generation: This signal chain generates the required envelope shaping signal. It comprises a number of elements to ensure that the correct signal is generated for the required operation of the amplifier and the prevailing signal conditions.
- Delay balancing: Delays through the various signal paths mean that the RF signal and envelope shaping signal each have their own delays. These need to be compensated for to ensure the RF envelope and the envelope tracking modulator / supply operate in synchronism so that the signal peaks and the voltage peaks from the modulator occur at the same time. With high modulation rates, even small delays can be significant.
- Envelope tracking modulator / supply: The envelope tracking modulator or power supply modulates the voltage to the power amplifier, and sometimes previous amplifiers as well to enable the amplifier to operate at its maximum efficiency point, or other point dependent upon the requirements.
An envelope tracking system requires a number of additional circuit blocks to be introduced into the overall transmitter. Envelope tracking has to be applied to the whole transmitter and not just the final amplifier because a feed of the envelope is required to drive the envelope tracking power supply. This means that unlike a Doherty amplifier which also provides improvements in efficiency, and can operate on its own, envelope tracking must be embedded into the transmitter.
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