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Active Matching: Z_s and P_s as funcion of frequency

Introduction

In mobile phone applications it often happens that the power amplifiers (PA) are far from ideal and that the maximum output power is achieved at a load impedance that significantly deviates from 50Ω. This means that many times it is worth the effort to match the antenna to the PA’s optimum load impedance rather than to 50Ω at the TX-part of the bands. This is commonly referred to as Active Matching.

BetaMatch has built-in features to facilitate matching and optimization for sources that have:

• Impedance that varies with frequency.
• Output power (or input sensitivity) that varies with frequency.
• Any combination of the above.

In order to perform this kind of matching it is necessary to first find out at what impedance the PA delivers maximum power (and the value of this maximum power).

Source Editor

The place to change Z_s and P_s in BetaMatch is in the source editor. The source editor can be opened by clicking on the source in the network plot. It can also be opened from either the View or the Component menus.

When first opened the source editor looks like this:

The source editor consists of 3 parts:

• The window on the left side is the editor where you can enter and edit the new source parameters.
• The middle column consists of controls to change and view the source.
• The right side shows the conjugate of current source impedance. This window is not editable (but it is still possible to select and copy text from this window).

The controls in the center row are from the top:

• [ → ]: Changes the current Z_s* to the values in the source editor.
• [ ⇄ ]: Swaps the data between the ‘editor window’ and the ‘ Z_s ^* window’.
• [ ← ]: Copies the content in the Z_s* window to the editor window.
• The tick boxes Zs* in Smith and Zs in Smith shows Z_s* and Z_s in the Smith Chart when ticked (these options are also available in the context menu of the Smith Chart).
• Format text: This will make the data easier to read by formatting it into evenly distributed columns. You can change the text size in the text windows by holding down the Control-key while rotating the mousewheel.

Note

The optimum load (Z_s*) is the value that is used to describe the source. Note that it is the conjugate of the optimum source impedance. This is by convention and is also convenient since normally the Z_s* is the value that is measured by most load-pull measurements.

The source is changed by entering rows in the editor window. Each rows consist of at least 3 numbers:

1. Frequency in MHz (f_i).
2. Real part of the impedance starting from f_i.
3. Imaginary part of the impedance starting from f_i.
4. Optional: Relative ‘output power’ in dB. If this value is omitted it defaults to 0dB.

The values given describes the source impedance and output power from that frequency until new values are given. The default row when the editor is opened:

0 50 0

means that Z_s* = 50Ω and that P_s = 0 dB (100%) from 0 MHz and on-wards. This line describes a constant default source. To change this a new row must be added.

Some Examples

Example 1:

  0      50    0
880  25  -25
915      50    0

The above describes a source with Z_s* = 25 - j*25 Ω in the frequency range 880 - 915 MHz and Z_s = 50Ω for all other frequencies. Since the there is no data in the 4th column P_s is assumed to be constant 0dB (100%).

Remember that it is the conjugate of P_s that is given in example 1 above. Z_s = 25 + j*25 Ω for frequencies in 880 - 915MHz.

Example 2:

0    50  0          0
880  50  0  -0.75
915      50      0          0

Example 2 describes a source with a constant Z_s* = 50Ω everywhere. The output power, P_s, is 0.75dB lower in the TX-part of the GSM900 band (880 - 915 MHz). When an optimization is run it will compensate for this and give a weight of 0.75dB to this part of the band. In the optimization results the 880 - 915 band will have higher efficiency than other frequencies.

An optimized curve with P_s as in ‘example 2’ run for GSM900 and GSM1800 bands is shown below. The thick line describes the actual output (with the varying output). The thin line shows the efficiency (as if the output was 0dB for all frequencies). Also notice that the efficiency is higher for the low band than for the high band. This is because the optimizer is compensating for the decrease in output power on the TX-part of the GSM900 band.

Example 3:

This example shows a combination of examples 1 and 2. Here both the impedance and the output power varies. Below is a figure to show what it looks like in the source editor when the data have been entered and transferred to the Zs* window with the ← button.

Tip

Although there is no load or save function in the source editor it is still possible to select, copy and paste text into and out from the editor window. If you have a more complex set of data you can save it in a normal text file and copy it into the source editor when you need it. Copy, cut and paste can be done via the context menu or with the normal shortcuts (ctrl-c, ctrl-x and ctrl-v respectively)

Tip

You can change the font-size in the source-editor by holding down the control-key while rotating the scroll-wheel.

Note

In BetaMatch the source is treated as a generator (transmitter), but it could just as well have been regarded as a receiver. The reason for choosing the ‘transmitter’ approach is purely practical since most of the time it is the amplifiers that deviate from 50Ω, whereas the receivers normally is close to this value.