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What is Vpeak | Role of Vpeak in Antenna Aperture Tuner selection

This page covers Vpeak basics in antenna tuner. It mentions role of Vpeak in antenna aperture tuner or tuning.

Introduction:
In antenna aperture tuning we are changing electrical length or effective size of the antenna. This changes position of its resonance to other frequency band i.e. altering the radiation pattern. The antenna aperture tuner helps in improving TRP (Total Radiated Power) and TIS (Total Isotropic Sensitivity) of the antenna. TRP is related to transmission and TIS is related to reception in antenna.

The antenna tuner can be simple resonant circuit as shown in the figure-1. This is T-type antenna tuner. The other types are π-type and L-type antenna tuners. It is aimed at proving impedance matching to 50 Ohm and non 50 Ohm parts of the wireless system as shown. Impedance where Vpeak is required to be adjusted is higher than its input/output impedances.

The good antenna design with proper aperture tuning can deliver following benefits.
• Wide Bandwidth
• Increase in data rate
• Lower power consumption
• Longer battery life
• Smaller footprint

What is Vpeak?

• The parameter Vpeak specifies maximum voltage antenna tuner can withstand and also deliver acceptable harmonic performance simultaneously.
• The antenna tuning is done by placing tuning element appropriately at a location along radiating structure of the antenna. The value of Vpeak depends on proximity of tuning element with respect to shorting pin of the antenna. Vpeak is lower if it is near to the shorting pin and Vpeak is higher if it is away from the shorting pin. Following formula are used to conver Power into Vpeak and vice versa.
• P = (Vrms)2/Z, where Z =50 Ohm
• Vpeak =Vrms * 1.414


Antenna aperture tuner
Image source: Application Note from Skyworks Solutions, Inc.

Vpeak role in antenna aperture tuner selection

➤Insufficient Vpeak leads to damage to the circuit. If the damage does not happen than also it can degrade TRP and TIS specifications. This leads to generation of excessive harmonic signals. Due to this mobile phone (or device) under development will fail certification tests (e.g. Radiated Spurious Emission) and consecutively delay the release of the product in the market. Moreover insufficient Vpeak leads to decrease in the antenna efficiency.
➤Many vendors mention optimum Vpeak value in their product catalogues with respect to harmonics specification.
For example: SPST antenna tuner (part number: SKY19250) from Skyworks specifies that harmonic performance of better than -36 dBm can be acheived if Vpeak used is 80 V.
➤Vpeak value depends on input power, antenna matching circuit topology and loaded Q of the resonant circuit. The figure-1 depicts graph of Vpeak versus frequency which depends on VSWR at the input of the circuit.

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