PIN diode Vector Modulators -

Fundamentals and Drive Requirements

Rev. V4

Application Note

AN3001

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Introduction

M/A-COM’s Chip Scale Package (CSP) vector

modulator platform offers a means of varying

attenuation and phase in a single surface mount

package.

These vector modulators offer linear

phase and minimal amplitude ripple in their bands of

operation.

Due to using PIN diodes as the active

devices, these vector modulators have high intercept

points. These vector modulators operate in tandem

with a dual linearizer, MADRCC0002 that has been

developed by M/A-COM.

Basic Vector Modulator Operation

The block diagram for the vector modulator is given

in Figure 1. The vector modulator consists of four

elements: an input quadrature hybrid, two voltage

variable attenuators (four-port quadrature hybrids

with both outputs terminated in PIN diodes), and an

output power combiner.

One of the keys to understanding how vector

modulators work is to understand that there is a

phase shift of 180

° when the impedance of the

diodes (for a single VVA) crosses 50 ohms.

For

diode impedance of greater than 50 ohms, there is

no phase change when the signal reflects off of the

diodes. This can also be noted on the Smith Chart.

Note that when the impedance of the diodes is less

than 50 ohms, there is no phase shift. The following

will describe the vectors in three cases. See Figure

2 for additional insight into how single vectors pass

through the vector modulator.

•

Case 1. If the diodes in the bottom VVA are set

to an impedance of 50 ohms, the bottom VVA

will be in a high loss state. The output of the top

VVA will have a phase of 0

° or 180°, depending

on the impedance of the terminating diodes.

See the “A” or “/A” labels.

•

Case 2. If the diodes in the top VVA are set to

an impedance of 50 ohms, the top VVA will be

in a high loss state. The output of the bottom

VVA will have a phase of -90

° or 90°, depend-

ing on the impedance of the terminating di-

odes. See the “B” or “/B” labels.

•

Case 3.

If the output of the top VVA has a

phase of 0° with a magnitude of 1, and the out-

put of the bottom VVA has a phase of 90° with

a magnitude of 1, the resultant vector will have

a magnitude of 1.414 (square root of 2) at an

angle of 45°.

Definition of RF Parameters

It is helpful to define some of the parameters used

to specify the electrical performance of a vector

modulator, so that the specifications are clearly

understood. Note that this type of vector modulator

is designed to have continuous phase shift over

360

°.

Reference Loss: This is the minimum loss where

all phases from 0

° to 360° are available. This is

measured at the center frequency of the specified

band.

Attenuation Range: This is the attenuation (relative

to the reference loss), where the performance of

the vector modulator is defined. For example, if a

vector modulator has an attenuation range of 10

dB, and a reference loss of 12 dB, the vector

modulator will operate over an absolute loss range

between 12 and 22 dB. Note that in the case of

SA90-0001, the vector modulator has a higher at-

tenuation range than the specified range, but the

attenuation is difficult to control, and the phase line-

arity and amplitude ripple start to degrade.

Frequency Band (MHz)

Band

Model Number

Model Number of Prototype with Driver

Availability

925 - 960

GSM

MAMDCC0005

MAMDCC0005-DC000

Q2 2002

1930 - 1990

PCS

MAMDCC0002

MAMDCC0002-DC000

In Stock

2040 - 2240

UMTS

SA90-0001

SA90-0001-DC000

In Stock

Table 1. Selection Guide for Vector Modulator