AN3001 Datasheet, PDF (1/7 Pages) M/A-COM Technology Solutions, Inc. – PIN diode Vector Modulators - Fundamentals and Drive Requirements

Application Note
AN3001
PIN diode Vector Modulators -
Fundamentals and Drive Requirements
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.
Rev. V4
• 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)
925 - 960
1930 - 1990
2040 - 2240
Band
Model Number
Model Number of Prototype with Driver
GSM
MAMDCC0005
MAMDCC0005-DC000
PCS
MAMDCC0002
MAMDCC0002-DC000
UMTS
SA90-0001
SA90-0001-DC000
Table 1. Selection Guide for Vector Modulator
Availability
Q2 2002
In Stock
In Stock
1
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