Design of a low cost vector modulator for signal processing applications in the frequency
range 1 to 200 MHz.
G. Bassato, R. Ponchia
INFN, Laboratori Nazionali di Legnaro
I. INTRODUCTION
In the last ten years the performance of many analog
components traditionally used in the field of low frequency
signal processing has increased so much that it became
possible to use them in applications that were exclusive
domain of RF devices. This note describes the design of a
vector modulator based on the AD834 four-quadrant
analog multiplier. This function has been used for the
realization of a low cost analog phase shifter in the
frequency range up to 200 MHz.
whose transmission phase shift is depicted in fig. 2. The
phase response of such device is clearly the same of an
analog multiplier: when the control voltage moves from
positive to negative values the signal phase is rotated
sharply by 180 degrees. This consideration suggested to
develop a vector modulator using an analog multiplier as a
voltage controllable attenuator.
II. EXPERIMENTAL SET-UP
A vector modulator is a device in which the input
signal is split into a pair of orthogonal components
normally designated as I (in phase) and Q (in quadrature).
The amplitude of each component is independently
controlled through a couple of linear attenuators; then the
output signal is obtained by summing in-phase the I e Q
components. So, assuming that VI and VQ are the control
voltages on I and Q ports, the amplitude of output signal is
given by (VI2 + VQ2)1/2, while the phase angle is given by
arctan(VQ/VI).
The figure 1 shows a simplified diagram of this device.
FIG. 2 Amplitude and phase response of an ideal biphase
attenuator
III. S THE AD834 MULTIPLIER
In real components, the implementation technology
depends on manufacturer’s choice. Splitting and
combining is in general done by means of hybrid networks.
The attenuator is in most cases implemented through a
double balanced mixer (Merrimac, Ma-COM). Olektron
Inc. makes use of a so-called biphase linear attenuator,
Analog Devices produces a four quadrant analog
multiplier (AD834) that matches an exceptional high
bandwidth (up to 500 MHz) with a good linearity and
constant phase response. The device has an input range of
+/- 1Volt and a trans-conductance transfer ratio given by:
XY/(1V)2 x 4mA, where X and Y are the input voltages.
The output is differential and appears as a pair of open
collectors; so, to generate a single ended groundreferenced voltage output an external current to voltage
conversion is needed. This can be done through a
wideband transformer or a balun; both solutions have been
realized and tested.
To build the vector modulator we need also a couple of
splitter/combiners. For this purpose we used Mini-Circuits
PSC and PSCQ series combiners (respectively for 0 and 90
degrees). These devices are relatively narrow-band, at
least if compared with the bandwidth of AD834 and RF
transformers, so they are the only components that have to
be substituted when changing the modulator frequency.
The transformers are Mini-Circuits T4-1.
The figure 3 shows the modulator basic scheme.
The center-tapped transformer provides the necessary
DC load to AD834 outputs and matches the impedance to
the PSC input. This arrangement performs well for
frequencies up to few tens of MHz, and then the transfer
gain drops down due to tranformer inductance.
phase shifter. This is a straightforward application for a
vector modulator. In fact, if we apply to the control ports I
and Q two voltage signals that track respectively the
functions cosϕ and sinϕ, the output remains constant in
amplitude but gets rotated by an angle ϕ. For the test we
used a network analyzer HP4195A and a home-made
board containing two laser-calibrated DACs (AD390) to
generate the sinϕ and cosϕ waveforms. We ran the tests at
80 and 160MHz; for each frequency we built two phase
shifters: one based on our modulator, the other based on
Olektron CPMs. The result of test at 80MHz is shown in
the figure 5. At 160MHz the behavior is substantially the
same: the phase error of the AD834-based device remains
lower than 2 degrees, that is acceptable for most
applications.
An effective way to broaden the modulator working
frequency is to use the transformer as a balun (see fig. 4).
In this scheme the current in transformer windings flows in
such a manner that the magnetic flux gets nulled and the
residual inductance is determined only by the series
inductance of the line. Two RL resistors are necessary to
provide the DC bias to the outputs. This configuration
provides a differential to single ended conversion with a
bandwidth increased by an order of magnitude.
Fig. 5 Comparison of phase error for an AD834 and an Olektron
P-CPM based phase shifter (f=80 MHz).
V. CONCLUSIONS
The AD834 voltage multiplier can be used effectively
to realize a low cost vector modulator in the frequency
range up to 200MHz. Its good linearity permits to build a
phase shifter that exhibits a phase error lower than
commercially available modulators. We plan to use the
circuit described above as the base for a digitally
controlled phase shifter. The only things we have to add
are, for each modulation port, a DAC and a look-up table
containing the sinus (or cosine) functions. Such device
could be an inexpensive alternative to the Merrimac digital
phase shifters we currently use in the RF control system of
ALPI linac.
IV. THE VECTOR MODULATOR AS PHASE
SHIFTER
A method to evaluate the device performance in terms
of linearity and phase response is to test its behavior as