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Merrimac offers an extensive line of manual, electronic,
digital and precision phase shifters for both systems and labo-
ratory applications covering frequencies from below 200 kHz
to beyond 3 GHz.
Mechanical Phase Shifters - PS* series
All Merrimac mechanical phase shifters incorporate a
lumped element quadrature hybrid together with a matched
pair of L-C networks to realize variable phase shifts. As seen
in Figure 1, variable L-C networks linked to output ports 2 and
3 of a quadrature hybrid act as sliding short circuits. Placed at
the output ports of the hybrid, these short circuits reflect
incident energy back towards the source. The reflected energy
appears at port 4, the isolated port, essentially unattenuated.
The
sliding short is varied across an electrical range by
varying the capacitors which causes the phase angle of the
reflected signal to vary by up to 2
f. By properly selecting the
L-C elements, a one-way phase range of 90° can be obtained
and an overall phase shift of 180° is realized. Phase shifts of
360° can be realized either by connecting a 0°/180° phase
reversing switch in tandem with a 180° continuously variable
phase shifter, or by connecting two 0°-180° continuously
variable types together on a common shaft.
Electronic Phase Shifters - PE* series
Electronically controlled, continuously variable phase
shifters operate similarly to the manual phase shifters de-
scribed above. The principal difference is that voltage variable
capacitors (varactors) are used instead of manually adjusted
capacitors. To achieve the desired phase shift range, identical
units are connected in series. Figure 2 illustrates a typical
electronically variable phase shifter.
Electronic phase shifters are available in a variety of pack-
ages and are suitable for use in systems requiring automatic
phase control, closed loop feedback networks and steering of
electronically scanned antennas.
Digital Phase Shifters - PT* & PW* series
This series of phase shifters is designed to be controlled
directly from TTL circuits and is available for center frequen-
cies from 10 MHz to 3 GHz. Phase shift is provided in binary
sequenced increments from LSB (least significant bit) to the
MSB (most significant bit). Units are designed in either digital
switched sections for the optimum absolute accuracy (PTM
series), or as a D/A converter driving analog phase shifters for
guaranteed monotonicity (PWM series).
Center Frequency
The center frequency depends on the type of quadrature
hybrid used. The low frequency limit of mechanical phase
shifters is set by the insertion loss of the resonant network and
the availability of manually variable capacitors with an ade-
quate max-min ratio. The high frequency limit is set by the
physical size of the variable capacitor. The high frequency
limit of electronic phase shifters is limited by stray capaci-
tances and circuit Qs.
PHASE SHIFTERS
Electronic & Mechanical; Analog & Digital
200 kHz to 3 GHz
GENERAL INFORMATION
Figure 1. Mechanical Phase Shifter Section
Figure 2. Electronic Phase Shifter Section
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M ERRIM AC / 41 Fairfield Pl., W est Caldwell, NJ, 07006 / 973-575-1300 / FAX 973-575-0531
Limitations and Trade Offs
21Mar96 VSWR and Bandwidth
Bandwidth is controlled by the type of quadrature hybrid
used. VSWR generally deteriorates at the band edges. If the
quadrature hybrid is optimally designed, VSWR is controlled
by the tracking of the capacitors (or varactors) in the resonant
network. For those units using a piston trimmer as the variable
capacitor, the plate-to-plate capacitance forms an additional
RF path which may result in poor VSWR when a broadband
quadrature hybrid circuit is used. Therefore, phase shift is
generally limited to 90° and bandwidths are limited to 10%
except on specially tailored units available on custom orders.
Phase Shift Range
For mechanical phase shifters, a limited number of variable
capacitor styles are available. This limits the options available
in custom designs. In contrast, for electronic phase shifters, the
range of phase shift is theoretically unlimited and useful range
is governed by the limits imposed by acceptable insertion loss.
Phase Shift vs. Control Characteristic
Since variable capacitors are not generally available with
unusual tapers except in very large quantities, custom charac-
teristics are generally unavailable. For electronic phase
shifters, the phase shift vs. control curve can be linearized
using a multi-section approach where only the linear portion
of each section is used. The cost of this approach is increased
insertion loss. Alternatively, a specific transfer characteristic
curve can be guaranteed from unit-to-unit for which the user
may then design a linearizing drive circuit.
Bias polarity for electronic phase shifters
Standard polarity is positive. However this may be reversed
to meet specific customer requirements.
Group Delay Flatness (phase linearity)
Phase shifters in which no inductors are used are phase
linear. However, the maximum phase shift obtainable is thus
limited to approximately 90°. If more phase shift is desired,
several stages can be cascaded in electronic phase shifters
provided the insertion loss is acceptable. This approach is used
in some digital phase shifters. In mechanical phase shifters,
limited availability of ganged variable capacitors preclude this
approach.
Power
The power handling capacity of mechanical phase shifters
is limited primarily by the Q of the L-C resonant network.
Typically, for a 180° unit, this network contributes loss of 0.5
dB or roughly half the total loss of the unit. At 10 watts input,
for example, approximately 1 watt must be dissipated by the
two inductors. High Q inductors are required for high power
designs and heat sinks may also be needed.
The power handling capacity of electronic phase shifters is
set primarily by the operating bias. At zero bias, most varactors
begin to rectify at 0 dBm. When this occurs, the unit insertion
loss rises and a reverse bias must be applied. Applying reverse
bias, however, reduces available phase shift range. Units can
be built to operate at levels as high as +10 dBm on special
order.
Figure 3. Typical Phase Shift vs. Rotation
Figure 4. Typical Phase Shift vs. Control Voltage
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M ERRIM AC / 41 Fairfield Pl., W est Caldwell, NJ, 07006 / 973-575-1300 / FAX 973-575-0531
21Mar96 Phase Shift Range, Manually Variable Models:
The minimum total phase shift, in electrical degrees, that
results from varying the manual control from minimum to
maximum settings. The phase shift range specified is that
phase shift available in addition to the fixed insertion phase
shift which is the inherent phase shift between the devices
input and output ports.
Phase Shift Range, Electronic (Analog) Models:
The minimum total phase shift, in electrical degrees, that
results from varying the control voltage through its full range
from minimum to maximum. The phase shift range of most
Merrimac electronic phase shifters is in a
negative direction in
degrees, (i.e. insertion phase decreases as control voltage
increases).
Insertion or Transfer Phase:
The difference in phase between input and output ports. For
mechanical and digital phase shifters, the insertion phase oc-
curs at the minimum setting. For the analog electronic phase
shifters, the insertion (minimum) phase occurs at the maxi-
mum control voltage.
Phase Shift vs. Frequency (Group Delay):
Merrimac phase shifters are not constant phase or devices
that are
flat with frequency. For any given setting of phase
shift, whether manual or voltage controlled,as frequency var-
ies, the value and slope of phase shift changes as a function of
frequency, as shown in Figure 5.
Phase Shift vs. Control Characteristic:
This parameter, applicable in particular to custom elec-
tronic phase shifters, is the deviation of the transfer function
(phase vs. volts) from the best fit straight line.
Control Voltage Range (Electronic Phase Shifters)
The DC voltage range required to shift phase from maxi-
mum to minimum. Additionally, this parameter sets the maxi-
mum voltage that may be applied to the device without
damage.
Phase Stability Versus Temperature (Figure 6):
The maximum rate of change in insertion phase at any
frequency over the specified temperature range.
Insertion Loss (Figure 7):
Figure 5.Typical Phase Shift vs. Frequency
Figure 7. Typical Insertion Loss
Figure 6. Typical Phase vs. Temperature
Figure 8. Typical Loss vs. Control Variation
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M ERRIM AC / 41 Fairfield Pl., W est Caldwell, NJ, 07006 / 973-575-1300 / FAX 973-575-0531
Parameter Definitions
21Mar96 The ratio of unrecoverable power (dissipated within the
circuit) to input power, expressed in dB, at a given frequency.
Insertion Loss Variation Versus Control Voltage:
The maximum change of insertion loss from all causes
experienced as the control input voltage is varied over its
specified range (Figure 8).
Settling Time (Electronic & Digital Phase Shifters):
The maximum time interval starting with application of the
control input until the output attaining 95% of its final value
including overshoot and ringing.
Modulation Rate:
The modulation rate is the inverse of settling time. For
phase shifters, the specified modulation rate is limited to about
1% of the center frequency . Phase modulators in the PMP and
PLM series have been designed for much higher rates of
modulation.
Phase Shift Accuracy, Digital Units:
The maximum phase error between the theoretical and