The RF topology for
this Digital Attenuator consists of a transmission
line with diode spacing optimized over a desired frequency
range and the digital to analog drive circuitry to
control the variable analog attenuator.
The key component is the choice of diode. It is an
important design consideration. It is mounted in a
shunt configuration with and without Q Spoiling Networks.
Diodes oriented in the same direction eliminate bi-directional
currents for improved temperature compensation. Using
chip diodes with a ribbon lead has its drawbacks.
In a high frequency broadband application, the inductance
of ribbon lead adversely affects the attenuation and
bandwidth. A Beam Lead Diode installed using a proprietary
technique reduces the series inductance and has a
significant improvement on performance. To minimize
adverse effects from biasing, the network is buried
as far as possible from the input and output ports.
In the RF design of a Digital Attenuator, the preferred
media is microstrip. Microstrip allows access to the
circuitry while being measured. This provides for
a more precise tuning capability. This technique yields
an overall improvement in both the optimization and
performance.
Even with the improvement in the microwave performance,
it is the control circuitry that provides the absolute
accuracy for the overall device. It has to be capable
of canceling the non-linear effect of the PIN diodes
and provide a monotonic linear control input slope
characteristic.
In order to achieve the absolute accuracy for the
Digital Attenuator, the digitally controlled section
of the Attenuator has a resolution capable of 64K
to control and compensate the Digital Attenuator.
For any desired resolution, the optimal performance
values for each Digital Attenuator is stored in the
driver's EEproms ready to be commanded from the external
Digital Control Input. This is accomplished by using
a computer with I/O and IEEE controller cards, G.
T. Microwave's proprietary program and a Vector Network
Analyzer. The computer's I/O port sets the external
control input for the desired digital attenuation,
then ramps the driver's 64K of resolution down the
dynamic attenuation range using another I/O port to
an internal control input. While ramping the Digital
Attenuator driver, the Vector Network Analyzer measures
each step and sends the data to the computer via the
IEEE Bus. When the optimal location is determined,
the computer programs the driver's EEproms for the
external control input count.
The Digital Attenuator being demonstrated is optimized
over a 9:1 bandwidth, 2.0-18.0 GHz with 105 dB of
dynamic attenuation range and 0.03 dB resolution,
12 BITs of TTL compatible binary logic and it is capable
of switching from any state to any state within 350
nanoseconds. The Insertion Loss is 5.0 dB, attenuation
flatness is +/- 1.0 dB to +/- 8% of the set attenuation
value and V.S.W.R. is 2.2:1. The Digital Attenuator
envelope is 3.0 x 2.0 x 0.75 inch. Using the techniques
described herein, the following test data illustrates
the typical performance that is achieved
Click
here to view Plot Responses
This technology hosts a variety of products, which
include, but are not limited to: BPSK, QPSK &
Vector Modulators, Phase Shifters and Phase Free Attenuators.
The models are offered with options that include:
digital control with up to 64K of resolution, linearized
or any desired control input slope characteristic,
narrowband optimized performance, temperature compensation,
video filtering and sub-assembly integration.
New modulation techniques will require technology
to demand a new generation of components. These components
will need an improved performance at a lower cost.
Now, industry can welcome the arrival of ultra-broadband
Digital Attenuators that will provide tomorrow's capability
today at G. T. Microwave, Inc., the leading edge in
performance. |
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