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High
Input Analog Bandwidth Track and Hold Amplifiers for Digital
Sampling and Radar Applications
By Loi Nguyen, Vice President of Technology, Inphi Corporation
Demands for higher performing, lower cost,
smaller size components for digital sampling scopes, automatic
test equipment (ATE), digital receivers, and radar systems
continue to grow as design engineers seek alternatives to
expensive, bulky analog/RF components. From higher input
analog bandwidth and sampling rates to requirements for
performance at DC to well over 10 GHz, manufacturers are
challenged to take advantage of innovative technologies
and processes to deliver. Indium Phosphide (InP) is the
latest technology that has been exploited to fill these
demands.
InP devices exhibit the highest cutoff frequency,
a figure-of-merit that measures the speed of a transistor,
among all semiconductor technologies in production today.
As a result, circuits made in InP, even those with relaxed
geometry of 1.0-mm, outperform similar circuits made in
traditional Gallium Asernide (GaAs) and Silicon Germanium
(SiGe) with much smaller geometries (e.g., 0.18-mm). For
high-speed applications, InP technology has been proven
to be a very cost effective solution, competing very well
with GaAs and SiGe technologies. Inphi Corporation, for
example, has shipped InP circuits in high volume since 2002,
and continues to develop advanced InP products to meet the
ever-increasing demands for high performance integrated
circuit solutions.
A new class of very high input analog bandwidth track and
hold amplifiers (THAs) has been developed by Inphi Corporation
for the test and measurement, ATE, digital receivers, and
radar systems. A THA is typically used as the "front end"
of an analog-to-digital converter (ADC) in high speed digital
sampling applications. The THA's primary function is to
track the input signal and hold its voltage constant during
the interval required for the ADC to perform the analog
to digital conversion. Today, most commercially available
ADCs are designed to have an input analog bandwidth of less
than 250 MHz. This eases the conflicting requirements of
high dynamic range and fast sampling rate, and ensures delivery
of high resolution components at low cost. By using the
THA as the front end of a low cost commercially-available
ADC, system designers can extend the input analog bandwidth
of the ADC from around 100 MHz to well over 12 GHz.
In such a design, an 18 GHz input analog
bandwidth THA can drive a commercial-off-the-shelf ADC with
100 MHz input analog bandwidth, resulting in a circuit that
offers a significant cost advantage over alternative approaches
and is readily available for a digital sampling signal analysis
test instrument. Table 1 compares the performance of an
InP THA (Inphi model 1821TH) against competing products
in GaAs and SiGe for high input analog bandwidth digital
sampling applications.
Track and hold amplifiers are also attractive solutions
for advanced radar systems. For this application, the input
radio frequency (RF) signal is typically a wideband analog
signal in the two to 18 GHz frequency range. Traditionally,
the signal is downconverted multiple times (typically three
times) to an intermediate frequency (IF) in the 100 MHz
range in order for it to be digitized by a narrowband ADC.
By varying the local oscillators (LO) frequencies in the
receiver chain, different parts of the RF signals can be
sampled and "channelized" to different ADCs. This is a rather
cumbersome design with multiple mixers, LOs, LO drivers,
gain blocks, filters, and ADCs, which add cost, size, and
weight to the radar.
Recent advances in ADC technology, however, offer an alternative
solution. By using a broadband ADC, it is now possible to
eliminate multiple mixers, LO drivers, gain blocks, filters,
and narrowband ADCs, as shown in Figure 1. The role of the
THA is to extend the input analog bandwidth of the ADC as
well as to improve the system dynamic range and linearity.
In a comparison of the single-tone total harmonic distortion
of a National Semiconductor broadband ADC (model ADC081500)
with and without the InP THA at a 1.5 GHz sampling clock,
significant improvements, five to 10 dB, were obtained with
the InP THA "front end" over the entire frequency range
from 100 MHz to three GHz.
In summary, a new class of high input analog
bandwidth, high sampling rate THAs (Figure 2) is now available
for test and measurement, automatic test equipment, digital
receivers, and radar systems applications. These THAs offer
system designers attractive solutions to directly capture
and digitize high bandwidth signals at GHz frequencies,
which result in higher performance, lower cost, smaller
size, and lower weight systems.
Inphi Corporation
www.inphi-corp.com
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Keeping the Right Perspective on Timing
