Beam agility derived from the flexibility of this architecture allows
for frequency hopping and beam scan patterns that are not predictable (good for avoiding jamming).
Solid state amplifiers have advanced to the point where TR
modules can be light and compact while transmitting the high
power needed for longer range radars. Gallium Nitride (GaN) is
an enabling technology due to its power-handling and thermal
capabilities. The conversions between analog and digital domains
(DAC/ADC) will soon be located directly within the TR module
itself (see Figure 1) putting the digital signal right next to the
antenna, simplifying the overall design of the system.
There are many challenges associated with the test and calibration of phased array antennas. The more elements that make up the
array, the longer the time it takes to fully characterize the antenna.
In a phased array antenna with hundreds or thousands of elements,
where it is necessary to characterize each element in a relative way
to the others, the ability to accelerate the test by using multiple
coherent measurement channels is a significant benefit.
The element-to-element phase and magnitude (gain) errors of the
Figure 3. Measuring relative phase in the presence of noise.
various components in the array antenna are significant limitations
to its overall performance. Since phase is used to steer the beam in
a phased array, the errors introduced by the misalignment of the radiating elements must be calibrated out so that the antenna operates
efficiently and accurately. Next, we’ll focus on the static phase and
gain errors across elements and describe new methods for producing
a set of calibration measurement data to correct for these errors.
Depending on the signal, there are two possible methods to
analyze the cross-channel response by measuring relative phase and
gain. The first method is a narrowband approach that uses a swept or
stepped tone and a narrowband receiver to measure one frequency at
a time and perform cross-channel computations in the time domain.
However, this method is limited to narrowband measurements.
The second method uses a broadband stimulus and a wideband
receiver to measure all frequencies simultaneously and compute
the cross-channel spectrum. The ideal measurement solution has
the flexibility to use both methods. A wideband digitizer with
Digital Down Conversion (DDC) provides this flexibility and is a
unique solution because of its adjustable bandwidth. Let’s look
more closely at DDC and its benefits for this application.
As shown in Figure 2, a hardware-based DDC is a two-stage
digital signal processing block that processes data taken directly from the analog to digital converter (ADC) at full sample
rate. Then, after frequency translation and decimation, the data
is stored as complex I&Q samples to the digitizer’s memory. A
DDC can also be created in software, but the consequence is that
software DDCs run much slower and rely on the data at full ADC
sample rate to be first off-loaded from the digitizer for processing.
So what can the DDC do for you? As part of a digitizer it allows
you to isolate the signal of interest then improve the SNR and dynamic range within the bandwidth of the signal of interest by reducing the amount of integrated noise. It also extends the amount of signal capture memory or reduces the amount of data that needs to be
transferred for a given duration. Since there is less data to analyze, a
DDC can reduce the workload of the post processing algorithms.
The DDC improves sensitivity for phase and amplitude measurements by reducing noise in the time domain. The noise density remains the same, but less noise gets integrated into the measurement
as the span is reduced. Figure 3 depicts multiple waveforms and
how, at a given threshold, we can visualize the phase differences
between these waveforms. Determining the actual time that a noisy
waveform crosses through the threshold can be difficult. The three
plots show how it is more precise to determine where the waveform
crosses the threshold with reduced noise on the waveform.
Figure 4 shows a generic block diagram of a test system used in
the test and calibration of array antennas. The challenge is to measure relative phase and amplitude of radiating elements in a phased
array. It is often desirable to test multiple pairs of elements at a time
to accelerate test speeds when dealing with large arrays containing
hundreds or thousands of elements. The signal path is from left to
right, originating with the antenna array under test. The signal
travels through a path of several stages of conditioning and down
conversion. The goal is to take the RF/µ W signal from the antenna
elements and frequency translate it down to an IF that is within
the BW of the digitizer. To maximize the use of dynamic range of
the digitizer, it is typical to amplify or attenuate the signal to levels
that fall close to the full scale range of the digitizer being used. In
some cases a low-pass filter is also employed for image rejection.
The digitizer is the back-end of the signal measurement chain.
When testing a phased array antenna configuration, it is often
desirable to test multiple pairs of elements in parallel to accelerate
test speeds. Therefore, a multichannel digitizer with phase coherent