IEEE Geoscience and Remote Sensing Magazine - June 2017 - 64

Level-Shifting Amplifier

Oscillator 10 GHz

10-GHz BPF
VCO

fREF

PFD

Loop
Filter

+
Differential
Amplifier

BPF

Compensation
Voltage Ramp

DDS
f
t
600-900 MHz
AWG
v

∑

VTUNE

2-8-GHz BPF

Coupler

Mixer

To
Transmitter

÷20
Frequency Divider

fIgURE 8. A simplified system block diagram of the VCO-based Snow Radar. PFD: phase/frequency detector.

Normalized Power (dB)

0
-10
-20
-30
-40
-50
67.4

67.5
67.6
67.7
Beat Frequency (MHz)

67.8

fIgURE 9. The measured system response of the VCO-based radar

using a 2.81- ns optical delay line.

modulation and frequency nonlinearity, as discussed in the
"System Effects and Their Removal" section.
VCO-BASED SYSTEM
While the YIG oscillator-based system produced reasonable
results from surface-based platforms, it was not suitable for
measurements obtained onboard fast-moving aircraft.
One of its main drawbacks was that the relatively slow sweep
rates that are obtainable with YIG oscillators affected the
coherency of the radar return when operating from fastmoving platforms. To improve the sweep rate, the YIG
oscillator was replaced by a solid-state VCO driven by a PLL
with fast tracking and fast settling capabilities. One of the
64

earlier VCO-based implementations of the chirp generator
produced a linear chirp with 6 GHz of bandwidth in 1 ms
[45]. The sweep rate was subsequently improved to 250 μs for
a bandwidth of 1 GHz [46], 4.5 GHz [21], and 6 GHz [47].
Figure 8 shows a simplified block diagram of the VCObased chirp generator with dual compensation [48]. A
voltage ramp is generated with a 16-b arbitrary waveform
generator (AWG) operating at 250 MS/s. The voltage ramp
is conditioned (filtered, amplified, and dc-offset) to provide coarse corrections for the nonlinear voltage-frequency
characteristic of the VCO. A sample of the 12-18-GHz VCO
signal is frequency-divided by a factor of 20 and compared
with a digitally generated reference-chirp signal in the 600-
900-MHz range. The output of the phase detector is passed
through a difference amplifier followed by a passive-loop filter. This stage provides fine corrections by phase-locking
the 12-18-GHz chirp to the 600-900-MHz reference, which
results in further improvement of the chirp linearity. Both
coarse and fine corrections are applied to a summing amplifier that drives the VCO. Precise-timing alignment between
the reference chirp and the dc-correction waveform is critical for the proper implementation of this technique, and it is
achieved through an iterative calibration process. To obtain a
chirp in the 2-8-GHz range, the VCO output is mixed down
using a 10-GHz fixed oscillator locked to the radar system's
main clock. Figure 9 shows the measured system response
of the VCO-based system with 6 GHz of bandwidth and
250-μs sweep time. The response was obtained using a synthetic target built with a fiber optic line with a measured delay of 2.81 ns. The near-range sidelobes are approximately
20 dB below the main peak, which corresponds to nearly
0.009% linearity [47].
ieee Geoscience and remote sensing magazine

june 2017

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - June 2017