Firefly-II). The DDS 1 GHz clock signal is generated using a Phase-Locked Loop (PLL) oscillator that
uses the 100 MHz system clock as reference. All GPRI-II timing and reference signals are derived from
the 100 MHz reference oscillator such that the radar is fully synchronous.
Programming of the DDS is via a 3-wire serial protocol. An small micro-controller handles this
protocol and communicates with the Instrument Computer over an RS-232 serial line. The DDS
produces a chirp in the frequency range 100 to 300 MHz. This chirp is first heterodyned to the
frequency range 900 to 1100 MHz. The first local oscillator (LO) is a 1200 MHz Phase-Locked Loop
operating above the desired 900-1100 MHz sideband.
The microwave LO is a phase-locked Dielectric Resonator Oscillator (DRO) operating at 8.1 GHz. The
output chirp spanning 17.1 to 17.3 GHz is then filtered, then amplified and then feed to the antenna. An
isolator in the output path provides a constant load for the input and output RF amplifiers. In the
receiver, the signals from the 2 receiver antennas are amplified, filtered, and mixed down to 900-1100
MHz using the DRO LO signal. We use an MMIC active mixer with integral LO amplifier (Hittite
HMC570LC5) for this function.
The IF signal is then further amplified and mixed with the original transmitted chirp (FM-CW). A
portion of the chirp signal spanning 900 to 1100 MHz is amplified and used to demodulate the received
signal. Digitally programmable attenuators are used to set the receiver gain. The second mixer is a
high-level MMIC active mixer with integral LO amplifier (Hittite HMC686LP4). The baseband
demodulated signals lie in the range 0.30 to 2.5 MHz. The frequency of the demodulated signals is
proportional to distance from the radar.
These two baseband channels are amplified and digitized at 6.25 MHz by 14-bit analog-to-digital
converters (ADC). These ADCs are in the front-end of the ETTUS USRP-2 Software Defined Radio
(SDR) that streams data samples over raw Ethernet to the system instrument computer.
One of the significant advantages of the GPRI-II design is the ability to change the output operating
frequency by changing the LO frequency. Another important feature is the use of gain modules with
constant gain over the passband in the IF module.
The microwave electronics use MMIC modules on co-planar-waveguide boards rather than
connectorized modules as in the GPRI-1. This major advantages including
1. Elimination of connectors and cabling, leading to reduction in losses, less connectors and
shorter transmission lines
2. Approximately 3 times less power consumption leading to substantially less heat generation
3. Lower cost, typical MMIC components cost a fraction of the connectorized component but
require a custom microwave coplanar waveguide circuit board and custom enclosure.
4. Simpler mechanical construction, parts are integrated on the circuit board. Reduced size and
mass elimination of heat-sinks.
5. Better performance, amplifier efficiency is about 2.5 times better than connectorized versions.
Better noise figure due to lower loses.
Microwave components from Hittite are the basis of the GPRI-II up- and down converters. The
HMC710LC5 is an IQ frequency up-converter and the HMC570LC5 is the matching receiver IQ down-
converter. These chips contain a 2 local oscillator (LO) amplifiers, a X2 frequency multiplier, a 90
degree splitter, 2 balanced mixers, and an RF amplifier packed within a 5x5 mm square package. The
up-converter is used to heterodyne the 0.9 to 1.1 GHz chirp to 17.1 to 17.3 GHz. The down-converter is
used in the receiver to amplify and translate the receiver input signal down to 0.9 to 1.1 GHz. The
