IEEE - Aerospace and Electronic Systems - September 2019 - 40

The Iris Deep-Space Transponder for the SLS EM-1 Secondary Payloads
control. Similarly, the low-noise amplifier (LNA) and
preselector filter have been packaged into a module to
facilitate placing the front-end electronics physically
closer to the receive antenna to reducing the overall
system NF.
At the input of the receiver slice is a switch filter
bank composed of low temperature cofired ceramic filters and a hermetically packaged RF switch. This circuitry provides the receiver with four RF input ports
which can interface with various external devices
including LNAs and block converters. The filtering in
this switch filter bank provides further isolation from
the transmitter along with the rejection of any strong
out-of-band interferers. The frequency translation in the
receiver is realized with an image reject mixer (IRM)
with greater than 35 dB of image rejection. The filtering
and IRM work together to improve the overall receiver
sensitivity via rejection of the amplified image noise
which folds over into the receive band following the
conversion. A low-pass filter at the output of the IRM
attenuates the LO leakage through the mixer. The
remaining IF chain is composed of surface acoustic
wave filters used to reduce the system noise power,
fixed gain amplifiers, and VVA which make up the
adjustable gain control of the receiver.
The exciter slice houses the main TCXO, and the
receiver and transmitter PLOs. Each PLO is composed of
a PLL frequency synthesizer with an active integrator and
a low phase-noise VCO. The receiver PLO is fed to the
receiver slice using a short RF cable. The transmitter
PLO, on the other hand, remains on-board, and with further filtering and amplification, drives the LO port of the
IQ modulator. Following the modulator is a switch filter
bank used to improve spectral purity and reject signal content in the receive band in addition to other neighboring
bands. The switch allows for interconnection of up to four
RF signal chains. These RF chains can include anything
from SSPAs and antennas to block up-converters for
example.
Figure 10 shows a photograph of the assembled
receiver and exciter assemblies. To realize a miniaturized
RF assembly with high gain and a variety of highfrequency sources, aluminum enclosures separate each
distinct frequency and high-gain circuitry into RF cavity
compartments to avoid electromagnetic interference
(EMI) and cavity oscillations. These compartments are
formed between the aluminum cover and the exposed
ground planes of the printed circuit boards (PCB). EMI
gaskets improve electrical conductivity and assure a tight
RF seal at the interfaces. In addition to the enclosures,
there are high-density ground via arrays dispersed
throughout the PCB to help with isolation, and additional
wrap-around ground metallization on the PCB edges help
to reduce external EMI. These packaging techniques not
only improve the system RF performance but also
40

Figure 10.
RF side of the X-band receiver assembly (left) and X-band exciter
assembly (right).

improve the system thermal performance by reducing the
thermal resistance from the device junctions to the spacecraft thermal radiator. We consider these RF packaging
techniques safeguards against radiative parasitic which
take energy away from our intended RF signals and
manifest them in the form of undesired spurious and oscillations. In addition, we also protect our signal from conductive parasitic by implementing impedance-controlled
lines, high isolation bias networks, careful distribution
of RF gain, and direct current power conditioning isolation
using low drop-out regulators. In the end, these circuit
design and packaging techniques allow us to realize a
robust module with the repeatable performance that lends
itself to standard manufacturing processes and reduces
the need for excessive hardware tuning during slice level
testing.

Power Supply Design Description
Previous versions of the power supply are mission-specific designs with lower ionizing dose requirements
(e.g., 2.9 krad for Mars trajectory), but with EM-1 missions baselining longer mission durations, some redesign was necessary to increase the radiation tolerance.
Space-grade radiation-hardened switching converters
are typically bulky hybrid devices that require large
volume and mass. Instead, converters built on bipolar
technology were chosen for their immunity against
destructive latch-up from singular events caused by
highly energetic particles. However, to understand tolerance against total radiation dosage, selected candidate
devices were radiation tested (high-dose rate, 48 rad/s)
with a Cobalt-60 source up to 50 krad with a sample
size of at least three devices. Table 4 lists the commercial-off-the-shelf (COTS) devices that were tested and
their results. Many devices exposed functional failures
before reaching the target radiation level and were eliminated from design consideration.
Most secondary voltages can be generated using
buck converters and linear regulators given they are at
lower voltages than the bus input voltage to the unit.

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2019

IEEE - Aerospace and Electronic Systems - September 2019

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