IEEE - Aerospace and Electronic Systems - September 2019 - 35

radio architecture allows easy reconfiguration of the
radio for other frequencies such as S- and Ka-band with
interchangeable radio frequency (RF) front-end slices.
The uplink signal is first digitized by the analog-todigital converter (ADC) after down-conversion and amplitude leveling by the voltage variable attenuator (VVA) in
the front-end electronics. The tracking loops implemented
on Iris are fully digital second-order phased-lock loops
(PLL) with configurable loop bandwidth (LBW) settings.
The coherent turn-around approach takes the derived
frequency from the uplink carrier signal and feeds this to a
direct digital synthesizer (DDS) to tune the downlink

phased-lock oscillator (PLO). This approach provides a
simple method with fewer components that allows miniaturization while maintaining the necessary frequency agility from the transponder. The main oscillator is a 50-MHz
temperature compensated crystal oscillator (TCXO). This
reference signal is also used to generate the local oscillator
(LO) frequency for block down-conversion in the receive
chain. A simplified block diagram of this architecture is
shown in Figure 3, and Table 2 lists some key specifications of the Iris V2.1 transponder.

RADIO DESIGN
This section provides a brief design description of the hardware, software, and firmware elements that comprise the
SDR. The hardware elements are broken into the digital processing, the RF front-end electronics, and the power supply.

Table 1.

SLS EM-1 CubeSat Missions Using the Iris Transponder

Figure 1.
Iris version 1 (INSPIRE) and version 2 (MarCO) transponder stacks.

Mission name

Figure 2.
Iris V2.1 assembled flight transponder stack.

SEPTEMBER 2019

Target
destination

Max range

LunaH-Map

Lunar

$1 Mkm

Lunar IceCube

Lunar

$1 Mkm

Lunar
Flashlight

Lunar

$1 Mkm

ArgoMoon

Heliocentric

N/Ay

CubeSat for
Solar Particles

Heliocentric

$15 Mkm

BioSentinel

Heliocentric

$84 Mkm

Near Earth
Asteroid
Scout

Asteroid

$180 Mkm

y Mission details not available.

IEEE A&E SYSTEMS MAGAZINE

35



IEEE - Aerospace and Electronic Systems - September 2019

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