X/X/Ka Deep Space Transponder & Ka-Band TWTA for Bepi-Colombo Mission to Mercury

BepiColombo Technology Status Review

ESTEC, 22-23 February 2005

Radio Frequency Subsystem

BepiColombo RFS: Functional Block Diagram

The Radio Frequency Sub-System (RFS) for the BepiColombo mission is located onto the Mercury Planetary Orbiter (MPO) and includes:

– the Ka-Band Translator (KaT) – the TT&C Equipments.

DST

X-BandDiplexer

X-BandTWTA Antenna

TWTAKa-Band

Ka-BandTransponder

Down-link TLM Up-link TLC (to CDS)(from CDS)

749 F1880 F1

3344 F1

294 F

315 F

F1 ≈ 9.5 MHz F ≈ 109 MHz

X/X/KaOutput Filter

TT&C Subsystem20 Watt

20 Watt

Ka-Band Communication for Deep Space MissionsThe next frequency allocation for deep space communications is Ka-bandwhich offers a four-fold performance advantage over X-band due to the increased directivity of the down-link beam.

This benefit can be used either:– to increase the data return to the Earth by a factor of four – to reduce the spacecraft antenna or RF power.

From the radio-science point of view, passing to Ka-band allows reducing the plasma effects on the radio link and increasing the signal-to-noise ratio in the carrier tracking loop with a substantial improvement of the Doppler shift measurement accuracy.

XKa

Ka

Ka

X

Besides, the use of a multi-frequency link such as X/X/Ka allows the plasma noise compensation as utilized in the frame of the Cassini mission.

X/X/Ka DST Architectural Design

ALENIA SPAZIO Deep Space Transponder HeritageThe progress of Digital Signal Processing techniques and the improvements of the Very Large Scale Integrationtechnologies allow the implementation of digital modem for Deep Space Transponder (DST).Accordingly, a new class of DST

based on digital architecture has been developed by Alenia for the ESA Deep Space Missions Rosetta, Mars Express and Venus Express.

Alenia DST architecture includes the Receiver Section (with S-band/X-band front-end and digital demodulation capabilities), the S-band Transmitter and the X-band Transmitter.

Alenia DST: FM Model

Architectural Approach to X/X/Ka DST Design (1/2)The proposed X/X/Ka DST architecture is derived from ALENIA SPAZIO DST Transponder platform.

The digital-based design makes the ALENIA SPAZIO DST architectures well suited for X/X/Ka applications. In fact, ALENIA SPAZIO design allows substituting the S-Band with the Ka-Band Transmitter leaving almost unchanged all the other functions included in thesignal-processing core.

In addition, the proposed X/X/Ka DST architecture includes some fundamental improvements with respect to the previous designsuch as:

– Frequency agility– Short-loop receiver architecture based on single-IF and sub-sampling– Regenerative and transparent ranging channels– Support for DOR– Advanced modulation scheme capabilities– Optimization of the RF and digital modules hardware implementation

Architectural Approach to X/X/Ka DST Design (2/2)The X/X/Ka DST is based on a digital architecture; this solution allows meeting the functional requirements with the following advantages with respect to a fully analogue solution:

– Receiver reconfigurability according to the received signal input power;– Easy implementation of narrow loop bandwidths;– Inclusion of data demodulation capability (subcarrier tracking circuit, bit-

synchronizer);– Data rate flexibility with easy matched filtering implementation;– Interface optimization based on MLC and DS16;– Design flexibility due to software tuning of signal processing algorithms;– Direct digital frequency synthesis (DDFS);– Digital modulation capabilities

Rx Front-End IF Section(13F1)

Rx Section Frequency Generation

TCXO

Ext.USO

RF

Sw

itch

From X-band Diplexer(749 F1)

1st IF Section(33F1)

X-Tx Section Frequency Generation

To X-band SSPA

32F1

2nd IF Section(110F1)

77F1

Tx Front-End(880F1)

770F1

1st IF Section(33F1)

BB Section(F1)

To Ka-band TWTA

32F1

2nd IF Section(264F1)

231F1

Tx Front-End(3344F1)

3080 F1

736F1

4F1

X-Band Receiver Module

X-Band Transmitter Module

Ka-Band Transmitter Module

DC

/DC

Converter M

oduleDigital Module

To ADCAGC Control Rx Clock

X3X4

From Digital Module

BB Section(F1)

From Digital Module

Fractional-NSynthesiser

RF

Sw

itch

Fractional-NSynthesiser

Tx Clock

X/X/Ka DST Block Top-Level Block Diagram

32F1

32F1

X/X/Ka DST Frequency Plan (1/2)The X/X/Ka DST frequency plan exploits the Fractional-N synthesis approach to generate the base frequency F1.

The base frequency is then used to generate the LO frequencies both on the receiver and the transmitter side by means Integer-N synthesizer.

– This solution allows to achieve very good performance in terms of phase noise, Allan deviation and coherent turn-around functionality.

– From a technological point of view, the proposed approach is very attractive in terms of mass, power consumption and hardware complexity.

– The X/X/Ka DST includes also the switching circuit for internal Thermally Compensated Oscillator (TCXO) or external Ultra-Stable Oscillator (USO) reference frequency selection.

– Optionally, the internal TCXO can be replaced with a low-power OCXO capable to provide better performance in terms of phase noise and short-term stability (the selected TCXO and OCXO have the same package).

X/X/Ka DST Frequency Plan (2/2)Digital Section

TCXO(OCXO)

DDS

Kx

F1

F0

Rxall-digital

PLL

Fd

Ftx=3344F1+Kk Fd

77F 1

736F1

X-Band Transmitter Section

Frequency Generation Section

AGC

SAWFrx=749F1+Fd

Receiver Analogue Section

ADC13F1-Fd

4F1

32F 1

F1+KX FdFtx=880F1+Kx Fd

Frequency Generationbased on Integer-N

4F1

DDS

4F1

770F

1

231F

1

32F 1

KK

F1

Ka-Band Transmitter Section

F1+KK Fd

SAW

SAW

x4

3048

F 1or

External USO

Fractional-NSynthesis

PhaseMod.

Ka-Band DORTones

DORTones

÷432F1 8F1 2F1

8F1

x3

Receiver Analogue Module (1/2)

Fractional-N Synth. Hybrid

Front-End Hybrid

LO Generation

IF Hybrid

TCXO & Regulation

Digital Clock Generation

Receiver Analogue Module (2/2)IF chain is implemented in a single-chip providing the required gain and AGC capabilities.

The IF-on-Chip (IFOC) is under development using Utsi technology that is well suited for space application.

IFOC Final Layout

Chip size: 2.846mm x 2.849 mm

Bond Pad opening: 60um

Minimum spacing: 20um

Number bond wires: 61

6 bit32 dB Step

Atten

5 bit -16 dBStep Atten

Serial Interface

SAW filterIL=

BW=Fo=

16 dB StepAtten

6

1IF 2IF 3IF

Differential out enable

IF out

IF out IF IN

SerialInterface

3

5

Digital Module (1/3)The X/X/Ka DST signal processing core is based on the TTC Modem presented in a companion paper.

The TTC Modem digital platform includes innovative features such as:– Short-Loop (based on Phase Rotator) and Long-Loop based architectures;– Autonomous carrier acquisition by local sweep;– Advanced tracking capabilities (suppressed and residual carrier, 2nd and 3rd-

order loops);– Advanced data demodulation (for NRZ and SP-L) with lower implementation

losses;– Transparent and Regenerative Ranging channels;– Embedded Micro-controller working as a sequencer in order to manage the

transponder configuration (no external Microprocessor is needed);– Possibility to change sampling frequency according to the mission phase in

order to optimise the performance and reduce the power consumption;– Convolutional and Turbo-Coding;– All-digital modulation capabilities including advanced format such as GMSK,

SRRC-OQPSK and the traditional residual carriers modulation scheme (i.e. PM/BPSK/NRZ, PM/SP-L).

Digital Module (2/3)

Carrier Demodulation

SubcarrierDemodulation

X-BandTx-DDS

Bit Synchroniser

Rx-FPGA

Rx Clock( 4F1)

TC Data, TC Clock, Squelch

ADC

TM Data for X-Band Down-link

Rx

Dig

ital B

oard

Carrier Lock

Ka-BandTx-DDS DAC

TxD

igita

l Boa

rd

Tx-FPGA

CoherenceProcessor

Stro

be P

ulse

Sca

led

Loop

Err

or

Sca

led

Loop

Err

orRangingChannels

DAC

TM Data for Ka-Band Down-link

To Ka-Tx Module

To X-Tx Module

Tx Clock( 4F1)

fromX-Rx Module

RF AGC Control(to X-Rx Module)

Bus

Single Wire

1553RTU OBDH Bus

Digital Signal Processing Core (3/3)Note that the Digital Module is shared between Rx and Tx, but only at physical level. In fact, two independent power supplies will feed the receiver and transmitter circuitry independently. The only electrical connection between Tx and Rx circuitry is given by the reference clock for the coherent mode and the raging signal. It is possible to switch the Tx reference frequency from USO to TCXO or vice-versa without affecting the receiver status. The receiver can be synchronized with TCXO or USO as well. The selection of the Rx and Tx frequency reference is accomplished by means the dedicated RF switches commanded by the relevant HPC.

Space Qualified FPGAFPGA is now a viable alternative to ASIC also for space equipment. In particular, Actel supports a new FPGA family (i.e. RTAX-S) which is well suited for on-board applications.

– The RTAX-S devices have been already used by Alenia Spazio in the frame of Cosmo and Koreasat 5 space programs.

For high-reliability applications requiring hermetic packages, CCGA packages have become increasingly popular as an alternative to Ceramic Ball Grid Array (CBGA) packages for applications requiring very high-density interconnections and higher board level reliability.

– The CCGA uses a high-temperature solder column instead of a high-temperature solder ball to create a higher standoff for more flexible interconnection, and to achieve a significantly increased thermal fatigue life of the package solder joint.

– CCGAs are very robust packages, though the columns are more susceptible to handling damage than the balls.

Receiver State Diagram (1/2)

S1 Signal Detection: The signal detection is based on two center frequency detectors (one for the Narrow-Band and one for the Wide-Band), together with a non-coherent AGC. This architectural solution allows closing the carrier loop bandwidth only when the carrier is inside the loop pull-in. When one of the two center frequency flag is active the relevant Carrier Acquisition state is entered.

S2N/S2W Carrier Acquisition Narrow/Wide: In this state the carrier tracking loop and the lock detection algorithms are performed, along with a coherent AGC. After locking the carrier, the processing continues with the Sub-carrier Acquisit3ion Narrow/Wide state.

S1

S2N

S3N

S4N

S2W

S3W

S4W

E1-2

E2-1

E2-3E3-4

E4-3

E1-2 E2-1

E2-3

E3-4

E4-3

E3-1

E4-1

E3-1

E4-1

E3N

-3W

E4N

-4W

E3W

-E3N

E4W

-4N

Receiver State Diagram (2/2)S3N/S3W Sub-carrier Acquisition Narrow/Wide: The telecommand tracking loops and lock detection algorithms are performed during this state, together with the carrier tracking loop and the carrier lock condition verification. When the telecommand is locked, the processing continues with the Signal Tracking Narrow/Wide state, while if the carrier lock is lost the Signal Detection state is entered.

S4N/S4W: Signal Tracking Narrow/Wide: During this state, the carrier and telecommand tracking loops and the lock condition verification algorithms are executed; a squelch algorithm is executed to verify the quality of the received signal.

Narrow-Band to Wide-Band and Wide-Band to Narrow-Band Transitions:Transition between the N.Band to W.Band configuration and vice-versa are allowed to cope with variation of signal power and dynamic during the carrier tracking phase.

S1

S2N

S3N

S4N

S2W

S3W

S4W

E1-2

E2-1

E2-3E3-4

E4-3

E1-2 E2-1

E2-3

E3-4

E4-3

E3-1

E4-1

E3-1

E4-1

E3N

-3W

E4N

-4W

E3W

-E3N

E4W

-4N

X-Band Transmitter Module

1

Fract.-N Synth. HybridFract.-N Synth. Hybrid3rd LO Generation

1st IF Card2nd IF Card

(to Ka-Tx)

(to Ka-Tx)

Output Section

Ka-Band Transmitter Module

1

1st IF Card

1st LO

2nd LO

2nd IF Card

Output Section (hybrid) DOR Tones Card

Mass & Power BudgetsMass Budget

Module WeightDigital Module 500 g

X-Band Receiver Module 550 gX-Band Transmitter Module 600 gKa-Band Transmitter Module 700 gDC/DC Converters Module 650 g

Miscellanea 300 gTotal 3300 g

Power Budget (preliminary):– Rx only: 11 W– Rx + X-Tx: 17 W– Rx + X-Tx + Ka-Tx: 20 W

Physical Dimensions: – X-axis: 215 mm– Y-axis: 176 mm– Z-Axis: 125 mm

Ka-Band TWTA

IntroductionAs baseline the Ka-Band TWTA for BepiColombo is based the already existing tube TH4606C designed and manufactured by Thales biased by a customised EPC provided by Galileo Avionica.

The TH4606C offers:– Efficiency and mass characteristics: the TWT was designed for high efficiency

and low mass.– Dual frequency performances

Ka-Band TWT TH4606C Main FeaturesTwo different operative configurations can be considered for theTH4606C in the frame of the BepiColombo application:

– The TWT used at the maximum possible RF output power (~35 W). This means that the Ka-Band TWTA power efficiency is maximized.

– The TWT used at 6 dB of input back-off . This means that intermodulationproducts are minimised.

On the basis of the trade-off carried out during Phase 1 of the contract, it turns out that a single Ka-Band TWTA can be used for both the 3344F1(i.e. DST) and the 294F (i.e. KaT) signals.

The Ka-Band exciter inside the X/X/Ka DST will include an ALC circuits capable to control the Ka-Band TWTA input power over 10 dB of dynamic range.

– In this way, it will be possible to select the Ka-Band TWTA operating point according to the mission status (i.e. single-carrier or dual-carrier operations) bymeans Memory Load Command.

Ka-Band TWT TestingIn the frame of the EM phase, the TWT characterisation versus BepiColombo requirements will be performed including both “standard” tests and “special” tests tailored on radio-science requirements.

Figure Note

Input / Output Phase Shift

The test aims to evaluate theinput/output phase shift at saturationover any 10 °C within the operatingtemperature range over any 400 kHzwithin the operating bandwidth.

Group Delay Stability atsaturation

The test aims to evaluate the groupdelay stability over any 10 °C withinthe operating temperature range overany 200 Hz within the operatingbandwidth.

Group Delay Calibration

The test aims to evaluate the TWTAranging delay (standard ranging) in afrequency band of +/- 8 MHz aroundthe carrier. The calibration isperformed for the whole range oftemperatures, Doppler values, powersupply bias values and input signallevels and shall include calibrationerror as well.

Ranging Repeatability

The group delay measurement for anyspecific set of signal and/orenvironmental parameters shall bewithin the calibration accuracyrequirement. This requirement appliesto test set-up used for Group DelayCalibration.

Dual Frequency OperativeMode

The test aims to evaluate the Carrier-to-Intermodulation products ratiowhen operating in dual frequencymode at IBO=0.5 dB (i.e. saturation)and IBO=-5.7 dB.The test is performed considering twounmodulated carriers at 3344F1 and294F and the repeated applying theWBRS modulation (20 MHz tone) tothe 294F carrier.