Wednesday, July 24, 2024

A Re-Engineered Approach to Standard Transponders for Space Applications

Parent Category: 2017 HFE

By Dr. Kamaljeet Singh


This article demonstrates a re-engineering approach in a standard transmitter and receiver to eliminate extensive tunability and temperature variation effects associated with standard transponders. This approach is based on a mixer and filtering rather than use of a conventional step recovery diode (SRD) to achieve the desired performance. An associated limitation with SRDs is the lack of availability of certain models, batch-to-batch variation, and temperature dependency.  The SRD approach requires long turnaround time involved in tuning the elements to achieve desired performance at printed circuit and module package level. This article proposes the concept of employing a mixer along with a cavity filter that can be implemented at almost any frequency band. A synthesized version can be agile enough to implement for multiple missions, providing added advantage to the proposed approach. Furthermore, replacing the front-end and high power chain with MMIC based topologies can drastically reduce the physical footprint of the existing system and ensure long term availability of the components.


RF sub-systems are critical components in spacecraft as they provide the link to payload operations. The main challenge is to have a reproducible, reliable and robust system at various frequency bands which requires standardization and operational awareness to achieve desired performance. Modules contain many active circuits for which there are batch-to-batch and performance variations. This necessitates an effort to compensate for parasitics through tuning[1]. Systems for GEO-missions consist of transmitters, receivers, associated antennas and feed systems. A transponder is basically a system which receives a signal, amplifies it, changes its frequency, and re-transmits it. The main components in existing C and Ku-band telemetry and tracking (TTC) transponders are: transmitter, receivers, TCXO, main antenna, null filling antenna, uplink coupler, downlink coupler, preselect filter, global horn, and modulator for Ku-band beacon. The main function of on-board satellite communication systems is to transmit telemetry signals, receive tele-command signals, and transmit payload data which is further enumerated below:

  • Transmit telemetry data from the spacecraft to ground
  • Receive and demodulate the command signals transmitted on a selected uplink carrier
  • Demodulate the tone ranging information transmitted from the ground and re-modulate for tracking purposes

 1705 HFE transponders01

Figure 1 • Standard Overview of TT&C Transponder.

Transmitter packages transmit telemetry data and ranging information at (C/Ku-band) from the spacecraft to ground as a phase modulated downlink.  Receiver packages receive and demodulate the command signal transmitted on an uplink carrier from a ground station at C/Ku-band. Also, it demodulates the tone ranging information from the transmitted up-link carrier.

Frequency Band

Uplink Frequency

Downlink Frequency


148-149.9 MHz

137-138 MHz


2.025-2.11 GHz

2.20-2.29 GHz


5.9-6.4 GHz

3.7-4.2 GHz


14-14.5 GHz

11.7-12.2 GHz

Table 1 • Tentative Frequency Bands for TT&C Transponders.

The VHF band was used for earlier missions but present satellites are using S and C-band frequency moving towards use of Ku/Ka-band [2]. This article details the proposed implementation of a mixer instead of SRD multiplier to overcome the limitations imposed by the existing transponder, namely of the receiver and transmitter. The presented scheme can be easily implemented without making major modifications to the existing system.

Challenges in the RF Sub-system

The major challenge in RF sub-systems is to meet the specifications at the desired frequency. This is because the frequency varies for each project and accordingly the tuning of the components and matching plays an important role in meeting the desired specifications. Several design trade-offs need to be carefully considered such as the Step Recovery Diodes used for a frequency multiplier chain, and specific process for mounting a Temperature Compensated Local Oscillator (TCXO) [4]. Tuneable and selectable components (mostly passive) need to be carefully selected and tuned to achieve optimum performance in terms of gain, amplitude flatness, modulation index, harmonic and spurious suppression, etc. The variation in performance necessitates a systematic approach to diagnose and rectify the problems. Further, active devices are also prone to frequent failure due to improper handling and biasing. Some of the failures encountered in RF packages are:

  • Device failure with initial biasing: due to high voltage/shorting phenomena
  • Batch to batch variations in active devices such FETs, BJTs, and SRDs: matching/tuning is performed to compensate
  • Grounding Problems: Improper grounding results in oscillation
  • Varied behaviour in passive components: component replacement and failure analysis
  • Failures in thermal cycling, vibration tests etc.: non-destructive analysis and replacement

Apart from the above, as the RF frequency is increased, external effects and connection tab/wire lead parasitics, mutual coupling, radiation and associated losses increase considerably in RF packages. Also, proper care must be taken to mitigate heat dissipation in power amplifiers circuits to avoid thermal runaway. The most critical component is the multiplier chain and any small deviation from expected function needs to be rectified to avoid overall degradation of system performance.

Transponder Description

Transmitter for GEO Mission

The transmitter section consists of five sections consisting of various components such as FETs, a quadrature hybrid, SRD, capacitors and resistors as shown below:

  • Temperature controlled crystal oscillator (reference source)
  • Multiplier.
  • Modulator.
  • Power Amplifiers.
  • Power Supply and Interface circuitry.

A standard transmitter as shown in Fig 2 utilizing a times 12 frequency multiplier chain with amplification to achieve the desired specifications.

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Figure 2 • Standard Transmitter Topology.


Receiver for GEO Mission

A receiver in C and Ku-band is a dual conversion superheterodyne receiver.  The main blocks for the receiver are:  low noise amplifier (LNA) (MIC based) , microstrip and SAW filter, multipliers, and amplifiers.  The receiver printed circuit board (PCB) consists of a combination of different materials such as RT/duroid®, TFG, Glass epoxy, and Alumina substrates consisting of 18 boards. These support and interconnect components such as HEMTs, transistors, SRDs, SAW filters, and mixers.

The standard receiver as depicted above faces challenges such as: incorporation of a new reference source due to frequency agility in each satellite, swapping out a RF system needs tuning and re-qualification, on-board interference from nearby RF sources, non-availability as an off-the-shelf system, and obsolete devices.  Further MMIC chips are introduced to the design for implementing the miniaturized version front-end of the above system.

Table 2 provides an overview of the existing topologies showing that the main critical portion is the multiplier chain requiring considerable labor intensive time consumption in tuning.  The overall time burden also depends on the selection and segregation of the components, workmanship, problem debugging expertise, matching with downstream circuit chains, etc.



Time Consumption

Mitigation Techniques 


Spurious components


Stable source with low spurious components




Proposed approach

Problem diagnosis

Fab related/component


Listing of frequently occurring problems

Table 2 • Critical Aspects in Standard Transponders.

Multiplier Topologies

A frequency multiplier is basically employed for signal generation at higher frequencies due to superior noise performance and stability.  Planar topologies can be realized using diode or transistor based topologies.  Diode-based multipliers are employed for higher order multiplication because transistor based topologies require higher power drive and are less efficient. Also, an added advantage associated with diode multipliers is their robustness in challenging environmental conditions and insensitivity to electro-static device related failures.  Most prominent diode multipliers are p-n diodes, varactors, step-recovery diodes. SRDs are employed for higher order multiplication due to their inherent strong non-linearity, which has a propensity to generate harmonics[3].

1705 HFE transponders03

Figure 3: Standard Receiver (Superheterodyne approach) Topology.

The main limitation of SRD-based multipliers is the inherent non-linearities resulting in batch to batch variation in the devices. As shown in Figure 4, the source is followed by a low pass filter to avoid unnecessary interactions and to improve stability with diode matching which is realized at low input frequencies by often employing lumped inductors and capacitors. The impulse generator circuit consists of an inductor in series with a shunt capacitor at the diode input, followed by a resonator circuit to enhance the desired output frequency following the diode.  The realization of these circuits is carried out employing tunable capacitors which cater to the proper matching and operation of the multiplier chain. A sensistor for self-bias is also incorporated with a capacitor to remove parametric instabilities associated with the circuit and to minimize slight changes in power that otherwise result in degraded performance. Again, for emphasis, the circuit often requires tunability and extensive optimization at substrate and package level over various power level inputs and temperatures to achieve the desired results.

1705 HFE transponders04

Figure 4 • SRD Based Multiplier.

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Figure 5 • Proposed Transmitter Topology with Minimal Changes.

Proposed Synthesizer Based Transponder

The proposed configuration is based on the harmonic generation in a mixer replacing SRD.  A mixer also produces multiple harmonics. The main challenge in using a mixer is to achieve precise frequency selection which can be carried out by employing a cavity or other high Q based bandpass filter for harmonic selection.

  • FPGA based demodulator is used for both FM and PSK demodulation.
  • A PLL based synthesized source is used for carrier generation for Local Oscillator.

The major advantages of the proposed topology are: replacement of SRD multiplier subassemblies with mixer based frequency multiplication (no tuning required); synthesized based version, active multiplier for frequency multiplication (up to Ku-band implementation); MMIC amplifier (single assembly) for low power mode, and an internally matched FET for high power mode. The same concept can be extended for Ka-band (30 GHz) where a frequency agile synthesizer employing low phase noise local oscillators is employed in conjunction with a direct digital frequency synthesizer (DDS) up-conversion technique to generate a linear FM signal.  This DDS concept is to be implemented in place of a PLL which has relatively high phase noise and slow switching speeds. The DDS can generate linear and agile frequency chirps.

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Figure 6: Proposed Receiver Topology with Minimal Changes (example at C-band).


The proposed methodology of incorporating a mixer instead of a SRD-based frequency multiplier is easily implementable and has potential for realization with a shorter production turnaround time. The implementation requires minimum modification of the existing topologies and needs less realizable time due minimal dependence on circuit tuning. The concept can expand for Ku-band also which needs incorporation of the active multiplier only to achieve lower order multiplication. The added advantage of the concept is removal of temperature variability associated with the SRD based approach. Turnaround time is lessened due to elimination of tunability and incorporation of MMIC chips. Development of future systems that cater to high power, high bandwidth, Ka-band satellites and ground systems for point-to-point connectivity requires a paradigm shift in the system configuration with incorporation of re-configurability embedded within. Ka-band systems will accommodate wider-bandwidth modulation schemes and higher data rates, which must include  achieving mm-wave power levels and good RF power added efficiency (PAE), frequency phase stability, and phase noise. Similarly, other aspects such as amplitude and phase equalization across GHz-wide operating channels, digital modulation and demodulation performance need to be critically examined along with the concept of RF-CMOS to realize active and passive circuits on the single chips such as mixers and amplifiers.

Acknowledgement: Sincere thanks to the technical team at ASTRA and its Director, ISAC for his trust and encouragement.


[1] K. Singh, R Selvi, A. V. Nirmal, S. V. Sharma,” RF sub-systems: Productionization perspective and challenges,” ESSRI Conference, ISRO, Bangalore-2016

[2] K Singh, AV Nirmal, SV Sharma,” Indigenous realization of satellite sub-systems through industry participation: Giant leap towards Make and Made in India by ISAC,” Annual Technical IETE Convention, Delhi 2016

[3] K. Singh, R. Ramsubramanian, S. Pal,” Self bias SRD based frequency multiplier for satellite based applications,” China-Japan Microwave Conference, 2006, China

[4] Suman R. Valke, T. S. Narayanan, K. S. Sudhira, M .Neelavathy, D. John and  K. N. S .Rao,” A New Miniaturized Version of C-band TT&C Transmitter for INSAT,” Spacecraft Technology Journal ,1997, pp 32-38

About the Author

authorDr  Kamaljeet Singh obtained an M. Tech (Microwaves) from Delhi University in 1999 and was awarded his PhD in 2010. He worked in the area of geo-receivers at C-band and initiated the development of various RF components. He was posted in 2006 to Semiconductor Laboratory near Chandiagrh in 2006 where he worked in the area of RF-MEMS. There he initiated development of various sensors and productionized physical sensors along with re-establishment of  a 6” MEMS fabline. Dr. Singh currently works at the ISRO Satellite Centre and is involved in product ionization of S-band TT &C transponders.  He is a Fellow of IETE, Life member of ASI & Punjab Academy of sciences, and a member of IEEE. He has published more than 85 articles in various journals/conferences, and is a reviewer of many journals. He is recipient of young scientist awards from INAE, IETE & ISRO. His book, RF Principles and Applications in RF-MEMS Switches is under publication, and he also filed for a patent.

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