Unlocking Space: The Vital Role of RF Baluns in Satellite and Commercial Space Applications

In the vast expanse of satellite and commercial space applications, technology serves as the engine propelling us into uncharted territories. As the demand in both public and private sectors continues to grow, the realm of space-based communication technologies is seeing unprecedented growth. One pivotal component allowing for this transformation in communication and signal processing in space is the RF balun. State-of-the-art space-grade baluns have emerged as unsung heroes in this extraterrestrial landscape, facilitating the coherent exchange and processing of data and signals amidst the cosmic chaos the system finds itself in.

The term balun is a portmanteau of balanced and unbalanced, indicating that a balun will transition between a balanced (also called ‘differential’) transmission line where opposite currents both travel in transmission lines and an unbalanced (also called ‘single-ended’) transmission line where the return current travels in the ground. This means that a balun has equal power outputs just like a power divider, however, it has a 180° phase difference between outputs. This phase difference allows for differential signaling for common mode noise reduction as well as interfacing with state-of-the-art ADCs. 

Figure 1: Simplified Balun Operation

In space, the harsh and unforgiving environment underscores the requirements of high reliability and robust manufacturing of all components in the communication signal chain from both an electrical performance perspective as well as a mechanical performance perspective. This dual emphasis ensures not only superior electrical performance but also resilience in the face of challenging mechanical and environmental conditions. Ultimately the goal is enabling high performance in the most demanding space environments, however, a natural question at this point is: what does a high-performance balun look like, and how can we guarantee this performance in space?

Balun Figures of Merit – Electrical Performance:

The evaluation and adoption of a given balun for a space application entails a rigorous examination of its electrical figures of merit before even considering the space-qualified nature of the balun. These metrics, encompassing aspects such as frequency coverage, balance, common mode rejection ratio, and more, serve as quantitative measures for assessing the efficacy of these components. In this section, we break down the figures of merit for baluns covering what matters when adopting a balun into a standard communication system, and in the subsequent section what is needed for space-based technologies to adopt a balun.

Frequency coverage: As with all RF/microwave circuits, each performance metric is only valid across some specified bandwidth. Increasing the bandwidth from octave, to decade, to multi-decade without sacrificing performance is a major challenge. In general, Marki baluns can be divided into two types. Those with magnetic coupling perform below 10 MHz to GHz range, and those with only capacitive coupling have low-end performance limited to about 1 GHz but can operate up to millimeter wave frequencies.

Balance: The most important performance criterion is how close the balanced outputs are to having equal power and 180° phase, called balance. Phase balance is the measure of how closely the inverted output is to 180° out of phase with the non-inverted output, usually given in degrees. It is the most critical parameter for many balun applications. The typical phase balance for standard microwave baluns is ±15° max and ±10° typical, while high-performance Marki baluns approach ±5° max and ±2° typical. Related to phase balance, amplitude balance is specified in dB as the match between output power magnitudes. Low-performance baluns have an amplitude balance of about ±1.5 dB max and ±1 dB typical, while Marki products approach ±.5 dB max and ±.2 dB typical. Optimizing this critical spec is what allows Marki baluns to be used in the interface of high-frequency ADCs to minimize second harmonic distortion (HD2) since HD2 is a function of balance at the ADC interface.

Common Mode Rejection Ratio: If two identical signals with identical phases are injected into the balanced ports of the balun (called ‘common mode’ or ‘even mode’ signals), they will be either reflected or absorbed. The amount of attenuation this signal will experience from the balanced to unbalanced port is called the common mode rejection ratio (CMRR) and is expressed in dB. It is determined by the vectorial addition of the two signals and therefore is dependent on the amplitude and phase balance of the balun. The relationship between amplitude balance, phase balance, and CMRR is shown in Fig. 2. As a rule of thumb, a 0.1 dB improvement in amplitude balance will improve the CMRR by the same amount as a 1° improvement in phase balance. A low-performance balun will have 15-20 dB of CMRR, while Marki baluns can achieve 25-55 dB of CMRR. 

Figure 2: Common Mode Rejection Ratio in dB as Function of Amplitude and Phase Balance

Insertion and Return Loss: A lower insertion loss and higher return loss will mean more power available for downstream functions, an improved dynamic range, and less distortion of signals in previous stages of the system. In a balun without isolation, as in a reactive splitter, the return loss of balanced ports will be different for common mode and differential mode signals. In an ideal balun without isolation, the common mode signal would be perfectly reflected, with a return loss of 0 dB, while the differential signal would pass through completely with a return loss of -∞. To properly characterize this effect one can use mixed-mode S-Parameters instead of standard S parameters to determine how the device will operate with differential inputs.

Balanced Port Isolation: Usually referred to simply as isolation, this has the same meaning as in other power dividers and couplers, namely the insertion loss from one balanced port to the other in dB. Most baluns do not offer high isolation because the even mode is reflected instead of being properly terminated with a resistive load. The exception is 180° hybrid circuits, where the even mode is output to a port that can be resistively terminated. 

While other parameters like turns ratio, DC/ground isolation, group delay flatness, etc contribute to the multifaceted evaluation of RF balun performance, the figures of merit outlined above often serve as the 'headline specs' and are crucial in providing a first-pass validation of the device. When coupled with the device's ability to withstand the rigors of space, they empower RF designers to comprehensively assess a given balun for suitability in space applications.

Balun Space Requirements:

Once an RF designer has a balun selected that meets the electrical requirements, comes the challenge of validating its ability to provide the same electrical performance in the harsh space environment. In space, reliability is non-negotiable. Component selection and qualification are pivotal to mission success. Often, catalog off-the-shelf baluns are chosen for their proven terrestrial performance, however, to ensure their performance in the challenging conditions of space, a meticulous qualification process, known as 'up-screening,' becomes imperative. This process involves subjecting COTS baluns to a battery of tests that elevate them to the stringent standards required for space missions. At Marki, we recognize the diverse needs of space missions and are ready to collaborate on tailored qualification plans. With experience and success in Class H, Class K, and Class S qualifications, we have navigated the complexities of various screening protocols. Whether following MIL-PRF or NASA standards, our commitment is to work seamlessly with our clients to meet their specific screening requirements. Regardless of the qualification standard followed, there are some tests that nearly always come up when dealing with space qualifications.

Thermal shock testing is a critical aspect of up-screening, subjecting baluns to rapid and extreme temperature fluctuations that mimic the harsh conditions of space. This ensures the structural and functional integrity of the baluns under thermal stress that they may see in flight. Vibration testing is another integral part of the up-screening protocol. Given the relentless vibrations experienced during launch and throughout a space mission, space-qualified devices must prove their ability to withstand these forces while maintaining both functionality and structural integrity. Another important aspect of screening devices for space use is burn-in and life testing. Burn-in and life testing simulate the extended mission life in space, subjecting baluns to prolonged operational periods at elevated stress levels to prove long-term reliability in the intended environment. This testing is crucial for ensuring the longevity necessary for sustained performance in space missions. 

By subjecting COTS baluns to these rigorous up-screening processes, RF engineers can confidently assure the reliability and survivability of these components in the unforgiving environment of space.

Baluns for Space and Terrestrial Applications:

Our broadband surface mount baluns are most frequently the balun technology adopted by our space customers. Typically, these baluns find themselves at the interface between high-speed digital converters and heterodyne transmission systems. In this circumstance, designers are replacing what was previously the final IF transmission stage for these heterodyne converters. In this application, the most important spec for the balun is the phase balance. An improvement from 12 degrees of phase balance to 3 degrees of phase balance can improve the even-order dynamic range of the ADC by more than 10 dB! Matching a wideband analog to digital converter (ADC) to a single-ended source is a difficult challenge, especially when using super-Nyquist sampling (at frequencies above the fundamental Nyquist zone). A differential amplifier at the front end will add noise and degrade linearity, while a balun will provide voltage gain without adding noise (an ADC responds to voltage, which will be √2 higher or more depending on the impedance ratio at the differential outputs of a balun). The input impedance of an ADC is typically much higher than 50Ω, generally in the kΩ range meaning a higher output impedance balun will generally match better to an ADC input than a 1:1 balun. 

An example ADC matching circuit is shown in Figure 3 below. It involves AC coupling capacitors, parallel resistors, and inductive lines to match the capacitive, high-impedance ADC load to the transmission line input. It also includes series resistors to limit any amount of charge injection coming from the ADC’s internal sampling structure back into the analog system. Due to the band-limiting nature of the ADC, it is often necessary to use a balun that is a much wider band in a pure 50 Ω system than the required system bandwidth.

Figure 3: ADC Matching Circuit Using Balun

Another important application of baluns for space is driving differential antennas. There is an abundance of literature on this application from amateur ham radio to NIST papers. As in typical differential signaling, the rejection of common mode current is the most important metric for an antenna feed balun, although performance also requires proper impedance ratios and matching to the antenna. 

The final application we will discuss in this article is an extremely high-volume application. This would be the use of baluns to create balanced devices such as push-pull amplifiers and balanced mixers. Push-pull amplifiers work by splitting the signal into a positive and negative version with a balun, amplifying them, and then recombining the signals with another balun. One advantage of this scheme is that the saturated output power can be doubled. Alternatively, the input power to each amplifier can be reduced by half for a given output power, significantly reducing the distortion products created by higher input powers. Another benefit is that this scheme will dramatically reduce the second-order distortion outputs of the amplifier. The second-order and all other even-order distortion products will be identical in both amplifiers, while the fundamental will be out of phase. Therefore, all even-order products will be canceled in the output balun, while the odd-order products pass through. It is this even product cancellation that can be used to dramatically reduce spurious products in balanced devices like a double-balanced mixer. In this structure not only are the even order distortion products of both the RF and LO canceled out, but also the fundamental of the LO will be canceled out traveling to the RF and IF ports. Marki Microwave has been using this technique to design the world’s best mixers for many decades. Owing to our vast experience designing baluns for mixer applications, Marki Microwave can now offer discrete baluns to meet the most demanding requirements. 

Summary and Closing Remarks

Because the term ‘balun’ encompasses a wide range of devices and applications, the information on them is scattered and confusing. In this article, we have clarified that baluns can be thought of as differential power dividers. Their most important characteristic is how well balanced they are in power, and how close to 180° out of phase their balanced ports are. Baluns can be used for many applications to transition between single-ended and differential signals and to cancel common mode noise and signals. As we look to the future, the significance of baluns extends beyond terrestrial applications. The challenges of space demand even greater precision, pushing the boundaries of balance improvement, power handling, reduction in size, complexity, and cost while still maintaining survivability in the challenging environments of space. Baluns are poised to play an increasingly pivotal role in critical communication applications, ensuring reliable performance in the ever-expanding frontier of space exploration.