# A Novel UWB Pulse Expander Using an Integrated Microstrip Splitter, Delay Lines, and a Combiner
###### Abstract  
Passive pulse shaping at frequencies above 1 GHz is mainly achieved through frequency-domain processing using filters. Unfortunately, the conventional frequency domain approach does not allow precise control of the impulse response of the filter, therefore, setting limitations to the pulse shaping accuracy. Sub-nanosecond pulse expansion that preserves steep pulse transitions is one of the ultra-wideband (UWB) applications where frequency domain approaches do not provide satisfactory results. This paper proposes a highly innovative approach based on time-domain signal processing using a set of parallel microstrip delay lines connected in a network accompanied by a splitter at the input and a combiner at the output. The proposed design, analogous to finite impulse response (FIR) filters in digital signal processing (DSP), provides fine-grained control over time-domain characteristics and supports the implementation of complex functions, including pulse expansion. This paper presents a detailed analysis of previous work and theoretical considerations regarding the advantages and limitations of UWB pulse time-domain processing. Moreover, detailed HFSS simulations of components, such as a microstrip pulse splitter, delay lines, a combiner, and their combinations, are presented. Finally, the results of the experimental validation of the device, fabricated on an FR-4 substrate, are presented. Technology for effective implementation of a pulse splitter, delay lines, and a pulse combiner, as well as their matching, can be considered as key findings of the given research. Limitations associated with matching and delay estimation for pulsed UWB signals are highlighted. Footnote †: journal: Applied Sciences  
In many communication systems, minimizing power consumption in transmission circuits is a crucial concern, and is achievable using short pulses. For instance, pulse position modulation (PPM) with very short pulses stands out as a modulation technique that reduces energy consumption, making it attractive for users with constrained transmission energy budgets [16]. However, demodulating PPM signals with picosecond or sub-picosecond pulses pose challenges, which can be addressed through time interval measurements with picosecond resolution [17]. Modern event timers, boasting accuracy within several picoseconds [18], prove effective for PPM demodulation. However, stable detection requires input pulse durations of several hundred picoseconds or sub-nanosecond time intervals [19]. Numerous approaches exist to extend pulses in time without significantly distorting their waveforms. However, extending pulses in time alters their frequency spectrum, which can be accomplished using various filters. Passive or active filters with appropriate magnitude and group delay responses are commonly employed for this purpose [20]. Among these, all-pass filters (APFs) are widely used in digital signal-processing applications [21]. For example, [22] discussed a second-order fully differential voltage-mode active APF topology, including dispersive delay structures and controlled magnitude for analog signals. Despite their utility, APFs come with the downside of circuit complexity. While DSP is often preferred due to its flexibility and programmability [23; 24], analog signal processing (ASP) remains a competitive alternative, especially in energy-efficient systems where simplicity is critical [25]. DSP, however, suffers from drawbacks such as costly analog-to-digital and digital-to-analog conversions, increased power consumption, and higher bandwidth requirements compared to analog devices performing the same functions. The waveform of the pulse to be expanded and its spectrum are crucial factors in choosing filters. Simple microstrip filters can extend unipolar trapezoidal pulses with a 50 ps time duration, as shown in [16; 26]. In these papers, third-order Bessel-type low-pass filters (LPFs) are designed, fabricated, and tested experimentally.