Picosecond Pulse Labs has introduced a new phase-matched balun. The Model 5310 uses proprietary technology that provides exceptional phase and amplitude matching (±0.5° phase match from 500 MHz to 2 GHz; ±0.1dB amplitude match from 100 MHz to 3.5 GHz). This performance represents a significant improvement over traditional transformer-based balun designs (see Figures 1 and 2). Note that the phase and amplitude matched performance of the Model 5310 is not only significantly better, it also covers a frequency range that is approximately double that of a transformer-based balun.

Single-ended-to-differential conversion has historically been achieved with either a transformer-based balun or an active part such as a single-ended-to-differential amplifier. Both of these approaches have deficiencies.

Transformer-based baluns have an inherent limitation due to the frequency dependant performance of the transformer core materials used (phase and amplitude matching is a function of frequency). In addition, transformer-based baluns have a performance roll-off that makes them unusable at higher frequencies. Active parts such as amplifiers introduce jitter and noise that also cause performance issues.

Picosecond Pulse Labs’ products are designed with a very different approach. The company has extensive design and manufacturing experience with ultra-broadband techniques and has applied this knowledge to design the Model 5310 to address the deficiencies of transformer-based baluns.

The Model 5310 uses a novel, proprietary balun structure that is extremely well matched. This balun structure differs from traditional transformer-based baluns in that it splits the signal into two paths with a resistive broadband power divider and utilizes a proprietary inverter design comprised of a hybrid of coax cable and conventional transformer designs (see Figure 3).

This matched broadband inverter design leverages techniques originally developed by Picosecond Pulse Labs for the inversion of fast pulse signals.

Inverting fast pulse signals requires design considerations (ultra-broadband performance) that cannot be achieved with traditional transformer-based baluns. It should be noted that since the Model 5310 uses a resistive power divider, the insertion loss is greater than with transformer-based baluns, approximately 7 dB instead of 1 dB.

However, this can be compensated for with gain at other points in the system architecture. Another important point is that this architecture requires that the impedance ratio be 2:1, that is, 50 Ω at the input and a 100 Ω differential output.

Baluns and single-ended-to-differential conversions are key functions in many electronics applications. For example, state-of-the-art analog-to-digital converters (ADC) are designed for use with differential inputs. Utilization of differential signals with these ADCs improves key performance parameters such as linearity and spurious free dynamic range (SFDR). Consequently, when the available input is single-ended, some mechanism of converting the signal to differential with a minimum amount of distortion is required. If significant distortions are introduced, such as phase or amplitude mismatches, they will degrade the differential signal and the analog-to-digital conversion process. In general, phase mismatch is a greater problem than amplitude mismatch.

This is because transformer-based baluns typically have poor phase mismatch and good phase matching is needed to suppress undesirable harmonics. For example, a phase mismatch reduction from 10° to 1° will result in an improvement of second harmonic suppression of approximately 30 dB.

Baluns also play an important role in test and measurement. Differential test equipment (signal sources, vector network analyzers and oscilloscopes, for example) is typically expensive and frequently unavailable. Therefore, an accurate means of single-ended-to-differential conversion and vice versa is very useful for the characterization of differential signals and components with more commonly available single-ended test equipment. Good matching of phase and amplitude produces excellent signals for applications such as serial data testing where distortions show up as artifacts in eye diagrams and similar measurements. It should be noted that for serial data testing, the low frequency performance limitations of a balun must be considered since long strings of consecutive ones or zeros require good low frequency performance.

In addition to the Model 5310, Picosecond Pulse Labs offers two ultra-broadband balun products with greater bandwidths at the expense of moderately reduced matching performance. These two products, the Model 5315A and the Model 5320B, have bandwidths of 200 kHz to 17 GHz and 5 kHz to 11 GHz, respectively.

Picosecond Pulse Labs, Boulder, CO
(303) 209-8100, www.picosecond.com.

RS No. 306