It is sometimes required to take a single ended or differential AC source and distribute it to more than one DUT or other element. One example would be to distribute a differential AC test signal being generated in an ATE environment to either multiple test sites or ports of mult-channel DUT’s. Often these resources might be limited, and distributing them differentially will retain the balanced signal path required to retain the HD2 suppression in the original signal.
While numerous RF passive and active splitters are available, these might not match the necessary frequency band or have limited gain settings. The simple solutions shown here will terminate a differential or single ended source while driving multiple output paths with exceptional load isolation. The approach can also provide fine scale gain matching with 1dB response flatness to >200Mhz. The combination of an input balun, and very wideband Fully Differential Amplifiers (FDA’s), can provide customizable active splitters for ATE or communications applications.
AC or RF Signal Distribution Options
Simple passive splitters are widely available. These can provide the widest bandwidth but often require well-defined loads on each output, and they always come at the cost of some insertion loss. The simplest 50Ω single ended splitter is shown in figure 1. If every port is terminated in 50Ω, all three ports will see 50Ω source impedances at the cost of a 6dB insertion loss from the input of R2 to each load (12dB loss from AC1 to each load).
Figure 1. Example 50Ω splitter as drawn for a network analyzer response measurement.
A wide range of RF active splitters are also available, particularly for the CATV applications. These tend to be single ended with minimal gain from the input to parallel outputs. These also normally prefer each output be terminated in the specified impedance to deliver the intended signal distribution to all ports (ref. 1).
Extending the inverting summing design to transformer coupled FDA splitters
A generally useful circuit using FDA’s is to couple through an input balun to the two terminating gain resistors (Rg) as shown in figure 2. In this implementation, the 2-Rg resistors feed into a differential virtual ground due the high differential loop gain of a wideband FDA devices like the ISL55210 (ref.2). The balun gives a zero power single to differential conversion while the FDA provides an input termination, gain, and isolation for the input impedance to the output load. It is normally best to leave any balun centertap unconnected in this genre of application circuit.
Figure 2. Single channel, differential FDA with Balun input.
forms the output side termination impedance for the input balun. Assuming those are fixed at the required impedance to achieve an input match at Vi
, the overall gain can then be adjusted by changing the Rf
elements. Neglecting the transformer insertion loss, the voltage gain from the input of the balun to the differential output pins is shown in figure 2. This topology also offers numerous noise figure and harmonic distortion suppression advantages (ref. 3&4)
Since wideband FDA’s also provide nearly zero ohm output impedance, the load does not enter into this input matching giving a path to exceptional load isolation if Fig. 2 is adapted to parallel FDA’s. One direct way to provide multiple output paths is to simply fan out the differential signal at the output of Fig. 2 to multiple loads.
Often, these loads are doubly terminated transmission lines on ATE boards where that loading will then add up in parallel. While this can work in some applications, the heavy loading of parallel loads will rapidly degrade the harmonic distortion of the signal delivered to the parallel loads. As an option, consider splitting the signal on the input side with independent FDAs dedicated to each line.