Raymond Ho
Electronic Design
Most function generators and network analyzers have one port to provide the output signal. If a differential signal is needed, then you may need to acquire a network analyzer with two output ports at great cost. Of course, there are alternate solutions in generating a differential signal without acquiring expensive equipment. The two most popular solutions include the use of a transformer or a power splitter.
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| Figure 1. | A high output-voltage swing is possible with minimal power to the two op amps using this differential output circuit. The output voltage swing can be nearly twice as much as that from a circuit using a single-supply voltage of 12 V, useful for xDSL drivers. |
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Why do we want to generate a differential signal? In the case of an xDSL driver, two amplifiers are configured to take advantage of higher output swing with minimal power to the amplifiers (Fig. 1). If the amplifiers operate at +12 V, a single output swing may be limited by the supply rails (10 V p-p), while an output swing taken differentially can be almost twice as much (18 V p-p diff.). Note that this is the output we’re referring to and not the input. The input signal driven by the splitter is V p-p/gain.
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| Figure 2. | The differential circuit of Figure 1 requires a transformer. It is the least expensive way of obtaining a differential output voltage from that circuit. |
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Using a transformer to generate a differential signal is the least expensive approach (Fig. 2). Measuring the transformer’s bandwidth is one important aspect, because we don’t want the setup to have less bandwidth than the amplifier. Figure 3 shows the frequency response of two transformers with different bandwidths. The curves show that only transformer 2 is capable of measuring an amplifier with a 30-MHz bandwidth, while transformer 1 is limited to 4 MHz. An alternate solution in generating a differential signal is to use signal splitters. The table presents the specifics of the transformer and splitter.
| Table 1. | Transformer and signal sputter/combiner summary | ||||||||||||||||||||||||||||||||||
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One advantage in using a signal splitter is that no milling of the printed-circuit board (PCB) is necessary. Extra time will be needed to do the PCB layout versus a simple cable connection with the signal splitter. Of course, signal splitters are more expensive compared to the cost of a transformer and a PCB.
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| Figure 3. | The choice of the correct transformer shown in Figure 2 is important to ensure a proper frequency response when driving a pair of xDSL lines. Note that transformer 2 has a better frequency response than transformer 1. |
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This is the common case of time versus money. If one can spare $60, I would recommend using a signal splitter as a source to generate a differential signal. A signal splitter can guarantee its bandwidth versus using a transformer where the user needs to verify the bandwidth. In either case, the one-output-port network analyzer can now have two output ports with the use of a transformer or a signal splitter.
