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Problem

Physical transformers can not pass DC through from input to output, but AWRDE transformers will let DC pass through.

Solution

AWRDE has 3 transformers which will transform DC, these transformers are:

  • XFMR

  • XFMRTAP

  • XFMRTAP2

Adding an LC network will prevent the DC signal from going through, but allow for each winding to act as a DC short circuit. Following figure displays the output and input voltage waveforms of the 4:1 ideal transformer before adding the capacitor. The output voltage still contains the DC component of the input.

Figure below shows the schematic setup to address this issue. As shown in the schematic, a pair of 100 pF capacitors in series and a pair of 100 nH inductors in shunt are connected to the output and input of the transformer to act as a bias tee. The DC current is redirected through the inductors, rather than going through the transformer. This schematic is designed for the XFMR transformer element but a similar approach can be used for other two transformer elements in AWRDE. The small source resistor was added to limit the DC current through the inductor L1.

Figure below is the output voltage for this schematic. Notice that the DC component of the waveforms is filtered and the waveforms are centered around zero. The output voltage is plotted using two different simulators: Harmonic Balance and HSPICE Transient. The transient simulator takes into account that the capacitor and inductor take time to reach their steady state DC voltage and current. For example, a very large capacitor (i.e. 1e6 pF) will take a long time to reach steady state, but if the capacitor is too small, it will block low frequency signals. Therefore selecting the values for the capacitors and inductors is critical, but ONLY if the transient simulator is used. Harmonic balance simulation shows the steady state response, so very large values will not cause problems.