QEX

to find out what was really going on. The availability of PSpice4 makes it possible to easily do experiments that would be difficult or impossible without lots of expensive, exotic test equipment. This paper reports the results of my simulations of diode doubly balanced mixers (DBM) with various RF phase-shift networks.

The DBM Alone

Let's first connect a single DBM in the way it's tested by the manufacturer. The drive-level specification of 7 dBm is the power that is delivered to a 50 i! load connected in place of the DBM; it is not the actual power delivered to the DBM. A 50 il oscillator is isolated from the DBM by a 10 dB pad to make sure the source impedance facing the DBM is 50 Î2 resistive at all frequencies. The manufacturer tests the DBM this way so that his test data can be replicated by a customer using standard commercially available test equipment: 7 dBm is 500 mV RMS across 50 ii.

Fig 1A shows the LO drive current waveform for the manufacturer's test conditions. Notice that there is no current until the instantaneous source voltage exceeds the diode threshold voltage and that then the current waveform consists of sine-wave tips. To better understand the effect of LO drive variations, the drive voltage is varied ±10% to get three different curves in this, and subsequent, figures.

Fig IB shows the IF and RF voltage waveforms. We can see that the path between the IF and RF ports is open for appreciable time intervals twice in

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