Question of the Month Answers, October 2006

Published November 17, 2006

You be the expert: The answers to the October Question of the Month are below

Arun Kumar has submitted this month's question:

Dear Harlan,
I am involved in Schottky diode mixer design at microwave frequencies. For accurate design, I want to characterize the diode with the help of the text fixture and network analyzer. As the junction resistance varies with LO power level for characterization of the diode how much LO power should I give? Some literature says that the LO power should be such that the rectified current from the diode is in the order of 1 to 1.5 mA. How then should I measure the rectified current through the diode, which is mounted in the test fixture? Please help.

From: Scott Wartenberg, Booz Allen Hamilton

Dear Arun,
It sounds like you are characterizing a Schottky diode for use in a mixer design. In such a case, it is best to characterize the diode at the RF power it will see during circuit operation. Set the input power to the lowest RF power that does not significantly modulate the junction yet still produces enough RF drive for a measurement with adequate dynamic range. Depending on whether it is a high- or low-barrier Schottky, this is between -25 to -40 dBm, respectively. Reliably measuring a diode's AC rectified current is difficult even using precision source/monitor units, force/sense triaxial cables and high frequency bias tees. With no DC applied (as with a zero-bias Schottky), significant current flows only during positive AC voltage swings. During negative voltage swings, some capacitance is needed to continue sourcing an RMS current. External capacitance withstanding, the diode's junction capacitance is very small (~100fF). Driving the input RF power hard enough will eventually result in a rectified current of 1-1.5 mA RMS, but the peak voltage may damage the device. To improve your measurement, a few points are worth mentioning. At low current/voltage, a Schottky diode is highly reflective, its return loss near the right edge of the Smith Chart. Almost all of the -25 to -40 dBm power will be reflected back, making fine measurement resolution tricky. To increase measurement sensitivity, many vector network analyzers (VNA) come with a configurable S-parameter test set. This breaks out access to the forward (source output) and reverse (receiver input) signal paths. Inserting a low-noise amplifier in the reverse path improves the VNA receiver's dynamic range. An attenuator or isolator in the forward path ensures diode mixing products do not corrupt the RF source (remember S-parameters are single-tone measurements). Inserting a DC blocking capacitor at the VNA port also protects the receiver. A power flatness calibration is recommended. The alternative to this is creating a nonlinear Schottky diode model that is a function of both applied DC bias and RF power. Not a bad idea, just more work.

From: Sandeep Satav, EMI-EMC Centre

Dear Arun,
Put a pure resistive load (frequency independent, low inductance) for diode in test fixture. Provide LO power as you mentioned. Rectified current thus produced can be measured by sensing the change in temperature due to RMS heating of load due to rectified current. You will require a temperature sensor, which has low thermal mass or may be a fiber optic-based non-contact type (heat sensing by infra red emission and not through conduction) temperature sensor. Hazardous current generated in the electro-explosive devices due to exposure to electromagnetic radiations (kHz to GHz range) are estimated in a similar way.

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