Diode Matched AM Demodulator

Overview

In this laboratory we will use T-line matching techniques to match to a diode. The diode will be the demodulation element in a microwave receiver circuit-- essentially, a very high frequency crystal radio. The following figure shows a block diagram of the system.

Each group will design the matching network for the diode demodulator shown in the figure below and then insert their diode demodulator into an AM modulated communications system to test its performance. The not-to-scale figure below shows the basic layout and placement of the diode and chip capacitor. The diode we will use is the Hewlett Packard HSMS-2850 Surface Mount Zero Bias Schottky Detector Diode. Data 1 and Data 2 contain the data and application sheets.

Procedure

Notice that this system has two parts that make it a little more complicated than the previous arrangements. You are to match to a diode rather than a resistor and there is a capacitor connected to ground. Use the following steps to handle these new issues.

1. Calculate the width of the Microstrip line. This can be done using the Line Calc tool. Use the following values for your calculations:
• Er  =  3.357
• H  =  30 mils    (substrate thickness)

2. Capacitor selection. The value of the capacitor is chosen so it can be approximated as a short circuit (~ 1 to 3 ohms) at 6 GHz and an open circuit at audio frequencies. You will be given a chip capacitor for use in your circuit. Calculate its impedance at the audio and microwave frequencies.

3. Diode equivalent circuit. In order to match the diode impedance you must calculate its value from the SPICE model equivalent circuit. The equivalent circuit appears as shown below:

Use the following values in your impedance calculations: operating frequency of 6 GHz, L₁=2 nH, R₁=20 ohms, R₂= 9 K ohms, C₁=0.08 pF, and C₂=0.16 pF.

4. Because the capacitor essentially grounds the 6 GHz signal, you can model the circuit as shorting at the capacitor for RF frequencies. Although there is some reactance, the audio end feeds into an audio frequency amplifier, where the 6 GHz signal will likely find many other capacitive paths to ground.

Circuit Simulation

Using the values determined in the previous section, create a schematic for your matching network, then run a simulation on it. Be sure to include the proper gaps for attaching your chip capacitor and diode. Typically 50 mils is about right, but you may want to measure to be sure. The TA's have the components and calipers required for such a measurement, or you can look at the dimensions given on the data sheets (the “Data 1” link at the top of this page).

Layout

Before following the instructions for exporting and milling, you need to generate the layout first.  This can be done by "Layout->Generate/Update Layout..."  After this is done, you may have to readjust some of your transmission lines in the Layout window.  Then, follow the "exporting and milling" instructions as outlined on the lab page.

Send to be Milled

Email your  *.gbr files to ee361@byu.edu

Test (2 weeks later)

Retrieve the milled component, assemble, test and report.