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Design Notes
Passive FET Mixer with Lumped Element Diplexer
This is a design for a passive FET mixer. It demonstrates harmonic balance simulation of a mixer, including conversion loss and large signal reflection coefficient measurements over a frequency range.
Overview
This mixer uses the lumped element "Diplexer" subcircuit to separate the RF and IF signals. The LO signal drives the FET between ON and OFF states. To optimize the ON/OFF ratio, some gate bias is applied.
The mixer operates in a reflection mode, signals applied to the FET channel are reflected by a time varying reflection coefficient. The FET channel under the control of the LO signal switches between a low resistance state and a high resistance state.
Diplexer Schematic
The "Diplexer" circuit contains the "IF_Filter" and "RF_Filter" sub-circuits. It isolates the IF and RF ports from one another, but allows transmission between these ports and the common port, which is connected to the drain of the FET.
Modeling and Simulation
The harmonic balance simulation for the mixer is set up as follows:
- Tone 1 applied to PORT1 defines the LO frequency and power. The tone information is set by double clicking on the PORT to open the Element Options dialog box and then choosing the Port tab. This frequency is swept from 1300 MHz to 1400 MHz GHz which is defined by right clicking on the "FET_Mixer" schematic and choosing Options and then the Frequencies tab.
- Tone 2 applied to PORT2 defines the RF frequency and power. The frequency is defined explicitly by the equation "_FREQH1 + 500MHz", thus it leads LO frequency by 500MHz. The system variable "_FREQH1" returns the current simulation frequency of tone 1. The power has been set to "-10dBm"
Conversion loss is calculated using the large signal s-parameter measurement, LSSnm. The frequency of the output signal at PORT_3 (IF) is specified as:
-1 * tone1 frequency + 1 * tone2 frequency, or
-1300 MHz + 1300 + 500 MHz = 500 MHz
and the input signal frequency at PORT_2 (RF) is:
0 * tone1 frequency + 1 * tone2 frequency, or
0 GHz + 1300 + 500 MHz = 1800 MHz
because the tone 2 frequency is set up by the equation "_FREQH1 + 500MHz"
Large signal gamma is measured using the Gcomp measurement at PORT_2 (RF), with the same frequency setting as the input signal in the conversion loss measurement, above.
FET Mixer IF Output Power Graph
This graph show's the IF output power of the mixer vs. frequency.
Diplexer Transmission Graph
The diplexer's s-parameters are displayed in a rectangular graph that is included in the Diplexer schematic.