Design and Characterization of Microstrip Impatt Oscillator

Abstract: In this work An IMPATT oscillator circuit at Ka band has been designed using inverted microstrip circuit.The Design of the oscillator consist of an active element(IMPATT) which converts dc power to RF power and a passive element with the electronic circuits for dc bias for the device to operate, providing the em enviroment at the device terminal to initiate oscillation and optimizes the oscillators output power and efficiency. The observed frequency of oscillation is 36.4 GHz with a maximum output 0f 1.472 mW.

DESIGN Design of Microstrip Impatt Oscillator- Microstrip oscillator circuit generally consist of an impedance matching network and a resonant circuit for stabilizing the oscillations.The resonant circuit can be a metal resonant cap (1), Impatt diode resonant line section(2) or a dielectric resonator(3), resonant cap mounting of Impatt diodes for oscillations in one of the simple low cost techniques.The active devices used here was a GaAs IMPATT diode with its frequency of operation in the Ka band.
The diode specifications are listed below Diode part number-IMPATT F14-DI-350-420, Maximum Current- 200mA, Output power 511 mW, Oparating frequency 35.2 GHz.
The details of the substrate used are given below.
Material : RT duriod 5880, Permittivity : 2.2, Tan : 0.0009,Thickness : 10 mil The 10 mil substrate is chosen considering mounting of the diode. As seen from the case style of the diode, with a 10 mil (0.254 mm) substrate the diode chip can easily be mounted on the substrate and will be flushed with the substrate, giving a smooth surface finish.
Inverted microstrip line has been chosen due to following advantages, Inverted microstrip lines achieve a lower loss (or higher Q) than possible with microstrip line. Further this lines have a much lower effective dielectric constant (compared with that of a microstrip line), and performance being less sensitive to dimensional tolerances at high frequencies.Further the wide range of impedance values achievable using these lines makes them particularly suitable in realizing filters.The inverted microstrip retains the advantage of the suspended microstrip in terms of achieving larger strip dimensions and lower dissipative losses. Furthermore it reduces the radiation loss in contrast to that in an open suspended microstrip.
The resonator should be designed so that it matches the diode impedance and it should not radiate rather than guide the power to 50  line. Using PCADD (software package) and Microfil (standard line length calculator) and considering the effective dielectric constant to be 1.01 for inverted configuration, the dimensions of the oscillator circuits are shown in Table I.

Table I Dimension of Oscillator Circuits Resonator section Taper section Radius of the resonator 4.09 mm Initial Width 4.96 mm Length 0f 20 line 4.41 mm Final Width 1.56 mm Width of 20 line 4.96 mm Length 2.24 mm Gap for surface mount capacitor 0.3 mm. Low pass filter Length of 50  line 3.0 mm Width of I st section 1.5 mm Width of 50 line 1.56 mm Width of 2nd section 2.4 mm