Fabrication Of Oscillators Using Microwave Monolithic Integrated Circuits (MMICs)

Integrated circuits are a necessity when manufacturing electronic devices. Usually the circuit board and component chosen depends upon the frequency of operation. But usually typical printed circuit board served its purpose in the case of television, radio, modems, etc. But in earlier times during the rise of need of miniaturizing and optimizing electronics integrated circuits were used. These were implemented on printed circuit boards. At microwave frequencies, these boards accounted for heat sink, power loss problems. It was used in Microwave Ovens, RADAR, Spectroscopy, etc. So for these purpose microwave monolithic integrated circuits (MMIC) were developed. MMIC is also an integrated circuit used at microwave frequencies (300 MHz-300 GHz). Some of its salient features are:

• It is used for frequency mixing, noise immunization, frequency switching.
• When cascading an IC external matching networks are required, but here both inputs and outputs are matched in the MIMC itself – hence it’s very compatible.
• It uses ‘gallium arsenide’.
• It can withstand high temperatures and also support high voltages.

So our ultimate goal will be on mapping a standard electronic circuit, for that purpose we take an oscillator to be mapped into MIMC.

Okay! Let’s choose colpitt’s oscillator, it’s best suitable for working at microwave frequencies. Its operating frequency is determined from value of inductance (L) and capacitance(C) and it uses feedback from a voltage divider. Its circuit schematic is

The value of resistance, capacitance has been designed from the design equations. These are obtained by applying ‘Kirchhoff’s voltage laws [KVL]:

• Vcc=10v ,  Ic=1ma , B=200,  Ve=1v,
• Rc=Vcc/2Ic
• R1=Vr1/10Ib
• R2=Vr2/9Ib
• Re=Ve/Ie

The oscillator’s frequency is determined by:

F=1/ (2*pi (sqrt (L*C))

 

So now our oscillator is ready, we need to now implement in hardware.

Now we have to choose a very large of frequency, because MMIC supports only ‘RADIO FREQUENCIES’, here we have chosen 40MHZ as our frequency of operation which would suffice. Next MMIC is compatible only with lumped elements, so we have to consider only lumped components.

Lumped Systems are the ones in which the electrical properties such as R, L, C are confined to a small space in a circuit which are confined to high frequency applications.

 

Initially we take a Microstripline PC Board is taken and it is etched or Heat-soldered to make isolated spaces for placing a component. So let’s take a resistance for example,

 

The figure shows a resistor which on both the leads are bent and are soldered to the PC board at ‘IN’ and ‘OUT’ where the other two terminals are grounded as shown in the figure. The MIMC board cannot withstand high voltages, hence a low conductance path need to be installed for the terminals which need to be grounded, hence thin foils of ‘copper’ or ‘brass’ is taken which is wrapped around the leads to be grounded and holes are drilled at places where the leads need to be grounded. Semiconductor Materials for ‘MMIC Board Manufacturing’ such as GaAs, InP can be used. There are different types of boards at different power that can be selected:

• Low power (1 dB) with high noise figure.
• Medium power (4 dB) with moderate noise figure.
• High power (10 dB) with moderate noise figure.
• High power (17 dB) with moderate noise figure.

There are also different material packages available, here we chose Microstripline Boards. This is of 85-millimetre low cost package with High frequency performance.

 

Before these even the MMIC circuit can be simulated using ADS software [advanced system for design software]which can be simulated on ‘layout’ stages for simulating the results. I have referred to the links

http://www.arrl.org/files/file/Technology/microwave/8702023.pdf

 

I hope this fabrication technique opens scope for interested engineers in Electronics and I have to my fullest extent outlined the fabrication technique and this holds good for a sound implementation in hardware in the field of radio frequencies.