Application: How C and L are Used in FSK WLAN Circuit
We've talked a lot in class about ways capacitors and inductors are used in practice. Here is an example of how they are used in a microwave engineering circuit. This circuit is provides a wireless communication link between two computers. Computers use data in digital format (1's = 5V and 0's = 0V), but we can't just send 5V and 0V wirelessly through the air. Some sort of coding scheme is needed to convert the digital 1's and 0's into a signal that can be transmitted wirelessly. There are many different coding schemes. This circuit uses Frequency Shift Keying. The Frequency is used to code 1 or 0. This circuit transmits a 2.4GHz sine wave to represent a 0 and a 2.6 GHz sine wave to represent a 1.
Now... let's take a tour through this circuit. The videos give you a basic tour of the circuit and the several ways L and C are used in it.
You won't know how to design all of these pieces yet... You'll learn about the transmission lines in ECE3300, the FSK coding in Wireless Comm, the antenna in Antenna Design, and the rest of the circuit in Microwave Engineering (all senior year breadth electives). For the insanely curious, I'm including links to the various details of this circuit, but please don't think you should understand all of the stuff in those links YET!
FSK WLAN Antenna (uploading now)
- The transmission line in this circuit is a microstrip. It is modeled, as all other transmission lines can be, as an RLGC circuit.
- The antenna is a quarter-wavelength monopole antenna.
- The Amplifier has a DC blocking capacitor to prevent the DC that is powering the circuit from getting in to the higher frequency (AC) circuit. This circuit uses the 'inner only' model.
- The Amplifier has a choke (AC blocking inductor) to prevent the AC wireless signal from getting into the DC power supply.
- The splitter is a T-junction power divider with quarter wave transformers for impedance matching.
- Bandpass filters allow one frequency or a band (range) of frequencies to pass through. They block all other frequencies and are made up of various combinations of L and C. The microwave version of L and C combinations can be implemented (as in this case) with coupled lines.
- The diode detector is a voltage rectifier followed by a single capacitor used as a voltage integrator.
Want to know more? Here is a paper that describes the circuit in detail.