Keyless entry systems are becoming increasingly prevalent at least in the automobile industry. A keyless entry system primarily includes a transponder and a base station. The transponder communicates with the base station wirelessly. Security codes are employed to prevent unauthorized communication as for example, a base station will respond to requests only from a keyless entry key programmed to be associated with that particular base station. With keyless entry system, a mechanical key to open doors, for example, is not needed. The key typically includes a few buttons that are preprogrammed or can be programmed to send a preprogrammed set of instructions to the base station to perform specific operations. For example, a button may be programmed to open or close automobile doors or start the engine.
The keyless entry systems key typically includes a transponder that communicates with the base station. The transponder includes an electronic circuit that sends and receives communication signals to/from the base station that is located inside the automobile. The term automobile is used generally. The description provided in this document may equally apply to other types of machines as well. The base station is programmed to communicate the control systems of the automobile to effectuate performance of requests received from the transponder.
The communication between the transponder and the base station typically requires active stimulation of oscillation in a resonance circuit that is incorporated in the transponder. The resonance circuit typically is or includes a LC oscillator. The LC oscillator includes a coil (L) and a capacitor (C). The LC oscillator may also include a resistor to tune the Q-factor of the LC Oscillator.
Oscillators are circuits that generate a continuous voltage output waveform at a required frequency with the values of the inductors, capacitors or resistors forming a frequency selective LC resonant tank circuit and feedback network. The LC oscillators frequency is controlled using a tuned or resonant inductive/capacitive (LC) circuit with the resulting output frequency being known as the Oscillation Frequency. By making the oscillators feedback a reactive network the phase angle of the feedback will vary as a function of frequency and this is called Phase-shift.
Oscillators convert a DC input (the supply voltage) into an AC output (the waveform), which can have a wide range of different wave shapes and frequencies that can be either complicated in nature or simple sine waves depending upon the application. Oscillators are also used producing either sinusoidal sine waves, square, saw tooth or triangular shaped waveforms or just a train of pulses of a variable or constant width. LC Oscillators are commonly used in radio-frequency circuits because of their good phase noise characteristics and their ease of implementation.
Typically the active stimulation of oscillation in the resonant circuit is implemented by connecting one side of the coil fix to supply and pulling down the other side towards ground. This setup is a single ended configuration of the coil.
In a transponder that is used in Passive Keyless Entry systems, typically includes an active receiver with high sensitivity. The single ended coil configuration is potentially susceptible to noise coupling, e.g., due to startup of a processor (CPU).