What is a filter?
An electrical circuit that allows certain frequencies of a signal to pass through while reducing or completely blocking other frequencies
Signal Definition
A signal is an electrical quantity (voltage or current) that carries useful information
Noise Definitions
Noise is any unwanted electrical signal that interferes with the useful signal
Gain Definitions
Gain describes how much a signal is amplified or reduced by a circuit
Gain Equation
Gain = Voltage out / Voltage In
What is different about filter gain than amplifiers?
Filter gain depends on frequency, unlike amplifiers where gain is usually constant
Ideal Filter characteristics
- Has a perfectly sharp boundary
- in the pass band, all signal pass through unchanged
- In the stop band, no signal pass through at all
Real Filter characteristics
- Transition band where single gradually reduces
- A finites slope (not perfectly vertical)
Low-pass Filter
Passes low frequencies and blocks high frequencies
High-pass Filter
Passes high frequencies and blocks low frequencies
Band-pass Filter
Passes only a specific range of frequencies and blocks both lower and higher frequencies
Band-stop filter
Blocks a range of frequencies and allows frequencies above and below that range to pass
Notch Filter
A very narrow band-stop filter designed to block a single frequency
Frequency behaviour of resistor (R)
- Resistance does not change with frequency
- Cannot filter on its own
Frequency behaviour of a capacitor (C)
Capacitors have capacitive reactance, which depends on frequency
What does capacitive reactance mean?
- At low frequency current is blocked
- At high frequency current is allowed to pass
What is a RC filter?
Uses a resistor and a capacitor
RC High-Pass Filter
- Capacitor blocks low frequencies = low output
- Capacitor passes high frequencies = high output
RC Low-Pass Filter
- At low frequencies, capacitor blocks = output is high
- At high frequencies, capacitor passes current to ground = output is low
What is the cut-off frequency?
The point where the signal starts to be significantly reduced
How can the cut-off frequency be defined?
- -3dB = gain has dropped by 3 decibels
- Half-power points = output power is 50% of input
- 0.707 points = output voltage is 70.7% os input
Formula for RC filters
f -3dB = 1/2piRC
What is a decibel?
Logarithmic unit used to express rations
What is the gain at -3dB?
0.707
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Echocardiography73
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Transducer and Dynamic Response23
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Basic Circuit Concepts Part 118
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Potential Drivers (basic circuit concepts part 2)25
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AC, Frequency Response and Interference38
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Amplification27
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Filtration37
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Electrical Safety and Medical Devices42
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Digitisation51
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Radiation Protection33
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Medical Imaging57
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ECG Electronics37
