Audio Filters

This circuit uses a switched capacitor filter IC from National Semiconductor to filter signals with frequencies higher than the 3KHz needed for voice audio. The schematic includes an audio amplifier that is designed to drive a standard audio head phone. The circuit is described in more detail in the receiver section of Dave Johnson`s Handbook of Optical Through the Air Communications...

The three-amplifier implementation of the state-variable filter in Figure 1 provides for second-order bandpass, highpass, and lowpass responses. The strength of the circuit, however, is in the bandpass response (VOUT/VIN), in which it`s easy to achieve high gain (G) and high Q. These two characteristics are important in applications in which selectivity is a key parameter in the filter. The application value of the circuit becomes even greater when DPPs (digitally programmable potentiometers) control and vary the bandpass filter`s center frequency, f0, and passband gain, G...

Designers originally conceived equal-element filters as all-pole microwave bandpass filters that provide minimum center-frequency insertion losses for specific values of resonator-unloaded Q (Reference 1). All resonators of the equal-element bandpass filter operate at the same loaded Q. For LC filters, the equal-element filter has another advantage. In the lowpass prototype, all inductors have the same value, and all capacitors have the same value. This minimum number of circuit elements provides design simplicity and reduces filter cost...

Using a modified equal-element design for a lumped-circuit lowpass filter has several advantages over the well-known equal-element design (Reference 1 and Reference 2). The modified design exhibits superior passband performance with only modest degradation of stopband selectivity. Moreover, the modified design is simple and easy to manufacture. You can extend the modified equal-element design to highpass LC filters...

A variable notch filter with both high and low pass filters. At first glance this circuit looks fairly complex, but when broken down,can be divided into high pass and low pass filter sections followed by a summing amplifier with a gain of around 20 times. Supply rail voltage is +/- 9V DC. The controls may also be adjusted for use as a band stop (notch) filter or band pass filter...

Although you can obtain universal, resistor-programmable switched -capacitor filters that are configurable as notch filters, most cannot operate at bandwidths higher than 100 kHz. Further, the typically 16- to 20-pin packages do not include a continuous -time, antialiasing filter to prevent spurious signals from appearing at the output. By using an eight-pin, dual operational amplifier and an eight-pin, switched-capacitor bandpass filter, you can construct a notch filter (Figure 1). IC2, a TLC082 is a dual BiCMOS op amp, replacing the older JFET-input stage with lower noise CMOS but retaining the bipolar output for high drive capability...

Notch filters remove a single unwanted frequency from an input signal. They are also a vital component of pulse-shaping networks, such as time-averaging filters. You can tune a state-variable filter over a wide range by changing the time constants of its integrating amplifiers (References 1, 2, and 3). Textbooks focus on its high-pass, bandpass, and lowpass outputs, but they sometimes fail to note that subtracting the bandpass output from the input signal creates a notch filter...

Described here is a very inexpensive solution to many phono-controlled applications like remote switching on, for instance, or activating a camera, tape recorder, burglar alarms, toys, etc. The circuit given here employs a condenser microphone as the pick-up. A two-stage amplifier built around a quad op-amp IC LM324 offers a good gain to enable sound pick-up upto four metres. The third op-amp is configured as a level detector whose non-inverting terminal is fed with the amplified and filtered signal available at the output of the second op-amp...

White noise (the sound you hear when a TV is tuned to a non-existent station) has a frequency characteristic which raises the power level by 3dB with each increasing octave, and is not suitable for response testing (and will probably blow your tweeters as well). By combining a 3dB / octave filter and a white noise source, we can get a very good approximation to "perfect" pink noise, where the power in the octave (for example) 40 to 80Hz is exactly the same as in the octave 10kHz to 20kHz...

The technique applies to highpass filters. You derive a composite highpass filter by using m-derived terminating half-sections with one or more constant-k interior full sections. Classic image-parameter design used m-derived half-sections with m=0.6 for best passband impedance matching (in other words, high input and output return losses)...

Unlike lowpass, bandpass, and other magnitude-altering filters, allpass filters can shift the phase of a signal without affecting its amplitude. As you sweep the variables from zero (dc) to infinity, the sign of H(s) changes from plus to minus, indicating a change in phase from 0 to 180°. You can realize this transfer function in two wideband transconductance amplifiers (WTAs). The circuitry inside the dashed lines in Fig 1 is one allpass network.A WTA`s transfer function is IOUT8VIN/Z, where 8 is simply an internal constant and Z is an external gain-setting component connecting the WTA`s Z+ and Z- pins. The transfer function for voltage amplification is VOUT/VINIOUTxZOUT. Most applications require a resistive Z...