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Tuneable bandpass filter - TF/04

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Manually tuneable bandpass filter • 1Hz/11kHz tuning range • variable selectivity Q = 5, 20, 50 • insertion loss 0dB

Two pole, manually tuneable state variable bandpass filter. Tuning range 1Hz/1kHz bymeans of 11 : 1 continuously variable ten-turn dial and 4 decade frequency multiplier. 10% range overlap eliminates deadband.

Damping (VQ) is selectable, and should be maximised (Q=5) to facilitate tuning, thence reduced sufficient for waveform purity. Passband gain is unity, normalised bandpass responses are shown in Fig.1.


Note that second order filters produce a damped oscillatory response to variations both in input signal amplitude and centre frequency, thus imposing constraints on useage.

Applications are therefore limited to frequency analysis and signal recovery pertaining to quasi steady state data.

Tuning rate (secs./octave) is proportional to 1/f and to Q ; low frequency analysis necessarilycomprises a low sweep rate, low Q search.


The TF/04 is shown schematically in Fig.2. Two cascaded integrators are driven by a summing amplifier driven in turn by feedback components from each integrator together with the input signal itself.

The second order bandpass transfer function has centre frequency º 1/tand damping º ß.The input is reduced prorata to ß prior to summing in order to maintain unity gain at resonance.