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Active filter
Weak signals require special attention. The techniques of
separating signal from noise vary depending on the nature of the signal
and of noise. There are no general easy prescriptions.
When the frequencies of the signal and of the noise differ, one way
to increase the signal-to-noise
(S/N) ratio is to restrict the bandwidth of the amplifier in such a
way that only the signal frequencies are transmitted.
This principle is illustrated using an active filter device.
The AF100 universal active filter is a versatile active
filter device. It has high-pass (HP),
low-pass (LP), and band-pass (BP) outputs simultaneously
available and an uncommitted summing amplifier for making notch
filters. The centre frequency is tunable from 200 Hz to 10 kHz with
two resistors. The quality factor (Q) is variable from 0.01 to 500 by
changing two additonal resistors. The AF100 can be used in either an
inverting or a non-inverting configuration.
-
- These external resistor values should give a centre frequency of
Hz and a Q of slightly greater than unity.
Connect the FG output to and use the scope in the two-channel
mode to observe both the FG output
and the bandpass output of the filter. Connect the FG TTL output to the
digital counter for a readout of frequency.
Set the FG for a 1V peak-to-peak sine wave. Observe the bandpass output
as the FG frequency is varied through the centre frequency, .
- What happens to the bandpass output at ?
- To measure accurately, switch the scope to produce an -plot
(Lissajous figure) of filter output vs. filter input. At the centre
frequency the bandpass output should be exactly 180 out of phase
with the input signal. Use the Lissajous figure to adjust the FG exactly
to the centre frequency (see Experiment 1, and/or Malmstadt p.43
or Brophy p.63, for a discussion of Lissajous figures).
- Now switch the scope back to the dual trace mode and measure the
peak-to-peak output voltage of the bandpass filter as a function of FG
frequency over a range of Hz to kHz. Record 10-15 values in this
range including several near .
- Calculate and plot the filter gain in dB vs. log frequency.
- From the graph, determine the rolloff rate of the filter in dB/decade, on
both sides of .5.1 Comment on the values you obtain.
- Now connect the scope to the low-pass filter output. Convince yourself
that the device acts as a low-pass filter. Accurately measure and record
the dB frequency where gain
,
and the phase shift at the dB frequency.
- Repeat for the high-pass filter output.
- To get a filter with a higher Q, use
k and
k. Set
the FG to give a sine wave with
V. Observe the
bandpass output.
- Measure and plot the gain in dB vs. log frequency for the high-Q
bandpass filter.
- Estimate the Q of the two bandpass filters you have investigated. Q can be
measured as the ratio of the centre frequency of the bandpass output to
the bandwidth (the difference in frequency between the upper and the lower
dB points).
- Return the AF100 to the low-Q state, (
k,
k).
Vary the feedback resistors and measure the centre frequency of the
bandpass output.
|
, predicted |
, measured |
% error |
10k |
|
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|
50k |
|
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|
200k |
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Next: Notch filter
Up: Active filters and tuned amplifiers
Previous: Active filters and tuned amplifiers
For info, write to: physics@brocku.ca
Last revised: 2007-01-05