Lab Experiments with AoE (Art of Electronics)

 

DL1GO German Amateur Radio Station, DOK C12, Loc JN58AT, CQ 14, ITU 28

 

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Audio Bandpass

Problem Statement:
Design a passive audio RC bandpass for the frequency range between 1500 – 8000 Hz. The external load to the bandpass is greater or equal to Rext_load  = 1M.  

 

The Circuit:
An excellent introduction to the subject, and a lab setup is described on page 103 in the reference book:

Thomas C. Hayes, Learning the Art of Electronics, Cambridge University Press, 2016

A simulation of the passive RC bandpass can be found in EveryCircuit with user “rigo59”:

http://everycircuit.com/app

The bandpass is designed as a cascade of a RC high-pass and a RC low-pass filter. The circuit plan of the bandpass is shown in Fig. 1.

 

Lab Setup:
The lab setup with a breadboard is shown in Figure 2 and the impulse response of the bandpass filter on a digital scope in Fig. 3. The generator is set to sinewave with amplitude of 4V, (Vss = 8V).

 

Experiment 1:
The design of the second stage of the bandpass consists of a low-pass with the 3dB frequency f3dB = 16.000Hz = ( 2 x 8000Hz). The value of the resistor is set to RLP = 100k, because it is a factor 10 lower than the 1M external load (see reference book for the details of a “factor 10 rule”). The value of the capacitor is calculated with the formula f3dB = 1 / ( 2 * pi * R * C). For the practical use an appropriate value is 100pF.

The response of the low-pass filter in the frequency domain in the range of 490Hz to 30.000Hz is shown in Fig. 143.

 

Experiment 2:

The design of the first stage of the bandpass consists of a high-pass filter with the 3dB frequency f3dB = 750Hz = ( 1/2 x 1500Hz). The value of the resistor is set to RHP = 10k, again because of the “factor 10 rule”. The value of the capacitor is calculated with the formula f3dB = 1 / ( 2 * pi * R * C). For the practical use an appropriate value is 20nF.

The response of the high-pass filter in the frequency domain in the range of 490Hz to 16.000Hz is shown in Fig. 142.

 

Experiment 3:

The bandpass is realized as a cascade of a high-pass filter (1st stage) and a low-pass filter (2nd stage). The impulse response in the frequency domain in the range 490Hz – 30.000 Hz in presented in Fig. 144.

 

144-circuit        

Figure 1: Passive RC bandpass designed for a frequency range between 1.5kHz and 8kHz

 

 

 

 

Figure 2: Lab setup of the bandpass filter on a breadboard

 

 

 

 

Figure 3: Lab setup with digital scope showing the frequency domain of the bandpass filter

 

 

 143

Figure 143: Low-pass filter in the frequency domain range 490Hz - 30.000Hz (Experiment 1)

 

142

 

Figure 142: High-pass filter in the frequency domain range 490Hz - 16.000Hz (Experiment 2)

 

 

 

144

 

Figure 144: Bandpass filter in the frequency domain range 490Hz - 30.000Hz (Experiment 3)

 

                   

 

Created by dl1go on March 28, 2016 - 22:00 |  Lab Experiments with AoE