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As part of my hobby, I wanted to design and understand a circuit involving an op amp inverting summer followed by a multiple feedback bandpass filter.

The goal was to feed two signals, one 250mV peak 1kHz sine wave, and one 250mV peak 2kHz sine wave into the signal summer, then view the output from summer, and then feed that signal to a bandpass filter to isolate the 1kHz signal again. Finally, view the output of the bandpass filter to see how clean the signal is.

The circuit is shown below.

enter image description here

I have simulated the circuit in Multisim and elsewhere and the results are what I expected.

I built the circuit on breadboard, applied two signals from two separate function generators and tried the view the various signals on my oscilloscope (Hantek DSO5102P).

I can't seem to get stable waveforms to view.

I tried to adjust trigger to CH1, CH2, EXT, but nothing seems to work right. I even tried auto set but the oscilloscope can't stabilize the waveforms.

  • Source 1: 250mV peak, sine,1khz
  • Source 2: 250mV peak, sine,2khz
  • Trigger settings: CH1 or CH2, edge type, rising edge, auto mode, DC coupling

1st image is shown with CH1 (yellow) connected to signal source 1, CH2 (blue) connected to signal source 2. Trigger is on CH1.

enter image description here

2nd image is shown with CH1 (yellow) connected to signal source 1, CH2 (blue) connected to signal source 2, Trigger is on CH2.

enter image description here

3rd image is CH1(yellow)connected to output of first op amp, CH2 (blue) connected to output of second op amp. Trigger on CH1.

enter image description here

4th image CH1 connected to output of first op amp. Trigger is on CH1.

enter image description here

I have checked to make sure the connections are correct and they are. The output of the op amp summer should look like the graph shown below. The output of last op amp should look like a 1khz sine wave.

enter image description here

Edit:

The two function generators: HP 3312A function generator and HP 8111A pulse/function generator

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    \$\begingroup\$ Most function generators have a trigger or clock output/input. This is used to synchronize them. If you don't connect that cable to synchronize them you'll get a random phase drift between your signals. \$\endgroup\$
    – user1850479
    Commented Sep 26, 2021 at 15:57
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    \$\begingroup\$ Even with the clock in/out connected, you may still need to set the function generators up properly, one to output a synchronization signal and the other to use that synchronization clock instead of its own clock. \$\endgroup\$
    – Hearth
    Commented Sep 26, 2021 at 15:58
  • \$\begingroup\$ I tried using one function generator to trigger the second (I even tried using the second to trigger the first generator), however this doesn’t solve the problem. \$\endgroup\$
    – Leoman12
    Commented Sep 27, 2021 at 2:35

2 Answers 2

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The two separate function generators are not synchronized in any way. The generated 1 kHz and 2 kHz are not in any phase or frequency relation to each other, so the resulting waveform reflects just that, a sum of 1 kHz and 2 kHz sine waves that are not in perfect phase sync.

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  • \$\begingroup\$ I did use one function generator to trigger the second function generator but there was no change. \$\endgroup\$
    – Leoman12
    Commented Sep 27, 2021 at 2:36
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I even tried auto set but the oscilloscope cant stabilize the waveforms.

Not until both 1 kHz signal and 2 kHz signals are frequency locked as a perfect 1:2 ratio will this happen. Might I suggest you use your o-scope triggering in single shot mode rather than using it to display multiple images over and over again: -

enter image description here

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