HOW-TO GUIDE
How to Compare Input and Output Signals on an Oscilloscope
By the OhmSavvy Research Team ·
1Connect CH1 to the input and CH2 to the output
Feed the circuit a known test signal, usually a sine from a function generator at a frequency inside the range the stage is meant to handle. Clip the CH1 probe tip to the input node and the CH2 probe tip to the output node. Both ground clips go to the circuit's ground, the same ground the generator returns to. Trigger on CH1, since the input is the reference that everything else is judged against.
2Match the probe and channel settings on both sides
A comparison is only as good as its weakest channel setup. Use the same probe type on both sides, set each channel's attenuation to match its probe (10x probe, 10x channel), and compensate both probes against the scope's calibration output. Start with the same V/div on both channels so equal heights really mean equal voltages; you can change one later if the output is much larger, as long as you account for it. Keep both on DC coupling when you want to see clipping against a supply rail, and switch the output to AC coupling only if a DC bias pushes it off screen.
3Overlay or stack the two traces
With both channels centered on the same zero line, the traces sit on top of each other and any difference in height or timing is obvious. That overlay works well for gain. When the shapes get busy, use each channel's vertical position to stack them, input in the upper half and output in the lower half, so you can study each waveform without the other one in the way. Stacking is also kinder to inverted outputs, which otherwise cross the input trace twice every cycle.
4Read the gain and convert it to decibels
Measure the same quantity on both channels, peak-to-peak for example, then divide: gain = Vout ÷ Vin. In dB that becomes 20 × log10(Vout ÷ Vin). On the overlay below the input peaks at 1 division and the output at 2, both at 1 V/div, so the input is 2 Vpp, the output is 4 Vpp, the gain is 2 and the stage provides about +6 dB.
5Spot inversion, clipping and filter attenuation
Inversion shows up as the output peaking while the input bottoms out, as in the stacked view above. Clipping appears when you push the input up and the output stops growing: its tops and bottoms flatten against the supply rails while the input stays a clean sine. Turn the input down until the flat tops disappear to find the largest undistorted swing.
With a filter, sweep the generator frequency and watch the output shrink relative to the input; the frequency where the ratio falls to about 0.707 is the −3 dB point. The output usually shifts in time as well, which you can turn into degrees with our phase difference guide. For fast logic, the same two-channel setup also shows propagation delay, the time from an input edge to the matching output edge. Many scopes can also plot CH1 minus CH2 as a math trace, which shows the difference between two signals directly; see how to subtract two signals with scope math.
Gain to dB reference
| Vout ÷ Vin | Gain in dB | What it usually means |
|---|---|---|
| 0.1 | −20 dB | Strong attenuation, such as a filter well into its stopband |
| 0.5 | −6.0 dB | Output half the input, such as a 1:1 resistive divider |
| 0.707 | −3.0 dB | The conventional cutoff point of a filter |
| 1 | 0 dB | Unity gain: a buffer or voltage follower |
| 2 | +6.0 dB | Output twice the input |
| 10 | +20 dB | A typical single gain stage |
| 100 | +40 dB | High gain, often split across two stages |
Troubleshooting an input/output comparison
| Symptom | Likely cause | Fix |
|---|---|---|
| Gain off by exactly 10x (20 dB) | Probe attenuation and channel setting do not match | Set each channel to the probe's 1x or 10x position |
| Output trace drifts while input is stable | Trigger set to CH2, or output frequency differs | Trigger on CH1; check the stage is not oscillating |
| Square-wave input shows overshoot or rounding | Probe not compensated | Compensate both probes on the calibration output |
| Output sits off screen | DC bias on the output | Use the channel position control or AC coupling on CH2 |
| Large hum on one trace only | That probe's ground clip is loose or on a different ground | Return both clips to the same circuit ground point |
One ground, not two
Mains-powered circuits
Scopes for input/output comparisons
Two channels are the minimum for this job, since both traces must share one trigger and one timebase. A built-in generator saves a separate box for driving the input, and extra channels let you watch an intermediate stage at the same time.
Scopes for comparing two signals
100MHz, 2 channels, with a single-channel 25 MHz generator built in to drive the circuit input.
50MHz and 4 channels, so you can probe input, output and two points in between on one screen.
100MHz, 4 channels and 1 GSa/s, with headroom for faster amplifier and filter stages.
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Frequently asked questions
Why do I use 20 log and not 10 log for voltage gain?
Decibels are defined from a power ratio as 10 × log10(P2/P1). Power in a given resistance goes with the square of voltage, so the voltage version becomes 20 × log10(Vout/Vin). Use 20 log when you are working from two voltage readings on the scope, which is almost always the case here.
Should I compare peak-to-peak or RMS readings?
Either works as long as you use the same measurement on both channels. Peak-to-peak is easiest to read off the grid, but it includes any noise spikes. RMS from the Measure menu tends to be steadier on a clean sine. Mixing the two, such as Vpp on CH1 and RMS on CH2, gives a ratio that is wrong by a fixed factor.
Can I compare input and output with a one-channel scope?
Only for steady, repeating signals and only for amplitude. Measure the input, move the probe to the output, and measure again without changing the source. You lose the shared time reference, so inversion and phase shift cannot be seen reliably. Two channels make the comparison far quicker and more trustworthy.
My output is bigger than expected by exactly 10 times. What happened?
Almost certainly a probe attenuation mismatch. One channel is set to 1x while a 10x probe is attached, or the other way round, so its reading is off by a factor of ten, which is 20 dB. Check the attenuation setting in each channel menu against the switch on each probe.
Do I need a signal generator to compare input and output signals?
Not always. Any steady, repeating signal the circuit already handles, such as audio from a phone or a sensor output, lets you compare the two sides. A generator helps because it gives a clean sine at a known frequency and amplitude, so gain and distortion are easy to judge. Some bench scopes include one; otherwise a basic function generator or a microcontroller PWM pin will do.
