Need help with validation of test setup - Page 1
Forum section: electronics / beginners

author: werek
author_rank: Newbie
timestamp: July 05, 2025, 12:06:28 pm
content: Hi,To give a bit of background on my experience: I completed technical high school, but my career shifted toward software development. Now, after 20+ years, I've decided to dip my toes back into electronics, starting with some simple testing. I've bought a bit of equipment for this and am currently working through typical test cases. At the moment, I’m looking to perform a Bode plot using my oscilloscope.I've read several articles on this topic, but most of them focus on power supply circuits, which usually require an isolation transformer on the AFG output for safety. However, my test subject is the preamp section of an audio amplifier (which I’d like to preserve), so understanding how it operates is important.I’ve already tested the inputs and outputs with a multimeter and confirmed they share a common ground (negative lead). Since the oscilloscope also shares this ground, connecting it shouldn't cause any short circuits. Based on this, it seems safe to proceed without using a separate isolation transformer on the AFG output—just a 50Ω load passthrough.Here’s the planned signal path for the test:1. Oscilloscope (with integrated AFG) is powered via battery (not connected to mains); the amplifier is powered by mains voltage.2. AFG generates a signal within the audio spectrum (20 Hz – 20 kHz).3. That signal is split into two RCA outputs—one goes back to the oscilloscope, the other to the amplifier’s line-in.4. The signal going to line-in is split into left and right channels.5. The “sub out” from the amplifier is connected to the second oscilloscope input via an RCA cable and BNC-to-RCA adapter.6. Power on the oscilloscope, then the amplifier, set the volume to maximum, perform the tests, and plot the Bode characteristics.7. Power off the amplifier, then the oscilloscope.That said, since it's been over 20 years since I last worked with electronic circuits, I'm not entirely confident in my test setup. I’d appreciate your thoughts—any suggestions or improvements—on whether this approach is sound and whether it will give me reliable results across the 20 Hz to 20 kHz sweep. I’m specifically looking to see if there’s any low-pass filtering, signal attenuation, or EQ shaping happening.For connections, I’m using BNC-to-RCA adapters and RCA splitters after the AFG (with the 50Ω output load applied).

author: BillyO
author_rank: Super Contributor
timestamp: July 05, 2025, 08:45:22 pm
content: Should work, but at these frequencies I don't think you have to worry too about 50Ω loading.

author: TimFox
author_rank: Super Contributor
timestamp: July 05, 2025, 09:17:53 pm
content: The 50-ohm resistor loading the AWG won't hurt anything, and the voltage calibration of the DHO914S assumes a 50-ohm load on the AWG.

author: BillyO
author_rank: Super Contributor
timestamp: July 05, 2025, 09:27:54 pm
content: Quote from: TimFox on July 05, 2025, 09:17:53 pmOf course it will not hurt anything, but I doubt the results, especially if your doing a Bode plot, will be much different. But yes, it the rigorous thing to do.