Waveform generator to oscilloscope - weird output above certain frequencies - Page 1
Forum section: electronics / beginners

author: newtekuser
author_rank: Frequent Contributor
timestamp: July 20, 2023, 03:36:34 am
content: I recently picked up a B&K 120MHz arbitrary generator to "test" my TDS5054 oscilloscope by checking that the waveforms generated show correctly on the receiving end. After watching Dave's YT video on the exact subject I purchased a T-shape BNC connector and terminated it at one end with a 50ohm plug on the generator side, the other end on the oscilloscope is unterminated and running over a 3ft 50ohm coax cable.The channel on the oscilloscope is also set for 50ohm, the generator has a High-Z as well as a 50ohm setting which does not make any difference in the shape of the signal.What I am seeing is that only signals below 1MHz show up correctly, more so if it's a square wave and more so if I play around with the amplitude setting on the generator. After trying a few values above 1MHz I can't get a signal to show up properly. Each time I'm generating a new waveform I am using the "auto-set" feature on the oscilloscope.My question is, do I need to adjust the amplitude on the generator when increasing and decreasing the frequency to show it correctly on the oscilloscope?Trying to determine if my scope is bad, the new to me signal generator or my methodology which I'm hoping for.

author: ataradov
author_rank: Super Contributor
timestamp: July 20, 2023, 03:58:12 am
content: Don't use autoset, it may get confused and set the wrong thing. Autoset is the most useless thing in the scope, waste of a button. Just adjust the settings manually to the expected ranges. That way you will have double confirmation that setup is correct.Also, attach some captures of what you are seeing and what are generator settings at that time.

author: radiolistener
author_rank: Super Contributor
timestamp: July 20, 2023, 04:03:20 am
content: Quote from: newtekuser on July 20, 2023, 03:36:34 amthis is incorrect. T-fork with 50 Ω terminator should be on oscilloscope side. You're needs terminator on cable side which receive RF energy. In your case oscilloscope is receiver so it's side should be 50 Ω, but since oscilloscope input has 1 MΩ, you're needs to add 50 Ω dummy load on oscilloscope side.

author: ataradov
author_rank: Super Contributor
timestamp: July 20, 2023, 04:06:12 am
content: TDS5054 has 50 Ohm input already. But for 1 MHz and 1m cable, it should not matter that much. Although what is "show up properly" in this context is not clear.

author: newtekuser
author_rank: Frequent Contributor
timestamp: July 20, 2023, 05:51:41 am
content: Not sure what I was doing wrong before but I can see sine, square, sawtooth waves even in the tens of MHz now, before I was seeing mostly noise. The generator caps at 50MHz for square though.For squares, they’re not very flat above 10MHz and the amplitude setting does not seem to be reflected on the scope side. For example, a 50MHz square wave with 50mVpp only reads 35mVpp at the scope end.I’ll keep in mind for where the termination needs to be, thanks! Did not matter in my case though. The default was set to 1M ohm on the scope and I set it to 50ohm manually.

author: ataradov
author_rank: Super Contributor
timestamp: July 20, 2023, 05:58:29 am
content: This is where properly matching and terminating everything starts to matter. With your setup all you need to do is set 50 Ohm termination on the scope and 50 Ohm output on the generator. No additional hardware is needed, just use a direct cable.What type of cable do you have? Is it something reasonable or random Chinesium?How does this setup perform with higher amplitudes? Does the loss remain constant?

author: newtekuser
author_rank: Frequent Contributor
timestamp: July 20, 2023, 06:03:57 am
content: Quote from: ataradov on July 20, 2023, 05:58:29 amChinesium I suppose, got it on amazon: “Eightwood BNC Male to BNC Male Adapter Low Loss RG400 Cable 3 Feet for CB Ham Radio Antenna Oscilloscope Spectrum Analyzer Signal Generatorhttps://a.co/d/3tvaUSd”Will have to do more experiments tomorrow to check how the amplitude loss compares at different values.

author: ataradov
author_rank: Super Contributor
timestamp: July 20, 2023, 06:17:16 am
content: This does not look too bad actually. It still something with completely unknown performance, but it could have been so much worse.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 20, 2023, 09:40:42 am
content: Quote from: newtekuser link=topic=3857I do not know the specific details of your generator but it is not uncommon for the output impedance to be 50 Ohm always and the High-Z setting just changes the displayed output level.

author: radiolistener
author_rank: Super Contributor
timestamp: July 20, 2023, 03:09:07 pm
content: Quote from: wasedadoc on July 20, 2023, 09:40:42 amGenerator output usually has 50 Ω impedance (include Chinese FY6800, PSG9080 and other) but some audio signal generators may have 600 Ω or both - 50 Ω and 600 Ω together (on separate connectors or with manual impedance switch).Also note that signal generator setting for Hi-Z or other value doesn't affect actual output impedance, its just affects amplitude value on the signal generator display...There is no sense to put terminator on signal source, because terminator is needed to avoid reflection from the end of cable so there is no sense to put terminator on the signal source side - it will reduce amplitude and cannot help to fix something, because coax cable with proper termination on receiver side is already has 50 Ω impedance on the input.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2023, 03:32:30 pm
content: Almost all high-frequency signal generators have an internal 50 ohm source impedance (possibly with an internal 50 ohm matched attenuator for low-level outputs), and the "high-Z" output voltage is twice that of the 50 ohm level.One interesting exception is a type of pulse generator, e.g. -hp- 8012B, which is natively a current output, with an internal 50 ohm load resistor that can be switched in and out, where the output voltage is calibrated for both 25 and 50 ohm total load resistance.For best results, one switches in the internal resistor and terminates the other end of the cable with 50 ohms, to obtain 25 ohms total load.For highest output voltage, one can terminate only at the far end of the cable.https://bama.edebris.com/download/hp/8012b/HP8012B.pdf

author: newtekuser
author_rank: Frequent Contributor
timestamp: July 21, 2023, 01:32:26 am
content: Connected the coax cable directly between the generator and scope and now the amplitude matches perfectly on the scope

author: BillyO
author_rank: Super Contributor
timestamp: July 22, 2023, 02:30:14 am
content: Quote from: newtekuser on July 20, 2023, 05:51:41 amYou are at the limit for the generator. The wave form is not going to look like a perfect square wave. Show us 30MHz and 40MHz.