Should I terminate it? - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: February 17, 2022, 12:34:54 pm
content: Hi folks.Building a simple 4-channel square waveform generator for a test bench, I had the idea of putting on the front of the panel, apart from the 4-outputs, a selectable BNC output that, by pressing a switch, it automatically selects one of the 4 outputs and connects it to the BNC connector.So the user can connect an external oscilloscope and keeping unbusy the 4 outs, can observe one of the 4 output waveforms selected.Now the question:should I terminate the BNC output with any resistance (1Meg?) or keep the Signal flapping in the breeze as Dave usually says?a 50Ohm termination would affect the main output- So should I pass it to a buffer op-amp before the BNC and terminate it?I post the schematic.Any helps appreciated.Best regardsSteve

author: Terry Bites
author_rank: Super Contributor
timestamp: February 17, 2022, 01:22:05 pm
content: You dont need to terminate it. You're only dealing with a few MHz. You can easily put 100mA through this switch but the CD4024 can only manage a few mA, a 50R temination will hammer that flat.I'd put a 47R reistor in series the anlaog switch end and use the 'scopes 1Meg internal ternination.

author: Houseman
author_rank: Regular Contributor
timestamp: February 17, 2022, 10:42:56 pm
content: Quote from: Terry Bites on February 17, 2022, 01:22:05 pmReally thank you for your reply.By the way the CD4024 is used only for selection process, its output feeds only the A0 A1 of the real switch permitting the 4 signal CH1 to CH4 to be multiplexed to the single output.But as you say why did you tell "only dealing with a few MHz?" should be higher frequencies be terminated indeed? Are you saying a non terminated output connection could catch unwanted noise and couple it to the internal circuitry at higher frequencies?Best regards. Thanks a lot.Steve

author: TimFox
author_rank: Super Contributor
timestamp: February 18, 2022, 12:13:04 am
content: The 50 ohms from the source to the output jack, feeding a 50 ohm cable into a high impedance (1 megohm) load is called "series termination", and is a common configuration for signal-level connections, since it consumes less power.If the load is truly high, and the 50 ohm source impedance is a good match to the 50 ohm cable, then a 1 V pulse feeding the 50 ohm output resistor will result in an initial edge of 0.5 V at the output connector, which will encounter a positive reflection at the load, boosting it back to 1 V. The reflected 0.5 V pulse then travels back through the cable to the 50 ohm source, where it should be absorbed, and everyone is happy.Double termination, with a 50 ohm source and 50 ohm load helps protect against mismatches, at the cost of only half the available voltage. A small mismatch at the source and load will result in a very small "echo" at the load, since the small reflection from the slightly wrong load will hit the slightly wrong source and be reduced further before it travels back to the load.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 18, 2022, 12:13:34 am
content: As you go to higher frequqncies and/ or longer lines, then signal integtrity matters far more. If you fail to correctly set the sending and recieving impecances correcly the line will have releflections (power going back towards the generating end and a whole load of phase and frequncy distortion. Thre line will also radiate like a badly desined antenna.. A low frequencies over short lines the problems are les pronounced. Saying that, because telephone lines are km long, careful termination is applied- especially as they now carry broadband.Read up on transmission line therory one cold and lonely night.

author: Houseman
author_rank: Regular Contributor
timestamp: February 18, 2022, 09:19:54 am
content: Quote from: Terry Bites on February 18, 2022, 12:13:34 amReally thank You Terry.. The fact is not reading transmission line theory... the fact is really understand it and apply it to real circuit, othewhise it's like formulas that are flying around with no-real sense for me.Anyway thank you so.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2022, 04:29:59 pm
content: Quote from: Terry Bites on February 17, 2022, 01:22:05 pmIncorrect. Repetition rate does not matter; edge rate does, and you can break it down to frequencies using Fourier analysis if you want. The schematic is illegible (or does not contain the actual generator part at all) but I assume whatever the actual square wave source is, can go to hundreds of MHz easily, if not GHz.Series termination is pretty much necessary if you want to avoid overshoot and ringing. So add that 50R in series with the output.