Square wave distortion - Page 1
Forum section: electronics / projects

author: and101
author_rank: Contributor
timestamp: September 02, 2016, 07:45:51 pm
content: HiHopefully someone can help me with a problem I am having. I am designing a 10MHz frequency standard based on one of the FE-5680A standards you find on ebay. The output from the FE-5680A is passed though a low pass filter and an op-amp before going to a distribution board that has several sine wave outputs all buffered though op-amps and four square wave 3V outputs. The sine-wave outputs work fine, they give a nice clean signal, but the square wave output is distorted with a spike of around 12V at each end of the wave form as you can see on the screen shot.To convert the sine wave to square wave I am using an LTC6957-3 from Linear. I designed the circuit based on an application note on their website. The only additions I made to the circuit were to add a dual schmitt-trigger buffer on each of the outputs so the circuit would be able to provide four individually buffered outputs. The voltage coming into the distribution board is 0.5V RMS so that should be within the limits of what the LTC6957-3 can take.The signal is the same on either side of the buffers so I don't think it is those that are causing the problem. Any help with this problem would be appreciated. I have included a picture of the PCB design with the square wave converter highlighted by the red box in case it is a layout problem.

author: edavid
author_rank: Super Contributor
timestamp: September 02, 2016, 08:10:04 pm
content: If you are using scope probes, try connecting directly to the 50ohm outputs instead.Also, try shorting out C37.

author: nuno
author_rank: Frequent Contributor
timestamp: September 02, 2016, 08:35:28 pm
content: And take the measurement with the GND spring (or even shorter connection if you can), not the GND lead.If you have other probes connected to the scope, disconnect them. At least in my Rigol that makes a small difference.

author: Fungus
author_rank: Super Contributor
timestamp: September 02, 2016, 08:49:23 pm
content: Definitely a problem with the probing. I suspect long wires.Can you take a photo of how you're connecting the probe to the signal generator?

author: and101
author_rank: Contributor
timestamp: September 02, 2016, 09:02:37 pm
content: Thanks for the suggestions. I just tried measuring the output using a 50 ohm coax with a BNC on one end and an SMA on the other instead of a scope probe and the peaks have gone but the waveform now has a bit of a dip at the start. Shorting out C37 didn't appear to make any difference. I will try fitting the boards into a case tomorrow to see if it gives a cleaner signal when everything is grounded properly.I haven't done much with analogue RF before as most of my previous projects have been digital so I am still learning what can go wrong when working with high frequencies.

author: Fungus
author_rank: Super Contributor
timestamp: September 02, 2016, 09:28:36 pm
content: Quote from: and101 on September 02, 2016, 09:02:37 pmWhenever you see a spike of voltage that's higher than the power supply's voltage you need to think "inductance".Quote from: and101 on September 02, 2016, 09:02:37 pmIt's not the rate of arrival of the pulses that's the problem. You'd still have exactly the same nasty shape even if you only hadone rising edge.Don't continue until you understand that. It's a very important point.When you look at a signal you need to think in terms of the component frequencies of that signal, not how often the trace goes up and down on the screen. A single square edge is still the sum of an infinite number of sine waves even if it never goes down to GND again.

author: tatus1969
author_rank: Super Contributor
timestamp: September 02, 2016, 09:41:28 pm
content: Quote from: and101 on September 02, 2016, 09:02:37 pmThe output impedance of your generator does not match the 50R wave impedance of your cable. This causes reflections bounce forth and back ->https://en.wikipedia.org/wiki/Impedance_matchingIf you add a 50R resistor in series with the LTC6957's output that will disappear. Also make sure your scope is in 50 ohms mode. But all that depends on how you want to use your generated signal. If you intend to use it to feed coax, then that'll be the way to go. If you want to produce a low ohmic TTL signal or alike, then the 50R resistor is inappropriate. Most proably the spike that shows on your scope is a measurement problem as mentioned. Have a look here:http://teledynelecroy.com/doc/passive-probe-ground-lead-effectsEDIT: I see in the schematic that you have 50R resistors. But that value is probably not correct because it does not take the output resistance of the 74LVC's into account. As they are highly nonlinear and depend on load, production spread and temperature (in the range of 20~50 ohms), I would not recommend doing it this way anyway. Better use proper high speed RR opamps. And did you have your scope in 50R termination mode? If it does not have that feature as mine, here's a nice inline terminator:https://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2016, 09:46:09 pm
content: So you've learnt that providing the correct loading to the outputs instead of probes is nearly giving you the correct waveform. What's lacking is the drive capability of the gates to produce a squarer waveform.You'll probably need to parallel 2 or more gates to get a squarer waveform.

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2016, 10:55:36 pm
content: Quote from: tautech on September 02, 2016, 09:46:09 pmFrom this thread:https://www.eevblog.com/forum/beginners/fast-edge-oscillator-traces-length/Like this:

author: calexanian
author_rank: Super Contributor
timestamp: September 03, 2016, 04:30:47 am
content: +1 for interaction with the probe/leads and improper termination. That is where I would start. also I might look into video amplifier/ driver chips over TTL for driving external loads. They are generally more stable.

author: and101
author_rank: Contributor
timestamp: September 03, 2016, 12:22:01 pm
content: After solving the problem with the scope probe I tried bridging all four outputs from the buffers, replacing the 50R resistors with 200R versions and the output is now closer to a square wave. There is still a slight dip on the rising edge but I don't think it will cause a problem. At the moment I only need a single square wave output for my SDR so losing the other three isn't a problem and all of the sine wave outputs work so for now I will use the existing circuit and then possibly redesign it in the future if I need more outputs. The frequency standard has been designed to be modular with separate boards for power, filtering and distribution so I can redesign the output stage without having to scrap the rest of the project.I also noticed that my scope doesn't have a option for setting the impedance to be 50 ohms internally so I will order a BNC 50? straight through terminator to fit at the scope end of the cable which will hopefully help with keeping the signal intact.Thanks for all of your help.

author: tatus1969
author_rank: Super Contributor
timestamp: September 03, 2016, 01:41:34 pm
content: thumbs up where we have been helpful