Generating tri level sync signal. - Page 1
Forum section: electronics / projects

author: Dubbie
author_rank: Unknown
timestamp: November 13, 2017, 08:49:04 am
content: I have a project where I need to generate a tri level video sync signal using a microcontroller.The signal starts at 0v then goes -300mV then +300mV then back to 0vI have 3.3 and 5 volts available. What clever ideas do you guys have for this? Preferably with the minimum of external components. I have a couple of ideas myself but am certain someone else will have a better one! I’d like to avoid any RC settle time on startup if possible. The camera input impedance is 75ohms I believe.

author: Ian.M
author_rank: Super Contributor
timestamp: November 13, 2017, 09:37:15 am
content: First you need a well regulated negative rail. Then you need to generate a signal with the required amplitude into a 75R load. Is it acceptable to generate double the amplitude with a source impedance of 75R so the amplitude is correct when it is loaded? If so, if your MCU has enough output drive capability (about +/-10mA), it may be possible to generate it with a four resistor network (actually five resistors as you'll need a series one to trim the source impedance to 75R if you use readily available preferred values).

author: tggzzz
author_rank: Super Contributor
timestamp: November 13, 2017, 10:15:26 am
content: Video often has a spec that is something along the lines of +1V -0.5V relative to the mean level, or similar. If that is the case you may be able to AC couple the output and avoid a negative rail.If it isn't the case then you will need a negative rail; DC-DC converter modules are available.

author: Dubbie
author_rank: Unknown
timestamp: November 13, 2017, 10:39:09 am
content: Quote from: Ian.M on November 13, 2017, 09:37:15 amThis was one of my plans to try. the numbers worked out on paper. I have struggled to find much on the net to answer the question of if I can rely on the end device pulling it down to the correct voltage. I can’t see why not because a video signal can go up to 1V anyway. The negative... not so sure.

author: Dubbie
author_rank: Unknown
timestamp: November 13, 2017, 10:42:50 am
content: Quote from: tggzzz on November 13, 2017, 10:15:26 amI do have a DAC on the uC that I can use. I did wonder about buffering it with an op amp and ac coupling it.

author: Ian.M
author_rank: Super Contributor
timestamp: November 13, 2017, 10:52:04 am
content: What's the total pulse duration? If its short enough, you may be able to AC couple it without excessive droop. It may need a shunt resistor at the output to provide a DC path. You can clamp the output with a N-MOSFET and a series Schottky diode during startup to suppress the startup transient and keep it close to zero while the coupling cap charges.Otherwise, if you *really* need a low source impedance +/-0.3V signal it gets *FUGLY* and *COMPLEX* - e.g negative supply rail, resistor network and video OPAMP to do the level shifting and buffer the output.If it isn't subject to a NDA, tell us exactly what this sync signal is for and must connect to. Its lilkely that there is at least one user here familiar with industry standards for the interface in question unless its really obscure.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 11:00:05 am
content: Quote from: Dubbie on November 13, 2017, 10:42:50 amHow fast is the DAC?What sort of rise/fall times are required on the +/-300mV signal?

author: Ian.M
author_rank: Super Contributor
timestamp: November 13, 2017, 11:54:43 am
content: AC coupling idea, with pre-bias resistors before the coupling caps so they charge slowly while the I/Os are tristated to avoid startup transients.

author: Dubbie
author_rank: Unknown
timestamp: November 13, 2017, 01:27:55 pm
content: Quote from: Ian.M on November 13, 2017, 11:54:43 amThat looks pretty good Ian, although after looking at the required rise times I might have to get a bit fancier on the uC output!To try to answer some of your questions hero999 and Ian:AFAIK The rise time should technically be 53nS with the duration of each pulse 538nS. I don’t know how or if this changes for faster frame rates.The reason I need to build this is I need to have variable speed control over the frame rate of a professional digital movie camera (a RED). It does have a gpio sync input that I am currently using, but it needs huge performance margins which limits the frame rate I can achieve. In theory using genlock (tristate sync) I can get right to the performance limits of the camera with control over the exact frame timings. This is a one off project.After looking at the DAC, it is way too slow. I think I could probably only get around 2uS risetime from it.I suspect the camera is probably not very fussy about the exact risetimes and durations because it is completely digital and there seems to be no reason to rigorously enforce these that I can think of. In my application I can tolerate a lot of jitter.One plan I have if I don’t get anywhere, is to borrow a professional sync generator from someone and measure its output as best as I can with my 200Mhz scope.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 02:13:39 pm
content: If the MCU's output rise/fall time is too slow, then how about adding a buffer, such as the 74LVC1G125, to speed it up?http://www.ti.com/lit/ds/symlink/sn74lvc1g125.pdfIf the rise/fall time of the MCU is too slow for the buffer, then you could add a Schmitt trigger, such as the 74LVC1G17.http://www.ti.com/lit/ds/symlink/sn74lvc1g17.pdfNote that the above options both add a delay to the output, which will need to be taken into consideration.

author: Dubbie
author_rank: Unknown
timestamp: November 13, 2017, 09:28:31 pm
content: I measured the rise time of the MCU at 8nSI will see if that works for now, by building the circuit. if not, I will add the Schmitt buffers which are on order.I'll also have to put together a 75ohm to 50ohm adapter for my scope.Thanks a lot for your input.

author: Dubbie
author_rank: Unknown
timestamp: November 14, 2017, 01:45:21 am
content: Thanks Ian,I built your design (I changed the 10uF to 1uF because the pulses were so short.)It works perfectly!Thanks again for your input.