How to make a programmable Pulse Source? - Page 1
Forum section: electronics / projects

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: December 09, 2024, 12:49:24 pm
content: Hi,I need a pulse source just Like a function generator, I want to control the output voltage between 10mV to 6V or higher, also Pulse width should be minimum 10ns and frequency could be varies between 100Khz and 5 Mhz or more.I wanted to go DDS way but what ever IC or design materials are available, I think I would need to use a JEDEC based DAC that again will require a heavy FPGA to work. I need to know if there is any simpler solution for this. Or Is there any method similar to the one's used in generic Chinese function generators like : JDS6600.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: December 09, 2024, 03:00:57 pm
content: So far in your requirements there is not really stated how fast your DAC in MSamples per second needs to be.May be you want / can get away with quite slow DAC to set amplitude, but need fast circuitry to switch output on/off?

author: peter-h
author_rank: Super Contributor
timestamp: December 09, 2024, 03:09:22 pm
content: Is this a one-off or production? There are many many ways to do it.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 09, 2024, 07:30:28 pm
content: A classic Jim Williams design treasure for a "Nanosecond Pulse Width Generator" can be found in AN98.https://www.analog.com/en/resources/app-notes/an-98.html

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: December 10, 2024, 06:46:00 pm
content: Quote from: Andree Henkel on December 09, 2024, 03:00:57 pmSlow DAC would Not be able to provide faster Rise and fall time

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: December 10, 2024, 06:46:44 pm
content: This is going to be in production, but not in huge volumes.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: December 10, 2024, 06:48:26 pm
content: Quote from: Terry Bites on December 09, 2024, 07:30:28 pmThank you for pointing to it, but the problem is I want to have a control on the amplitude, My application requires it to increase slowly.

author: jbb
author_rank: Super Contributor
timestamp: December 10, 2024, 07:54:02 pm
content: Could you sketch out what waveform you want to generate, showing shape, amplitude, slopes etc?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 10, 2024, 08:44:21 pm
content: Quote from: Arsh ahmad on December 10, 2024, 06:48:26 pmYou have not given rise/fall times, or load driven, or if you need to vary the pulse width/duty cycle, or the granularity needed in V or T axes .Assuming this is a digital pulse, you can drive an analog switch style output stage from a DAC power supply, to vary the amplitude of a fast pulse.DDS generation will have jitter on pulses, which most would consider undesirable.A small MCU with a PLL PWM stage will manage PCLK/N frequency generation without jitter, which may be fine for 100k-5MHzIf you need linear or fine control over pulse width, and analog monostable style output stage would give that.An alternative approach would be a fast, wide hysteresis comparator as a oscillator with user control of rise and fall ramps.Even a 74LVC14 with 2 pots, 2 diodes and 1 cap could give you something useful.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 11, 2024, 11:37:42 am
content: Yes, gain control will be a big issue.A VGA of some kind will be needed for linear ramps, maybe an analog multiplier (active mixer) or voltage controlled RF attenuator.If you can live with a stepped output voltage then a gain stage followed by a switched attenuator or PGA would be the way to go.To amplify a 10nS pulse, the gain stage will need to have a large signal BW > tr*0.35 if rise times are to be preserved. At a 10ns pulse width you'd want at least 1ns, so >350MhzPrecise (or not) voltage clamping could be an option.TI, AD etc make fully differential wideband amps with clamping control and they might be just the job. Add some modest gain after this stage get your full output range.An ALC loop could be implemented too. You will need some real pcb voodo to get your pulser to work.See what I'm imagining might work below, sims ok in microcrap with ideal passives and no load. Yeah, I know, so don't complain guys.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: December 11, 2024, 02:14:18 pm
content: Take a look at the schematic for the HP 8112A pulse generator for some ideas. It has all the features you mention and is largely built around discrete components (DACs, glue logic, amplifiers) with overall control by a micro. The service manual is readily available on line here:https://www.manualslib.com/download/1765782/Hp-8112a.html

author: Terry Bites
author_rank: Super Contributor
timestamp: December 11, 2024, 02:59:23 pm
content: Dead link

author: ftg
author_rank: Frequent Contributor
timestamp: December 11, 2024, 03:23:17 pm
content: Removing the "." dot at the end of .html in the url makes it work and not dead.So ".html." --> ".html"

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: December 11, 2024, 03:23:36 pm
content: Quote from: Terry Bites on December 11, 2024, 02:59:23 pmTry this:https://www.manualslib.com/manual/1765782/Hp-8112a.html

author: Marco
author_rank: Super Contributor
timestamp: December 11, 2024, 05:07:36 pm
content: Quote from: Arsh ahmad on December 09, 2024, 12:49:24 pmThe resolution is the needed specification. If you want 100 ps resolution, that's an expensive DAC.If the pulse width and amplitude is constant during the pulse train it makes more sense to make a fast monostable and trigger it.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 11, 2024, 05:22:51 pm
content: Your dot mate, not mine.....