555 Time Marker Generator - Page 1
Forum section: electronics / projects

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 30, 2017, 11:53:48 pm
content: I need a time marker/marker generator for calibrating scopes and what not, but I don't want to spend any money if I can just build one out of simple parts.The problem is I'm unsure of the precision of such a simple device. I'll be adjusting it with an external frequency counter, and the frequencies are going to be low (in the kilohertz). What would be the maximum frequency precision I could achieve with a simple 555 circuit though and would it even be worth it?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 01, 2017, 12:33:06 am
content: Digital scopes don't need it, analog scopes can be happy with 1% to 3% accuracy of the time marks. Problem is that "real" time mark generators have highlighted pulses, do you want that too?For example, 1 pulse out of 10 will be higher amplitude so you can trigger on it, as well as having a separate trigger output.Take a look at the manual for the Tektronix 2901 Generator. It's from the 1960s so it'll use relatively simple stuff.You can use a 10MHz crystal and some jellybean logic chips and get better results than my 2901, except for the high frequency outputs.You'll want more than a few KHz output if you plan on checking the 1us per division and faster settings.I've used my 2901 on my Rigol and the pulses line up exactly on all ranges... and if they didn't, all I could do is tell the scope to auto-cal, I guess.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2017, 04:54:40 pm
content: Quote from: Cyberdragon on June 30, 2017, 11:53:48 pmThe 555 timer is completely unsuitable for this. It's far too inaccurate, : 1% at most, even with an expensive precision timing capacitor.Use a crystal oscillator instead.

author: rstofer
author_rank: Super Contributor
timestamp: July 01, 2017, 05:12:37 pm
content: Quote from: Hero999 on July 01, 2017, 04:54:40 pmAnd you would still need a frequency counter (which might be part of the scope) to see what the 555 is doing.I would get a 10 MHz crystal oscillator (maybe 3.3V) and drive into a series of 4 bit counters. They could be decimal counters so you could string as many as you need to get to the lowest frequency of interest. The thing with decimal counters is that the bits are not symmetric in high/low (obviously) so you might want to divide by two using the decimal output as a clock into a D flop.An even more elegant way of doing it is to set up the dividers in a CPLD. You could then get a 100 ns pulse (one clock wide) at any divisor you want. You could even use some kind of selector switch to get the proper frequency.

author: Cyberdragon
author_rank: Super Contributor
timestamp: July 01, 2017, 09:27:29 pm
content: I am using a counter, I said so in the original post.Wait, one of you said 1-3% is fine, and and another said I need better. But, yeah, a 555 appears to be highly unstable when generating short pulses. I could resort to using a logic circuit, but how precise is an arduino?

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2017, 09:44:12 pm
content: You might find some guidance from this thread:https://www.eevblog.com/forum/testgear/time-mark-generator-for-calibration-analog-oscilloscope-(make-or-buy-help)/Check the specs of mark time gens from manuals and datasheets online to get some idea of what you need to mimic.For analogue scopes you don't need anything insanely accurate as measurements are derived from the graticules but you'd prefer some time marks with very low jitter for repeatable checks and adjustments.Any reasonable modern AWG should be good enough for low BW CRO"s.

author: Ian.M
author_rank: Super Contributor
timestamp: July 01, 2017, 10:20:38 pm
content: Quote from: Cyberdragon on July 01, 2017, 09:27:29 pmAssuming a 'classic' Atmel ATmega based Arduino, provided you are using a hardware timer module to generate the output pulse, as accurate as the crystal providing the ATmega MCU clock. +/- 30PPM is a typical crystal accuracy spec. Other types of MCU *should* offer similar accuracy, but you need to consult the MCU datasheets on a case by case basis to confirm that.Unfortunately many models of genuine Arduinos use ceramic resonators that aren't very accurate compared to a crystal - an order of magnitude or two worse is not unusual. Don't be fooled by models like the Uno that have a crystal for their USB interface chip but only a ceramic resonator for the main MCU.