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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Hi all, I'm building a little signal generator with slightly-variable-away-from-50% duty cycle. What I'm trying to do is allow the signal generator to operate without the need to use a scope to determine the duty cycle.
I will be avoiding microcontrollers for this task. I don't need a numerical readout -- an LED bargraph display will be fine, and I'll design a driver for it.
What I'm looking for is ideas on how to convert the signal generator's duty cycle into a voltage.
Ideally, I'd like this to work between 500kHz and 15MHz, or thereabouts. Thoughts?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi BP, Assuming a "square wave" output with CMOS or TTL logic level, a simple low pass filter will average the voltage to a DC value proportional to the duty cycle. (i.e. an RC low-pass with a cutoff freq around 50kHz should do the trick.) As long as the peak output voltage and waveform shape from your sig gen stays the same over the full frequency range, you should be able to accurately calibrate such a circuit.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Thanks Bill! I was worrying an RC filter earlier, and was concerned with hopeless nonlinearity to the working frequency. With careful component choice, I think you've given me a neat solution. I'll let you guys know how it goes.
Another suggestion I had from a friend was to use the signal generator to make a current source with an inductor from the output, and then load the current source with a parallel RC (the time constant being large enough that ramping at the lowest frequency of interest is not significant).
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi matt
I also vote for RC filter + LM3914 or similar, it's by way simplest solution.
Resolution will be 10% that way, so if you are goint to change DC only little I'm not sure if you'l get a good readout from it... What IC are you using as signal generator?
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Hi Firkragg, I'm trying not going to use the LM3914, A) because it's expensive, and B) because I may wish to have much finer increments than 10%. Sure, they can be cascaded, but for now I'll enjoy the thrill of doing it with discrete components.
The duty cycle won't be variable from 0-100%... I'm looking more at 25% - 75%, if that. The IC I'm using as a "signal generator" is going to be just a 4046, since they're cheap as chips. To alter the duty cycle, I'm building a carefully calibrated Pulse Squisher(tm) =P
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I don't really see how an RC filter should not work for this; Matt, please explain. Another fun way would be to use a "slow" component like a small lightbulb to integrate over the signal and then e.g. use a photodiode to read out the averaged signal.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The energy output of a light bulb would be RMS, but for a square wave it is possible to convert the RMS value to average. This is because all the voltage values are known since the voltage can only be 0 or a constant V.
average = (RMS*RMS) / V
So some trickery with an op-amp or two should be able to give the correct output.
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measuring duty cycle with discrete logic components
