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Function Generator question

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IamSmooth

Sat Sept 02 2006, 09:06PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I can precisely control the frequency and duty cycle with my function generator. I also know that for a basic 555 circuit the reistance (Ra and Rb) and capacitance across the proper pins control the frequency and duty cycle. How does the function generator vary these parameters while keeping the other fixed with just two knobs (frequency knob and duty cycle knob)?

Using the 555 as an example, I would like two knobs to smoothly control frequency and duty cycle:
If one wants the frequency fixed, the duty cycle is varied by changing values of Ra and Rb (diode across pins 6&7). I could use a three-pin trim pot for this. How then, could I smoothly vary the frequency? I don't think there are variable capacitors with a sufficient range. Would I have to link two variable resistors to change simultaneously across 6&7 and 8&7?

Avalanche

Sat Sept 02 2006, 10:30PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

You basically need to not use a 555, or at least don't take the output from pin 3

To vary your duty cycle independent of the frequency, or vice versa, you need two things - a sawtooth or triangle generator (you can take the signal from the timing capacitor on the 555) and a comparator. Altering the frequency of the sawtooth is as simple as altering the frequency of your oscillator, whatever your are using - vary either the capacitor value or the resistor. To alter the duty cycle you change your reference voltage that you are using on the comparator - so it swings high or low at a different voltage on each cycle of the sawtooth, resulting in a change of duty cycle, or pulse width, depending on where the switching takes place. I'm probably not the best at explaining things, but I hope that makes some sense. The TL494 basically does this, it might help if you look at the functional block diagram in the TL494 datasheet.

IamSmooth

Sat Sept 02 2006, 10:53PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Maybe the eassiest solution would be to use two 555 timers. One would provide an input trigger which can control the frequency. THe second 555 can be used to vary the duty cycle.

Hazmatt_(The Underdog)

Sat Sept 02 2006, 11:06PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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you might want to have a look at the ICL8038. There is a NTE cross to it, I forget the number. It is one of the chips that generates sine/triangle/square with duty and VCO. Its sorta like its own function generator all in one.

It looks very promising sometimes, but it can be picky depending what your VCO input voltage range is. Sometimes you have to set limits for min/max voltage so that it doesn't stop oscillating.

Anyways, that chip and a counter setup will give you basically a commercial generator to 2MHz.

IamSmooth

Sat Sept 02 2006, 11:11PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I don't actually need a chip to solve the problem. I aminterested in how one would design a simple circuit from basic parts to control both frequency and duty cycle. THis is one of those "I want to know how to do it" things.

Again, just to be clear, I would like two control knobs to control frequency and duty cycle independent of each other.

One solution might include a 555 astable for frequency control and a monostable 555 for duty cycle. The problem is that I need to make sure the monostable pulse duration is always shorter than the astable period.

Avalanche

Sat Sept 02 2006, 11:56PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

IamSmooth wrote ...

Maybe the eassiest solution would be to use two 555 timers. One would provide an input trigger which can control the frequency. THe second 555 can be used to vary the duty cycle.

Yes you can do that. The only problem with that is when you adjust the potentiometers in a way that the trigger pulse to the second timer is longer than the overall frequency will allow. If you do it, it's best to aim for the shortest trigger pulse duration possible, like less than 1uS, which would mean using a 7555, the CMOS version of the 555.

If you want to do it yourself, you can create a simple oscillator with an inverter chip and use a comparator for the duty cycle, or simply use a TL494 with a pot on the timing resistor, and another acting as a potential divider on the input to the PWM comparator. I know what you mean about wanting to make everything yourself from scratch, but then you soon realise what a lifesaver things like comparators are!

IamSmooth

Sun Sept 03 2006, 12:16AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Avalanche wrote ...

If you want to do it yourself, you can create a simple oscillator with an inverter chip and use a comparator for the duty cycle, or simply use a TL494 with a pot on the timing resistor, and another acting as a potential divider on the input to the PWM comparator.

How does one use a comparator to control the duty cycle for a variable timing input?

Edit:
How is this?

HV Enthusiast

Sun Sept 03 2006, 03:49AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Just use two 555 timers. One controls PRF, and the other pulsewidth, so you can control frequency and duty cycle independently.

Bennem

Sun Sept 03 2006, 06:52AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Take a look at Steve Wards site,
under 'DRSSTC interrupter'.
for an example of two 555 timer ics controlling
both frequency and pulse width independently.

Carbon_Rod

Sun Sept 03 2006, 08:54AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

I am not sure where this thread was headed so I will answer the initial question.

If you want to know about waveforms generation look at the LM8038 analog chip design (rather simple actually.) Why the duty cycle does not change (well at least in theory it shouldnâ€™t) the fundamental frequency of operation should be immediately obvious.

You may want to look at linear control designs in oscillators and amplifiers. Some clever folk simply used a â€œstereoâ€ pot to adjust the parameters and or remained balanced with crude internal calibration (as even good op-amps can have bizarre shifts with temperature.) If you find a schematic you will see complexity made precision instrumentation expensive.

The j-fet amps they have now with excellent slew rates and rather astonishing gains. Some are also anti-DC-biasing.

When DSP, chopper controlled analog amps, and SOCs arrived you can imagine how much designers were relieved -- almost everything is digital these days as its cheaper to build.

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