rohlf@unc.cs.unc.edu (John Rohlf) (03/29/89)
Given an externally generated square wave of varying frequency(audio range) and varying amplitude, what is the best/ easiest way of converting it to a sawtooth of the same frequency and amplitude?
jewett@hpl-opus.HP.COM (Bob Jewett) (03/30/89)
> Given an externally generated square wave of varying > frequency(audio range) and varying amplitude, what is the best/ > easiest way of converting it to a sawtooth of the same frequency > and amplitude? One solution: Find a VCO that has a unit-amplitude sawtooth as an auxiliary output. Phase lock the VCO to the input square wave. To get the amplitude right, full-wave rectify the square wave and apply the result and the sawtooth to an analog multiplier.
till@lucid.com (Don Tillman) (03/30/89)
From: rohlf@unc.cs.unc.edu (John Rohlf) Date: 28 Mar 89 23:08:54 GMT Given an externally generated square wave of varying frequency(audio range) and varying amplitude, what is the best/ easiest way of converting it to a sawtooth of the same frequency and amplitude? Have a ramp preset to the square wave's high voltage level at the square wave's rising transition. Slew the ramp down from that point. Electrically adjust the slew rate so that the ramp crosses zero at the square wave's falling transistion. This could track correctly after a cycle or two. The ideas I threw out both involved generating a voltage proportional to the square wave's frequency first: either use that voltage to adjust the ramping rate from the square wave's high voltage or drive a voltage controlled integrator. Neither way is real accurate.
mark@motcsd.UUCP (Mark Jeghers) (03/30/89)
In article <7476@thorin.cs.unc.edu> rohlf@unc.cs.unc.edu (John Rohlf) writes: > > Given an externally generated square wave of varying > frequency(audio range) and varying amplitude, what is the best/ > easiest way of converting it to a sawtooth of the same frequency > and amplitude? A couple of methods of varying complexity/quality/cost: A. Simple capacitor discharge setup. Load the square wave output with a diode and a capacitor (and maybe a small resistor) such that the cap charges more or less instantaneously and discharges slowly. The problem is that the quality is low and it works over a small range of frequencies. B. A more complex version of "A", using a transistor to control the discharge rate of the cap. Use the transistor as a "throttle", varying the discharge rate in inverse proportion to the frequency. Complications: you must have a voltage or current available that varies with the frequency, also, I don't recall the exact wiring of the parts to achieve this effect. C. The D/A approach. Start with a square wave many octaves higher than what you want as your output. Feed it to a binary counter, 7 or 8 bits wide for instance. Feed that to an A/D converter and sawtooth will be output. It will contain staircases which can be filtered out. Using more bits will reduce the staircases in size, but also require a higher initial frequency. A square wave whose frequency matches the sawtooth can be taken directly off the lowest frequency bit of the binary counter. Corrections/expansions/comments upon this are welcome. Mark Jeghers Motorola Computer Systems
henry@utzoo.uucp (Henry Spencer) (03/31/89)
In article <65200002@hpl-opus.HP.COM> jewett@hpl-opus.HP.COM (Bob Jewett) writes: >Find a VCO that has a unit-amplitude sawtooth as an auxiliary output. It's trivial to build such a thing, by the way: an integrator integrating the input, a comparator for "reached the top", and a one-shot feeding an analog switch to reset the integrator. -- Welcome to Mars! Your | Henry Spencer at U of Toronto Zoology passport and visa, comrade? | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
strong@tc.fluke.COM (Norm Strong) (04/01/89)
In article <1989Mar30.164612.28073@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: }In article <65200002@hpl-opus.HP.COM> jewett@hpl-opus.HP.COM (Bob Jewett) writes: }>Find a VCO that has a unit-amplitude sawtooth as an auxiliary output. } }It's trivial to build such a thing, by the way: an integrator integrating }the input, a comparator for "reached the top", and a one-shot feeding an }analog switch to reset the integrator. Yes, the problem is trivial as stated. Let's see if we can make it more interesting: How would you make the circuit so that the sawtooth starts at zero on the leading edge of the square wave, and reaches maximum just as the square wave falls. Let this be a steady state repetitive waveform. Manual adjustments are not permitted. Now, isn't that more interesting? -- Norm (strong@tc.fluke.com)
henry@utzoo.uucp (Henry Spencer) (04/02/89)
In article <7518@fluke.COM> strong@tc.fluke.COM (Norm Strong) writes: >}>Find a VCO that has a unit-amplitude sawtooth as an auxiliary output. >} >}It's trivial to build such a thing, by the way: an integrator integrating >}the input, a comparator for "reached the top", and a one-shot feeding an >}analog switch to reset the integrator. > > How would you make the circuit so that the sawtooth starts at zero on >the leading edge of the square wave, and reaches maximum just as the square >wave falls. Let this be a steady state repetitive waveform. Manual >adjustments are not permitted. > >Now, isn't that more interesting? No, not very, actually: this is what phase-locked loops are for. Barring unusual situations like very high frequencies, a $5 4046 kills this problem dead. (Why do you think I was building a sawtooth VCO?) Sometimes you need a bit of ingenuity in the surrounding circuitry to get the details right, but automatically synchronizing a generated waveform with an external one is basically a solved problem. Getting the amplitude to match is actually the hardest part of the original problem. The advent of one-chip phase-locked loops, analog switches, and high-quality op amps has reduced a lot of such analog signal-processing problems to trivialities. It only gets sticky when the cheap, convenient chips are ruled out by demanding requirements like very high speed or very high precision. -- Welcome to Mars! Your | Henry Spencer at U of Toronto Zoology passport and visa, comrade? | uunet!attcan!utzoo!henry henry@zoo.toronto.edu