On 4/23/2015 8:12 PM, whit3rd wrote:> On Thursday, April 23, 2015 at 4:30:48 PM UTC-7, Phil Hobbs wrote: > [about linear V-F conversion] > >> Well, maybe in the circular-cow SPICE flatland. >> >> If you can make a _physical_ HC4046 F-V converter with 1% linearity over a range of at least four octaves, I'll donate... > > Oh, you might lose that bet. To make a V-F converter, one would have to use an > op amp to servo Vvco so that Vfollower = Vinput, but you said F-V. > To make a linear F-V converter doesn't take a second chip. > > The point here, is that all '4046 devices use a very linear current-steering ICO, programmed > by an input current source (and mirrors). By "ICO", I mean a current-controlled > oscillator, it isn't internally a VCO at all. The input current source, though, is > a not-very-linear voltage-to-current converter (with input voltage range and gain > errors). You are concerned with a nonlinearity that is almost entirely in the > voltage-current conversion stage, which is nulled by use of the follower transistor. > > If one were very concerned with linearity, one could fix the bias Vvco and instead > of a resistor to program the current source, use a precise current sink at the > resistor pin, so that matching with the follower transistor is left out of the > error budget. >Okay, do it. It's easy with a metal-gate 4046, very difficult with the screwed-up HC versions. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Frequency-to-voltage converter, PLL with an AD650
Started by ●April 21, 2015
Reply by ●April 23, 20152015-04-23
