At 12:39 PM 3/7/2010, you wrote: >On Sun, 07 Mar 2010 12:58:57 +0100, "Dario Greggio" >said: > > Xiaofan Chen ha scritto: > > > > > Then there are clones like Exar XR4151 which might be > > > what you want. > > > > wow how old!! must've been the 70s... > > > > Now that I think of it, can't one use a PIC and make up something? One > > with pin-compatible power pins... > >Hi Dario, > >Yes, the age of the parts is what's really terrifying :) > >A PIC could be used, but I wonder what the relative accuracy would be >using a PIC with its internal oscillator or external RC. > >Cheers, > >Bob Does it have to be a "chip"? I'd just use a VCO made with op-amps. One dual and a transistor will do it, provided you pick one that works well enough as a comparator (fast enuf and no nasty diodes across the inputs). The other op-amp works as a +/- integrator and the transistor switches +/- controlled by the comparator. A TL431 or whatever for the reference. Could be < $1 in parts. Temperature stability will be predictable and primarily determined by the integrator capacitor tempco. Of course if you're willing to modify the other end of the design there are better ways to communicate across a single opto than frequency that may become practical if you have a microcontroller on both sides. >Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist