VCO Update

gstopp at gstopp at
Tue Aug 15 01:37:18 CEST 1995

     Hi again all,
     Now that I'm back in the office after the weekend I'm taking advantage 
     of some spare moments here in the lab attaching huge numbers of 
     high-speed digitizing probes onto a lowly little analog vector-board 
     thing that hardly puts a dent in the vast bandwidth capabilities 
     I got sync to work on the VCO. Alls I did was to add an external A.C. 
     coupled input to the current-reversing comparator to resistor-mix it 
     in with the triangle wave from the integrator buffer. This way an 
     external edge causes the comparator to reach its switching threshold 
     before the integrator trips it.
     It behaves on the scope like you would expect - if the VCO is at or 
     below the frequency of the sync input then the VCO's triangle wave is 
     at a reduced amplitude at the same frequency as the sync input. As the 
     VCO frequency rises above the sync input, the waveforms start to get 
     more complicated. The strength of the sync effect is determined by the 
     amplitude of the sync input. At lower levels a kind of "soft sync" 
     effect is achieved, where the VCO will sync to the sync input only if 
     it is near a whole number frequency ratio (at least it looks that 
     All this on the scope, of course. Haven't heard it yet.
     Anyway at this point I think it would be a good idea to put together a 
     schematic and parts list, which is a little different from the 
     Electronotes original. At least this will be an example of a 
     high-performance VCO built from discrete components that are readily 
     available. So far the cost comes out to:
     3 x TLO82 (LF353N)    = 3 x .95   = $1.90 
     2 x CA3080N (LM3080N) = 2 x 1.72  = $3.44 
     2 x 748 (LM748N)      = 2 x 1.05  = $2.10 
     1 x MAT-03 (AD)       = 1 x 8.50  = $8.50 
     1 x MPF102 FET        = 1 x .56   = $1.12
     Total                              $17.06
     plus tax plus circuit board plus resistors, caps, etc. all prices for 
     single quantities your mileage may vary member FDIC void where 
     Next: create new schematic and parts list, and scan it into a .GIF or 

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