i clone one ramp generator from
http://www.hanssummers.com/spectrumanalyser/sasweepgen.html

but i use lm358 instead of tl084.

here is my final ramp:

in testing via scope, a broken ramp sawtooth wave observed:

both raising and falling edge broken at a same voltage level(both ic5 pin2/pin14 have same problem ). it confuse me. i try disconnect the log 10kpot form 6V reference (disconnect the 82k resistor also). broken is missing. adding decouple caps to 6V reference does not help.

what's wrong with the circuit?

here is the sweep result : a crystal pot at 19Mhz , a notch corresponding to the broken of sawtooth wave.

 

At what voltage level is it "broken"?

 

Looks like it might be cross-over distortion in the IC. Have you tried a different LM358?

Is this a single ended supply?