On Sat, 13 Mar 1999 15:29:40 -0500 Sean Breheny writes: >Yes,I am breadboarding this,and that may be part of the problem,but I >have >taken special care since this is such a high-bandwidth circuit. I have >a >bypass cap (O.1uF) directly from Vss to Vdd,and the actual Vss and Vdd >pins >are connected directly to the power supply. The major source of trouble is stray capacitance from the first inverter's input to ground and to the output. It is impossible to avoid this in a breadboard, since the contact under the board is a large piece of metal. It may be feasible to bend that pin away from the chip and solder a "flying" junction. Capacitance from the input to ground can make the circuit unstable. That's why "shorting out the input" with a large capacitor didn't work. The amplifier's gain and bandwidth is too high to keep the input under control. You actually get infinite gain connecting a low-impedance source directly to the input. Model the inverter amplifier as an op-amp with the + input connected to an internal reference voltage. I'd connect a resistor directly to the input (for low stray capacitance) then back to the signal source. The ratio of the feedback resistor to this resistor approximately controls the gain. The second amplifier may not be helping any. Since the timer input of a PIC has a schmitt-trigger, as long as you are reaching the thresholds it will count OK. The input signal doesn't have to be rail to rail. But the extra very high gain of the second amplifier is generating a large signal near the first amplifier that could feed back. So try it with just one amplifier stage. >It almost seems to me that the inverter is acting as if it has a small >amount of hysteresis You may be right, I never had good results with HC04. I like the CD4069UB much better, but it probably doesn't work up to 50 MHz. or that I am exceeding its phase margin due to >parasitic (or added in the test case of the 0.1uF cap above) >capacitance. I >don't see how this is avoidable. The suggestions I made above try to avoid that by minimizing stray capacitance, and isolating the source capacitance from the amplifier with a resistor. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]