0
\$\begingroup\$

I want to generate a clock for a z80 CPU using a crystal oscillator but somewhere between 8 and 13.5mhz the voltage drops and with 20mhz it's about 1 volt peak to peak. I have tried numerous circuits including the one below using a 74hc14 and a 74hcu04 with the same results , what could I be doing wrong? one of the circuits I tried

\$\endgroup\$
  • 1
    \$\begingroup\$ What are you using to measure the voltage? If scope, what is the bandwidth of the scope and probe? \$\endgroup\$ – AlmostDone Apr 5 '18 at 14:18
  • \$\begingroup\$ I'm using an analog scope that can go up to 35mhz and I have some cheap eBay probes at 10x and they write 100mhz 600v on them \$\endgroup\$ – C32 Apr 5 '18 at 14:20
  • 1
    \$\begingroup\$ I suspect what you're seeing is the 35MHz bandwidth coming into play. \$\endgroup\$ – AlmostDone Apr 5 '18 at 14:21
  • \$\begingroup\$ Would they advertise it as a 35 MHz scope if it altered the results on just 14 MHz? Or could it need calibration? \$\endgroup\$ – C32 Apr 5 '18 at 14:24
  • \$\begingroup\$ Look at the graph of a single pole, low pass rolloff characteristic. If your scope and your probe combined are 35MHz BW, that means your displayed amplitude will be 70% of the actual amplitude for a 35MHz signal. Perhaps someone reading this on more than their phone can answer properly with some graphics or a circuit sim. \$\endgroup\$ – AlmostDone Apr 5 '18 at 14:31
2
\$\begingroup\$

what could I be doing wrong?

Your schematic show a schmitt trigger as the oscillator's inverter - this is incorrect - it should be a fast unbuffered inverting device for this circuit to work correctly and accurately.

\$\endgroup\$
  • \$\begingroup\$ Isn't the 74hcu04 unbuffered? I believe I read somewhere that it is. Anyway, the problem was my probes . Apparently higher frequencies can result in lower voltage reading \$\endgroup\$ – C32 Apr 5 '18 at 17:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.