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4hv.org :: Forums :: General Science and Electronics
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4Mhz square wave generator

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Sulaiman
Sun Feb 21 2016, 07:04AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Check your user manual regarding your oscilloscope probes because

oscilloscopes usually have a direct input input impedance of around 1 MOhm // 15 pF
a x1 probe just adds to the capacitance, say 1 MOhm // 45 pF
a x10 probe has lower input capacitance, say 10 pF

so always use your probe in x10 mode, unless the signal is to low a voltage to see clearly on the 'scope.
to reduce high frequency loading of the circuit being 'scoped.

before using the probe on x10 you need to adjust it using the small screwdriver slot on the body of the probe where it plugs into the 'scope, see your manual for details.

Given the above, it is quite likely that your 'scope probe in

. x1 mode loaded the circuit enough to eliminate the spikes/ringing/overshoot
. x1 mode has a lower frequency response than your 'scope, e.g. 7 MHz so cannot 'see' the overshoot
. x10 mode may be showing overshoot that is not actually there as the probe has not been adjusted

these things come with experience, you will soon catch up !
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ZakWolf
Sun Feb 21 2016, 07:24AM
ZakWolf Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Sulaiman wrote ...

Check your user manual regarding your oscilloscope probes because

oscilloscopes usually have a direct input input impedance of around 1 MOhm // 15 pF
a x1 probe just adds to the capacitance, say 1 MOhm // 45 pF
a x10 probe has lower input capacitance, say 10 pF

so always use your probe in x10 mode, unless the signal is to low a voltage to see clearly on the 'scope.
to reduce high frequency loading of the circuit being 'scoped.

before using the probe on x10 you need to adjust it using the small screwdriver slot on the body of the probe where it plugs into the 'scope, see your manual for details.

Given the above, it is quite likely that your 'scope probe in

. x1 mode loaded the circuit enough to eliminate the spikes/ringing/overshoot
. x1 mode has a lower frequency response than your 'scope, e.g. 7 MHz so cannot 'see' the overshoot
. x10 mode may be showing overshoot that is not actually there as the probe has not been adjusted

these things come with experience, you will soon catch up !

Thanks Sulaiman, I know how to adjust the probes with a screw driver and I had them properly tuned.

So I will switch back to x10 and just add a load resister to help dampen the output more.

Is there a better (faster or more stable) tunable schmitt-trigger oscillator that I should use or is the SN74HC14N going to be ok to drive IXDD614CI.

EDIT:the 74ac14n is the faster version of the two
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Conundrum
Mon Feb 22 2016, 05:39AM
Conundrum Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Yup. AC is faster.

Would you not be better off with a can oscillator with thermal feedback?
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Sulaiman
Mon Feb 22 2016, 05:30PM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
An OCXO (oven controlled crystal oscillator) is overkill don't you think ?
Temperature changes will alter the inductance of air core inductors much more than the frequency of the crappiest crystal oscillator

Ultimately I suspect a pll solution will be best.
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