Zero Hertz or Infinity

Discussion in 'Homework Help' started by TheSpArK505, Jan 21, 2015.

Sep 25, 2013
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Hi everyone
I wonder what is the frequency of a DC signal? Basically i know its zero Hertz DA!! But what about infinity frequency for DC >>>
And how do Passive elements behave with the variation of frequency?

2. WBahn Moderator

Mar 31, 2012
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How many cycles does a DC signal make per second?

The only way you could claim a non-zero frequency would be to claim a zero amplitude, in which case the distinction between AC and DC and what frequency it is at is intrinsically meaningless since the signal is identically zero.

As for how passive elements behave at different frequencies, first you start with the constitutive equations for the elements (i.e., the defining relationships between voltage and current for that component). For a resistor that's Ohm's Law, V=IR. What are the constitutive equations for an inductor and capacitor?

3. crutschow Expert

Mar 14, 2008
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Infinity frequency is the polar opposite of DC (0 Hz).

4. joeyd999 AAC Fanatic!

Jun 6, 2011
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And, to be completely pedantic, flicker (1/f) noise pretty much guarantees there is really no such thing as DC.

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5. MrChips Moderator

Oct 2, 2009
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Like how can a cop give you a ticket for not stopping at a STOP sign? What is the definition of "coming to a complete stop"?

6. joeyd999 AAC Fanatic!

Jun 6, 2011
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0 Kelvin?

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7. MrChips Moderator

Oct 2, 2009
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DC is 0Hz. You gotta measure the period!

8. joeyd999 AAC Fanatic!

Jun 6, 2011
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Is this a reply to my 0K post? If so, you asked "what is the definition of coming to a complete stop."

Well, a complete stop implies zero kinetic energy. This occurs only at 0K (vacuum energy not considered here). Thus, my answer in non-time based units.

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9. MrChips Moderator

Oct 2, 2009
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Sorry. No, I was replying to the DC problem.

As for "Complete Stop" we have a mountain here and I swear it moved since yesterday.

10. joeyd999 AAC Fanatic!

Jun 6, 2011
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Give it time. It'll move.

11. MrAl Distinguished Member

Jun 17, 2014
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Hi,

Yes zero Hertz is considered DC, but infinite frequency is often considered to be zero time, or saying it differently, zero time is often considered infinite frequency.

So we have zero frequency is like DC, and zero time is like infinite frequency.
We also end up with infinite time is like zero frequency.

12. WBahn Moderator

Mar 31, 2012
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When you say zero time I'm assuming you are talking about the period.

Well, what's the period of something that never changes? Infinite.

Actually, in both cases it is undefined except in the limit.

13. MrChips Moderator

Oct 2, 2009
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velocity = Δd/Δt
Hence zero velocity is in the limits Δd→0/Δt→∞

Similarly,

frequency = n/Δt
Hence DC is in the limits n→0/Δt→∞

Sep 25, 2013
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Thanks all for your answers . It seems that it needs a PROOF to find out

15. WBahn Moderator

Mar 31, 2012
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To find out what? How passive components behave at different frequencies?

Try doing what I suggested and start from the constitutive equations. The "proof" is only a few lines long.

It might help if you told us what the homework problem is you are trying to solve.

16. MrAl Distinguished Member

Jun 17, 2014
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Hi,

Maybe it would help if we looked at at filter circuit and it's response for AC and DC and at various times.

17. joeyd999 AAC Fanatic!

Jun 6, 2011
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A filter will have a different response tomorrow than today?

18. MrAl Distinguished Member

Jun 17, 2014
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Hi,

Ha ha, over milliseconds or microseconds, not really days

19. WBahn Moderator

Mar 31, 2012
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If it's nonstationary (which of course means it's nonlinear).