# what could be the minimum frequency of digital signal ?

Discussion in 'General Electronics Chat' started by munna007, Jan 10, 2011.

1. ### munna007 Thread Starter Active Member

Jul 23, 2008
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hello frends.....this question is asked to me in interview.....i am unable to find its answer....can u please tell me ????

2. ### Audioguru New Member

Dec 20, 2007
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The opposite of an infinite frequency.
One cycle each zillion light-years.

3. ### hgmjr Moderator

Jan 28, 2005
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As audioguru has indicated there is no practical limit to the length of the period of a squarewave. Is there a particular low frequency in which you are interested?

hgmjr

4. ### Papabravo Expert

Feb 24, 2006
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The practical answer is zero or DC. I don't see why this should be such a challenging question.

5. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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Maybe the interviewer was interested in the minimum digital sampling frequency needed, i.e., Nyquist.
John

6. ### russ_hensel Distinguished Member

Jan 11, 2009
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Actually the period cannot exceed the age of the universe. If you have a signal at 0 hz, you may have to wait a while, or a long while or a longer while to see if it really is 0.

7. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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If sampling, then at least twice the highest frequency you want resolution of.

For standard gates, DC, in the "ideal sense", 0 Hz for ∞.

8. ### Papabravo Expert

Feb 24, 2006
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The point is that any DC signal including GND fits the literal definition of "periodic" in the sense f(x) = f(x+2∏) for all x from -∞ to +∞. I don't have to wait for the end of the universe, DC is DC and constant by definition.

Last edited: Jan 10, 2011
9. ### tom66 Senior Member

May 9, 2009
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Light years measure distance, not time.

10. ### #12 Expert

Nov 30, 2010
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Audioguru is allowed these due to his great age and wisdom.

11. ### Audioguru New Member

Dec 20, 2007
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OK, I'll correct myself.
The time it will take you to walk the distance a light beam travels in a zillion years.

12. ### tom66 Senior Member

May 9, 2009
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Make it infinity and you've got a deal.

13. ### russ_hensel Distinguished Member

Jan 11, 2009
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Yes and for all frequencies. But you do not know it is DC if you do not wait, it may simply be a long period square wave.

14. ### Papabravo Expert

Feb 24, 2006
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I agree if you are conducting an experiment. The original question asked for a minimum frequency. Zero frequency is one possible answer if you allow a constant value to be included in the set of periodic waveforms. The other possible answer is some infinitesimally small frequency ω that approaches zero from the right, call it 0+, and as has already been observed, you will have to wait a long tome to see it return to its original value.

15. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Think about giving that answer in an interview though, especially if they actually were looking for the Nyquist limit.

16. ### CDRIVE Senior Member

Jul 1, 2008
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I would have given the identical answer.

17. ### munna007 Thread Starter Active Member

Jul 23, 2008
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in digital singla processing by prokias its written that frequency of digital signal varies from -1/2 to +1/2 hz.is it write ????

Jan 18, 2008
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19. ### Papabravo Expert

Feb 24, 2006
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Not quite. In the continuous domain there is no concept of negative frequency. In the digital domain there are two thing going on:

1. A "band-limited signal" has a certain bandwidth. That is there is a minimum and maximum frequency. Signals are often represented by centering the band about the vertical axis. So negative frequency is that part of the band below the "center frequency"
2. Frequencies in cycles per second(Hz.) or radians per second are normalized with respect to the sampling frequency givina a range of [0..1]*Fs or [-0.5..0.5]*Fs, where Fs is the sampling frequency. The sampling frequency needs to be at least twice the frequency of the highest frequency in the signal being sampled.