higher harmonics are attenuated by the tank selectivity

Discussion in 'Homework Help' started by screen1988, Apr 4, 2013.

Mar 7, 2013
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Hello, I need help again.
it is still crossed coupled oscillator.
Please see my attched file. Here is a short description in my book.
With large input swings, M1 and M2 experience complete swiching in a short transition time, injecting nearly square current waveforms into the tanks[Fig 8.12b].Each drain current waveform has an average of Iss/2 and a peak amplitude of Iss/2. The first harmonic of the current is multiplied by Rp whereas higher harmonics are attenuated by the tank selectivity.
The bold is what I can't understand, could you go into detail this?

• Square.JPG
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2. vk6zgo Active Member

Jul 21, 2012
677
85
Actually,there is no such thing as a "first harmonic"!

If the fundamental frequency of the signal is f1,then 2f1 is the "second harmonic ",3f1 is the third harmonic,& so on.

The tuned circuit (I hate the term "tank",but that's just me) does not look like a resonant circuit for any other frequencies than the one it is tuned to,so the harmonics,will be attenuated.

screen1988 likes this.
3. WBahn Moderator

Mar 31, 2012
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I'm pretty sure that they are actually using the term "first harmonic" in a technically correct way, namely that the first harmonic IS the fundamental and I think that's what they are referring to here. As you say, everything except the fundamental will be attenuated and so whatever frequency that are sying won't be pretty much has to be the fundamental.

It's when people construe it to mean the first frequency higher than the fundamental, namely 2f, that they are off the reservation. But for two signals to be harmonically related one just has to be an integer multiple of the other and '1' is an integer.

But I agree that not using the term at all is the better choice if you are trying to minimize the potential for confusion.

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4. vk6zgo Active Member

Jul 21, 2012
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As you say,it is "technically correct",but there is more to usage than that.

The distinction between fundamental & harmonics is a time-honoured one in the RF world,& I always believed,throughout Electronics.

The usage of the term "first harmonic" in a text book may tend to confuse the beginner.

Mar 7, 2013
310
3
Thanks, I see it now. Now please see my attached file. With tuned oscillator, at frequency ωo Vout/Vin is maximum but at other frequency the gain Vout/Vin is not entirely equal to zero. Therefore, the output will have many frequencies not only ωo. Is it right? If so how can I get rid of this?

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6. WBahn Moderator

Mar 31, 2012
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You can't. No realizable filter will completely reject all unwanted frequencies. You just need to make a filter that attenuates unwanted signals "good enough" for you needs.

7. WBahn Moderator

Mar 31, 2012
18,089
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I agree. One of the sad facts is that many (even most) textbooks are not written by people that have any significant experience in the "real world" but, rather, by people that have spent little time outside the halls of acadamia. Thus they are not always adequately in touch with said "real world".

I'm of a spilt mind on the issue of even mentioning terms like "first harmonic" to students. On the one hand, you do open the door for confusion. On the other hand, if students think that a given term always means the same thing and that there is universal agreement and usage, then they are set up for miscommunications when they encounter the term being used another way. Thus it can be argued that they should be made aware when there are conflicting usages of terms and phrases so that, if nothing else, they are more apt to pay attention to the context of usage to at least verify that the terms are being used in the way they expect and to hopefully catch quickly when they aren't.

In the few times that I have had to deal with this particular case, I define the Nth harmonic as the component at N*f and I did point out if N=1 that this could, in principle, be called the "first harmonic" but that it was more properly referred to as the "fundamental". My hope was to fix a mathematical interpretation to the name so that they would be less likely to slip and call the signal at 2f the "first" harmonic because they were thinking that "harmonics" have to be at stricty higher frequencies and, since 2f is the first of these, it must be the "first harmonic".

8. t_n_k AAC Fanatic!

Mar 6, 2009
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Your attached schematic isn't a tuned oscillator. Clearly, if there is a defined linear relationship between input and output, the circuit is meant to be a representation of a tuned amplifier.