ASN9 - 3rd Harmonic & QSD: Difference between revisions
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[[Jodi Hodge|Back to my home page]] |
[[Jodi Hodge|Back to my home page]] |
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'''1. How does Third Harmonic Sampling work? (''The [http://www.groups.yahoo.com/group/softrock40 Soft Rock] receiver uses it.'')''' |
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[[Nick Christman|<b><u>Nick Christman</u></b>]]<br/> |
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'''Third Harmonic Sampling''' |
'''Third Harmonic Sampling''' |
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The ''Softrock 40'' software defined radio (SDR) uses a special technique when sampling the data -- this technique is referred to as '''3rd Harmonic Sampling'''. The purpose of 3rd Harmonic Sampling is to eliminate the need to use a high frequency oscillator which is useful economically and physically. (In other words, a high frequency oscillator is expensive and is often incompatible with "cheaper" switches.) However, nothing in this world is perfect and because of this imperfection, Third Harmonic Sampling have a signal loss of <math>\textstyle 20 \log_{10} (\frac{1}{3}) = -9.54 db</math> (Max Woesner). |
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The cosine series is made up of a infintie sum of harmonics |
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So how does Third Harmonic Sampling work? Well, let's take for example a very basic SDR (similar to the one designed in Electronics II). In the most basic SDR there is a local oscillator that outputs a wave in the form of a square wave. Now, this square is not the product of 21st century sorcery, but instead it is the result of an "infinite" sinusoidal series that is evaluated at the desired local oscillation frequency. This sinusoidal series is refereed to as a Fourier series and is defined as |
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\sum_{n=1}^{\infty} \frac{1}{n} cos(n \omega t) = cos(\omega t) + \frac{1}{2}cos(2\omega t) + \frac{1}{3}cos(3\omega t) + \frac{1}{4}cos(4\omega t) + \cdots </math> |
\sum_{n=1}^{\infty} \frac{1}{n} cos(n \omega t) = cos(\omega t) + \frac{1}{2}cos(2\omega t) + \frac{1}{3}cos(3\omega t) + \frac{1}{4}cos(4\omega t) + \cdots </math> |
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Of course, the segment of this series we are particulary interested in is the third harmonic, or <math>\textstyle \frac{1}{3}cos(3\omega t)</math>. Notice the constant <math>\textstyle \frac{1}{3}</math> in front of the cosine -- this is where the 9.54 db loss originates from (mathematically). From here, we simply use the third harmonic cosine as our local oscillator frequency. |
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The third harmonic is the third term |
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<math> \frac{1}{3}cos(3\omega t)</math>. |
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The benefit of using the third harmonic for sampling in a circuit rather than than the first harmonic is that it creates an oscillation that is three times as fast. SoftRock -40 software defined radio for example uses third harmonic sampling as a technique to obtain faster oscillation. In addition, third harmonic sampling is that it is cheaper to buy a low frequency oscillator and the parts driven by one. The disadvantage is that it results in power loss of -9.54dB.See how Softrock uses Third Harmonic Sampling[http://www.groups.yahoo.com/group/softrock40 Soft Rock] |
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A Quadrature Sampling Detector (QSD) is used to create a cosine (or sine) waveform from a established sine (or cosine) waveform. (I say "sine or cosine" because the output waveform is simply sinusoidal -- there is really no way to distinguish between sine and cosine, we can only distinguish between <math>\textstyle O \deg \mbox{ and } 90 \deg</math>.) |
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Describe how third harmonic sampling and QSD (Quadrature Sampling Detector works. |
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A Quadrature is output signal that is a phase off from the output signal. For example, in a software define radio two signals that are generated from two mixers are phase shifted 90 degrees. One signal is the desired signal and the second is the quadrature signal that is used for quadrature sampling. |
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For this assignment I will look third harmonic sampling and QSD as used by SoftRock-40 software defined radio. For SoftRock they needed a high frequency oscillator to provide the necessary signal to mix with the signal coming from the antenna. The reason this oscillation needed to be so high was due to there design which used one oscillator to create both the sin and cos waveform for there mixer. To do this they needed a oscillator with a frequency four times higher than the frequency they wished to sample. Getting an oscillator at these speeds is both difficult and expensive so they came up with a clever way to use the third harmonic form their oscillator to achieve the frequency necessary. |
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Further information on Quadrature Sampling |
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It is known by using Fourier Series a square wave can be made up of a series of cos wave i.e. <math>\sum_{n=1}^\infty \frac{1}{n} cos(n \omega t)</math>.<br><br> |
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SoftRock decided that they could use the 3rd harmonic of this i.e <math>\frac{1}{3}cos(3\omega t</math> to get the required oscillation without the hassle and cost of using a high frequency oscillator. Stated another way they need an oscillator that has a frequency three time slower than the one originally needed. The disadvantage to this is that the signal strength is a third smaller or around a 9.5 dB loss. |
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Latest revision as of 13:50, 19 December 2009
Third Harmonic Sampling
The cosine series is made up of a infintie sum of harmonics
The third harmonic is the third term
.
The benefit of using the third harmonic for sampling in a circuit rather than than the first harmonic is that it creates an oscillation that is three times as fast. SoftRock -40 software defined radio for example uses third harmonic sampling as a technique to obtain faster oscillation. In addition, third harmonic sampling is that it is cheaper to buy a low frequency oscillator and the parts driven by one. The disadvantage is that it results in power loss of -9.54dB.See how Softrock uses Third Harmonic SamplingSoft Rock
Quadrature Sampling
A Quadrature is output signal that is a phase off from the output signal. For example, in a software define radio two signals that are generated from two mixers are phase shifted 90 degrees. One signal is the desired signal and the second is the quadrature signal that is used for quadrature sampling.
Further information on Quadrature Sampling