ASN9 - 3rd Harmonic & QSD: Difference between revisions
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'''Third Harmonic Sampling''' |
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Describe how third harmonic sampling and QSD (Quadrature Sampling Detector works. |
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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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The cosine series is made up of a infintie sum of harmonics |
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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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<math> |
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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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\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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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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'''Quadrature Sampling''' |
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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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Further information on Quadrature Sampling |
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[http://www.flex-radio.com/News.aspx?topic=publications SDR-1000 QEX] |
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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