9 - Third harmonic sampling

Describe how third harmonic sampling and QSD (Quadrature Sampling Detector) works.

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.

It is known by using Fourier Series a square wave can be made up of a series of cos wave i.e. $\sum _{n=1}^{\infty }{\frac {1}{n}}cos(n\omega t)$.

SoftRock decided that they could use the 3rd harmonic of this i.e ${\frac {1}{3}}cos(3\omega t$ 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.

SoftRock also uses QSD (Quadrature Sampling Detector) This lets you break the incoming signal into two parts one part the real part of signal (cos) and the other the imaginary part (sin). SoftRock achieved this by using four switches and capacitors. This switching mechanism switches through the 4 switches at the same frequency as the sampling frequency the first and third switches part of sin (($0^{\circ },180^{\circ }$) and the second and fourth switches yield the cos ($90^{\circ },270^{\circ }$). The capacitors are there to hold the amplitude of the signal for quarter of the cycle while switch is there then waits for the switch to come around again. This then establishes the cos and sin part of the incoming signal. This type of detector is named the Tayloe Detctor.