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- Asymetry in transmission through the splitter
- We can average the output of the phase shifting circuit at +/- 50MicroVolts
- The Phaseshifter seems to impact the InPhase Output : problem. Why?

- Attenuation of the phase shifter : -1,5mV
- Dependance on bias voltage

- Not dependant on the frequency of the RF source considering the frequency range used

Source : N>Cecile>Experiment 2304201>Experiment For the theory data N>Cecile>Experiment 2304201>Calculations (Excel File) Source : N>Cecile>Experiment 2304201>Experiment For the theory data N>Cecile>Experiment 2304201>Calculations (Excel File)

- We can clearly see that above -40 dB of attenuation, the output of the mixer isn't valid.

- What is the limiting parameter now? the input power of the mixer or the noise?
- The mixer has a linear behaviour if we assume that the LO Input power is fixed, so to increase the limit RF input power we just have to increase the LO Input power. Obviously this cannot work all the time, so what's the limit of this technique?

Source : N>Cecile>Experiment 2704201 It means mainly two things : - First, we will have to wait a certain amount of time before taking the measurements during the calibration of the demodulation circuit with the 4T Measurements. -Second, when we will use the demodulation circuit on the device to detect the quick shifts in the resistance, we will have to remind ourselves that there is a delay between what occurs and what we are measuring.

- Like what is expected, Vr has twice the frequency of the modulation source
- If we compared the value of Vr_amp = 3.35/2 = 1,67 mV to the one we got from a simulated circuit Vr_amp_th = 1,617mV we can conclude that the demodulation circuit behaves approximately how it is supposed to work.
- Problem : the outphase is much more important than the inphase Vpp_Out/Vpp_In = Tan(Phi) where Phi is the phase shift between the RF source and the modulated signal, then we got Phi = 68.84 Degrees which is an important Shift. Where?

- We know have to determine if the phase shift just appears because the LO power was to low, and what is the limiting input RF Power.
- The demodulation circuit was indeed working.

(group velocity) Vg = c * VFwhere c is the speed of light, and VF is the velocity factor wich is given by VF = 1/SQRT(EpsilomR _Dielectric). In our case, the dielectric is the PTFE which gives us VF= 0,695 and Vg = 2,102 E+8 m/s. Finally if the delay is only due to the cables we would have Vg * 2.85 E-8 = 6 m of cables ! The delay must be due to the phase shift induced by the mixers.

The frequency of the oscillations is 12.38 MHz.

Accuracy indicator : 5Tau = 3,5 E-7 s

- The demodulation circuit only work on square waves of a frequency under 1 MHz. Indeed, there is not only a delay between trhe modulation and the demodulated signal, but the demodulation circuit also smooth the slope of the square.

The changes in S11 have to linger more that 5 E-7s to be detected by the demodulation circuit

- The frequency of the oscillations is constant and equals 12 MHz. It is not dependant on the frequency of the modulation source, nore the carrier frequency (for a range going from 330MHz to 350MHz).

terminated in its characteristic impedance, namely, 50 ohms. Many times, the impedance

match is expressed in terms of

easily attainable using the chart given in Section 0. Most of the filter models shown in this

handbook are designed to present a good impedance match in the passband and a highly

reflective impedance match in the stopband. Typically the VSWR in the center of the

passband is better than 1.2 to 1 and the VSWR in the stopband is typically 18 to 1, very

highly reflective. In these models, both filter ports present a good impedance

match in the passband and stopband. The

In order to limit the reflection by the Low Pass FIlters into the Mixer, it is better to use a Constant Impedance Filter with the appropriate IF range.

- From the Phase Delay trace we can interpret the 10 MHz trace we got previously. We are right in the middle of the Resonance in the Phase Delay, meaning that the 10MHz wave is considerably slowing down and gives us this big delay/smoothing.

- From the Return loss trace, we can see that the resonant frequency of the Low Pass is indeed around 12 MHz. Because for a certain frequency the higher the return loss is, the higher the transmission through the Low Pass is, which is the definition of a resonance frequency.

- Try to find a high performance constant impedance low pass filter, even a 250MHz low pass filter can do the trick as soon as the up side band frequency is twice the frequency of the carrier (660MHz), and we don't have any resonance frequency.
- Perform a numeric demodulation, via IGOR for instance

- Can we calculate how long after a discontinuos change in the input these phase shifts will affect us for?i.e. what do they do to a suqare wave signal?
- David Reilly suggests that the band pass filters can introduce a phase shift. MiniCircuits do makeband pass filters with extremely small group/phase delays - we should look into those.
- MiniCircuits/Analog devices also make demondulators. Perhaps we should look into a discrete component I&Q demodulator?e.g. http://www.analog.com/en/rfif-components/modulatorsdemodulators/ad8348/products/product.html
- We could also borrow from Bob that big demod circuit he has, and compare the behaviour to our discrete components setup that Cecile has made.

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Topic revision: r19 - 09 Sep 2014, RoyLi

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