This topic has moved to RFDemodulationCircuitTroubleShooting

-- AlexHamilton - 30 Apr 2010

Troubleshooting of the demodulation circuit.


The purpose of this tests was to determine the impact of the phase shifter in the circuit, and the dependance regarding to the input parameters.

Dependance on the Voltage Bias

The reason why we use this Bias Voltage range is because it corresponds to the extreme values used to get a -90Degrees phase shift with frequencies going from 330 to 350 MHz.


V average

  • 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

Dependance on the Frequency of the Source

VBias= 3.85V

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


Limiting power input

Test Circuit



Experimental trace


Comparison with theory



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

  • 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?

Phase Shift

I also noticed by checking the modulated signal that themixer induced a slight phase shift between the modulation and the modulated signal, depending on the frequency of the modulation.


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.

Demodulation Circuit

Tests with the completed demodulation circuit, and a mixer as a modulation source

*RF Source kiss

Frequency : 342,5 MHz

Power : 10dBm

*Modulation Source kiss

Frequency : 10MHz

Power : -20dBm

Offset : 0,5V (doesn't count because the mixer only works from 10MHz up to 1GHz


Where f4 = Vr = SQRT ( V_In(t)^2 + V_Out(t)^2 )

Notes :
  • 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?


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This topic has moved to RFDemodulationCircuitTroubleShooting

-- AlexHamilton - 30 Apr 2010
Topic revision: r4 - 29 Apr 2010, AlexHamilton
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