Tueftler
Newbie level 4
Dear electronic friends,
I am designing a multi-channel diode sensor PCB with high-frequency excitation (500 kHz - 10 MHz) and lock-in technique. The diode is excited with a high-frequency bias voltage and the current is measured via TIA. The light ("gate") of the diode modulates the amplitude and phase of diode current with a broad spectral range (DC - 100 kHz).
The design facts in short:
- Adjustable high-frequency excitation of the diode bias voltage: 500 kHz < f_c < 10 MHz
- Broadband diode current to be measured via TIA (amplitude and possibly phase): DC < f_m < 100 kHz
- Transimpedance amplifier (TIA) and gain-stage are both specified for 100 MHz
- ADC converter with max. 50 MSPS
- Opal Kelly XEM7310 FPGA with 200 MHz clock
Question (1) Which lock-in technique is the best for measuring amplitude and phase (for fc=10 MHz)? Analog rectifier, analog mixer or digital mixer?
I have simulated all possible techniques and came to some interesting conclusions:
- The digital lock-in realization resulted in a perfect calculation of the signal amplitude (multiplication with sin and cos of reference frequency, low-pass filters and root-mean square summation), ONLY if the measured signal has a CONSTANT phase shift compared to the reference excitation signal. When the phase changes slowly over time, it will be included in the in the calculated signal amplitude. Is there any clever way to obtain a perfect amplitude that is not affected by phase changes?
- When the modulated signal is rectified and smoothed with a capacitor and resistor, the result looks very good if the time-constant of capacitor and resistor is appropriately tuned (about fc/10). Conveniently, the obtained signal amplitude is NOT affected by temporal changes in the phase between measured signal and reference excitation signal. The disadvantage is that the phase information is completely lost, and a different smoothing capacitor/resistor has to be used for different excitation frequencies.
- A digital lock-in realization is more accurate than an analog one, because one can calculate a perfect sine wife without distortions. But, this requires a very good ADC bandwidth and dynamic range. What is if the ADC sampling frequency is only 5 times larger than the excitation frequency (carrier frequency)? Will it be still more accurate than its analog companion?
Question (2) Excitation better with a DAC generated sine wave or with a sine wave generator (AD9833)?
It seems that the sine wave for the sensor excitation must be perfectly harmonic. Noise near the excitation frequency (carrier frequency) f_c might disturb the modulated signal f_m especially at very low frequencies (because f = f_c + f_m after modulation). That means a DAC with 5-10 samples per period would lead to significant distortions, right? So, I would have to use the AD9833 for high-quality sine wave generation and somehow synchronize it with the FPGA?
Thanks so far!
I am designing a multi-channel diode sensor PCB with high-frequency excitation (500 kHz - 10 MHz) and lock-in technique. The diode is excited with a high-frequency bias voltage and the current is measured via TIA. The light ("gate") of the diode modulates the amplitude and phase of diode current with a broad spectral range (DC - 100 kHz).
The design facts in short:
- Adjustable high-frequency excitation of the diode bias voltage: 500 kHz < f_c < 10 MHz
- Broadband diode current to be measured via TIA (amplitude and possibly phase): DC < f_m < 100 kHz
- Transimpedance amplifier (TIA) and gain-stage are both specified for 100 MHz
- ADC converter with max. 50 MSPS
- Opal Kelly XEM7310 FPGA with 200 MHz clock
Question (1) Which lock-in technique is the best for measuring amplitude and phase (for fc=10 MHz)? Analog rectifier, analog mixer or digital mixer?
I have simulated all possible techniques and came to some interesting conclusions:
- The digital lock-in realization resulted in a perfect calculation of the signal amplitude (multiplication with sin and cos of reference frequency, low-pass filters and root-mean square summation), ONLY if the measured signal has a CONSTANT phase shift compared to the reference excitation signal. When the phase changes slowly over time, it will be included in the in the calculated signal amplitude. Is there any clever way to obtain a perfect amplitude that is not affected by phase changes?
- When the modulated signal is rectified and smoothed with a capacitor and resistor, the result looks very good if the time-constant of capacitor and resistor is appropriately tuned (about fc/10). Conveniently, the obtained signal amplitude is NOT affected by temporal changes in the phase between measured signal and reference excitation signal. The disadvantage is that the phase information is completely lost, and a different smoothing capacitor/resistor has to be used for different excitation frequencies.
- A digital lock-in realization is more accurate than an analog one, because one can calculate a perfect sine wife without distortions. But, this requires a very good ADC bandwidth and dynamic range. What is if the ADC sampling frequency is only 5 times larger than the excitation frequency (carrier frequency)? Will it be still more accurate than its analog companion?
Question (2) Excitation better with a DAC generated sine wave or with a sine wave generator (AD9833)?
It seems that the sine wave for the sensor excitation must be perfectly harmonic. Noise near the excitation frequency (carrier frequency) f_c might disturb the modulated signal f_m especially at very low frequencies (because f = f_c + f_m after modulation). That means a DAC with 5-10 samples per period would lead to significant distortions, right? So, I would have to use the AD9833 for high-quality sine wave generation and somehow synchronize it with the FPGA?
Thanks so far!