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Demodulator circuit problem at high frequency

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Daniel20048

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Hi

I try simulate demodulator based on full wave. The carrier frequency is 8MHz, Amplitude 1V. The modulated signal frequency is 100Khz, level is 100mV.
I attach my schematics (simulation created by LTSPICE,), and waveform at de-modulator output.(Zoom out and zoom in)

The input signal is differential and U4 modify it to single ended, this is vout1.
Vout2 is the output of the full wave demodulator comprised of U1,U3, and you can see the signal is distorted.
I use LTC6247, gbw=180MHz, slew rate is 90V/us.
So I think the decvice fit.
The diodes are BAT54.
At lower frequency (Carrier frequency=0.8MHz) VOUT2 look O.K.
Please advise if this is circuit problem or devices selection.

Thanks!!!
 

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Borber

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It seems that you forgot a 500 ohm resistor between ground and noninverting input of U3 (expected noninverting gain is 1 and not 2).
 

Daniel20048

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It seems that you forgot a 500 ohm resistor between ground and noninverting input of U3 (expected noninverting gain is 1 and not 2).


Hi, I tried add the resistor- it didn't help.
U3 work as differential amplifier for the signals from U1, so adding this 500 ohm resistor between ground and non-inverting input of U3 will not help.
I also can say that Vout2 look better when decrease the frequencies (FC,FM) by 10 factor.
Anyway the LTC6247 and the BAT54 diode should work at 7MHz!
Dan
 

FvM

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I don't understand what you exactly mean with distorted signal. Which waveform do you expect at the rectifier output?

A demodulator should have a filter that removes the carrier frequency residuals at the output. Where is it?
 

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