Function Opamp

[nuclear213]

Hello everyone,

after I got such great help the first time I asked, thanks again for that, I have another problem.

I wanted to play a bit with wireless power transfer and found a, what I think, great dev kit
https://www.we-online.de/web/de/electronic_components/produkte_pb/demoboards/wireless_power/design_kit_200_w/wireless_power_200wkit_page.php
from Würth Elektronik. The coupling and the power transfer is not that difficult to understand however they also include a AM transmitter and receiver. That is where I have my problem.

The transmitter side is still ok to understand, they just switch in some extra caps in the resonant circuit, changing the resonance frequency and thus changing the amplitude.

The receiver-side is what I do not understand and sadly I can also not find anything relevant on google about it. As far as I understand the Zenner D16 is there just to lower the Voltage by 20V. Then we have an envelope detector with D14, R45 and C56 and an AC-coupling with C54. After that I would guess we have the diodes to limit the voltage to -Vfwd and Vcc-Vfwd.

But then I do not understand the function of the first opamp. Does anyone have a pdf or a book recommendation for that? I have not seen such a configuration before.

[iMo]

The C55 R43 R40 form an low-pass with time constant t1 - that creates the floating "reference level".

The parts at the inverting input form a "shaper".

The opamp will compare the average ref level at the noninverting input with the actual modulation level at the inverting input, thus it recovers the digital signal off the input independent of the signal's amplitude (it compares the "fast modulation changes" against the "average reference level").

The "longest period td" of the digital modulation signal (1/0 changes) should be much smaller than the t1.

PS: this circuit was pretty popular in 70ties as we were building the DIY radio-control for models. The signal coming from the receiver (with varying amplitude carrying the digital modulation signal) passed that opamp, and the output TTL/CMOS level was then fed into a shift register. The outputs of the shift register were wired to the servo's PWM inputs.. 

[nuclear213]

Thanks for your reply.

I am a bit embarrassed that I did not see the RC-filter there. But that helps a lot.

However I tried to simulate the same design in LTspice and it just would not work. Even used the same opamps (AD8642). I can see the modulation at TP1 however the opamps do not output anything.
I tried 100kHz and 150kHz, as those are the frequencies used in their dev kit for the carrier signal and a simple 5kHz rectangle signal. Lowering or rising the amplitude of the AM signal does not change anything.