A few notes on the above: LC's were designed for 1 MHz but work better at 950 kHz, thus the chosen frequency. C8 is ~2.7 nF, rectifier caps are 10 nF, noise cap (farthest right) is 10 uF. The load is a couple of regulator and oscillator IC's. Diodes are 1n4148's, NMOSFET is 2n7000. I've got a question mark drawn for the gate input since I've basically just measured a reduced amplitude version of my current Vds signal there whereas it should be zero (the output of an LTC6990 when its power is too low to turn on), so that's a bit confusing.
I'm transmitting across a ~30 Vpp sine wave from a Class E amplifier that isn't well tuned but it appears to be sufficient. The problem is essentially this: with the MOSFET removed, my inductive link is excellent (nearly 1:1 waveform shapes and amplitudes transmit across) and all my IC's turn on and do their jobs (regulators, op-amps, and the modulation oscillator work great)... Yellow channel: Vin on Tx side. Blue channel: output of oscillator on Rx side, which goes to MOSFET gate.
...but with the 2n7000 installed I get an odd "rectification" and significant reduction in amplitude (down to ~1.6 V). You can see this in the waveforms below: Yellow channel: Vin on Tx side. Blue channel: Vin on Rx side (or across C8, or Vds, all are the same nodes).
I thought maybe there was self turn-on occurring so I tried some caps between the gate and source (470 pF and then 1.5 nF) but there was no change. The FET does appear to be on most of the time (with the exception of the 200-300 ns where voltage can actually rise across C8), and I'm sure some of this is due to Vgs surpassing Vt when the source side is going negative but the voltage doesn't seem exactly right (datasheet for 2n7000 says Vt can be as high as 3 V but should be more like 0.8 V). The time constant of the Vin rise/fall suggests the ripple tracks more with the transmitted flux than with any kind of RC-induced decay.
I have also pulled the 2n7000 off, replaced it with a new one, and gotten the same behavior; I also tested the one I pulled off with a simple LED test circuit and it still works.
As a note, I have seen a circuit in this configuration (layout and IC selection) work, with slightly different LC's a cap values.
Any thoughts here would be greatly appreciated, feeling like I'm missing something very basic.
Forgot to clarify what I was expecting (and how the previous iteration of this circuit worked). The Rx should like something this at the drain node: That shows up back in the Tx due to load modulation, then I filter it and measure the modulation frequency. Ignore the numbers in the simulation, intention is just the basic layout.
Based on Russell and Bruce's suggestions, I attempted to make use of two series FETs in the configuration shown here: You'll note that I left the 3.3 V oscillation since I can't do anything about that at the moment (working from a PCB and limited components so can't change regulators right now). I did include the series resistances Bruce mentioned (LTSpice doesn't show the coil series R but I did add it).
However, I'm back to seeing a modified Vin (Vds) at the node where the oscillator output joins the two gates: I can tell that this isn't the oscillator output because it's the same frequency as the input AC (oscillator isn't configured for that frequency range) and the amplitude is too low (should be GND to V+, which is 3.3V).
For experimentation's sake, I tried two more things: connecting the node between the two source pins to GND (didn't change anything) and also just connecting the LTC output directly to the top of Crx (could see the modulation on the Rx side now but the load change was too minimal to be seen back on the Tx coil).
Next I'll try cutting the trace on the source side and placing a series FET there, as suggested below.