I've designed following circuit: 
But let's completely forget the MOSFET and only focus on the optocoupler.
The signal provided to the input of the optocoupler (yellow) has following characteristics:
- Amplitude Modulation of type ASK / OOK (On-Off Keying). It's inverted though: when there is presence of modulation, it means a logic 0, when there is absence of modulation it means 1.
- Carrier frequency 50kHz
- \$V_{min}\$ and \$V_{max}\$ when carrier is present: -2.5V ; +2.5V
- \$V_{RMS}\$: 1.9V
Using an oscilloscope, and by trial and error, I succeeded to find the best R1 resistor value for my application.
Here are my results
With \$R1 = 4.7k\$
| \$R1\$ | \$R2\$ | \$V_{E}\$ |
| 4.7 | 8.33 | \$[0V ; 0.62V]\$ |
| 4.7 | 14 | \$[0V ; 1.04V]\$ |
| 4.7 | 24 | \$[0V ; 1.69V]\$ |
| 4.7 | 26 | \$[0V ; 1.84V]\$ |
| 4.7 | 30 | \$[0V ; 2.05V]\$ |
| 4.7 | 40 | \$[0V ; 2.68V]\$ |
| 4.7 | 41 | \$[0V ; 2.75V]\$ |
| 4.7 | 42 | \$[0V ; 2.8V]\$ |
| 4.7 | 43 | \$[0V ; 2.86V]\$ |
| 4.7 | 44.7 | \$[0V ; 2.96V]\$ |
| 4.7 | 46.7 | \$[0V ; 3.02V]\$ |
| 4.7 | 47 | \$[0V ; 3.05V]\$ |
| 4.7 | 50 | \$[0V ; 3.1V]\$ |
With \$R1 = 1k\$
| \$R1\$ | \$R2\$ | \$V_{E}\$ | \$t_{f}\$ |
| 1 | 40 | \$[0V ; 3.24V]\$ | |
| 1 | 30 | \$[0V ; 3.24V]\$ | 408 µs |
| 1 | 20 | \$[0V ; 3.19V]\$ | 272 µs |
| 1 | 10 | \$[0V ; 3.14V]\$ | 140 µs |
| 1 | 8.3 | \$[0V ; 3.14V]\$ | 117 µs |
I noticed that 4.7k is a too high value. Because when I use this resistor, the signal at the optocoupler phototransistor output behaves like a resistance whose value is much higher than my pull-down resistor value R2 of 10k! Leading to a voltage that is floating not far from GND instead of going up to VCC when modulation is present. This forces to choose crazy pull-up resistor for R2. When R1 = 4.7k, circuit starts to work only when choosing R2 = 47k onwards. BUT this is not acceptable: this makes the phototransistor very slow and I can see the pink curve become dirty (slow rise/fall time of 833µs).
With R1 = 2k however, I'm able to get back to a decent R2 = 10k without any problem. Optocoupler latency is much lower (in the order of 100µs).
The limit when doing my tests was: with 2.2k it still works, with 3.3k it already doesn't work anymore. I chose 2k for a safety margin, and I didn't choose 1k because I want to keep the impedance on the bus as high as possible.
My questions
1) How do I know what \$I_{f}\$ do I need?
I found R1 value by trial and error, but I'm now trying to understand how could I have rationally found that value with the datasheet and computations.
I just know that I want to be as far away from \$I_{f} max\$ as possible to:
- decrease consumption
- preserve LED lifetime.
But otherwise I have no idea how to select my target \$I_{f}\$.
Here is an attempt to reverse-engineer my "good value" of R1. Tell me what I'm doing wrong here:
$$ I_{f} = \frac{V_{RMS} - V_{f}}{R1} = \frac{1.9 - 1.2}{2000} = \frac{0.7}{2000} = 350µA $$
This current sounds extremely low to me. Am I doing something wrong when taking the RMS value of the signal? In a diode, if I understand correctly, the current flowing through it will change its forward voltage.
But I'm lost when my values are not like the datasheet shows. Maybe \$V_{f}\$ is extremely low when \$I_{f}\$ is 350µA, and so my computations are not correct?
2) Frequency response
Can you help me understand why this optocoupler marvelously filters out my 50kHz carrier frequency?
As I understand it, if we forget the MOSFET for a moment, \$R_{L}\$, my "load", is 10k. But what is my frequency to consider? Should I consider 50kHz? Or should I double that to 100kHz because there are 2 LEDs in the opto and each of them will rectify my signal?
Also, the graphic only shows frequency response when \$V_{CE} = 2mA\$. How are we supposed to interpolate this if we don't have the same value?
3) Optocouplers with 1 diode and 2 diodes
When an optocoupler schematics on a datasheet shows 2 diodes. Is it always 2 LEDs to make it AC-friendly? Or can it happen that it's only one LED and one diode?
If I understand correctly the LTV-814 has 2 LEDs, but I'm unsure because it appears no where on the datasheet. They only put the arrows on one LED.
