I am working on hacking a sonar for some time now (long story here:https://forum.arduino.cc/t/interfacing-fish-finder-take-2/679629/57 ) and finally have a tested working solution but there is one missing piece that I need help on: the tuned inductor. At the moment, I use the inductor that is embedded on the hacked device and there is no way to tell what characteristic it have and I destroyed it while trying to unsolder it from the board. I want to select and buy one but don't know where to start and would be very happy to have some help with this.
To work and from what I understand, the inductor need to transform a 5v input into a 70 volts p-p output. The input frequency generate by PWM match the output frequency that is : 200 khz. The resonant factor Q should be about 48 based on the transducer characteristic I have found so far. The diode + resistor and capacitor in the top left part of the schematic have a charging time constant of about 0.2 sec and keep the input inductor voltage high between pulse. Schematic below.
edit: since it was destroyed anyway, I unwinded the inductor. The transducer side have 355 turns, exactly 2.5m long of thin wire coil (about 0.05mm). The variable side was made of two 15 cm long wire (0.1 mm) and they each have 15 turns for a total of 30 turns. It was winded on the exterior of the other coil.
You need to know the capacitance of the transducer - this is a resonant transformer (two loosely-coupled inductors), with the secondary in resonance with the capacitance of the transducer at 200kHz. Resonant frequency is 1 / (2pisqrt(L*C)).
These days such resonant transformers are much harder to source.
The capacitance of the transducer is 2.2 nF and the frequency is 200 kHz.
Then I isolate L in the equation and obtained 287 uH for the secondary.
What to do for the primary?
Also when we select an inductor on digikey, they don't specify if it is the secondary or primary Henry value. By analogy a 100k linear pot will give you between 0 and 100 k ohm. The 287 uH secondary inductor is not the variable inductance in this case.
Digikey inductors are unlikely to include any IF transformers like this. BTW its quite possible the one you have has built-in capacitors too - if you can find a datasheet for the existing component you'll be in a much more informed position. It appears to be a TOKO 113CN-2249, but this is pretty ancient tech these days. And 200kHz is a rare frequency for RF circuits where IF freqs are commonly 455kHz and 10.7MHz
Yes, there is a capacitor hidden under the device. It is 21 pf but I doubt of the ability of my multimeter to report a realistic value for such low farad. It is connected between the 2 external pin of the primary inductor (NC pin and the pin connected to the mosfet but is not shown in my schematic above since I discover it later).
Since it is connected to the NC pin (not connected pin), I am not certain it do anything here?
Unfortunately, there is no datasheet. They are most probably custom made. I am looking for a not so far solution if it ever existed.
