Depends on the current. On the Voltage vs. Current curves select an operating current, read the voltage and Re=V/I.

Approximatelly inverselly proportional to the current
until at very high current values the series resistance
dominates.

<nbucska@pcperipherals.com>

Roman,

when you said small you really meant small!

Just to clarify, are you talking about using the schottky diode reverse biased like a varicap, changing the bias to change the capacitance, thereby resonating the loop at 433MHz?

Yes I would use the schottky diode reverse like a varicap.

The smallest mechanical trimmer is quite big (TZR1/Murata: 1,7 x 1,5 mm) and has a high ESR (about 0,5 Ohm). A small microprozessor with InCircuit programming and DA output is on board so it should be easy to change the bias.

I should need a ESR about 0,2 Ohm at 100mA/433Mhz/0...1,8V bias.

Roman

Roman,
    I don’t think you will have much luck with this idea. I checked out the HP/Agilent schottky diodes on the web. They give a linear equivalent model for SPICE and these have resistances of around 8 ohms (for HMPS-282x and HSMS-281x for example). How relevant these models are to the reverse bias situation is not clear, but it is so far above the values you need, I don’t hold out much hope.

I guess the problem is that if you use an uncharacterised parameter like this then it may work on one batch but never work again. Presumably a standard varicap has too much capacitance swing for your application. You could stick a small cap in series with the varicap to reduce the capacitance swing.

Roman:
If you want to use a diode as VARICAP, you bias it
backwads, i.e. current=0. The resistance depends on the
dielectric loss in series with C and || with the leakage current.

<nbucska@pcperipherals.com>

When logbook wrote, I will have about 8 Ohms - that will be much to much. 0,1...0,3 Ohms would be the maximum.

Thank you very much for your answers!!! It was my first time in this forum and I hope that I can help other people as good you did.

Roman