Variable capacitor for output stage of reader device (RFID)

[GeniusRus88]

Hello guys,

I'm trying to design reader device for RFID applications. But important thing is that frequency will be variable. For this purposes I can use to options: either for different frequencies use separated output stage with chosen value of capacitor or use one output stage and somehow vary capacitance. What is also important: i use microcontroller, so everything should be controllable by digital signals.

Here is a schematic of my output stage: https://obrazki.elektroda.pl/5741186500_1358509986.png
For more detailed info, this is application note of NXP, how to calculate all parameters of such ouput stage: **broken link removed**

And i found out that only one capacitor we should tune in order to maintain correct impedance matching. This is C1 (https://obrazki.elektroda.pl/5741186500_1358509986.png)

The value of it changes between 200 pF and 350 pF. How and by which methods i can vary it?
I can propose varicap, MOSFET capacitor.

But for me it's not clear, because i have in this output constantly changing sinusoidal voltage.
Therefore, how can i get different values of capacitance and be able to vary it by means of digital signals from microcontroller?
Thank you for your help, Gena

 

[tony_lth]

you can use one series cap or shunt cap.
Such as shunt one fixed 190pF cap and a var cap that can vary from 10pF~160pF.

 

[GeniusRus88]

Thank you for your reply!
I already tried several times to use varactor diode, but no results. Probably I use it not in a proper way.
Could you check my schematic?

https://obrazki.elektroda.pl/8804994800_1358756464.png

I'm doing simulations in LTSpice. Varactor diode is BB201 with maximum capacitance 133pF at 0V bias voltage and 40pF at 5V bias voltage. (This is according to specification of this device and my calculations too)

But I can't check exact value of this varactor in this software and i used indirect methods.
Since i know which value of total capacitance should be for certain frequency, i can say what value of this shunt varactor is.
and results are different, moreover, value of varactor is around 50pF at 0V and changes to 40pF at 5V.

Where is my error?

Thanks, Gennady

 

[kspalla]

You need to apply negative voltage right?

 

[GeniusRus88]

No, in such structure, we get capacitance from reverse biased varactor D1. Therefore, we should apply positive voltage on cnt.
And this is very famous usage of varactor diodes. Why it doesn't work i don't know....

 

[kspalla]

Try this below.
Remove C3 and C7.
Add 10uH choke for cntl before 47000 resistor.