433.92 MHz series capacitor microstrip

Hi all,

I am currently designing a board which is a part of my final work. It is in fact LNA based on Infineon's BFP405 BJT. The demand is to use microstrip technology to design matching at input and output. I used double L sections in order to BP filter. Now i have a problem with series capacitor of roughly 180pF (which is standard lumped cap value). I am asking is that value obtainable in relatively small size microstrip or not.

The next question is, If I decide to use lumped cap, how should I connect it with the following series microstrip inductor? Simply by solderinf without thinking of transmission line discontinuities, etc.

The similar problem is with making shunt shorted stub. Do I have to make some bends at the end, or just to solder capacitor to the ground via via ))?

Please don't tell me it is ridicoulous to make microstrip at this frquency, cause it is not in my domain.

Many thanks

-You can not obtain 180pF with MS techniques because it's a huge value for this technique...
-If you carefully design your MS circuit, discontinuity shouldn't be a problem except if you work above 10GHz or more.You've very already been working at relatively low frequency.That's OK..
-For shunt stub, if there isn't DC voltage, it's better to tie your stub to the ground with few vias to decrease the parasitic inductance.If there is a DC voltage, you should use a blocking capacitor with appropriate value which works much below its resonance frequency.

Thanks BigBoss.

-There is DC through shunt stub. If we assume that shunt stub will be realized as rectangle. One side of the rectangle will be on the side of "HF" path, and on the oposite side I have to connect bypass capacitor to the ground and to route DC voltage. The thing I don't really understand is practical realization. Should I put capacitor in the middle of the side of stub? If I wanr to route DC through the stub, via for example 20mil line, where should I connect DC feedline and stub?

- So, with the frequency of 433.92 MHz, I don't have to care about discontinuities? I can freely make stub cascades.

Tnaks again

You can use AWR or ADS to simulate.
I used to place the cap pad directly on the middle of arc stub. The position is not so accuracy. You can tune the position of capacitor in practical.
If you use rectangle, the otherside normally is open.

I suppose you missunderstood me.
I have shorted shunt transmission line, but I also want DC current through it in order to set bias point of BJT.
Transmission line will be rectangular, and because it is short circuited, I have to put capacitor at one end of the line. The question is, should it be on the middle or not. And, is it OK to short connection of the line and cap with the DC feedline?

surely you can do it.
Normally I select the substrate so that the trace width is the same as the cap pad.
I used to place the cap pad in the middle of trace, with a 0.1mm overlap. The cap position can be finely tuned in real.