J
Joerg
- Jan 1, 1970
- 0
Tom said:So what you're saying is that, with 1.3pF in the diode pair (at ZERO
bias), you have 2pF in the single pad, 0.8x0.9mm? So the ground plane
is less than 0.02mm (less than a mil) under that pad? You obviously
don't have a diode problem, you have a layout problem. You're right,
cutting the diode capacity to ZERO isn't all that much help, and it's
also not very realistic. On the other hand, it's also obvious you
didn't look very closely at the package for the part I suggested; on
the same scale (same ground plane less than a mil away), its pads
would be around 0.3pF each. Even with your (apparently) ridiculously
thin dielectric, its pads can be integrated into a 50 ohm line with
very little effect. Even with a SOT23, though, protecting a
microstrip on FR4 with 20 mil dielectric should not introduce any
discontinuity; the pad "capacitance" (well under 2pF anyway) is just
integrated into the line impedance. An 0.8mm trace width (the width
of a SOT23 pad) 0.5mm (20 mils) above a plane at Er=4.7 gives you
about a 50 ohm line, at 0.12pF/mm. If the SOT23 straddles the line,
you can neck the line down a bit where it passes between the pins 1
and 3 to maintain 50 ohms. Then it becomes an issue only of diode
capacitance, and it does seem to me that getting rid of nearly half
the capacitance is worthwhile.
Even if it WERE 3pF across 50 ohms, that's only a quarter dB or so at
500MHz. With the suggested diode-pair, at zero bias, you can get that
down to well under 0.1dB, and unless the design is remarkably clean or
simple, the rest of it won't be nearly that good.
That number came from datasheets and app notes and includes the SOT23
package as well. But as I said if the diode has a better package it
would lower the total. The HMPP in the MiniPak you suggested would
certainly have a better package. However, the other downside is that
it's around $0.40. A bit steep in this application versus the usual
$0.02 for a dual.