J
John Larkin
- Jan 1, 1970
- 0
Suppose I have a 1206 ceramic-core inductor on a pc board with nothing
else nearby. What would be the differential effect on inductance of
adding a ground plane on the opposite side of the board? Even more
important, how might it affect the TC?
I've got a 50 MHz oscillator, using a 150 nH inductor, some fixed
caps, a Maxim flecap (coarse tune), and a varicap (fine tune.) Last
board rev, we had it almost perfectly temperature compensated, thanks
to some N750 0603 caps that are special-ordered from Japan, almost
impossible to get in reasonable quantities and time frames. Some
copper was moved on the latest board rev, more copper close to the L,
and now the center frequency and TC are different, so I was wondering
if copper proximity could explain the difference.
Oh, does anybody know of a place to get stock surface-mount NTC caps?
We can software compensate out the new TC, so the thing works, but I'd
rather fix the inherent oscillator TC. If we apply a
linear-with-temperature compensation voltage to the varicap, we can
get a zero TC near room temp. But the varicap has its own, nasty TC
that varies with capacitance, so this compensation winds up with a
parabolic TC curve. OK, we could add a compensating polynomial in the
software, if we didn't mind spending a week or so getting that right.
RF requires a lot of patience.
John
else nearby. What would be the differential effect on inductance of
adding a ground plane on the opposite side of the board? Even more
important, how might it affect the TC?
I've got a 50 MHz oscillator, using a 150 nH inductor, some fixed
caps, a Maxim flecap (coarse tune), and a varicap (fine tune.) Last
board rev, we had it almost perfectly temperature compensated, thanks
to some N750 0603 caps that are special-ordered from Japan, almost
impossible to get in reasonable quantities and time frames. Some
copper was moved on the latest board rev, more copper close to the L,
and now the center frequency and TC are different, so I was wondering
if copper proximity could explain the difference.
Oh, does anybody know of a place to get stock surface-mount NTC caps?
We can software compensate out the new TC, so the thing works, but I'd
rather fix the inherent oscillator TC. If we apply a
linear-with-temperature compensation voltage to the varicap, we can
get a zero TC near room temp. But the varicap has its own, nasty TC
that varies with capacitance, so this compensation winds up with a
parabolic TC curve. OK, we could add a compensating polynomial in the
software, if we didn't mind spending a week or so getting that right.
RF requires a lot of patience.
John