Depends on how you look at it. On the high end, dV/dC is a lot
higher than it is on the low end.
Varactor-tuned oscillator
Mar 18, 2006
81 Replies
ROFLOL.
Thanks,
- Win
On Sun, 19 Mar 2006 17:01:49 +0100, "Bill Sloman" Gave us:
Damn! Looks like you put that retarded fucktard in his place, eh!
None shall pass!
formatting link
Sure, but it would be interesting to have a varactor whose c-v graph curves ever upwards. Too bad they're all the opposite.
John
"John Fields" schreef in bericht news: snipped-for-privacy@4ax.com...
Then there is dV/dF, how does that look like over the entire range, assuming no stray capacitance...
Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)
Some of the hyperabrupt varactors have doping profiles that give nicely linear dF/dV over a goodly range.
John
"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...
Then reverse your varactor so as to run current through it. More current, more capacitance :-)
Thanks,
Fred.
I don't think so, since if you forward bias it it'll turn into a
resistor.
--- Good thing they are, though, or the universe as we know it would end.
To get around it, and get the slope to go in the direction you want, just put an inverting opamp in front of it.
+--[R]--+ | | Vc>---[R]--+--|-\ | | >-+--[R]--+ VREF>---------|+/ | [VARACTOR] | GND-- John Fields Professional Circuit Designer
It can\'t.
The highest capacitance occurs when the diode\'s depletion region is
the thinnest, which is at the low voltage end of the scale and, as
reverse voltage across the diode is increased, the thickness of the
depletion region will increase, increasing the "distance" between
the capacitor plates, lowering the capacitor\'s capacitance. The
relationship between capacitance change VS voltage change is nearly
logarithmic through most of the varactor\'s range and flattens out
near the diode\'s high-voltage limit, indicating that the greatest
capacitance change per unit voltage will occur at the high
capacitance - low voltage end.
_Any_ diode\'s capacitance will decrease as the reverse voltage
across it is increased, so it would be a simple matter to hook up a
1N400X to a power supply and actually measure delta C VS delta V and
put the matter to rest.
"John Fields" schreef in bericht news: snipped-for-privacy@4ax.com...
I think John wants to apply 8kV for 20 picoseconds, turning it into a 4700uF capacitor for a brief moment, or something like that.
Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)
You really think he will notice?
Bill Sloman, Nijmegen
With the 8kV supply looking like a quarter and the pulse being
applied to the varactor glued to it as they both free-fall through
the field of a superconducting magnet? ;)
Sounds kind of like that movie with Jodie Foster in it: "Contact".
"John Fields"
** Look - another reptile pokes its ugly head out .............. Phil
My pleasure. :-)
"John Fields" "Phil Allison"
** It does - you ass.See the BBY40 diode curve.
formatting link
The OP can *easily* diminish the C of his varactor diode by another 1 pF with additional bias voltage.
If he can ALSO diminish the stray parallel C to around 15 pF, then the minimum possible C drops from 20 to 19 pF.
This equates to a frequency increase of 2.5 % or 2.7 MHz at the top of the FM band !!!
Makes it possible to tune onto another 13 FM station spots.
If you knew ANYTHING about "varicap" FM tuners, then you might figure out why using the low capacitance end of the diode's range is very desirable.
....... Phil
On Sun, 19 Mar 2006 17:42:39 -0600, John Fields Gave us:
Did you ever get around to seeing "What The Bleep Do We Know", John?
You won't be disappointed.
It must be a *very* short film.
I doubt it.
Keith
"Phil Allison" schreef in bericht news: snipped-for-privacy@individual.net...
the
30 years ago, when I was ~18, I've repaired TV's for a while. Philips used a 33V zener, TAA550. The channel selector was made of simple switches with multiturn pots behind it. They went all the way up to 33V to get rid of the last possible pf of the varicap.
Thanks, Frank.
(remove \'q\' and \'.invalid\' when replying by email)
"Frank Bemelman"
** Fair enough - since the TV bands cover such a wide frequency range.OTOH, the FM broadcast band is relatively quite narrow.
....... Phil
Join the Discussion
Have something to add? Share your thoughts — no account required.
Didn't find your answer?
Ask the community — no account required