Varactor diode recommendation

[Paul Burridge]

Hi,

Can anyone recommend a suitable varactor? I've had a look through
Farnell's catalogue and nothing there has enough tuning range (maximum
quoted is about 8.5 ratio). I need 12+ with most of the Cvar taking
place in the 0 to -10V range and ideally covering 3 to 50pf. Rseries
not too critical ~1ohm will do. Too much to ask? You tell me.

Thanks for any suggestions (obtainable in the UK),

p.

 

[Tim Wescott]

8.5:1 is a pretty high ratio, I think you should be happy with it.

I assume you're building some sort of insane VCO with the World's Widest
Frequency Range -- why not just design for a swing of a bit more than 2:1
and put in a programmable 1:2:4:8 divider?

 

[Rene Tschaggelar]

Hi,

Can anyone recommend a suitable varactor? I've had a look through
Farnell's catalogue and nothing there has enough tuning range (maximum
quoted is about 8.5 ratio). I need 12+ with most of the Cvar taking
place in the 0 to -10V range and ideally covering 3 to 50pf. Rseries
not too critical ~1ohm will do. Too much to ask? You tell me.

What frequency range did you have in mind Paul ?

Rene

 

[Paul Burridge]

8.5:1 is a pretty high ratio, I think you should be happy with it.

Um, yeah, but I'm not.

I assume you're building some sort of insane VCO with the World's Widest
Frequency Range -- why not just design for a swing of a bit more than 2:1
and put in a programmable 1:2:4:8 divider?

I'm seeking a simpler solution. At a frequency (ceramic resonator
based oscillator) of 8.1Mhz., I require a tuning range of 70khz about
the centre frequency (35khz either side of 8.1Mhz, in other words).
Doesn't sound much, but the osc.is already pulled 100khz over its
fundamental of 8Mhz.

I've heard of these varactors termed "hyper-abrupt" type which
apparently have a very steep graphical plot between applied Vr and
junction capacitance (ie. small change in voltage bias causes large
change in capacitance) but haven't been able to turn up a source for
them. :-(

 

[Paul Burridge]

What frequency range did you have in mind Paul ?

Hi Rene,

See my follow up to Tim. 8.1Mhz +/- 35khz.

Thanks,
p

 

[Tom Twist]

Um, yeah, but I'm not.

I'm seeking a simpler solution. At a frequency (ceramic resonator
based oscillator) of 8.1Mhz., I require a tuning range of 70khz about
the centre frequency (35khz either side of 8.1Mhz, in other words).
Doesn't sound much, but the osc.is already pulled 100khz over its
fundamental of 8Mhz.

I've heard of these varactors termed "hyper-abrupt" type which
apparently have a very steep graphical plot between applied Vr and
junction capacitance (ie. small change in voltage bias causes large
change in capacitance) but haven't been able to turn up a source for
them. :-(

Paul

The diodes with the best tuning range I have seen is Philips
Semiconductors BB146 and BB147.

BB146 is for UHF, has a range of 17.5 - 23, typical 20

BB147 is for VHF, has a range of 35 - 43, typical 40

They were described in a press release 17. jan 1997.
But BB146 is now obsolete.

You can download Philips RF manual here:

http://www.semiconductors.philips.com/acrobat/other/discretes/philips_rf_manual_3rd_edition.pdf

A candidate is BB132 for VHF, range 26 for 0.5V - 28V

You can buy it from Elfa, www.elfa.se , article no: 70-326-91

Good luck

Tom

ps What is your application?

 

[John Larkin]

Hi Rene,

See my follow up to Tim. 8.1Mhz +/- 35khz.

Thanks,
p

Well, in a narrowband situation like this, you can put some inductance
in parallel with the varicap and achieve an infinite effective
capacitance ratio. More than infinite, actually, as you can swing from
net inductive to net capacitive as far as you like. Say, take a
varicap that can go 200-1200 pf; null out 700 pf with inductance;
what's left is effectively +-500 pF.

John

 

[Tim Wescott]

So your two posts were related. It's too bad you can't just get an 8.1MHz
resonator - then your problem would be easy to solve.

Most of the wide range varactors are hyper-abrupt, but I think the term has
lost it's marketing importance. Motorola used to make them; if they don't
still then On probably does. There's also Zetex (http://www.zetex.com).

I've seen references to widebanding crystals by resonating the package
capacitance with a suitable inductor; in theory this leaves you with
something that just looks like a series resonator. This should help your
ability to pull the frequency -- you ought to be able to pull it above the
parallel resonant point of the resonator, but you'll have problems with
temperature drift and other nasty effects which will increase the more you
thumb your nose at the resonator.

 

[gwhite]

Hi,

Can anyone recommend a suitable varactor? I've had a look through
Farnell's catalogue and nothing there has enough tuning range (maximum
quoted is about 8.5 ratio). I need 12+ with most of the Cvar taking
place in the 0 to -10V range and ideally covering 3 to 50pf. Rseries
not too critical ~1ohm will do. Too much to ask? You tell me.

Thanks for any suggestions (obtainable in the UK),

SMV1255

www.alphaind.com