Varactor bias

[Jason James]

Is it correct to say that a varactor-diode's bias is voltage dependent only,
with only rev-bias leakage current consumed from the bias network?

If you were coupling a 2v p-p AF signal,..would it be so that provided the
bias is greater than a volt,..the thing would work OK,..or is there a
linearity consideration where the V-diode has to be biassed at a set
voltage?

Tks,...Jason

 

[gcd]

Hi,
not sure what you're trying to do here.

Leakage current for a varicap is bound to be in the order of 10s to 100s of
nano amps, not a lot of leakage.

Varicaps are used in reverse bias and part of an oscillator cct, thus the
tuning voltage is typically applied through an inductor from a low
resistance drive. The dc control voltage being generated by a PLL phase
detector and loop filter.

Coupling a signal suggests a capacitor to couple the signal, ie ac coupled
onto a dc bias. If you were to do this the ac signal will have a mean
voltage of the dc bias, ie if the dc bias was 1 volt and the ac coupled
signal was 2v p-p then the reverse volts on the diode would go down to zero
and go up to 2v. You would not want to operate a varicap down that low,
perhaps say a minimum of 1/2v on a 10v diode,. check the datasheet of the
unit you want to use.

And of course, freq will not linearly change with the applied voltage, the
capacitance will ( assuming you stay within the linear part of the curve for
the diode) but not frequency.

Hope that's of some help ?

Greg



"Jason James" <vellicet@bigpond.com> wrote in message
news:2p7dosFhp03aU1@uni-berlin.de...

Is it correct to say that a varactor-diode's bias is voltage dependent
only,
with only rev-bias leakage current consumed from the bias network?

If you were coupling a 2v p-p AF signal,..would it be so that provided the
bias is greater than a volt,..the thing would work OK,..or is there a
linearity consideration where the V-diode has to be biassed at a set
voltage?

Tks,...Jason

 

[Jason James]

"gcd" <gcdmelbnoSPAM@austarmetro.com.au> wrote in message
news:cgsdvf$1t6n$1@austarmetro.com.au...

Hi,
not sure what you're trying to do here.

Leakage current for a varicap is bound to be in the order of 10s to 100s
of
nano amps, not a lot of leakage.

So they draw little current and can be biassed from a divider network (?)

Varicaps are used in reverse bias and part of an oscillator cct, thus the
tuning voltage is typically applied through an inductor from a low
resistance drive. The dc control voltage being generated by a PLL phase
detector and loop filter.

Coupling a signal suggests a capacitor to couple the signal, ie ac coupled
onto a dc bias. If you were to do this the ac signal will have a mean
voltage of the dc bias, ie if the dc bias was 1 volt and the ac coupled
signal was 2v p-p then the reverse volts on the diode would go down to
zero
and go up to 2v. You would not want to operate a varicap down that low,
perhaps say a minimum of 1/2v on a 10v diode,. check the datasheet of the
unit you want to use.

It was going to be an FM modulator with a multiplier stage next, to not only
increase exc-freq but increase deviation as well. My exp on FM Txs is
non-existent as you can see..

I liked the DSE wireless mike which used the single transistor osc output C
changes consistent with Ic changes to generate FM.

And of course, freq will not linearly change with the applied voltage, the
capacitance will ( assuming you stay within the linear part of the curve
for
the diode) but not frequency.

Hope that's of some help ?

Greg

Thanx,...it has been,...Jason

 

[gcd]

Hi,

Yes you can use a divider network to adjust the voltage and so adjust the
frequency manually.

Not being part of a PLL means the stability will not be the best but it will
work. just have an inductor in the lead out to the varicap to ensure the
bias network doesnt affect the Q of the oscillator.

Ignore my last sentence in that post, it's not quite right - i was writing
that late last night and gibberish filled my head

Cheers
Greg
---------
g c d [delete everything else] A.T. austarmetro.com.au

"Jason James" <vellicet@bigpond.com> wrote in message
news:2pe6ouFjsvggU1@uni-berlin.de...

"gcd" <gcdmelbnoSPAM@austarmetro.com.au> wrote in message
news:cgsdvf$1t6n$1@austarmetro.com.au...
Hi,
not sure what you're trying to do here.

Leakage current for a varicap is bound to be in the order of 10s to 100s
of
nano amps, not a lot of leakage.

So they draw little current and can be biassed from a divider network (?)



Varicaps are used in reverse bias and part of an oscillator cct, thus
the
tuning voltage is typically applied through an inductor from a low
resistance drive. The dc control voltage being generated by a PLL phase
detector and loop filter.

Coupling a signal suggests a capacitor to couple the signal, ie ac
coupled
onto a dc bias. If you were to do this the ac signal will have a mean
voltage of the dc bias, ie if the dc bias was 1 volt and the ac coupled
signal was 2v p-p then the reverse volts on the diode would go down to
zero
and go up to 2v. You would not want to operate a varicap down that low,
perhaps say a minimum of 1/2v on a 10v diode,. check the datasheet of
the
unit you want to use.

It was going to be an FM modulator with a multiplier stage next, to not
only
increase exc-freq but increase deviation as well. My exp on FM Txs is
non-existent as you can see..

I liked the DSE wireless mike which used the single transistor osc output
C
changes consistent with Ic changes to generate FM.

And of course, freq will not linearly change with the applied voltage,
the
capacitance will ( assuming you stay within the linear part of the curve
for
the diode) but not frequency.

Hope that's of some help ?

Greg

Thanx,...it has been,...Jason