Testing a varactor for capacitance while under use?

D

Dave

Guest
Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter across the varactor while it is tuned to the desired point
would show what capacitance it had at that point, but this would essentially
be hooking the meter between a positive voltage and ground, and that would
not be good for the meter... If anyone has any ideas, I would love to hear
them. I'm at a loss, and really don't want to blow my meter.

Thanks,

Dave
 
On Mon, 26 Apr 2010 22:24:59 -0500, "Dave" <db5151@hotmail.com> wrote:

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter across the varactor while it is tuned to the desired point
would show what capacitance it had at that point, but this would essentially
be hooking the meter between a positive voltage and ground, and that would
not be good for the meter... If anyone has any ideas, I would love to hear
them. I'm at a loss, and really don't want to blow my meter.

Thanks,

Dave
Click to expand...
Do you mean a C-meter? Ohm it out to see if it has a DC path. If it
does, just add a healthy series cap to block DC.

John
 
"Dave"

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter
Click to expand...
** Meter - what meter is that then ?

Few capacitance meters work when there are other components wired across the
cap under test.

Eg, the cap meter function on my DMM will not read the capacitance of my
scope probe when the scope is connected.



.... Phil
 
On Apr 26, 11:24 pm, "Dave" <db5...@hotmail.com> wrote:
Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit?  I would think that
hooking the meter across the varactor while it is tuned to the desired point
would show what capacitance it had at that point, but this would essentially
be hooking the meter between a positive voltage and ground, and that would
not be good for the meter...  If anyone has any ideas, I would love to hear
them.  I'm at a loss, and really don't want to blow my meter.

Thanks,

Dave
Click to expand...
How about putting it in the tuned circuit and hitting it with a
pulse... see at what frequency it rings at... you have a 'scope?

George H.
 
"George Herold" <ggherold@gmail.com> wrote in message
news:3f643fd4-2fab-447c-9e92-2c25d180be64@5g2000yqj.googlegroups.com...
On Apr 26, 11:24 pm, "Dave" <db5...@hotmail.com> wrote:
Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter across the varactor while it is tuned to the desired
point
would show what capacitance it had at that point, but this would
essentially
be hooking the meter between a positive voltage and ground, and that would
not be good for the meter... If anyone has any ideas, I would love to hear
them. I'm at a loss, and really don't want to blow my meter.

Thanks,

Dave
Click to expand...
How about putting it in the tuned circuit and hitting it with a
pulse... see at what frequency it rings at... you have a 'scope?

George H.

Hey George,

Thanks for the reply. I do have a scope, and I am already hitting it with a
pulse and tuning the circuit to increase the received pulse's amplitude, but
the signal disappears into the noise as I go up the frequency range and I am
wondering how close to my actual desired frequency the circuit is tuning to
before the signal disappears. Problem is I don't know what the capacitance
of the varactor is at any given point, and I am operating solely with the
inductance of the whip antenna and connecting wire due to the constraints of
the frequency I am trying to pick up (10KHz). My calculations tell me I
only need a few picohenries to make a resonant circuit at the required
frequency, and I believe the whip and connecting wire more than provide for
that.

Open to ideas, and appreciate the response.

Dave

PS: am now thinking that it is 60Hz noise that I am fighting, and am
thinking that if I ground the antenna somehow I can eliminate this. Will
work on that...
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:igpct519cs4bi17sufurrgase317glt124@4ax.com...
On Mon, 26 Apr 2010 22:24:59 -0500, "Dave" <db5151@hotmail.com> wrote:

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter across the varactor while it is tuned to the desired
point
would show what capacitance it had at that point, but this would
essentially
be hooking the meter between a positive voltage and ground, and that would
not be good for the meter... If anyone has any ideas, I would love to
hear
them. I'm at a loss, and really don't want to blow my meter.

Thanks,

Dave


Do you mean a C-meter? Ohm it out to see if it has a DC path. If it
does, just add a healthy series cap to block DC.

John
Click to expand...
Hey John,

Yes, either my LCR meter or my DMM, which has a capacitance test feature.
And I already am blocking DC at this point, so that I am only working with
the signal (and the noise that surrounds us all.)

Thanks,

Dave
 
"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:83n80mFgfoU1@mid.individual.net...
"Dave"

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter

** Meter - what meter is that then ?

Few capacitance meters work when there are other components wired across
the cap under test.

Eg, the cap meter function on my DMM will not read the capacitance of my
scope probe when the scope is connected.



... Phil
Click to expand...
Hmmm. My Mastech LCR meter has properly read the capacitance of caps still
in the circuit, and I believe my (can't remember the nameof the brand) DMM
has also. Assuming (hate that word) this is the case, is there any way to
measure the capacitance of a varactor while it is in use?

Thanks,

Dave
 
On Tue, 27 Apr 2010 14:37:32 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"Dave"

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter

** Meter - what meter is that then ?

Few capacitance meters work when there are other components wired across the
cap under test.

Eg, the cap meter function on my DMM will not read the capacitance of my
scope probe when the scope is connected.
Click to expand...
DMM cap ranges are usually pretty bad.

This is excellent:

http://www.aade.com/lcmeter.htm

John
 
In article
<FLqdneaC_50oQkvWnZ2dnUVZ_gidnZ2d@posted.internetamerica>,
db5151@hotmail.com says...
Thanks for the reply. I do have a scope, and I am already hitting it with a
pulse and tuning the circuit to increase the received pulse's amplitude, but
the signal disappears into the noise as I go up the frequency range and I am
wondering how close to my actual desired frequency the circuit is tuning to
before the signal disappears. Problem is I don't know what the capacitance
of the varactor is at any given point, and I am operating solely with the
inductance of the whip antenna and connecting wire due to the constraints of
the frequency I am trying to pick up (10KHz). My calculations tell me I
only need a few picohenries to make a resonant circuit at the required
frequency, and I believe the whip and connecting wire more than provide for
that.

Open to ideas, and appreciate the response.

Dave

PS: am now thinking that it is 60Hz noise that I am fighting, and am
thinking that if I ground the antenna somehow I can eliminate this. Will
work on that...




Picohenries? 10KHz? Fishy
Click to expand...

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
"Dave"

Hmmm. My Mastech LCR meter has properly read the capacitance of caps
still in the circuit, and I believe my (can't remember the nameof the
brand) DMM has also.
Click to expand...

** Yawnnnn...

Assuming (hate that word)..


** No - you absolutely love it.

Cos it is all you ever do.



.... Phil
 
"Jitt"

Picohenries? 10KHz? Fishy
Click to expand...

** Eeeeeyuup ....

We have ourselves a genuine live one here - folks.

Needs to be kept on the hook long as possible.




..... Phil
 
On Apr 27, 8:59 am, "Dave" <db5...@hotmail.com> wrote:
"George Herold" <ggher...@gmail.com> wrote in message

news:3f643fd4-2fab-447c-9e92-2c25d180be64@5g2000yqj.googlegroups.com...
On Apr 26, 11:24 pm, "Dave" <db5...@hotmail.com> wrote:

Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit? I would think that
hooking the meter across the varactor while it is tuned to the desired
point
would show what capacitance it had at that point, but this would
essentially
be hooking the meter between a positive voltage and ground, and that would
not be good for the meter... If anyone has any ideas, I would love to hear
them. I'm at a loss, and really don't want to blow my meter.

Thanks,

Dave

How about putting it in the tuned circuit and hitting it with a
pulse... see at what frequency it rings at... you have a 'scope?

George H.

Hey George,

Thanks for the reply.  I do have a scope, and I am already hitting it with a
pulse and tuning the circuit to increase the received pulse's amplitude, but
the signal disappears into the noise as I go up the frequency range and I am
wondering how close to my actual desired frequency the circuit is tuning to
before the signal disappears.  Problem is I don't know what the capacitance
of the varactor is at any given point, and I am operating solely with the
inductance of the whip antenna and connecting wire due to the constraints of
the frequency I am trying to pick up (10KHz).  My calculations tell me I
only need a few picohenries to make a resonant circuit at the required
frequency, and I believe the whip and connecting wire more than provide for
that.

Open to ideas, and appreciate the response.

Dave

PS: am now thinking that it is 60Hz noise that I am fighting, and am
thinking that if I ground the antenna somehow I can eliminate this.  Will
work on that...
Click to expand...
Hi Dave, I don't know much about antenna's and such. I don't think
you can ground the antenna though. If there is low frequency crap
getting in you can try a high pass filter.

The varactor spec sheet should give you some idea of the C as a
function of V. I was suggesting that if you wanted to measure C(V)
that you could let it ring with a known L and figure out C.. or look
at some RC time with a known R.

You might try some other variable caps see if you can get that to work
and then go back to the varactors once you have a better idea of the C
value you need.

George H.
 
On Apr 26, 8:24 pm, "Dave" <db5...@hotmail.com> wrote:
Does anyone know if one can test a varactor to see what capacitance it is
exhibiting while it is being used to tune a circuit?
Click to expand...
Characterize the varactor's C-V curve out of circuit, then
monitor the (average, DC bias) voltage on the varactor
(which can be done without disturbing the AC operating conditions).

Some varactor uses (frequency doubling) with high AC
excitation can't be usefully probed this way. That's because
the 'capacitance' is a linear characteristic, and varactors
are intrinsically not linear devices.
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top