Testing a varactor for capacitance while under use?
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From: Dave on 26 Apr 2010 23:24 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
From: George Herold on 26 Apr 2010 23:59 On Apr 26, 11:24 pm, "Dave" <db5...(a)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.
From: John Larkin on 27 Apr 2010 00:21 On Mon, 26 Apr 2010 22:24:59 -0500, "Dave" <db5151(a)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
From: Phil Allison on 27 Apr 2010 00:37 "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
From: Dave on 27 Apr 2010 08:59
"George Herold" <ggherold(a)gmail.com> wrote in message news:3f643fd4-2fab-447c-9e92-2c25d180be64(a)5g2000yqj.googlegroups.com... On Apr 26, 11:24 pm, "Dave" <db5...(a)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... |