Testing a varactor for capacitance while under use?
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From: Dave on 27 Apr 2010 09:00 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:igpct519cs4bi17sufurrgase317glt124(a)4ax.com... > 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 > 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
From: Dave on 27 Apr 2010 09:04 "Phil Allison" <phil_a(a)tpg.com.au> wrote in message news:83n80mFgfoU1(a)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 > > > > > > 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
From: John Larkin on 27 Apr 2010 10:07 On Tue, 27 Apr 2010 14:37:32 +1000, "Phil Allison" <phil_a(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. > > > DMM cap ranges are usually pretty bad. This is excellent: http://www.aade.com/lcmeter.htm John
From: Jitt on 27 Apr 2010 10:46 In article <FLqdneaC_50oQkvWnZ2dnUVZ_gidnZ2d(a)posted.internetamerica>, db5151(a)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 --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: Phil Allison on 27 Apr 2010 11:55
"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. ** Yawnnnn... Assuming (hate that word).. ** No - you absolutely love it. Cos it is all you ever do. .... Phil |