Gain peaking in driven shield
in [Design]
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From: Joerg on 6 May 2010 16:32 Michael A. Terrell wrote: > Joerg wrote: >> John Larkin wrote: >>> On Wed, 5 May 2010 19:57:02 -0700 (PDT), George Herold >>> <ggherold(a)gmail.com> wrote: >>> >>>> On May 5, 7:45 pm, John Larkin >>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> [...] >> >>>>> What sort of coax are you using? Can you reduce its capacitance? >>>> There is a cable which has ~12" of RG174 (? not sure that's the right >>>> number, small coax) and in the probe there is ~8" of SS coax from >>>> Lakeside cryo. I'm pretty much stuck with the cable I have. >>> You can make low-C coax, like they use in scope probes, with a tiny >>> wire inside a biggish shield, with some low density spacer like spiral >>> monofilament or something. >>> >> Can't see George's posts because he uses gmail but this may help: Older >> passive car antennas (sans integrated preamp) had very low capacitance >> coax in order not to detune the first AM filter and not to snuff out too >> much signal. But I am not sure where that can be bought without the >> antenna. Autoparts stores used to have it but that was >20 years ago. > > > It is nothing more than RG/62, 93 ohm coax. > The stuff I had was a whole lot higher than 93ohms. About 1/4" diameter, super-thin inner conductor. But I had bought that in Europe and it became hard to obtain after the advent of active antennas. Technically it wasn't pure-bred coax from a performance point of view because the inner conductor wasn't fully enclosed by dielectric. It had some slack in there and was a wee bit longer per ft than the jacket, in order not to break that thin wire. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Michael A. Terrell on 6 May 2010 19:48 Joerg wrote: > > Michael A. Terrell wrote: > > Joerg wrote: > >> John Larkin wrote: > >>> On Wed, 5 May 2010 19:57:02 -0700 (PDT), George Herold > >>> <ggherold(a)gmail.com> wrote: > >>> > >>>> On May 5, 7:45 pm, John Larkin > >>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> [...] > >> > >>>>> What sort of coax are you using? Can you reduce its capacitance? > >>>> There is a cable which has ~12" of RG174 (? not sure that's the right > >>>> number, small coax) and in the probe there is ~8" of SS coax from > >>>> Lakeside cryo. I'm pretty much stuck with the cable I have. > >>> You can make low-C coax, like they use in scope probes, with a tiny > >>> wire inside a biggish shield, with some low density spacer like spiral > >>> monofilament or something. > >>> > >> Can't see George's posts because he uses gmail but this may help: Older > >> passive car antennas (sans integrated preamp) had very low capacitance > >> coax in order not to detune the first AM filter and not to snuff out too > >> much signal. But I am not sure where that can be bought without the > >> antenna. Autoparts stores used to have it but that was >20 years ago. > > > > > > It is nothing more than RG/62, 93 ohm coax. > > > > The stuff I had was a whole lot higher than 93ohms. About 1/4" diameter, > super-thin inner conductor. But I had bought that in Europe and it > became hard to obtain after the advent of active antennas. I used to repair car radios when I was a kid. (late '60s) The coax said RG/62. A Delco engineer told me it was 93 ohms. RG/62 has a very thin center conductor in a polypropylene tube, and the center conductor is a very thin wire that zig-zags it way inside of the polypropylene tube. > Technically it wasn't pure-bred coax from a performance point of view > because the inner conductor wasn't fully enclosed by dielectric. The dielectric is mostly air. The polypropylene tube was intended to keep the braid's shape by pressing it against the outer jacket. Just like some CATV hardline with ceramic rings between conductors, or air filled hardline used for high power AM broadcast transmitters. > It had some slack in there and was a wee bit longer per ft than the > jacket, in order not to break that thin wire. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
From: Joerg on 6 May 2010 20:35 Michael A. Terrell wrote: > Joerg wrote: >> Michael A. Terrell wrote: >>> Joerg wrote: >>>> John Larkin wrote: >>>>> On Wed, 5 May 2010 19:57:02 -0700 (PDT), George Herold >>>>> <ggherold(a)gmail.com> wrote: >>>>> >>>>>> On May 5, 7:45 pm, John Larkin >>>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> [...] >>>> >>>>>>> What sort of coax are you using? Can you reduce its capacitance? >>>>>> There is a cable which has ~12" of RG174 (? not sure that's the right >>>>>> number, small coax) and in the probe there is ~8" of SS coax from >>>>>> Lakeside cryo. I'm pretty much stuck with the cable I have. >>>>> You can make low-C coax, like they use in scope probes, with a tiny >>>>> wire inside a biggish shield, with some low density spacer like spiral >>>>> monofilament or something. >>>>> >>>> Can't see George's posts because he uses gmail but this may help: Older >>>> passive car antennas (sans integrated preamp) had very low capacitance >>>> coax in order not to detune the first AM filter and not to snuff out too >>>> much signal. But I am not sure where that can be bought without the >>>> antenna. Autoparts stores used to have it but that was >20 years ago. >>> >>> It is nothing more than RG/62, 93 ohm coax. >>> >> The stuff I had was a whole lot higher than 93ohms. About 1/4" diameter, >> super-thin inner conductor. But I had bought that in Europe and it >> became hard to obtain after the advent of active antennas. > > > I used to repair car radios when I was a kid. (late '60s) The coax > said RG/62. A Delco engineer told me it was 93 ohms. RG/62 has a very > thin center conductor in a polypropylene tube, and the center conductor > is a very thin wire that zig-zags it way inside of the polypropylene > tube. > Ours looked similar, center conductor sort of meandering through there. But it was over 150ohms, we had very fancy gear at the RF institute where I had a side job designing stuff. I don't remember why I measured it but instantly wanted to buy some for myself. The institute couldn't sell me any but told me where to get it, and it was within bicycling distance. I believe the two 150ohm coaxes from this company are not twinax but not sure: http://www.generalwireproducts.com/index.php/Industrial-Twinaxial-Coax.html [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 6 May 2010 21:32 On Thu, 6 May 2010 09:48:57 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 6 Maj, 05:25, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 5 May 2010 19:57:02 -0700 (PDT), George Herold >> >> >> >> <ggher...(a)gmail.com> wrote: >> >On May 5, 7:45�pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Wed, 5 May 2010 09:26:32 -0700 (PDT), George Herold >> >> >> <ggher...(a)gmail.com> wrote: >> >> >I�m looking at the thermal noise from a resistor down the bottom of a >> >> >probe. �Cable and probe are maybe 1/2 a meter long. �I added a shield >> >> >line to try and reduce the capacitance between the �active� end of the >> >> >resistor and ground. �(The other end of the resistor is tied to ground >> >> >at the bottom of the probe.) >> >> >> >The driven shield seemed to work great at the higher impedance levels >> >> >1Meg and 100k ohms. (Though a more careful examination showed there >> >> >were some issues.) �When I tried 10k ohms there was some serious gain >> >> >peaking at the higher frequencies...above 100kHz. �I mucked about a >> >> >bit and made sure this wasn�t the common problem of an opamp driving a >> >> >capacitive load. >> >> >> >Late yesterday it struck me that there is capacitive coupling from the >> >> >shield back to the input. �I had been mistakenly thinking of the >> >> >shield as only a capacitance to ground. �The capacitance of the inner >> >> >conductor to the ground (Cig) is 60pF, from the inner conductor to the >> >> >shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF. >> >> >> >I was first using an opamp follower to drive the shield, but later >> >> >added a bit of gain...and then threw it away. >> >> >> > � � �+-----Cis--+ >> >> > � � �| � � � � �| >> >> > � � �| �|\ � � �| >> >> >+-----+--+ \ � � | >> >> >| � � � �| �>----+---+----+ >> >> >| � � �+-- / OPA | � | � �| >> >> >Rmeas. | |/ �134 | � R1 � Csg >> >> >Rmeas. | � � � � | � R1 � | >> >> >| � � �+---------+ � | � �| >> >> >| � � � � � � � � � GND � GND >> >> >GND >> >> >> >R1 was 50 ohms (to get rid of Csg ringing) >> >> >And then this, >> >> >> > � � �+-----Cis--+ >> >> > � � �| � � � � �| >> >> > � � �| �|\ � � �| >> >> >+-----+--+ \ � � | >> >> >| � � � �| �>----+-R4R4--+----+ >> >> >| � � �+-- / OPA | � � � | � �| >> >> >Rmeas. | |/ �134 | � � � R5 � Csg >> >> >Rmeas. | � � � � | � � � R5 � | >> >> >| � � �+--R3R3---+ � � � | � �| >> >> >| � � �| � � � � � � � � GND �GND >> >> >GND � �R2 >> >> > � � � R2 >> >> > � � � | >> >> > � � � GND >> >> >> >Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms. >> >> >> >This seems like it must be a known problem and I wondered if there are >> >> >any simple solutions. �I thought that a bit of inductance (L = >> >> >Cis*Rmeas^2) in the right place might help, but I only managed to make >> >> >a nice oscillator. >> >> >> >Thanks for any help or advice, >> >> >George H. >> >> >> You're trying to measure Johnson noise, right? >> >> >Yup, The gain peaking is a real pain. >> >> >> One problem with any such guarding/bootstrapping scheme is that there >> >> is a room-temp amplifier that picks up the signal and drives the >> >> guard, and it has noise of its own. In some cases that makes adding >> >> the guard a losing battle. >> >> >10 k ohms has 40nV/rtHz at room Temp...IIRC. >> >> Closer to 13. >> >> >The opa134 has 8nV of voltage noise. >> >> 10K makes 8 nV at around 100 Kelvin. >> >> >> >> >> The OPA134 may be a bit slow, especially driving Csg. >> >> >8 MHz GBP. �I only need it to work up to 1 MHz or so. �Do you have a >> >better opamp in mind? >> >> How about a BF862 jfet? 0.8 nv/rthz and very low current noise. No >> opamp can do that. Cheap, too. >> > >how low, doesn't something like ADA4898 get close? > > >-Lasse That's basically an LT1028 clone, with huge front-end bipolars. The price is a lot of current noise, 2.4 pA/rthz. Dump that into 10K and you have 24 nV/rthz. John
From: George Herold on 6 May 2010 21:38
On May 6, 9:54 am, MooseFET <kensm...(a)rahul.net> wrote: > On May 5, 9:26 am, George Herold <ggher...(a)gmail.com> wrote: > [....] > > > > > +-----Cis--+ > > | | > > | |\ | > > +-----+--+ \ | > > | | >----+---+----+ > > | +-- / OPA | | | > > Rmeas. | |/ 134 | R1 Csg > > Rmeas. | | R1 | > > | +---------+ | | > > | GND GND > > GND > > If the cable is very long, the simple capacitance model of the > cable fails. You need to start dealing with the inductance > too. Yeah, I've only got about 0.5m of cable at a few MHz max. Once the cable becomes a tranmission line you can't 'do' the driven shield trick. Can you? > > I have used the LSK170 to make a very low noise non-inverting gain > to make this work a little better. The noise put onto the cable > by your shield drive starts to win at some high frequency. > > --------------- > ! ! > [R] U1 [V] > ! !\ ! > !---+--------! >----+--+---- > in ---->! !/ ! > !---+--[R]------------- > ! R2 > [R] > ! R1 > > IIRC R1=300, R2=30 U1=gain of 100 Wow, not sure I really get that circuit. You've got the Jfet as a 'follower' and I assume I should hang the shield on the bottom of R1... (Correct me when I'm wrong) along with some resistance (?) to sink the current. The amp U1 is feeding the bottom of a voltage source and R which looks like a current source. But I'm not sure what R2 is doing? > > It really matters that the gain be 1.00 and the phase be 0.00 at the > frequency you need to measure. In my case, I have the option of > changing > the circuit between measurement bands. The circuit had a roll off at > about F*100 and F/100 The phase shift at high frequencies is certainly troublesome... Playing around with LTspice tonight it seemed that the peaking happened when I made the Cig capacitance too small. (Cig is only shown in Bill Sloman's correction to my circuit diagram.) Perhaps if the gain is less than 1.00 the gain peaking will be smaller? I'm being too greedy by driving the shield too hard. George H. > > > > > > > R1 was 50 ohms (to get rid of Csg ringing) > > And then this, > > > +-----Cis--+ > > | | > > | |\ | > > +-----+--+ \ | > > | | >----+-R4R4--+----+ > > | +-- / OPA | | | > > Rmeas. | |/ 134 | R5 Csg > > Rmeas. | | R5 | > > | +--R3R3---+ | | > > | | GND GND > > GND R2 > > R2 > > | > > GND > > > Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms. > > > This seems like it must be a known problem and I wondered if there are > > any simple solutions. I thought that a bit of inductance (L = > > Cis*Rmeas^2) in the right place might help, but I only managed to make > > a nice oscillator. > > > Thanks for any help or advice, > > George H.- Hide quoted text - > > - Show quoted text - |