Forums

Critic My Ltspice Model

Started by amdx November 29, 2011
  Sorry if this is a repost, I didn't see my first post.

  I'm looking for constructive criticism to create a better model and
help me with the concept I present about the stray capacitance.

The concept: the stray capacitance doesn't lower the input impedance
it just adds to the resonating capacitor, changing the resonant frequency.
  The 100meg is supposed to isolate the power supply. (normally very 
loose inductive coupling is used)
The 950K is just to provide a voltage divider to show me the input 
impedance.
  The experiment then removes the the 950k and places a 4pf stray 
capacitance to ground.
     Thanks, Mikek

Version 4
SHEET 1 1100 680
WIRE -192 112 -288 112
WIRE -80 112 -112 112
WIRE 0 112 -80 112
WIRE 80 112 0 112
WIRE 320 112 160 112
WIRE 448 112 384 112
WIRE 544 112 448 112
WIRE -288 128 -288 112
WIRE -80 128 -80 112
WIRE 0 128 0 112
WIRE 448 144 448 112
WIRE 544 144 544 112
WIRE -288 224 -288 208
WIRE -80 224 -80 208
WIRE -32 224 -80 224
WIRE 0 224 0 192
WIRE 0 224 -32 224
WIRE 544 224 544 208
WIRE -32 240 -32 224
FLAG -288 224 0
FLAG 544 224 0
FLAG 448 224 0
FLAG -32 240 0
FLAG 256 288 0
SYMBOL voltage -288 112 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value2 AC 4
SYMATTR InstName V1
SYMATTR Value SINE(0 20 1e6)
SYMBOL cap 528 144 R0
SYMATTR InstName C2
SYMATTR Value 4.8p
SYMBOL cap 320 96 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value .3p
SYMBOL res -208 96 M90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 100e6
SYMBOL cap -16 128 R0
SYMATTR InstName C4
SYMATTR Value 101p
SYMBOL ind -64 224 R180
WINDOW 0 -15 89 Left 2
WINDOW 3 -42 28 Left 2
SYMATTR InstName L1
SYMATTR Value 240u
SYMATTR SpiceLine Rser=.1
SYMBOL res 432 128 R0
SYMATTR InstName R1
SYMATTR Value 20meg
SYMBOL res 176 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 950k
TEXT -336 280 Left 2 !.ac lin 1e6 995000 1050000
TEXT 176 96 Left 2 ;A
TEXT 48 96 Left 2 ;B
TEXT -200 8 Left 2 ;The voltage at point A is 1/2 the voltage at point 
B, \nSo looking into the .3pf cap the impedance is about 950k.
TEXT -224 376 Left 2 ;If I remove the 950k and put a 4pf stray cap to 
ground it would just add\nto the 101pf cap in the resonating circuit. So 
I don't see how the\nstray capacitance will lower the input impedance 
when used to\n measure an LC circuit.
TEXT 48 464 Left 2 ;Except for the capacitor's loss resistance.
TEXT 256 128 Left 2 ;4pf\ns\nt\nr\na\ny
On Tue, 29 Nov 2011 09:21:48 -0600, amdx <amdx@knologynotthis.net>
wrote:

> Sorry if this is a repost, I didn't see my first post. > > I'm looking for constructive criticism to create a better model and >help me with the concept I present about the stray capacitance. > >The concept: the stray capacitance doesn't lower the input impedance >it just adds to the resonating capacitor, changing the resonant frequency. > The 100meg is supposed to isolate the power supply. (normally very >loose inductive coupling is used) >The 950K is just to provide a voltage divider to show me the input >impedance. > The experiment then removes the the 950k and places a 4pf stray >capacitance to ground. > Thanks, Mikek >
Why not do the traditional Q-meter thing, namely a peak rectifier? A schottky diode, a cap, and a DVM would be all you need. Something like a 0.5 or 0.2 pF schottky maybe. The Q-loading of the DVM would be tiny. John
Why don't you put pencil to paper and CALCULATE how each element
affects the impedance, rather than hacking away at it with a
simulator?  You might actually learn something ;-)

On Tue, 29 Nov 2011 09:21:48 -0600, amdx <amdx@knologynotthis.net>
wrote:

> Sorry if this is a repost, I didn't see my first post. > > I'm looking for constructive criticism to create a better model and >help me with the concept I present about the stray capacitance. > >The concept: the stray capacitance doesn't lower the input impedance >it just adds to the resonating capacitor, changing the resonant frequency. > The 100meg is supposed to isolate the power supply. (normally very >loose inductive coupling is used) >The 950K is just to provide a voltage divider to show me the input >impedance. > The experiment then removes the the 950k and places a 4pf stray >capacitance to ground. > Thanks, Mikek > >Version 4 >SHEET 1 1100 680 >WIRE -192 112 -288 112 >WIRE -80 112 -112 112 >WIRE 0 112 -80 112 >WIRE 80 112 0 112 >WIRE 320 112 160 112 >WIRE 448 112 384 112 >WIRE 544 112 448 112 >WIRE -288 128 -288 112 >WIRE -80 128 -80 112 >WIRE 0 128 0 112 >WIRE 448 144 448 112 >WIRE 544 144 544 112 >WIRE -288 224 -288 208 >WIRE -80 224 -80 208 >WIRE -32 224 -80 224 >WIRE 0 224 0 192 >WIRE 0 224 -32 224 >WIRE 544 224 544 208 >WIRE -32 240 -32 224 >FLAG -288 224 0 >FLAG 544 224 0 >FLAG 448 224 0 >FLAG -32 240 0 >FLAG 256 288 0 >SYMBOL voltage -288 112 R0 >WINDOW 123 24 124 Left 2 >WINDOW 39 0 0 Left 2 >SYMATTR Value2 AC 4 >SYMATTR InstName V1 >SYMATTR Value SINE(0 20 1e6) >SYMBOL cap 528 144 R0 >SYMATTR InstName C2 >SYMATTR Value 4.8p >SYMBOL cap 320 96 M90 >WINDOW 0 0 32 VBottom 2 >WINDOW 3 32 32 VTop 2 >SYMATTR InstName C3 >SYMATTR Value .3p >SYMBOL res -208 96 M90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R2 >SYMATTR Value 100e6 >SYMBOL cap -16 128 R0 >SYMATTR InstName C4 >SYMATTR Value 101p >SYMBOL ind -64 224 R180 >WINDOW 0 -15 89 Left 2 >WINDOW 3 -42 28 Left 2 >SYMATTR InstName L1 >SYMATTR Value 240u >SYMATTR SpiceLine Rser=.1 >SYMBOL res 432 128 R0 >SYMATTR InstName R1 >SYMATTR Value 20meg >SYMBOL res 176 96 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R3 >SYMATTR Value 950k >TEXT -336 280 Left 2 !.ac lin 1e6 995000 1050000 >TEXT 176 96 Left 2 ;A >TEXT 48 96 Left 2 ;B >TEXT -200 8 Left 2 ;The voltage at point A is 1/2 the voltage at point >B, \nSo looking into the .3pf cap the impedance is about 950k. >TEXT -224 376 Left 2 ;If I remove the 950k and put a 4pf stray cap to >ground it would just add\nto the 101pf cap in the resonating circuit. So >I don't see how the\nstray capacitance will lower the input impedance >when used to\n measure an LC circuit. >TEXT 48 464 Left 2 ;Except for the capacitor's loss resistance. >TEXT 256 128 Left 2 ;4pf\ns\nt\nr\na\ny
...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On 11/29/2011 9:43 AM, Jim Thompson wrote:
> Why don't you put pencil to paper and CALCULATE how each element > affects the impedance, rather than hacking away at it with a > simulator? You might actually learn something ;-) >
Yes simulators need to be used properly. In the wrong hands, a simulator can be used like a slot machine. With complex circuits, chances are the slot machine will pay off more than the simulator.
On Tue, 29 Nov 2011 09:21:48 -0600, amdx wrote:

> Sorry if this is a repost, I didn't see my first post. > > I'm looking for constructive criticism to create a better model and > help me with the concept I present about the stray capacitance. > > The concept: the stray capacitance doesn't lower the input impedance it > just adds to the resonating capacitor, changing the resonant frequency. > The 100meg is supposed to isolate the power supply. (normally very > loose inductive coupling is used) > The 950K is just to provide a voltage divider to show me the input > impedance. > The experiment then removes the the 950k and places a 4pf stray > capacitance to ground. > Thanks, Mikek >
Mikek (or is it really Mike K.? -- if you grew up in an English-speaking country I'd expect the latter, but "Mikek" sounds like a valid name in a Slavic or other "language I don't know" sort of way). You are suffering from a confusion of terms. The term "impedance" means the overall effect of resistance and reactance at a given frequency. So the stray capacitance at the amplifier input does, indeed, lower the impedance, because of the added susceptance of the cap. If, however, you replace "impedance" with "loading", and be sure to pay attention to resonance, then the extra capacitance affects the loading barely at all compared to if it wasn't there. The reason is because the conductance looking into the 0.3pF gimmick cap is low (it is limited by that cap's reactance); adding in the susceptance of the stray capacitance shifts the resonance, but the conductance stays largely the same (although it is slightly affected by the operating frequency shift), so the loading stays largely the same (and low). -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Tue, 29 Nov 2011 10:43:57 -0700, Jim Thompson wrote:

> Why don't you put pencil to paper and CALCULATE how each element affects > the impedance, rather than hacking away at it with a simulator? You > might actually learn something ;-)
Oh, Jim. Where's your sense of the Modern Way? Mikek -- he's right. You could churn and turn on a simulation for a week, and not learn as much as you'd learn spending an hour doing this symbolically with a pencil and paper. Calculate the complex admittance (that's the reciprocal of the impedance) of the circuit looking into the amplifier, with the stray capacitance as a variable. Now look at how the real part of that admittance changes (or doesn't) with changes in stray capacitance, and how that real part of the admittance changes with the (slight) changes in resonant frequency induced by the amplifier input capacitance. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Tue, 29 Nov 2011 13:02:29 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Tue, 29 Nov 2011 10:43:57 -0700, Jim Thompson wrote: > >> Why don't you put pencil to paper and CALCULATE how each element affects >> the impedance, rather than hacking away at it with a simulator? You >> might actually learn something ;-) > >Oh, Jim. Where's your sense of the Modern Way?
I probably use a simulator more than anyone else here. And I've been doing it since 1977. But, anytime I hit a quirk, I fall-back to hand calculations to make sure I understand what's going on.
> >Mikek -- he's right. You could churn and turn on a simulation for a >week, and not learn as much as you'd learn spending an hour doing this >symbolically with a pencil and paper. > >Calculate the complex admittance (that's the reciprocal of the impedance) >of the circuit looking into the amplifier, with the stray capacitance as >a variable. Now look at how the real part of that admittance changes (or >doesn't) with changes in stray capacitance, and how that real part of the >admittance changes with the (slight) changes in resonant frequency >induced by the amplifier input capacitance.
Wonder if "mikek" knows how to do an AC sweep (in LTspice) and observe real and imaginary parts (or magnitude and phase)? ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Tue, 29 Nov 2011 13:02:31 -0700, Jim Thompson wrote:

> On Tue, 29 Nov 2011 13:02:29 -0600, Tim Wescott <tim@seemywebsite.com> > wrote: > >>On Tue, 29 Nov 2011 10:43:57 -0700, Jim Thompson wrote: >> >>> Why don't you put pencil to paper and CALCULATE how each element >>> affects the impedance, rather than hacking away at it with a >>> simulator? You might actually learn something ;-) >> >>Oh, Jim. Where's your sense of the Modern Way? > > I probably use a simulator more than anyone else here. And I've been > doing it since 1977. > > But, anytime I hit a quirk, I fall-back to hand calculations to make > sure I understand what's going on.
Yup. Simulators are such seductive tools to the beginner, because they seem so useful. "Seem", of course, being the operative word. If you know exactly what the limitations of the simulation are, they're very handy -- but if you can't do the basic work by hand first, then they ain't gonna help!! -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Tue, 29 Nov 2011 13:02:29 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Tue, 29 Nov 2011 10:43:57 -0700, Jim Thompson wrote: > >> Why don't you put pencil to paper and CALCULATE how each element affects >> the impedance, rather than hacking away at it with a simulator? You >> might actually learn something ;-) > >Oh, Jim. Where's your sense of the Modern Way? > >Mikek -- he's right. You could churn and turn on a simulation for a >week, and not learn as much as you'd learn spending an hour doing this >symbolically with a pencil and paper. > >Calculate the complex admittance (that's the reciprocal of the impedance) >of the circuit looking into the amplifier, with the stray capacitance as >a variable. Now look at how the real part of that admittance changes (or >doesn't) with changes in stray capacitance, and how that real part of the >admittance changes with the (slight) changes in resonant frequency >induced by the amplifier input capacitance.
It's sure easier to simulate that than do all the math. I often simulate DC stuff, like voltage dividers, rather than grinding it out with a calculator. As far as training your intuition goes, getting feedback in a few seconds is a lot better than getting feedback after minutes of calculations. John