when is a capacitor not (quite) a capacitor
Hi all - anyone seen or been bitten by this?? I'm working on a middling-sized circuit, nothing fancy, all passives and library ADI opamps, inamps and diff drivers.? Implementing a little gain and (as you'd expect from me) some filtering functions, including some series LC traps close to the inputs.? It's one of those circuits that's on the edge of not finding a DC op point (24.0.12 on this work machine, 24.1.x deffo not ready for salaried work IMHO).? Series traps are from signal nodes to ground.? So there's a capacitor on the branch between that node and ground.? So in principle it cannot participate in any DC op point calculations.? But...
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I get to the point where, slowly dropping down the inductor's series resistor (most tests with internal but external seemed similar), I would cross a threshold where it couldn't find an op point.? Quite abruptly; for instance, Rs >=65 gave an op point, Rs <=64 and lower did not. (alternate solver; didn't test normal).? So, clearly, the NAM must have some DC path across that cap.? Tried changing the Gmin setting from default 1E-12 to 1E-14.? Success!? Now all realistic configurations give me an operating point.
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This is something that could be fixed (hi @mstokowski and @mborn) - by making sure that Gmin==0 across any capacitor that's not otherwise set up to be different, _just_ during the op point search.? Of course the DC solver needs to be robust to hanging branches so there might need to be some topology trim/prep, solving a slightly different matrix (the caps in series case will probably be irritating).? So not trivial, just simple... I have a sneaking feeling that some of my older op point troubles could have been fixed up this way.
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Another way of doing this would be to have a flag for a capacitor to explicitly set Re (shunt admittance) to zero - but that's much more work for the user.? Better for the code to do it inherently (perhaps with a setting choice box).
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Of course 24.1.5+ might already have fixed this, I can't try that until at least this evening.
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Hope this is interesting and relevant -- K
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PS (1) how do you stop the .log file from popping up at the end of each sim now, annoying and (2) where did the little display of the key for which .step plotted to which color trace go?
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Re: AC Analysis - show Mag in dBuV versus Freuqncy
This is very OT, but dBm is very widely
applied to 50 ohm circuits in the RF field.
On 2025-04-14 12:10, Jerry Lee Marcel
via groups.io wrote:
No, it's not. dBm applies only to 600 ohm circuits. If you
want independant on load, you must use dBu (or dBV).
--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only
If something is true: * as far as we know - it's science *for
certain - it's mathematics *unquestionably - it's religion
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Re: AC Analysis - show Mag in dBuV versus Freuqncy
No, it's not. dBm applies only to 600 ohm circuits. If you want independant on load, you must use dBu (or dBV).
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Re: AC Analysis - show Mag in dBuV versus Freuqncy
To be very precise, 0.775 V is approximately equivalent
to 1mW in 600 ohms, which is sqrt(600x0.001) = 0.774596...
That is the explanation of the voltage value.
On 2025-04-14 11:39, Goran Finnberg via
groups.io wrote:
John Woodgate:
>dBu? is decibels referred to 0.775 V, which is 1 mW in
600 ohms.
0
dBU is not 1mW in 600 ohms.
0 dBU is 0,775 V irrespective of impedance.
0 dBm is 1 mW in 600 ohms.
--------------------
Best regards,
Goran Finnberg
The Mastering Room AB
Goteborg
Sweden
E-mail: mastering@...
Learn from the mistakes of others, you can never live long
enough to
make them all yourself.??? -?? John Luther
(\__/)
(='.'=)
(")_(") Pyret, Ranglet, Aron, VovVov, Nero, Smurfen &
Pussin:RIP
--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only
If something is true: * as far as we know - it's science *for
certain - it's mathematics *unquestionably - it's religion
|
Re: AC Analysis - show Mag in dBuV versus Freuqncy
Bruce:
>0 dBm is 1mW independent of the load resistance. https://en.wikipedia.org/wiki/DBm
In audio and telephony, dBm is typically referenced relative to the 600-ohm??commonly? used in telephone voice networks, while in radio-frequency work dBm is typically referenced relative? to a 50-ohm impedance.
-------
Best regards,
Goran Finnberg
The Mastering Room AB
Goteborg
Sweden
E-mail: mastering@...
Learn from the mistakes of others, you can never live long enough to
make them all yourself.??? -?? John Luther
(\__/)
(='.'=)
(")_(") Pyret, Ranglet, Aron, VovVov, Nero, Smurfen & Pussin:RIP
|
Re: AC Analysis - show Mag in dBuV versus Freuqncy
0 dBm is 1mW independent of the load resistance.
?
Bruce
On 14/04/2025 22:39 NZST Goran Finnberg via groups.io <mastering@...> wrote:
?
?
John Woodgate:
?
>dBu? is decibels referred to 0.775 V, which is 1 mW in 600 ohms.
? 0 dBU is not 1mW in 600 ohms.
?
0 dBU is 0,775 V irrespective of impedance.
?
0 dBm is 1 mW in 600 ohms.
?
?
--------------------
Best regards,
Goran Finnberg
The Mastering Room AB
Goteborg
Sweden
E-mail: mastering@...
Learn from the mistakes of others, you can never live long enough to
make them all yourself.??? -?? John Luther
(\__/)
(='.'=)
(")_(") Pyret, Ranglet, Aron, VovVov, Nero, Smurfen & Pussin:RIP
|
Re: AC Analysis - show Mag in dBuV versus Freuqncy
Le 14/04/2025 ¨¤ 01:02, Zivko Petru via groups.io a ¨¦crit?:
In LT spice AC analysis You are instructed to use 1Vac as input.
Actually, 1Vac is meaningless. Could be 1V rms, 1V peak or 1V peak-to-peak. The dB value is always referred to the input voltage, whatever the unit.
In order to have the results expressed in dBuV, the amplitude must be 1u.
?
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dbu on the other hand is referenced to 1 uV.
dBu is an audio measurement reference, corresponding to an rms value of 0.775V
dB referred to 1uV is dBuV
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Re: AC Analysis - show Mag in dBuV versus Freuqncy
John Woodgate:
>dBu? is decibels referred to 0.775 V, which is 1 mW in 600 ohms. 0 dBU is not 1mW in 600 ohms.
0 dBU is 0,775 V irrespective of impedance.
0 dBm is 1 mW in 600 ohms.
--------------------
Best regards,
Goran Finnberg
The Mastering Room AB
Goteborg
Sweden
E-mail: mastering@...
Learn from the mistakes of others, you can never live long enough to
make them all yourself.??? -?? John Luther
(\__/)
(='.'=)
(")_(") Pyret, Ranglet, Aron, VovVov, Nero, Smurfen & Pussin:RIP
|
Re: Creating a dirt-simple subcircuit component
FYI - not a bad idea to use a custom,?small symbol for a 0 V voltage source, to use either as an ammeter or a net isolator.? Other group members did that already.? Easy to make your own.? Of course it adds the overhead of a custom symbol, which you need to remember to include when sending someone your schematic.
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Andy
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Re: Creating a dirt-simple subcircuit component
There are dynamic range issues with very small resistors.
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Pre-LTspice, I saw simulations with very small resistors where voltages drifted a few volts beyond the supply rails.
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Andy
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Re: Creating a dirt-simple subcircuit component
On Sun, Apr 13, 2025 at 04:11 PM, John Woodgate wrote:
Why not?
On 2025-04-13 23:58, eetech00 via groups.io wrote:
I don't recommend the use of tiny valued resistor as jumpers.
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Because a zero valued voltage sources' behavior is basically transparent to the simulator.
A small valued resistor in not. It still exhibits behavior in the circuit as resistor and could
affect high frequency simulations or behavior of models that contain tiny valued parasitic components.
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Re: AC Analysis - show Mag in dBuV versus Freuqncy
dBu? is decibels referred to 0.775 V, which is
1 mW in 600 ohms. dB?V, or better dB(?V), is decibels referred
to 1?V.
On 2025-04-14 00:02, Zivko Petru via
groups.io wrote:
?
dbu on the other hand is referenced to 1 uV.
--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only
If something is true: * as far as we know - it's science *for
certain - it's mathematics *unquestionably - it's religion
|
Re: Creating a dirt-simple subcircuit component
Why not?
On 2025-04-13 23:58, eetech00 via
groups.io wrote:
I don't recommend
the use of tiny valued resistor as jumpers.
--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only
If something is true: * as far as we know - it's science *for
certain - it's mathematics *unquestionably - it's religion
|
Re: AC Analysis - show Mag in dBuV versus Freuqncy
Hello Ray.Koo,
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Since you are looking for attenuation characteristic of the filter,? your base reference is immaterial.
Namely it could be dB with respect to any value... say boiled but not peeled potatos .
It is just a ratio value of input versus output or the other way around output versus input with a change in the sign in front of the number.
The is the wisdom of dB or B? or dBu or dBm or dBc or dBa with respect to an appropriate? reference.
In LT spice AC analysis You are instructed to use 1Vac as input. Therefore the output will be specifies in dB with respect to that 1Vac.
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dbu on the other hand is referenced to 1 uV. As such it specifies voltage ratio not attenuation characteristic ( usual use of dBu and dBm as absolute value references ).
?So enjoy your results in dB and recognise it as the correct value, a ratio of input to output? not a signal level with respect to1 uV or 1 mW into 50 ohm or 1mW into 600 ohm.
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Re: Creating a dirt-simple subcircuit component
Hi
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I have two "jumpers" in my library both made from a single zero voltage source.
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One has a "shorting link" graphic.
One has a "current probe" graphic.
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I don't recommend the use of tiny valued resistor as jumpers.
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Re: Creating a dirt-simple subcircuit component
That¡¯s what I am using¡
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Show quoted text
From: [email protected] <[email protected]>
On Behalf Of BRUCE108
Sent: Sunday, April 13, 2025 2:45 PM
To: [email protected]
Subject: EXTERNAL: Re: [LTspice] Creating a dirt-simple subcircuit component
?
You can use a 0 Volt source as a jumper. This allows differen netnames on each side with 0 vols difference across the source. My distribution has a jumper2 symbol which is not a sub-circuit, but a custom symbol for a 0 Volt source.
The contens of the .asy file are:
Version 4
SymbolType CELL
LINE Normal 24 64 32 64
LINE Normal -24 64 -32 64
CIRCLE Normal -16 60 -24 68
CIRCLE Normal 16 60 24 68
ARC Normal -20 44 20 84 20 60 -20 60
SYMATTR Prefix V
SYMATTR Value 0
SYMATTR Description A wire jumper that has different node names for each terminal
PIN -32 64 NONE 0
PINATTR PinName +
PINATTR SpiceOrder 1
PIN 32 64 NONE 0
PINATTR PinName -
PINATTR SpiceOrder 2
Prefix V with a value 0 is a 0 volt source between two points
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Re: Creating a dirt-simple subcircuit component
You can use a 0 Volt source as a jumper. This allows differen netnames on each side with 0 vols difference across the source. My distribution has a jumper2 symbol which is not a sub-circuit, but a custom symbol for a 0 Volt source.
?
The contens of the .asy file are:
?
Version 4
SymbolType CELL
LINE Normal 24 64 32 64
LINE Normal -24 64 -32 64
CIRCLE Normal -16 60 -24 68
CIRCLE Normal 16 60 24 68
ARC Normal -20 44 20 84 20 60 -20 60
SYMATTR Prefix V
SYMATTR Value 0
SYMATTR Description A wire jumper that has different node names for each terminal
PIN -32 64 NONE 0
PINATTR PinName +
PINATTR SpiceOrder 1
PIN 32 64 NONE 0
PINATTR PinName -
PINATTR SpiceOrder 2
?
Prefix V with a value 0 is a 0 volt source between two points
?
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Re: Creating a dirt-simple subcircuit component
You can have zero ohm resistor in LTspice, but it doesn't do what you might expect.
If you use one, LTspice just ignores the symbol and the netlist shows that it replaced the resistor with a wire, so both "ends" are the same net.
-- Regards, Tony?
You cannot have a zero-ohm resistor in LTspice.? I've created a jumper with a 1f ohm resistance called MyJmpr.? The simple subcircuit is assigned it to the original jumper symbol and saved as MyJmpr.asy.? I have uploaded it to the Temp files as MyJmpr.zip.? I do like the idea of using a zero-volt voltage source.? You could do the same as I have done with the resistor and assign that as a subcircuit to the original jumper assembly and save the result as a new jumper.
John
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Re: Creating a dirt-simple subcircuit component
Yep!
I don¡¯r really like the Jumper symbol, so made it one as a resistor-sized box, marked ¡°0 ¦¸¡±
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Show quoted text
From: [email protected] <[email protected]>
On Behalf Of John Waugaman via groups.io
Sent: Sunday, April 13, 2025 11:32 AM
To: [email protected]
Subject: EXTERNAL: Re: [LTspice] Creating a dirt-simple subcircuit component
?
You cannot have a zero-ohm resistor in LTspice.? I've created a jumper with a 1f ohm resistance called MyJmpr.? The simple subcircuit is assigned it to the original jumper symbol and saved as MyJmpr.asy.? I have uploaded it to the Temp
files as MyJmpr.zip.? I do like the idea of using a zero-volt voltage source.? You could do the same as I have done with the resistor and assign that as a subcircuit to the original jumper assembly and save the result as a new jumper.
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Re: Creating a dirt-simple subcircuit component
You cannot have a zero-ohm resistor in LTspice.? I've created a jumper with a 1f ohm resistance called MyJmpr.? The simple subcircuit is assigned it to the original jumper symbol and saved as MyJmpr.asy.? I have uploaded it to the Temp files as MyJmpr.zip.? I do like the idea of using a zero-volt voltage source.? You could do the same as I have done with the resistor and assign that as a subcircuit to the original jumper assembly and save the result as a new jumper.
John
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John Woodgate
Bell, Dave
Tony Casey
John Waugaman