Forums

S-parameter curves in LTspice

Started by Jeroen Belleman May 9, 2014
Call me dense, but after many years of using LTspice,
during which many times I inserted return-loss bridges
into my circuits to measure S11, someone pointed out
to me that LTspice has a directive for that.

Just insert ".net V1 v(out)" somewhere into the schematic,
and an S11 trace will become available.

I'm still missing some of the finer points however: The
resulting S11 only makes sense with the directive above.
The docs seem to say that it should be ".net v(out) V1",
but then the resulting S11 makes no sense that I can see.
With the latter form, I also get the three missing S-
parameters, as well as H, Z and Y parameters. S21 makes
no sense either. I don't know yet about the others.
If I insert ".net I(R1) V1", with R1 the output load, S11
is OK again. I'm puzzled. Maybe someone can shed some
light on this?

For the curious, below is the circuit I've been playing
with. It's a constant impedance O(6) Bessel low-pass
filter. It's more an exercise in filter synthesis than
a practical design...

Jeroen Belleman

Version 4
SHEET 1 1468 680
WIRE -96 -80 -144 -80
WIRE 16 -80 -32 -80
WIRE 64 -80 16 -80
WIRE 176 -80 144 -80
WIRE 288 -80 256 -80
WIRE 400 -80 288 -80
WIRE 464 -80 400 -80
WIRE 608 -80 544 -80
WIRE 736 -80 688 -80
WIRE 848 -80 736 -80
WIRE 896 -80 848 -80
WIRE 1104 -80 976 -80
WIRE 16 -32 16 -80
WIRE 288 -32 288 -80
WIRE 400 -32 400 -80
WIRE 1104 -32 1104 -80
WIRE 736 -16 736 -80
WIRE 848 -16 848 -80
WIRE 16 96 16 48
WIRE 288 96 288 32
WIRE 400 96 400 48
WIRE 736 96 736 48
WIRE 848 96 848 64
WIRE 1104 96 1104 48
WIRE -144 176 -144 -80
WIRE -144 176 -496 176
WIRE -16 176 -144 176
WIRE 160 176 64 176
WIRE 256 176 160 176
WIRE 544 176 336 176
WIRE 592 176 544 176
WIRE 896 176 672 176
WIRE 1008 176 896 176
WIRE 1056 176 1008 176
WIRE 1056 192 1056 176
WIRE -496 208 -496 176
WIRE 160 208 160 176
WIRE 544 208 544 176
WIRE 896 208 896 176
WIRE 160 288 160 272
WIRE 544 288 544 272
WIRE 896 288 896 272
WIRE 1056 288 1056 272
WIRE -496 304 -496 288
FLAG 1056 288 0
FLAG 896 288 0
FLAG 544 288 0
FLAG 160 288 0
FLAG -496 304 0
FLAG 1008 176 out
FLAG 1104 96 0
FLAG 848 96 0
FLAG 736 96 0
FLAG 400 96 0
FLAG 288 96 0
FLAG 16 96 0
SYMBOL ind -32 192 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L1
SYMATTR Value 1.5124
SYMATTR SpiceLine Rser=0
SYMBOL ind 240 192 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L2
SYMATTR Value .8125
SYMATTR SpiceLine Rser=0
SYMBOL ind 576 192 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L3
SYMATTR Value .3785
SYMATTR SpiceLine Rser=0
SYMBOL cap 144 208 R0
SYMATTR InstName C1
SYMATTR Value 1.0329
SYMBOL cap 528 208 R0
SYMATTR InstName C2
SYMATTR Value .6072
SYMBOL cap 880 208 R0
SYMATTR InstName C3
SYMATTR Value .1287
SYMBOL res 1040 176 R0
SYMATTR InstName R1
SYMATTR Value 1
SYMBOL voltage -496 192 R0
WINDOW 3 -185 164 Left 2
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PULSE(0 1 1 1m 1m 1m)
SYMATTR Value2 ac 1
SYMATTR InstName V1
SYMBOL cap -32 -96 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value {1/1.06998}
SYMBOL res 0 -48 R0
SYMATTR InstName R2
SYMATTR Value 1
SYMBOL ind 48 -64 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L4
SYMATTR Value 2.44663
SYMATTR SpiceLine Rser=0
SYMBOL res 272 -96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 1.61635
SYMBOL cap 272 -32 R0
SYMATTR InstName C5
SYMATTR Value .501225
SYMBOL res 384 -48 R0
SYMATTR InstName R4
SYMATTR Value {1/.23388}
SYMBOL res 704 -96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 1.0087
SYMBOL ind 448 -64 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L5
SYMATTR Value 1.67931
SYMATTR SpiceLine Rser=0
SYMBOL ind 880 -64 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L6
SYMATTR Value .944529
SYMATTR SpiceLine Rser=0
SYMBOL res 1120 64 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R6
SYMATTR Value 3.55786
SYMBOL cap 720 -16 R0
SYMATTR InstName C6
SYMATTR Value .330091
SYMBOL res 832 -32 R0
SYMATTR InstName R7
SYMATTR Value {1/.397832}
TEXT 240 352 Left 2 !.ac dec 100 10m 10
TEXT 240 392 Left 2 !;tran 10
TEXT 248 304 Left 2 !.net v1 v(out)
On Fri, 09 May 2014 17:25:00 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

>Call me dense, but after many years of using LTspice, >during which many times I inserted return-loss bridges >into my circuits to measure S11, someone pointed out >to me that LTspice has a directive for that. > >Just insert ".net V1 v(out)" somewhere into the schematic, >and an S11 trace will become available. > >I'm still missing some of the finer points however: The >resulting S11 only makes sense with the directive above. >The docs seem to say that it should be ".net v(out) V1", >but then the resulting S11 makes no sense that I can see. >With the latter form, I also get the three missing S- >parameters, as well as H, Z and Y parameters. S21 makes >no sense either. I don't know yet about the others. >If I insert ".net I(R1) V1", with R1 the output load, S11 >is OK again. I'm puzzled. Maybe someone can shed some >light on this? > >For the curious, below is the circuit I've been playing >with. It's a constant impedance O(6) Bessel low-pass >filter. It's more an exercise in filter synthesis than >a practical design... > >Jeroen Belleman
I have no idea what that LTspice directive does, but maybe these appnotes will shed some light... <http://www.analog-innovations.com/CreateS-ParameterSubcircuitsForSpice.pdf>
> >Version 4 >SHEET 1 1468 680 >WIRE -96 -80 -144 -80 >WIRE 16 -80 -32 -80 >WIRE 64 -80 16 -80 >WIRE 176 -80 144 -80 >WIRE 288 -80 256 -80 >WIRE 400 -80 288 -80 >WIRE 464 -80 400 -80 >WIRE 608 -80 544 -80 >WIRE 736 -80 688 -80 >WIRE 848 -80 736 -80 >WIRE 896 -80 848 -80 >WIRE 1104 -80 976 -80 >WIRE 16 -32 16 -80 >WIRE 288 -32 288 -80 >WIRE 400 -32 400 -80 >WIRE 1104 -32 1104 -80 >WIRE 736 -16 736 -80 >WIRE 848 -16 848 -80 >WIRE 16 96 16 48 >WIRE 288 96 288 32 >WIRE 400 96 400 48 >WIRE 736 96 736 48 >WIRE 848 96 848 64 >WIRE 1104 96 1104 48 >WIRE -144 176 -144 -80 >WIRE -144 176 -496 176 >WIRE -16 176 -144 176 >WIRE 160 176 64 176 >WIRE 256 176 160 176 >WIRE 544 176 336 176 >WIRE 592 176 544 176 >WIRE 896 176 672 176 >WIRE 1008 176 896 176 >WIRE 1056 176 1008 176 >WIRE 1056 192 1056 176 >WIRE -496 208 -496 176 >WIRE 160 208 160 176 >WIRE 544 208 544 176 >WIRE 896 208 896 176 >WIRE 160 288 160 272 >WIRE 544 288 544 272 >WIRE 896 288 896 272 >WIRE 1056 288 1056 272 >WIRE -496 304 -496 288 >FLAG 1056 288 0 >FLAG 896 288 0 >FLAG 544 288 0 >FLAG 160 288 0 >FLAG -496 304 0 >FLAG 1008 176 out >FLAG 1104 96 0 >FLAG 848 96 0 >FLAG 736 96 0 >FLAG 400 96 0 >FLAG 288 96 0 >FLAG 16 96 0 >SYMBOL ind -32 192 R270 >WINDOW 0 32 56 VTop 2 >WINDOW 3 5 56 VBottom 2 >SYMATTR InstName L1 >SYMATTR Value 1.5124 >SYMATTR SpiceLine Rser=0 >SYMBOL ind 240 192 R270 >WINDOW 0 32 56 VTop 2 >WINDOW 3 5 56 VBottom 2 >SYMATTR InstName L2 >SYMATTR Value .8125 >SYMATTR SpiceLine Rser=0 >SYMBOL ind 576 192 R270 >WINDOW 0 32 56 VTop 2 >WINDOW 3 5 56 VBottom 2 >SYMATTR InstName L3 >SYMATTR Value .3785 >SYMATTR SpiceLine Rser=0 >SYMBOL cap 144 208 R0 >SYMATTR InstName C1 >SYMATTR Value 1.0329 >SYMBOL cap 528 208 R0 >SYMATTR InstName C2 >SYMATTR Value .6072 >SYMBOL cap 880 208 R0 >SYMATTR InstName C3 >SYMATTR Value .1287 >SYMBOL res 1040 176 R0 >SYMATTR InstName R1 >SYMATTR Value 1 >SYMBOL voltage -496 192 R0 >WINDOW 3 -185 164 Left 2 >WINDOW 123 24 124 Left 2 >WINDOW 39 0 0 Left 2 >SYMATTR Value PULSE(0 1 1 1m 1m 1m) >SYMATTR Value2 ac 1 >SYMATTR InstName V1 >SYMBOL cap -32 -96 R90 >WINDOW 0 0 32 VBottom 2 >WINDOW 3 32 32 VTop 2 >SYMATTR InstName C4 >SYMATTR Value {1/1.06998} >SYMBOL res 0 -48 R0 >SYMATTR InstName R2 >SYMATTR Value 1 >SYMBOL ind 48 -64 R270 >WINDOW 0 32 56 VTop 2 >WINDOW 3 5 56 VBottom 2 >SYMATTR InstName L4 >SYMATTR Value 2.44663 >SYMATTR SpiceLine Rser=0 >SYMBOL res 272 -96 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R3 >SYMATTR Value 1.61635 >SYMBOL cap 272 -32 R0 >SYMATTR InstName C5 >SYMATTR Value .501225 >SYMBOL res 384 -48 R0 >SYMATTR InstName R4 >SYMATTR Value {1/.23388} >SYMBOL res 704 -96 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R5 >SYMATTR Value 1.0087 >SYMBOL ind 448 -64 R270 >WINDOW 0 32 56 VTop 2 >WINDOW 3 5 56 VBottom 2 >SYMATTR InstName L5 >SYMATTR Value 1.67931 >SYMATTR SpiceLine Rser=0 >SYMBOL ind 880 -64 R270 >WINDOW 0 32 56 VTop 2 >WINDOW 3 5 56 VBottom 2 >SYMATTR InstName L6 >SYMATTR Value .944529 >SYMATTR SpiceLine Rser=0 >SYMBOL res 1120 64 R180 >WINDOW 0 36 76 Left 2 >WINDOW 3 36 40 Left 2 >SYMATTR InstName R6 >SYMATTR Value 3.55786 >SYMBOL cap 720 -16 R0 >SYMATTR InstName C6 >SYMATTR Value .330091 >SYMBOL res 832 -32 R0 >SYMATTR InstName R7 >SYMATTR Value {1/.397832} >TEXT 240 352 Left 2 !.ac dec 100 10m 10 >TEXT 240 392 Left 2 !;tran 10 >TEXT 248 304 Left 2 !.net v1 v(out)
...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 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 09/05/14 17:33, Jim Thompson wrote:
> On Fri, 09 May 2014 17:25:00 +0200, Jeroen Belleman > <jeroen@nospam.please> wrote: > >> Call me dense, but after many years of using LTspice, >> during which many times I inserted return-loss bridges >> into my circuits to measure S11, someone pointed out >> to me that LTspice has a directive for that. >> >> Just insert ".net V1 v(out)" somewhere into the schematic, >> and an S11 trace will become available. >> >> I'm still missing some of the finer points however: The >> resulting S11 only makes sense with the directive above. >> The docs seem to say that it should be ".net v(out) V1", >> but then the resulting S11 makes no sense that I can see. >> With the latter form, I also get the three missing S- >> parameters, as well as H, Z and Y parameters. S21 makes >> no sense either. I don't know yet about the others. >> If I insert ".net I(R1) V1", with R1 the output load, S11 >> is OK again. I'm puzzled. Maybe someone can shed some >> light on this? >>[...] >> Jeroen Belleman > > I have no idea what that LTspice directive does, but maybe these > appnotes will shed some light... > > <http://www.analog-innovations.com/CreateS-ParameterSubcircuitsForSpice.pdf>
Ah, there's an even simpler return-loss bridge in there than what I've been using. Thanks. In the mean time, I've been perusing some LTspice courses and hints, and they seem to prefer the ".net I(R1) V1" form, which indeed worked for me too. Presumably they had the same problem. ;-) Frankly, I don't even get why the *output* load should appear in there at all. S11 is the reflection from the *input*. Jeroen belleman
>"Jeroen Belleman" wrote in message news:lkis08$6i0$1@speranza.aioe.org...
>Call me dense, but after many years of using LTspice, >during which many times I inserted return-loss bridges >into my circuits to measure S11, someone pointed out >to me that LTspice has a directive for that.
>Just insert ".net V1 v(out)" somewhere into the schematic, >and an S11 trace will become available.
>I'm still missing some of the finer points however: The >resulting S11 only makes sense with the directive above.
The other option, is simply to use an alternative, but design equivalent, method of specifying circuit conditions. I do that here: http://www.kevinaylward.co.uk/ee/lna/lna.html To wit, drive the amp with the source, say 50 ohm, and make a statement on the the frequency range of say, 1db variance from -6db attenuation and, say +/-10 degs from 0, or whatever. The R.F. universe sits in a universe by itself. There is no technical reason to use and think in terms of s-parameters at all. This is a historical artefact. All design parameters can simply be reformulated in a way that ANY Spice can handle directly. So long as all blocks are consistently specified, it all cancels out in the wash. Its a bit like you yanks living in the past with your 105 deg F in TX, when the rest of the sensible world moved on to Centigrade in the last century. No one here converts to Fahrenheit to understand why its hot where jimmy T lives, we all know 45 degrees C has fried his brain a while back. Kevin Aylward B.Sc. www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice.
On Sat, 10 May 2014 09:08:38 +0100, "Kevin Aylward"
<ExtractkevinRemove@kevinaylward.co.uk> wrote:

>>"Jeroen Belleman" wrote in message news:lkis08$6i0$1@speranza.aioe.org... > >>Call me dense, but after many years of using LTspice, >>during which many times I inserted return-loss bridges >>into my circuits to measure S11, someone pointed out >>to me that LTspice has a directive for that. > >>Just insert ".net V1 v(out)" somewhere into the schematic, >>and an S11 trace will become available. > >>I'm still missing some of the finer points however: The >>resulting S11 only makes sense with the directive above. > >The other option, is simply to use an alternative, but design equivalent, >method of specifying circuit conditions. I do that here: > >http://www.kevinaylward.co.uk/ee/lna/lna.html > >To wit, drive the amp with the source, say 50 ohm, and make a statement on >the the frequency range of say, 1db variance from -6db attenuation and, say >+/-10 degs from 0, or whatever. > >The R.F. universe sits in a universe by itself. There is no technical reason >to use and think in terms of s-parameters at all. This is a historical >artefact. All design parameters can simply be reformulated in a way that ANY >Spice can handle directly. So long as all blocks are consistently specified, >it all cancels out in the wash. > >Its a bit like you yanks living in the past with your 105 deg F in TX, when >the rest of the sensible world moved on to Centigrade in the last century. >No one here converts to Fahrenheit to understand why its hot where jimmy T >lives, we all know 45 degrees C has fried his brain a while back. > >Kevin Aylward B.Sc. >www.kevinaylward.co.uk >www.anasoft.co.uk - SuperSpice.
Now be nice. 45&#2013266096;C is not hot, it's "toasty" :-D ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 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 5/10/2014 3:08 AM, Kevin Aylward wrote:
>> "Jeroen Belleman" wrote in message >> news:lkis08$6i0$1@speranza.aioe.org... > >> Call me dense, but after many years of using LTspice, >> during which many times I inserted return-loss bridges >> into my circuits to measure S11, someone pointed out >> to me that LTspice has a directive for that. > >> Just insert ".net V1 v(out)" somewhere into the schematic, >> and an S11 trace will become available. > >> I'm still missing some of the finer points however: The >> resulting S11 only makes sense with the directive above. > > The other option, is simply to use an alternative, but design > equivalent, method of specifying circuit conditions. I do that here: > > http://www.kevinaylward.co.uk/ee/lna/lna.html > > To wit, drive the amp with the source, say 50 ohm, and make a statement > on the the frequency range of say, 1db variance from -6db attenuation > and, say +/-10 degs from 0, or whatever. > > The R.F. universe sits in a universe by itself. There is no technical > reason to use and think in terms of s-parameters at all. This is a > historical artefact. All design parameters can simply be reformulated in > a way that ANY Spice can handle directly. So long as all blocks are > consistently specified, it all cancels out in the wash. > > Its a bit like you yanks living in the past with your 105 deg F in TX, > when the rest of the sensible world moved on to Centigrade in the last > century. No one here converts to Fahrenheit to understand why its hot > where jimmy T lives, we all know 45 degrees C has fried his brain a > while back. > > Kevin Aylward B.Sc. > www.kevinaylward.co.uk > www.anasoft.co.uk - SuperSpice.
I agree with you, Kevin. S11 means a lot less to me than Zin which is easy to plot in the simulation universe. However, Zin is not so easy to measure in our universe at very high frequencies, thus the S11 approach. John S
"John S"  wrote in message news:lklklc$cgd$1@dont-email.me...

On 5/10/2014 3:08 AM, Kevin Aylward wrote:
>> "Jeroen Belleman" wrote in message >> news:lkis08$6i0$1@speranza.aioe.org... > >> Call me dense, but after many years of using LTspice, >> during which many times I inserted return-loss bridges >> into my circuits to measure S11, someone pointed out >> to me that LTspice has a directive for that. > >> Just insert ".net V1 v(out)" somewhere into the schematic, >> and an S11 trace will become available. > >> I'm still missing some of the finer points however: The >> resulting S11 only makes sense with the directive above. > > The other option, is simply to use an alternative, but design > equivalent, method of specifying circuit conditions. I do that here: > > http://www.kevinaylward.co.uk/ee/lna/lna.html > > To wit, drive the amp with the source, say 50 ohm, and make a statement > on the the frequency range of say, 1db variance from -6db attenuation > and, say +/-10 degs from 0, or whatever. > >> The R.F. universe sits in a universe by itself. There is no technical >> reason to use and think in terms of s-parameters at all. This is a >> historical artefact. All design parameters can simply be reformulated in >> a way that ANY Spice can handle directly. So long as all blocks are >> consistently specified, it all cancels out in the wash. > >> Its a bit like you yanks living in the past with your 105 deg F in TX, >> when the rest of the sensible world moved on to Centigrade in the last >> century. No one here converts to Fahrenheit to understand why its hot >> where jimmy T lives, we all know 45 degrees C has fried his brain a >> while back.
>I agree with you, Kevin. S11 means a lot less to me than Zin which is easy >to plot in the simulation universe. However, Zin is not so easy to measure >in our universe at very high frequencies, thus the S11 approach.
I only live in the simulation universe now, the lower level staff can deal with the real world and report back the results :-) Kevin Aylward B.Sc. www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice.
On 5/10/2014 12:30 PM, Kevin Aylward wrote:
> "John S" wrote in message news:lklklc$cgd$1@dont-email.me... > > On 5/10/2014 3:08 AM, Kevin Aylward wrote: >>> "Jeroen Belleman" wrote in message >>> news:lkis08$6i0$1@speranza.aioe.org... >> >>> Call me dense, but after many years of using LTspice, >>> during which many times I inserted return-loss bridges >>> into my circuits to measure S11, someone pointed out >>> to me that LTspice has a directive for that. >> >>> Just insert ".net V1 v(out)" somewhere into the schematic, >>> and an S11 trace will become available. >> >>> I'm still missing some of the finer points however: The >>> resulting S11 only makes sense with the directive above. >> >> The other option, is simply to use an alternative, but design >> equivalent, method of specifying circuit conditions. I do that here: >> >> http://www.kevinaylward.co.uk/ee/lna/lna.html >> >> To wit, drive the amp with the source, say 50 ohm, and make a statement >> on the the frequency range of say, 1db variance from -6db attenuation >> and, say +/-10 degs from 0, or whatever. >> >>> The R.F. universe sits in a universe by itself. There is no technical >>> reason to use and think in terms of s-parameters at all. This is a >>> historical artefact. All design parameters can simply be reformulated in >>> a way that ANY Spice can handle directly. So long as all blocks are >>> consistently specified, it all cancels out in the wash. >> >>> Its a bit like you yanks living in the past with your 105 deg F in TX, >>> when the rest of the sensible world moved on to Centigrade in the last >>> century. No one here converts to Fahrenheit to understand why its hot >>> where jimmy T lives, we all know 45 degrees C has fried his brain a >>> while back. > > >> I agree with you, Kevin. S11 means a lot less to me than Zin which is >> easy to plot in the simulation universe. However, Zin is not so easy >> to measure in our universe at very high frequencies, thus the S11 >> approach. > > I only live in the simulation universe now, the lower level staff can > deal with the real world and report back the results :-) > > Kevin Aylward B.Sc. > www.kevinaylward.co.uk > www.anasoft.co.uk - SuperSpice.
In that universe, you are correct. It might be nice to visit the other universe occasionally to discover the difficulties encountered to report the results you are using. Nevertheless, I accept and understand your position on the matter. Cheers, John S
On 10/05/14 18:37, John S wrote:
> On 5/10/2014 3:08 AM, Kevin Aylward wrote: >>> "Jeroen Belleman" wrote in message >>> news:lkis08$6i0$1@speranza.aioe.org... >> >>> Call me dense, but after many years of using LTspice, >>> during which many times I inserted return-loss bridges >>> into my circuits to measure S11, someone pointed out >>> to me that LTspice has a directive for that. >> >>> Just insert ".net V1 v(out)" somewhere into the schematic, >>> and an S11 trace will become available. >> >>> I'm still missing some of the finer points however: The >>> resulting S11 only makes sense with the directive above. >> >> The other option, is simply to use an alternative, but design >> equivalent, method of specifying circuit conditions. I do that here: >> >> http://www.kevinaylward.co.uk/ee/lna/lna.html >> >> To wit, drive the amp with the source, say 50 ohm, and make a statement >> on the the frequency range of say, 1db variance from -6db attenuation >> and, say +/-10 degs from 0, or whatever. >> >> The R.F. universe sits in a universe by itself. There is no technical >> reason to use and think in terms of s-parameters at all. This is a >> historical artefact. All design parameters can simply be reformulated in >> a way that ANY Spice can handle directly. So long as all blocks are >> consistently specified, it all cancels out in the wash. >> >> Its a bit like you yanks living in the past with your 105 deg F in TX, >> when the rest of the sensible world moved on to Centigrade in the last >> century. No one here converts to Fahrenheit to understand why its hot >> where jimmy T lives, we all know 45 degrees C has fried his brain a >> while back. >> >> Kevin Aylward B.Sc. >> www.kevinaylward.co.uk >> www.anasoft.co.uk - SuperSpice. > > I agree with you, Kevin. S11 means a lot less to me than Zin which is > easy to plot in the simulation universe. However, Zin is not so easy to > measure in our universe at very high frequencies, thus the S11 approach. > > John S
Bridges are still the most accurate way to compare impedances, and S11 is just the what comes out of a bridge. One gets used to it. I tend to favour S11 for impedance close to the reference and Zin when it's far away, but that's just me. Using S11 at or close to DC tends to raise some eyebrows, but there is nothing against it, really. Jeroen Belleman
On 5/10/2014 4:05 PM, jeroen Belleman wrote:
> On 10/05/14 18:37, John S wrote: >> On 5/10/2014 3:08 AM, Kevin Aylward wrote: >>>> "Jeroen Belleman" wrote in message >>>> news:lkis08$6i0$1@speranza.aioe.org... >>> >>>> Call me dense, but after many years of using LTspice, >>>> during which many times I inserted return-loss bridges >>>> into my circuits to measure S11, someone pointed out >>>> to me that LTspice has a directive for that. >>> >>>> Just insert ".net V1 v(out)" somewhere into the schematic, >>>> and an S11 trace will become available. >>> >>>> I'm still missing some of the finer points however: The >>>> resulting S11 only makes sense with the directive above. >>> >>> The other option, is simply to use an alternative, but design >>> equivalent, method of specifying circuit conditions. I do that here: >>> >>> http://www.kevinaylward.co.uk/ee/lna/lna.html >>> >>> To wit, drive the amp with the source, say 50 ohm, and make a statement >>> on the the frequency range of say, 1db variance from -6db attenuation >>> and, say +/-10 degs from 0, or whatever. >>> >>> The R.F. universe sits in a universe by itself. There is no technical >>> reason to use and think in terms of s-parameters at all. This is a >>> historical artefact. All design parameters can simply be reformulated in >>> a way that ANY Spice can handle directly. So long as all blocks are >>> consistently specified, it all cancels out in the wash. >>> >>> Its a bit like you yanks living in the past with your 105 deg F in TX, >>> when the rest of the sensible world moved on to Centigrade in the last >>> century. No one here converts to Fahrenheit to understand why its hot >>> where jimmy T lives, we all know 45 degrees C has fried his brain a >>> while back. >>> >>> Kevin Aylward B.Sc. >>> www.kevinaylward.co.uk >>> www.anasoft.co.uk - SuperSpice. >> >> I agree with you, Kevin. S11 means a lot less to me than Zin which is >> easy to plot in the simulation universe. However, Zin is not so easy to >> measure in our universe at very high frequencies, thus the S11 approach. >> >> John S > > > Bridges are still the most accurate way to compare impedances, and S11 > is just the what comes out of a bridge. One gets used to it. I tend > to favour S11 for impedance close to the reference and Zin when it's > far away, but that's just me.
Oh yes, that's fine. I just never use the .NET directive in LTSpice because of the problem you brought up. I simply plot Zin, which is the same thing and easier to remember.
> Using S11 at or close to DC tends to raise some eyebrows, but there > is nothing against it, really.
I know. Now try to tell somebody that you can measure the power factor of a switching load on a battery. That will raise eyebrows, too. John S
> Jeroen Belleman