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Beginner's Question re LT Spice and RF Filter Design

 

Am designing a low pass filter for the 20m amateur band, code below. (My apologies if this violates group protocol. Advise and I will adhere in the future.)?
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I note that the display shows this model to initially be -6dB and wonder why. Again, truly a beginner. Links to appropriate material gratefully received.
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Thank you in advance.
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William, k6whp
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"Cheer up, things could get worse. So I cheered up and things got worse."
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On Sat, Mar 29, 2025 at 08:48 PM, k6whp wrote:
Am designing a low pass filter for the 20m amateur band, code below. (My apologies if this violates group protocol. Advise and I will adhere in the future.)?
Even better:? Why not read the group's guidelines on the main group webpage.
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You attempted to paste your schematic file into the message.? Never paste or attach any files into a group message.
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Andy
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On Sat, Mar 29, 2025 at 08:48 PM, k6whp wrote:
I note that the display shows this model to initially be -6dB and wonder why.
It's a pretty simple reason, which looks obvious after you realize it.
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Your signal source has a 1 (volt) amplitude and 50 ohm source impedance.? It is a Thevenin source, so you've got 1 volt behind the 50 ohm source resistance.? When that is terminated (into a 50 ohm load), it is a voltage divider that cuts the voltage in half, so that the terminal voltage across V1 is 0.5 volt, resulting in a nominal 6 dB loss.
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When using AC sources like this, set their amplitude to 2 (volts), so that they make 1 (volt) when terminated.
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Andy
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Andy,
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I did so instinctively and THEN read the group webpage. My apologies for the oversight.
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--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."
 

I uploaded your schematic file.? It is "RF_Filter_Chebyshev.asc" and it is currently in the Temp folder, in the group's Files section.
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Andy
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Andy,
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Thank you. I was to have uploaded the .asc file into temp per the group protocol but your answer will suffice. Appreciate the advisory and support.
--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."
 

Andy,
I saw that.? Am going through advisories and FAQ? - currently the basic beginner's tutorial- and come up to speed with any and all of relevance.
Appreciate the leg up!
--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."

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