Re: Question about blinky board input
Bob did that design and I assume it may have to do with the LNA. I've never read the datasheet on the transistor so am not aware of the recommendations.
I'm doing the redo for the new LaunchPad board to try another configuration on the AX5043 input. There is a great argument in the pre-amp world of where the selectivity should be. One opinion (and the common in products) is to amplify first then do the selectivity, the though being the LNA should pull in and amplify weak signals before selectivity. The other opinion is to filter first, then amplify to reduce intermodulation products. I'm of the second opinion and believe filters on both ends of the LNA are in order. I'm not certain if a low loss but less tight in bandwidth and slope should be on the first or if full filtering should be first with a small filter on the output to clean up any generated products.
Remember, preamps in general are to overcome system losses and not pull magic out of the air. Noise is amplified as well as any signals reducing the noise to signal ratio. Preamps are like SWR, subject to many misunderstandings.
You are correct that a shunt filter on the input would bleed off any charges that may (will) build up on the antenna.
Jim
I was looking at the blinky board receive section and I had a question. It has a shunt resistor for bleeding charge off the antenna and a series capacitor on the antenna input with a note that the shunt resistor will add some noise. I was wondering why it didn't have a small 2nd order shunt first filter there instead. Since it's shunt first, the inductor will bleed off any charge from the antenna. And it would provide a little filtration on the input, which would seem good to me.
Just more curious than anything.
-corey
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Thanks, this is great information.
On Fri, Feb 21, 2025 at 09:13:18PM -0600, Jim McCullers wrote:
Bob did that design and I assume it may have to do with the LNA. I've never read the datasheet on the transistor so am not aware of the recommendations.
I'm doing the redo for the new LaunchPad board to try another configuration on the AX5043 input. There is a great argument in the pre-amp world of where the selectivity should be. One opinion (and the common in products) is to amplify first then do the selectivity, the though being the LNA should pull in and amplify weak signals before selectivity. The other opinion is to filter first, then amplify to reduce intermodulation products. I'm of the second opinion and believe filters on both ends of the LNA are in order. I'm not certain if a low loss but less tight in bandwidth and slope should be on the first or if full filtering should be first with a small filter on the output to clean up any generated products.
I would agree with you, but it's just a hunch. It seems to me that signals mixing in the amp is something to be avoided. But that was just a hunch.
I would think that all this could be decided with science. I'm not sure why this is an argument. But I guess SWR is the same way, as you say below.
Remember, preamps in general are to overcome system losses and not pull magic out of the air. Noise is amplified as well as any signals reducing the noise to signal ratio. Preamps are like SWR, subject to many misunderstandings.
To me, the fact that you could pull a signal out of the air at picovolts and amplify it to a signal that's intelligible is pulling magic out of the air :-).
Thanks again,
-corey
You are correct that a shunt filter on the input would bleed off any charges that may (will) build up on the antenna.
Jim
I was looking at the blinky board receive section and I had a question. It has a shunt resistor for bleeding charge off the antenna and a series capacitor on the antenna input with a note that the shunt resistor will add some noise. I was wondering why it didn't have a small 2nd order shunt first filter there instead. Since it's shunt first, the inductor will bleed off any charge from the antenna. And it would provide a little filtration on the input, which would seem good to me.
Just more curious than anything.
-corey
pacsat-dev mailing list -- [email protected] View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/[email protected] To unsubscribe send an email to [email protected] Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org
participants (2)
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Corey Minyard -
Jim McCullers