Re: Question about blinky board input
Corey,
You can likely improve the performance of this system.
Jim’s comments are all good to keep in mind.
The first filter element is the antenna. We have to get the signal from the antenna to the gate of the device. Everything required to do this has impact on performance.
The antenna can accumulate static charge in initial handling and in space. A DC path at the antenna will help protect the sensitive gate of the FET.
Limiting the bandwidth of the input will reduce noise if your approach is not adding noise or attenuation at the frequency of interest. The goal is to improve the impedance matching between the antenna feed point and the FET gate. The FET is the first stage of gain and most important part of the receiver. After the FET you have more control of the circuit and impedance.
There can be an issue with temperature performance so the coupling circuit needs to be studied over temperature. The satellite will be moving in and out of the sun and can be spinning which can cause significant changes in temperature.
Getting the FET gate bias voltage correct over temp can be a issue also.
I say you can improve because I have recently seen preamps designed for radio astronomy which have such a low noise level the noise cannot be measured. It makes a smaller antenna compete with larger higher gain antennas. While the FET selection is important part of this the coupling between the antenna and the gate are just as important. The Blinky prototype is using a short piece of 50 ohm cable and connectors so the performance of this is ok for testing and prototype but not so good for actual operation in space.
Bob
On Feb 21, 2025, at 9:13 PM, Jim McCullers via pacsat-dev [email protected] 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.
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 Bob. Again, great information.
On Sat, Feb 22, 2025 at 11:16:48AM +0000, Bob Stricklin wrote:
Corey,
You can likely improve the performance of this system.
Jim’s comments are all good to keep in mind.
The first filter element is the antenna. We have to get the signal from the antenna to the gate of the device. Everything required to do this has impact on performance.
The antenna can accumulate static charge in initial handling and in space. A DC path at the antenna will help protect the sensitive gate of the FET.
Yes, and that happens on the ground, too.
Limiting the bandwidth of the input will reduce noise if your approach is not adding noise or attenuation at the frequency of interest. The goal is to improve the impedance matching between the antenna feed point and the FET gate. The FET is the first stage of gain and most important part of the receiver. After the FET you have more control of the circuit and impedance.
There can be an issue with temperature performance so the coupling circuit needs to be studied over temperature. The satellite will be moving in and out of the sun and can be spinning which can cause significant changes in temperature.
Getting the FET gate bias voltage correct over temp can be a issue also.
Yes, I was noticing that small changes in the capacitor and inductor values can make a pretty big difference in filter performance. You would need precision components.
I say you can improve because I have recently seen preamps designed for radio astronomy which have such a low noise level the noise cannot be measured. It makes a smaller antenna compete with larger higher gain antennas.
I've seen those, too, but they are generally liquid air cooled. For what they need, I guess it's required.
While the FET selection is important part of this the coupling between the antenna and the gate are just as important. The Blinky prototype is using a short piece of 50 ohm cable and connectors so the performance of this is ok for testing and prototype but not so good for actual operation in space.
Thanks again, this satisfies my curiousity. At least about this question :-).
-corey
Bob
On Feb 21, 2025, at 9:13 PM, Jim McCullers via pacsat-dev [email protected] 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.
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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Somewhere I have a report on LNAs and filtering. Buried in the info dump. I'll try to dig it out.
Antenna matching is equally important and similar to the PC-104 or connector situation we are not in the driver seat.
Jim WA4CWI
Corey,
You can likely improve the performance of this system.
Jim’s comments are all good to keep in mind.
The first filter element is the antenna. We have to get the signal from the antenna to the gate of the device. Everything required to do this has impact on performance.
The antenna can accumulate static charge in initial handling and in space. A DC path at the antenna will help protect the sensitive gate of the FET.
Limiting the bandwidth of the input will reduce noise if your approach is not adding noise or attenuation at the frequency of interest. The goal is to improve the impedance matching between the antenna feed point and the FET gate. The FET is the first stage of gain and most important part of the receiver. After the FET you have more control of the circuit and impedance.
There can be an issue with temperature performance so the coupling circuit needs to be studied over temperature. The satellite will be moving in and out of the sun and can be spinning which can cause significant changes in temperature.
Getting the FET gate bias voltage correct over temp can be a issue also.
I say you can improve because I have recently seen preamps designed for radio astronomy which have such a low noise level the noise cannot be measured. It makes a smaller antenna compete with larger higher gain antennas. While the FET selection is important part of this the coupling between the antenna and the gate are just as important. The Blinky prototype is using a short piece of 50 ohm cable and connectors so the performance of this is ok for testing and prototype but not so good for actual operation in space.
Bob
On Feb 21, 2025, at 9:13 PM, Jim McCullers via pacsat-dev [email protected] 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.
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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I was hoping we could talk about this a little bit in the meeting today, but Jim wasn't there, so I'll do it in email.
I've thought about it some more, and I don't think you have the issue with wideband amplification that you have on the transmit side. On transmit, if you send two signals, you have to half the power or it will clip. But there's not enough power here to worry about that, I don't think.
However, without an input filter, if you have a nearby transmitter transmitting at any frequency, it will overload the LNA on the receive input and desense it. I assume it's common to have multiple transmitters on different bands on a satellite, so I think some sort of filter on the input would be in order. I guess the bandwidth of the filter depends on how close the other frequencies are, but I'd be curious to know if there is any accumulated wisdom about this.
I'd be willing to try my hand at designing a filter, doing spice simulations, and picking parts. I guess you could use the same filter you have on the other side of the LNA, but that's a lot of money for $2 worth of parts.
I also realized that you have a similar problem on a DSP system where you have one receiver and are decoding multiple receive signals inside of what you receive. It's hard to do AGC with that kind of setup. It may not matter on a satellite because everything will be weak. Also, I've given up on using an ADC and DAC, it uses too many parts that are expensive. I've switched back to the audio CODEC where we can get ~200kHz of bandwidth. But the CODEC has a tremendously large dynamic range, around 100dB at the sample rates we are using. I don't know how they do that. But it will help a lot with signal power variance. Sorry, just rambling a bit.
-corey
On Sat, Feb 22, 2025 at 5:16 AM Bob Stricklin [email protected] wrote:
Corey,
You can likely improve the performance of this system.
Jim’s comments are all good to keep in mind.
The first filter element is the antenna. We have to get the signal from the antenna to the gate of the device. Everything required to do this has impact on performance.
The antenna can accumulate static charge in initial handling and in space. A DC path at the antenna will help protect the sensitive gate of the FET.
Limiting the bandwidth of the input will reduce noise if your approach is not adding noise or attenuation at the frequency of interest. The goal is to improve the impedance matching between the antenna feed point and the FET gate. The FET is the first stage of gain and most important part of the receiver. After the FET you have more control of the circuit and impedance.
There can be an issue with temperature performance so the coupling circuit needs to be studied over temperature. The satellite will be moving in and out of the sun and can be spinning which can cause significant changes in temperature.
Getting the FET gate bias voltage correct over temp can be a issue also.
I say you can improve because I have recently seen preamps designed for radio astronomy which have such a low noise level the noise cannot be measured. It makes a smaller antenna compete with larger higher gain antennas. While the FET selection is important part of this the coupling between the antenna and the gate are just as important. The Blinky prototype is using a short piece of 50 ohm cable and connectors so the performance of this is ok for testing and prototype but not so good for actual operation in space.
Bob
On Feb 21, 2025, at 9:13 PM, Jim McCullers via pacsat-dev [email protected] 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.
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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participants (3)
-
Bob Stricklin -
Corey Minyard -
Jim McCullers