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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