February 2025 - pacsat-dev - mailman.amsat.org

Re: Question about blinky board input
by Bob Stricklin 27 Feb '25

27 Feb '25

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 <pacsat-dev(a)amsat.org> 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 > > ----------------------------------------------------------- > > pacsat-dev mailing list -- pacsat-dev(a)amsat.org View archives of this > mailing list at > https://mailman.amsat.org/hyperkitty/list/[email protected] > To unsubscribe send an email to pacsat-dev-leave(a)amsat.org Manage all of > your AMSAT-NA mailing list preferences at https://mailman.amsat.org > > > ----------------------------------------------------------- > > pacsat-dev mailing list -- pacsat-dev(a)amsat.org > View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/[email protected] > To unsubscribe send an email to pacsat-dev-leave(a)amsat.org > Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org

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Re: pacsat dev mtg 2/27/2025
by Jim McCullers 27 Feb '25

27 Feb '25

Can’t make the meeting tonight. Work is progressing on LaunchPad V2 and receiver test circuit board. Jim pacsat dev mtg 2/27/2025 Feb 27, 2025, 7:00 – 8:00 PM (America/Chicago) Please join my meeting from your computer, tablet or smartphone. https://meet.goto.com/648535629 You can also dial in using your phone. Access Code: 648-535-629 United States: +1 (224) 501-3412 <tel:+12245013412,,648535629> Join from a video-conferencing room or system. Meeting ID: 648-535-629 Dial in or type: 67.217.95.2 or inroomlink.goto.com Or dial directly: 648535629(a)67.217.95.2 <mailto:[email protected]> or 67.217.95.2##648535629 Get the app now and be ready when your first meeting starts: https://meet.goto.com/install

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pacsat dev mtg 2/27/2025
by Bill 27 Feb '25

27 Feb '25

*pacsat dev mtg 2/27/2025* Feb 27, 2025, 7:00 – 8:00 PM (America/Chicago) *Please join my meeting from your computer, tablet or smartphone.* https://meet.goto.com/648535629 *You can also dial in using your phone.* Access Code: 648-535-629 United States: +1 (224) 501-3412 <tel:+12245013412,,648535629> *Join from a video-conferencing room or system.* Meeting ID: 648-535-629 Dial in or type: 67.217.95.2 or inroomlink.goto.com Or dial directly: 648535629(a)67.217.95.2 or 67.217.95.2##648535629 *Get the app now and be ready when your first meeting starts:* https://meet.goto.com/install

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Re: Question about blinky board input
by Jim McCullers 22 Feb '25

22 Feb '25

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 ----------------------------------------------------------- pacsat-dev mailing list -- pacsat-dev(a)amsat.org View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/[email protected] To unsubscribe send an email to pacsat-dev-leave(a)amsat.org Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org

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Question about blinky board input
by Corey Minyard 21 Feb '25

21 Feb '25

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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Re: Progress on DSP work
by Corey Minyard 20 Feb '25

20 Feb '25

On Tue, Feb 18, 2025 at 11:23 PM Cameron Castillo via pacsat-dev <pacsat-dev(a)amsat.org> wrote: > > Hi Corey, > > I'm not an expert in DSP, but I thought I'd ask some questions. Thanks Cameron. This is more analog, which is probably more up your alley. I'm a lot more familiar with DSP. > > Do you think moving to a low-IF architecture could assist with the filtering problem? Perhaps by moving the frequency up, you'd lose the need to filter down to so low a frequency. Perhaps centering the IF around 500 kHz or so would allow the low end of your stopband somewhere in the 100's of kHz range. It might be easier to design a filter for that than the baseband design. In combination with the downsampling you mentioned above, maybe you'd have an easier time? Well, I guess this is more of a DSP thing. I also don't completely understand what you are saying. I know it's common to have a 455kHz IF, but I'm not sure how it helps here. It would certainly be easier (well, I'd know more about what I'm doing) if this was in the digital realm, but it's not. For non I/Q D/A and A/D conversion, you have to filter the input signal below the lowest desired frequency (to avoid aliasing), mix it down, then lowpass filter it. For I/Q demodulation, it's quite a bit different. You don't have to filter out below the desired lowest frequency. Instead you mix down at the middle of the frequency range you are interested in, low pass filter, and since you have I and Q, you can use the negative frequencies. In the digital realm you can upconvert so that all the frequencies are positive. It's a little weird, but it does have some analog parallels in image rejection mixers. The chip that I found that was ideal (except for temp) was https://www.analog.com/en/products/ltc6602.html but AFAICT it's the only chip that does anywhere near what we would want. I'd also like to have some ability to have multiple possible parts for a job. > > Is there an integrated PLL in the signal processor? Looking at some dedicated DSPs on mouser, it seems that they all have integrated PLLs to meet stability criterion at the frequency of interest. Take a look at this datasheet on page 1476 where it is claiming that the stability of the 8 MHz internal reference is at +/- 1.5%, which would put the error at the maximum VCO frequency (64 MHz) at 960 kHz (by your calculation method). That still seems pretty significant to me. I also noted that they claim you could drive the reference with a crystal, which has much worse stability than a TCXO/OCXO. I think I'm misunderstanding what you're saying, but those are my thoughts. Unfortunately, I need 3 frequencies, one for IF, one for TX, and one for RX. I'm not so concerned about the PLL, but about the oscillator error. I'm not sure my calculation method is correct, either. For FM, I suppose some amount of error isn't too bad and you are having to doppler correct, anyway. I'm more wondering how more normal rigs handle this sort of thing. -corey > > Thanks, > > Cameron > > > ------------------------------------------------- > > Cameron Castillo > > KJ7ILB > > P: (503) 752-8877 > > -------------------------------------------------- > > > > On Tue, Feb 18, 2025 at 5:46 PM Corey Minyard via pacsat-dev <pacsat-dev(a)amsat.org> wrote: >> >> I'm continuing my learning process on the DSP board. I've learned a few >> things: >> >> Baseand filters are a pain. Well, most filters are a pain, but baseband >> filters are especially a pain. I couldn't find a pre-made filter that >> was really suitable. Well, I found one, but it didn't meet the >> temperature range. I have a design using op amps, but it uses 4 op amps >> to achieve a transition band that is about the same size as the >> passband for a stopband at -60db. That's 4 op amps, each for I and Q in >> each direction, so 16 total. At least you can get 2-4 in a package. >> The design simulates well in spice, but achieving a sharper filter >> appears to be very hard. If that's the case, we will have to sample at >> twice the cutoff frequency, basically losing half the bandwidth. So for >> a 200KHz bandwidth, we would have to sample at 400KHz and just throw >> away the top portion. This is a problem with narrow bandwidths; if you >> had a really wide bandwidth then you could use an LC filter and that >> would work better. An LC filter with 200KHz cutoff is rather... large. >> >> Second, I don't really understand clocks. In the designs I have done in >> telecom you generally just relied on the incoming clocks, but that's not >> radio. I found a chip from TI that is a PLL with a VCO and might do the >> job, a LMK05318B-Q1, though it's kind of power hungry. Or you could >> get programmable oscillators like the DSC8001CL5, but all of those only >> seem to go to 150MHz or so. There are fixed oscillators with higher >> frequencies. But the amount of error possible in the frequency seems >> pretty significant. >> >> But I don't understand about frequency stability. For instance, on the >> oscillators that the blinky board is using, you have 16MHz with 2.5PPM >> stability. IIUC, that would be +/- 40Hz. But when you multiply that up >> to 440MHz, that error would become 1100KHz, which seems pretty >> large. Am I understanding this correctly? Is this something to >> worry about? >> >> You could compensate in the digital side if you knew what the error was. >> >> Is there anyone I could discuss things with that knows about these sorts >> of things? Not much available on the internet. >> >> Thanks, >> >> -corey >> >> ----------------------------------------------------------- >> >> pacsat-dev mailing list -- pacsat-dev(a)amsat.org >> View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/[email protected] >> To unsubscribe send an email to pacsat-dev-leave(a)amsat.org >> Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org > > > ----------------------------------------------------------- > > pacsat-dev mailing list -- pacsat-dev(a)amsat.org > View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/[email protected] > To unsubscribe send an email to pacsat-dev-leave(a)amsat.org > Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org

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Re: Pacsat dev mtg 2/20/2025
by Chris Thompson 20 Feb '25

20 Feb '25

I am unable to make it tonight. Let me know if I have any actions. On Thu, Feb 20, 2025 at 4:54 PM Bill via pacsat-dev <pacsat-dev(a)amsat.org> wrote: > *Pacsat dev mtg 2/20/2025* > Feb 20, 2025, 7:00 – 8:30 PM (America/Chicago) > > > *Please join my meeting from your computer, tablet or smartphone.* > https://meet.goto.com/896764685 > > *You can also dial in using your phone.* > Access Code: 896-764-685 > United States: +1 (646) 749-3122 <+16467493122,,896764685> > > > *Join from a video-conferencing room or system.* > Meeting ID: 896-764-685 > Dial in or type: 67.217.95.2 or inroomlink.goto.com > Or dial directly: 896764685(a)67.217.95.2 or 67.217.95.2##896764685 > > > *Get the app now and be ready when your first meeting starts:* > https://meet.goto.com/install > > ----------------------------------------------------------- > > pacsat-dev mailing list -- pacsat-dev(a)amsat.org > View archives of this mailing list at > https://mailman.amsat.org/hyperkitty/list/[email protected] > To unsubscribe send an email to pacsat-dev-leave(a)amsat.org > Manage all of your AMSAT-NA mailing list preferences at > https://mailman.amsat.org > -- Chris E. Thompson chrisethompson(a)gmail.com g0kla(a)arrl.net

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Pacsat dev mtg 2/20/2025
by Bill 20 Feb '25

20 Feb '25

*Pacsat dev mtg 2/20/2025* Feb 20, 2025, 7:00 – 8:30 PM (America/Chicago) *Please join my meeting from your computer, tablet or smartphone.* https://meet.goto.com/896764685 *You can also dial in using your phone.* Access Code: 896-764-685 United States: +1 (646) 749-3122 <tel:+16467493122,,896764685> *Join from a video-conferencing room or system.* Meeting ID: 896-764-685 Dial in or type: 67.217.95.2 or inroomlink.goto.com Or dial directly: 896764685(a)67.217.95.2 or 67.217.95.2##896764685 *Get the app now and be ready when your first meeting starts:* https://meet.goto.com/install

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Re: Progress on DSP work
by Cameron Castillo 19 Feb '25

19 Feb '25

Hi Corey, I'm not an expert in DSP, but I thought I'd ask some questions. Do you think moving to a low-IF architecture could assist with the filtering problem? Perhaps by moving the frequency up, you'd lose the need to filter down to so low a frequency. Perhaps centering the IF around 500 kHz or so would allow the low end of your stopband somewhere in the 100's of kHz range. It might be easier to design a filter for that than the baseband design. In combination with the downsampling you mentioned above, maybe you'd have an easier time? Is there an integrated PLL in the signal processor? Looking at some dedicated DSPs on mouser, it seems that they all have integrated PLLs to meet stability criterion at the frequency of interest. Take a look at this <https://www.mouser.com/datasheet/2/268/dsPIC33AK128MC106_Family_Data_Sheet_…> datasheet on page 1476 where it is claiming that the stability of the 8 MHz internal reference is at +/- 1.5%, which would put the error at the maximum VCO frequency (64 MHz) at 960 kHz (by your calculation method). That still seems pretty significant to me. I also noted that they claim you could drive the reference with a crystal, which has much worse stability than a TCXO/OCXO. I think I'm misunderstanding what you're saying, but those are my thoughts. Thanks, Cameron ------------------------------------------------- Cameron Castillo KJ7ILB *P*: (503) 752-8877 -------------------------------------------------- On Tue, Feb 18, 2025 at 5:46 PM Corey Minyard via pacsat-dev < pacsat-dev(a)amsat.org> wrote: > I'm continuing my learning process on the DSP board. I've learned a few > things: > > Baseand filters are a pain. Well, most filters are a pain, but baseband > filters are especially a pain. I couldn't find a pre-made filter that > was really suitable. Well, I found one, but it didn't meet the > temperature range. I have a design using op amps, but it uses 4 op amps > to achieve a transition band that is about the same size as the > passband for a stopband at -60db. That's 4 op amps, each for I and Q in > each direction, so 16 total. At least you can get 2-4 in a package. > The design simulates well in spice, but achieving a sharper filter > appears to be very hard. If that's the case, we will have to sample at > twice the cutoff frequency, basically losing half the bandwidth. So for > a 200KHz bandwidth, we would have to sample at 400KHz and just throw > away the top portion. This is a problem with narrow bandwidths; if you > had a really wide bandwidth then you could use an LC filter and that > would work better. An LC filter with 200KHz cutoff is rather... large. > > Second, I don't really understand clocks. In the designs I have done in > telecom you generally just relied on the incoming clocks, but that's not > radio. I found a chip from TI that is a PLL with a VCO and might do the > job, a LMK05318B-Q1, though it's kind of power hungry. Or you could > get programmable oscillators like the DSC8001CL5, but all of those only > seem to go to 150MHz or so. There are fixed oscillators with higher > frequencies. But the amount of error possible in the frequency seems > pretty significant. > > But I don't understand about frequency stability. For instance, on the > oscillators that the blinky board is using, you have 16MHz with 2.5PPM > stability. IIUC, that would be +/- 40Hz. But when you multiply that up > to 440MHz, that error would become 1100KHz, which seems pretty > large. Am I understanding this correctly? Is this something to > worry about? > > You could compensate in the digital side if you knew what the error was. > > Is there anyone I could discuss things with that knows about these sorts > of things? Not much available on the internet. > > Thanks, > > -corey > > ----------------------------------------------------------- > > pacsat-dev mailing list -- pacsat-dev(a)amsat.org > View archives of this mailing list at > https://mailman.amsat.org/hyperkitty/list/[email protected] > To unsubscribe send an email to pacsat-dev-leave(a)amsat.org > Manage all of your AMSAT-NA mailing list preferences at > https://mailman.amsat.org >

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Progress on DSP work
by Corey Minyard 18 Feb '25

18 Feb '25

I'm continuing my learning process on the DSP board. I've learned a few things: Baseand filters are a pain. Well, most filters are a pain, but baseband filters are especially a pain. I couldn't find a pre-made filter that was really suitable. Well, I found one, but it didn't meet the temperature range. I have a design using op amps, but it uses 4 op amps to achieve a transition band that is about the same size as the passband for a stopband at -60db. That's 4 op amps, each for I and Q in each direction, so 16 total. At least you can get 2-4 in a package. The design simulates well in spice, but achieving a sharper filter appears to be very hard. If that's the case, we will have to sample at twice the cutoff frequency, basically losing half the bandwidth. So for a 200KHz bandwidth, we would have to sample at 400KHz and just throw away the top portion. This is a problem with narrow bandwidths; if you had a really wide bandwidth then you could use an LC filter and that would work better. An LC filter with 200KHz cutoff is rather... large. Second, I don't really understand clocks. In the designs I have done in telecom you generally just relied on the incoming clocks, but that's not radio. I found a chip from TI that is a PLL with a VCO and might do the job, a LMK05318B-Q1, though it's kind of power hungry. Or you could get programmable oscillators like the DSC8001CL5, but all of those only seem to go to 150MHz or so. There are fixed oscillators with higher frequencies. But the amount of error possible in the frequency seems pretty significant. But I don't understand about frequency stability. For instance, on the oscillators that the blinky board is using, you have 16MHz with 2.5PPM stability. IIUC, that would be +/- 40Hz. But when you multiply that up to 440MHz, that error would become 1100KHz, which seems pretty large. Am I understanding this correctly? Is this something to worry about? You could compensate in the digital side if you knew what the error was. Is there anyone I could discuss things with that knows about these sorts of things? Not much available on the internet. Thanks, -corey

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