Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Hi Neil, I agree with what Ron said. Also, note that even in FSK the phase is continuous, so I wouldn't call the instant frequency jump a "sharp transition" (but this is arguing about names, anyway). It's true that the GFSK version includes amplitude ramp-up and ramp-down at the start and end of the transmission to prevent the keying click associated with going to full TX power instantly. However, the shape of this ramp-up/down is not very critical, so even if this ramp is sent through a non-linear amplifier, it's OK. The amplifier would need to be really bad to transfrom the ramp into a large keying click. I believe that the following is true (at least mathematically): If a constant amplitude waveform is amplified non-linearly, then the spectrum of the amplified signal is very similar to the spectrum of the original signal (disregarding harmonic distortion that makes energy apppear near the integer multiples of the carrier frequency). This means that there is no noticeable spectral regrowth by sending an FSK or GFSK waveform through a non-linear PA. I don't know how the proof of this statement goes exactly, but I just simulated it with GNU Radio, comparing distorting a GMSK signal with BT=0.35 and another with large BT so that it looks like unfiltered MSK. So I don't aggree with "With a nonlinear amplifier well into saturation, the level of the energy away from the main tone will be boosted, so there is distortion of the relative amplitudes of the tone and sidebands." Best, Dani. El 13/1/20 a las 21:35, Neil escribió: > OK, agreed that the envelope in even a sharply-switched FSK is sort-of > constant at baseband (except during tone startup, when it goes from zero > to full amplitude in the first half-cycle), but with the sharp > transitions, some energy goes into the modulation sidebands created by > those transitions. The level of the main tone during a sharp transition > must drop as energy is being put into the sidebands generated by the > abrupt frequency change. With a nonlinear amplifier well into > saturation, the level of the energy away from the main tone will be > boosted, so there is distortion of the relative amplitudes of the tone > and sidebands. In a linear amplifier, the changes in level of the main > tones during frequency transitions will be reproduced faithfully and the > overall bandwidth will remain as per design. Also when operating close > to the edge of a filter passband, a fair amount of the energy in those > sidebands is removed during tone transitions. The amplitude variations > only happen around the time of a tone transition, so it depends on how > you define the granularity of your amplitude measurements. I should run > a proper test on the GFSK version. > > Anyway, none of this really matters unless you happen to be in line of > sight to a 30kW ERP station on 144MHz and you are trying to resolve a > signal 400Hz away that is 80dB weaker. > > The change to GFSK is *hugely* welcome as it has reduced the occupied > bandwidth of FT8 signals (even those using over-driven or > not-very-linear amplifiers) to a much more neighbourly level. > > Nice work by the OP. > > Neil G4DBN > > Average amplitude spectrum of an old-style FT8 signal at baseband. > > Spectrum after going through an SDR transmitter. Blue is a 15-second > average of the envelope, yellow is instantaneous. Some of the sidebands > have gone missing, so there is amplitude variation, at least very > briefly during tone transitions. > > > On 13/01/2020 19:48, Ron Economos wrote: >> >> There are no amplitude variations. Take a look at the figure from >> Daniel's link. What's being filtered is the *frequency* transitions. >> >> 4fsk >> >> Ron >> >> On 1/13/20 11:23, Neil wrote: >>> >>> The time-constant length (attack/sustain/decay times) is the root of >>> the original problem with ALC of course. Is there a way to store the >>> averaged envelope and remember it in a variable used to set the gain, >>> so you don't have to recalibrate every 15 seconds with each new >>> transmission, or syllabically with SSB voice? >>> >>> GFSK will work fine through a non-linear amplifier, but you lose the >>> benefits of the new smooth transitions. The amplitude variations are >>> only there during start, end and transitions. The old way used to >>> look like this in the frequency domain. That startup splat is more >>> than 2kHz wide. This is audio direct from WSJT-X to Spectrum Lab >>> within the same PC. >>> >>> I don't have an image of the new GFSK, but it was much cleaner when I >>> checked. The FSK transitions in this are done at zero crossing, but >>> even so, that represents a step change in slope and generates a >>> spread of frequencies. >>> >>> Neil >>> >>> On 13/01/2020 19:05, Daniel Estévez wrote: Hi Neil, As far as I know the new GFSK of FT8 and FT4 are still constant envelope, so they are tolerant to non-linear amplification (but don't read this as "immune to all sorts of
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Thanks for the correction Neil, I was sure FT8 was RRC filtered FSK, now I know better. Like Daniel though, I'm not sure why GFSK would require more linearity than FSK with abrupt phase transitions. I've never used GFSK, but I have used GMSK which is quite similar and it didn't require very high linearity. Adrian On January 13, 2020 7:23:38 PM UTC, Neil wrote: >The time-constant length (attack/sustain/decay times) is the root of >the >original problem with ALC of course. Is there a way to store the >averaged envelope and remember it in a variable used to set the gain, >so >you don't have to recalibrate every 15 seconds with each new >transmission, or syllabically with SSB voice? > >GFSK will work fine through a non-linear amplifier, but you lose the >benefits of the new smooth transitions. The amplitude variations are >only there during start, end and transitions. The old way used to look > >like this in the frequency domain. That startup splat is more than >2kHz >wide. This is audio direct from WSJT-X to Spectrum Lab within the same >PC. > >I don't have an image of the new GFSK, but it was much cleaner when I >checked. The FSK transitions in this are done at zero crossing, but >even so, that represents a step change in slope and generates a spread >of frequencies. > >Neil > >On 13/01/2020 19:05, Daniel Estévez wrote: >> Hi Neil, >> >> As far as I know the new GFSK of FT8 and FT4 are still constant >> envelope, so they are tolerant to non-linear amplification (but don't >> read this as "immune to all sorts of terrible clipping and >distortion"). >> See the bottom of page 4 in >> >> https://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf >> >> You raise a fair point that nowadays with SDR it is often enough to >> adjust the gain in an open-loop fashion, by monitoring the output >power >> and changing the gain until an appropriate level is found. This can >> often be done once in a set an forget fashion. I do that for my >QO-100 >> groundstation. >> >> However it's true that the gain of PAs can vary somewhat with >> temperature and frequency, so sometimes some sort of close-loop >> adjustment of gain (of an appropriately large time constant and >> everything to prevent distortion) would be better. >> >> Best, >> >> Daniel. >> >-- >Neil >http://g4dbn.uk/;>g4dbn.uk
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
El 13/1/20 a las 19:47, Neil escribió: > Alternative description of Automatic Level Control now we are in the > world of SDR: > > "ALC: An ancient and terrible use of amplitude feedback control by > boat-anchor radios which had insufficient control over their modulation > sources and had excessive gain. A cause of envelope distortion, > intermodulation products and general awfulness. A thing which should be > left firmly in the 1960s" > > FT8 (8-tone FSK with 6.25 Hz spacing and 6.25 symbols per second) was, > until recently a constant-amplitude mode, with all of the wideband > clicks at start and end of transmissions and wideband splats at > inter-symbol boundaries that entailed. Now it uses Gaussian FSK so the > amplitude element matters during start, stop and tone transitions, > reintroducing linearity as a requirement of the transmit chain. As it > has a known steady-state amplitude, all that is required is constant > linear gain. Closing a control loop which looks at an averaged envelope > of the signal to set the gain of an earlier stage to achieve a certain > output level at run time is a recipe for reintroducing some of the > nasties that have been removed by the move to GFSK. Calibrating the gain > versus output level would achieve the same effect but without that > unnecessary control loop. Hi Neil, As far as I know the new GFSK of FT8 and FT4 are still constant envelope, so they are tolerant to non-linear amplification (but don't read this as "immune to all sorts of terrible clipping and distortion"). See the bottom of page 4 in https://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf You raise a fair point that nowadays with SDR it is often enough to adjust the gain in an open-loop fashion, by monitoring the output power and changing the gain until an appropriate level is found. This can often be done once in a set an forget fashion. I do that for my QO-100 groundstation. However it's true that the gain of PAs can vary somewhat with temperature and frequency, so sometimes some sort of close-loop adjustment of gain (of an appropriately large time constant and everything to prevent distortion) would be better. Best, Daniel.
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Alternative description of Automatic Level Control now we are in the world of SDR: "ALC: An ancient and terrible use of amplitude feedback control by boat-anchor radios which had insufficient control over their modulation sources and had excessive gain. A cause of envelope distortion, intermodulation products and general awfulness. A thing which should be left firmly in the 1960s" FT8 (8-tone FSK with 6.25 Hz spacing and 6.25 symbols per second) was, until recently a constant-amplitude mode, with all of the wideband clicks at start and end of transmissions and wideband splats at inter-symbol boundaries that entailed. Now it uses Gaussian FSK so the amplitude element matters during start, stop and tone transitions, reintroducing linearity as a requirement of the transmit chain. As it has a known steady-state amplitude, all that is required is constant linear gain. Closing a control loop which looks at an averaged envelope of the signal to set the gain of an earlier stage to achieve a certain output level at run time is a recipe for reintroducing some of the nasties that have been removed by the move to GFSK. Calibrating the gain versus output level would achieve the same effect but without that unnecessary control loop. Neil G4DBN On 13/01/2020 17:57, Adrian Musceac wrote: Hi Marcus, I'm not a hardware guy, but I think it's a circuit that monitors voltage from the final amplification stage and adjusts gain of previous stages if the maximum level is exceeded. Present in all classic SSB transceivers. Perhaps somebody else can explain better. Adrian On January 13, 2020 5:09:12 PM UTC, "Müller, Marcus (CEL)" wrote: Hey Adrian, that's nice! Generally: you said you couldn't implement an ALC: what's an ALC? Best regards, Marcus Mon, 2020-01-13 at 16:40 +, Adrian Musceac wrote: Hi, I wrote a report about combining WSJTX and GNU radio with a PlutoSDR transceiver to achieve FT8 contacts on the 144 MHz amateur radio band. The short summary is that I achieved contacts at over 800 km and heard stations as far as 1000 km last month in excellent tropo propagation conditions with 20 Watts of output power. I thought you might be interested in reading this, but it's too long to paste here. The report can be read here: http://qradiolink.org/ft8-plutosdr-2meters.html Adrian YO8RZZ -- Neil http://g4dbn.uk/;>g4dbn.uk
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Hi Nate, thanks very much for the link, I will study the module as it might be just what I was looking for. FT8 is fortunately FSK over SSB, but for general SSB work this would be essential. Thanks, Adrian On January 13, 2020 6:06:17 PM UTC, Nate Temple wrote: >Hi Adrian, > >You may be interested in this OOT by drmpeg: >https://github.com/drmpeg/gr-cessb > >Based on this paper: >http://www.arrl.org/files/file/QEX_Next_Issue/2014/Nov-Dec_2014/Hershberger_QEX_11_14.pdf > >From the paper: "Generate SSB without the big overshoot peaks that make >ALC >necessary with conventional SSB modulators. " > >I'm not sure if this will help at all with FT8, but might be worth >exploring. > >Regards, >Nate Temple > >On Mon, Jan 13, 2020 at 9:59 AM Adrian Musceac >wrote: > >> Hi Marcus, >> >> I'm not a hardware guy, but I think it's a circuit that monitors >voltage >> from the final amplification stage and adjusts gain of previous >stages if >> the maximum level is exceeded. Present in all classic SSB >transceivers. >> Perhaps somebody else can explain better. >> >> Adrian >> >> On January 13, 2020 5:09:12 PM UTC, "Müller, Marcus (CEL)" < >> muel...@kit.edu> wrote: >>> >>> Hey Adrian, that's nice! >>> >>> Generally: you said you couldn't implement an ALC: what's an ALC? >>> >>> Best regards, >>> Marcus >>> >>> Mon, 2020-01-13 at 16:40 +, Adrian Musceac wrote: >>> Hi, I wrote a report about combining WSJTX and GNU radio with a >PlutoSDR transceiver to achieve FT8 contacts on the 144 MHz amateur >radio band. The short summary is that I achieved contacts at over 800 >km and heard stations as far as 1000 km last month in excellent tropo >propagation conditions with 20 Watts of output power. I thought you might be interested in reading this, but it's too >long to paste here. The report can be read here: http://qradiolink.org/ft8-plutosdr-2meters.html Adrian YO8RZZ >>>
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Hi Adrian, You may be interested in this OOT by drmpeg: https://github.com/drmpeg/gr-cessb Based on this paper: http://www.arrl.org/files/file/QEX_Next_Issue/2014/Nov-Dec_2014/Hershberger_QEX_11_14.pdf >From the paper: "Generate SSB without the big overshoot peaks that make ALC necessary with conventional SSB modulators. " I'm not sure if this will help at all with FT8, but might be worth exploring. Regards, Nate Temple On Mon, Jan 13, 2020 at 9:59 AM Adrian Musceac wrote: > Hi Marcus, > > I'm not a hardware guy, but I think it's a circuit that monitors voltage > from the final amplification stage and adjusts gain of previous stages if > the maximum level is exceeded. Present in all classic SSB transceivers. > Perhaps somebody else can explain better. > > Adrian > > On January 13, 2020 5:09:12 PM UTC, "Müller, Marcus (CEL)" < > muel...@kit.edu> wrote: >> >> Hey Adrian, that's nice! >> >> Generally: you said you couldn't implement an ALC: what's an ALC? >> >> Best regards, >> Marcus >> >> Mon, 2020-01-13 at 16:40 +, Adrian Musceac wrote: >> >>> Hi, >>> >>> I wrote a report about combining WSJTX and GNU radio with a PlutoSDR >>> transceiver to achieve FT8 contacts on the 144 MHz amateur radio band. The >>> short summary is that I achieved contacts at over 800 km and heard stations >>> as far as 1000 km last month in excellent tropo propagation conditions with >>> 20 Watts of output power. >>> I thought you might be interested in reading this, but it's too long to >>> paste here. >>> >>> The report can be read here: >>> http://qradiolink.org/ft8-plutosdr-2meters.html >>> >>> Adrian YO8RZZ >>> >>
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Hi Marcus, I'm not a hardware guy, but I think it's a circuit that monitors voltage from the final amplification stage and adjusts gain of previous stages if the maximum level is exceeded. Present in all classic SSB transceivers. Perhaps somebody else can explain better. Adrian On January 13, 2020 5:09:12 PM UTC, "Müller, Marcus (CEL)" wrote: >Hey Adrian, that's nice! > >Generally: you said you couldn't implement an ALC: what's an ALC? > >Best regards, >Marcus > >Mon, 2020-01-13 at 16:40 +, Adrian Musceac wrote: >> Hi, >> >> I wrote a report about combining WSJTX and GNU radio with a PlutoSDR >transceiver to achieve FT8 contacts on the 144 MHz amateur radio band. >The short summary is that I achieved contacts at over 800 km and heard >stations as far as 1000 km last month in excellent tropo propagation >conditions with 20 Watts of output power. >> I thought you might be interested in reading this, but it's too long >to paste here. >> >> The report can be read here: >> http://qradiolink.org/ft8-plutosdr-2meters.html >> >> Adrian YO8RZZ
Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band
Hey Adrian, that's nice! Generally: you said you couldn't implement an ALC: what's an ALC? Best regards, Marcus Mon, 2020-01-13 at 16:40 +, Adrian Musceac wrote: > Hi, > > I wrote a report about combining WSJTX and GNU radio with a PlutoSDR > transceiver to achieve FT8 contacts on the 144 MHz amateur radio band. The > short summary is that I achieved contacts at over 800 km and heard stations > as far as 1000 km last month in excellent tropo propagation conditions with > 20 Watts of output power. > I thought you might be interested in reading this, but it's too long to paste > here. > > The report can be read here: > http://qradiolink.org/ft8-plutosdr-2meters.html > > Adrian YO8RZZ smime.p7s Description: S/MIME cryptographic signature
