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Over-Driven FT-8 QRM #FT8 QRM #FT8

Tony
 

All:

The interference caused by over-driven FT-8 signals is concerning. The distortion that shows up on either side of the FT8 watering hole is often stronger than the signal one is trying to copy down the band.

The attached image shows an example of the bandwidth occupied by distortion products that are well beyond FT8 operating frequencies.

This has been going on for some time now and it seems most prevalent on weekends when there's lots of activity.

Am I the only one noticing this?

Tony -K2MO

Wa4kfz
 

I suspect too many hams are operating with the SSB compressor on. One advantage of the newer gear is that it supports a defined digital mode where the compressor is always off.

73,
Mark wa4kfz

On Sep 21, 2019, at 4:50 PM, Tony <DXDX@...> wrote:

All:

The interference caused by over-driven FT-8 signals is concerning. The distortion that shows up on either side of the FT8 watering hole is often stronger than the signal one is trying to copy down the band.

The attached image shows an example of the bandwidth occupied by distortion products that are well beyond FT8 operating frequencies.

This has been going on for some time now and it seems most prevalent on weekends when there's lots of activity.

Am I the only one noticing this?

Tony -K2MO




<20000813_201058.bmp>

Graham
 

Its only  single tone , no  intermod  products , don't matter where the  alc is set,  that's  harmonics  , due to the  aggressive  tone switching ,  mfsk  produces a  similar set of sidebands ,  but not  as bad ,,  wspr2  makes the same , but the  rate is  slow, so no one notices ..interesting  arrl  have issued a statement  , stating its not  compatible  with there  data  award's ?

73-Graham
G0NBD

Chuck K4RGN
 

I check my transmitted digital signals (FT8, etc) with a US$30 SDR USB dongle and SDRSharp freeware. The cheapest dongles don't receive below 25 MHz, but that doesn't matter. Put a dummy load on your transceiver, select 10 meters, send CQ's or VVV's in your chosen mode, and view your transmitted spectrum in SDRSharp. (This is a poor-man's spectrum analyzer.) Avoid overloading the dongle; if the dongle is close enough to your dummy load, you won't need to connect a whip antenna to the dongle.  

If you're splattering, it will be obvious in the SDRSharp waterfall display. With this setup a ham can easily adjust the audio levels between the computer and the transceiver to achieve a proper RF signal -- without having to wait for other hams to tell you that you're splattering. 

These SDR dongles are Swiss army knives for hams. I encourage everyone to get one, unless you have the luxury of a real spectrum analyzer in your shack. The dongles don't make calibrated measurements, but I seldom need calibration. Relative measurements are good enough. 

73, Chuck, K4RGN

Graham
 
Edited

Chuck,

If you  run one of the  RTL $10  dongles,  you  can select  ,  direct conversion in the  driver , and the  device  will  Rx from  ~~ 0 to  25  MHz  ish ,  it  cuts out the  front  end and just uses the  back  a/c convertor  

Usually  enough  stray  coupling to  work,  if not , you  can  attach a line to  the  board ,  if  you  google  RTL direct  conversion,  there are  various shots  of where to  connect  to  

Perhaps a  better  display  is the Spec-Lab  software ,  justlink the  SDR sharp  detector  audio  with  virtual  audio  cable , then you  have a  choice of  decoder  , I think  the  audio  b/w of the  sdr  sharp  system is  constricted , not  wide enough to  monitor  say VARA  FM 9600  mode ,  , which is often deployed  via constricted audio  b/w 

Scope probe and pc sound-card , with  Spec-lab , makes a very good audio  measuring  tool as well  , check the  data input audio  b/w against the  recovered off  air 
73-Graham
g0nbd

Tony
 

Graham:

Single-tone modulation shouldn't produce inter-mod products, but some of the FT8 signals behave that way as if there were 2-tones in play. Could you elaborate on aggressive tone switching and it's effect?

Tony -K2MO


On 9/22/2019 12:15 PM, Graham wrote:
Its only  single tone , no  intermod  products , don't matter where the  alc is set,  that's  harmonics  , due to the  aggressive  tone switching ,  mfsk  produces a  similar set of sidebands ,  but not  as bad ,,  wspr2  makes the same , but the  rate is  slow, so no one notices ..interesting  arrl  have issued a statement  , stating its not  compatible  with there  data  award's ?

73-Graham
G0NBD


Graham
 



Hi Tony 

May be  aggressive is a subjective  assessment , 

If CW tx , could be  described  as  Hard/Aggressive  Keyed ,  that  would  give  the  same  results ,  Ie  , Soft keying / envelope shaping ,  produces ,  less  side bands or  Click's , less  envelope shaping ,  produces a  much harsher / aggressive / wider  signal 

A good  comparison   is the  original  MFSK  mode  and,  say, ROS-HF  ,

In terms  of the  on-air presence ,  MFSK and ROS  are  ''identical''  both  use  single  tone ,  both change  the  tone  frequency  and  duration, as directed  by the  resolve of the  mode coding and modem design , both are  'base band  heterodyned systems ', each may transit via, none linear  systems [ OLIVIA  has a  phase component to  the modem  modulation scheme , and as such , requires linear amplification ]


But, where  as the  MFSK mode , produces a  set of near in ‘side  band's’ , [ even marked on the  spectrum  plot  in Patrick’s Multi-psk, as additional B/W ]   ,  ROS-HF can be seen to  be  clean  and  producing only  the  on air  tone  set .

The modem in ROS is  Phase continuous , may be other systems  are  also, but  , the  ROS system , embodies design features that  are  missing/not as developed   from other systems , ''Gray noise''  is the general  term 

The audio  level  supplied to the  Transmitter  , remains  constant  ,

 However ,  fluctuations  may be  observed on the  TX power meter , these ,  appear to  be  a by product of the  filtering  in the  TX .  the  abrupt  changes in Tone ,  appear to  be  subjected to  the filter  characteristics , phase shift , group  delay etc, which  in turn , adds   phase / time  distortion , the resolve of which is amplitude variation

 Racal  listed  various option fit  filters , some , voice ,  some  data , so i suspect the  problem was  known about  some time  back , two filters  in the  tx/rx  path ,  40 MHz  roofing and the  1.4 MHz pass/band , using the  widest possible  filter , can  reduce the  ‘side bands’, the DSP  compensates  for  disturbance ,  but ,  from a transmission aspect, its better to  maintain the  ''waveform''   

That probably  explains the  different  on-air  signals ,  interims of additional  side  bands  , from a  single tone , or at least that’s my general  observation , Tx [ and  unseen RX issues] serve to  degrade the  signal  quality , the  claim,  no filter is required , hold quite true , if the  DSP is  correctly  designed .. 

73-Graham
g0nbd



Tony
 

Graham:

Thank you for that explanation. I can see how the CW hard-keying analogy could apply to digital. If this is something inherent in FT8, the side-bands would worsen with stations running high-power.

I was surprised to see that some hams run near legal limit with FT8 and I'm not sure if the developers intended the mode to be used that way. 

Tony -K2MO


On 9/23/2019 2:46 PM, Graham wrote:


Hi Tony 

May be  aggressive is a subjective  assessment , 

If CW tx , could be  described  as  Hard/Aggressive  Keyed ,  that  would  give  the  same  results ,  Ie  , Soft keying / envelope shaping ,  produces ,  less  side bands or  Click's , less  envelope shaping ,  produces a  much harsher / aggressive / wider  signal 

A good  comparison   is the  original  MFSK  mode  and,  say, ROS-HF  ,

In terms  of the  on-air presence ,  MFSK and ROS  are  ''identical''  both  use  single  tone ,  both change  the  tone  frequency  and  duration, as directed  by the  resolve of the  mode coding and modem design , both are  'base band  heterodyned systems ', each may transit via, none linear  systems [ OLIVIA  has a  phase component to  the modem  modulation scheme , and as such , requires linear amplification ]


But, where  as the  MFSK mode , produces a  set of near in ‘side  band's’ , [ even marked on the  spectrum  plot  in Patrick’s Multi-psk, as additional B/W ]   ,  ROS-HF can be seen to  be  clean  and  producing only  the  on air  tone  set .

The modem in ROS is  Phase continuous , may be other systems  are  also, but  , the  ROS system , embodies design features that  are  missing/not as developed   from other systems , ''Gray noise''  is the general  term 

The audio  level  supplied to the  Transmitter  , remains  constant  ,

 However ,  fluctuations  may be  observed on the  TX power meter , these ,  appear to  be  a by product of the  filtering  in the  TX .  the  abrupt  changes in Tone ,  appear to  be  subjected to  the filter  characteristics , phase shift , group  delay etc, which  in turn , adds   phase / time  distortion , the resolve of which is amplitude variation

 Racal  listed  various option fit  filters , some , voice ,  some  data , so i suspect the  problem was  known about  some time  back , two filters  in the  tx/rx  path ,  40 MHz  roofing and the  1.4 MHz pass/band , using the  widest possible  filter , can  reduce the  ‘side bands’, the DSP  compensates  for  disturbance ,  but ,  from a transmission aspect, its better to  maintain the  ''waveform''   

That probably  explains the  different  on-air  signals ,  interims of additional  side  bands  , from a  single tone , or at least that’s my general  observation , Tx [ and  unseen RX issues] serve to  degrade the  signal  quality , the  claim,  no filter is required , hold quite true , if the  DSP is  correctly  designed .. 

73-Graham
g0nbd




Graham
 

Tony .

These modes  are  ' base band heterodyned'  as in the  generation  is  performed  at the  base band , in this case  audio  ,  the  ' waveform'   is then  heterodyned / mixed to the  wanted  frequency ,   the  opposite  of the  RX process ,  where  audio is  presented to the  DSP , after the  RX has recovered the  'Audio  wave' , Tx power should not  alter the  side band ratio  ,  as long  its  its linear , 

Alc could  provide a  leading edge over shoot  and tilt  of the  TX envelope , that's something,  the on/off  keying  , deployed in 
Opera  is  susceptible too , its necessary to  reduce the  drive to the  onset of  ALC to  prevent a  spike on the  leading  edge,if the  audio  keying  is used , 

Replacing the  CW-Key  on the  other hand ,  the  signal  utilises the  TX envelope  shaping , the  system was developed , using the  20 wpm  test signal from the  racal ma1723 as the  standard  'shape'  

Power wise ,  that's  a  unfortunate situation , the  concept of  narrow b/w  short burst signal is really  counter productive , as can be seen from the levels  deployed ,  multiple  access  at  low s/n  is best provided  by  the  appropriate  techniques , making  use  of the  SSB b/w channel ,  requiring a  second  layer in the  coding    applied to the  'modem' ie the  tone placement  routines . 

73-Graham
g0nbd