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value at center of filter skirt in SSB

K2KXK
K2KXK Member ✭✭
edited June 2020 in SmartSDR for Windows
When in USB or LSB mode, the filter skirt displays the low and high pass values as well as a center value.  For example in USB with the 2.7 filter selected, the skirt shows 100 at the low end and 2800 at the high end.  The meaning of these is clear.  It also shows 1.5k at the center-top of the skirt.  What does this value represent?  How is it calculated?  How do you use it?   Thanks.

Answers

  • Patrick
    Patrick Member ✭✭✭
    edited June 2020
    I think it may be the half power points (3db) of the filters created.   1.5K+1.5K = 3k-2.7K = 300Hz/2 = 150 Hz is ~ 50% point or half power.  I guess that is clear as mud.... 
  • N8AUM
    N8AUM Member ✭✭
    edited February 2020
    Doesn't the 3db point change depending on which setting is used in the filter settings ?
  • Larry - W9FLA
    Larry - W9FLA Member ✭✭
    edited February 2020

    Three of the numbers represent/describe the location of the filter parameters with respect to the carrier or zero frequency.  Two are the corner frequencies of 100 Hz and 2800 Hz.  The 2.7 kHz (2800 – 100) represents the filter bandwidth.  The top 1.5K number represents the location of the filter center frequency with respect to the carrier and shown with two significant figures.  It can be calculated as:

     

    Fl+(Fh-Fl)/2 i.e 100+(2800-100)/2=100+1350=1450 or approximately 1.5K





  • K2KXK
    K2KXK Member ✭✭
    edited February 2020
    Hi Larry.  Thanks for the great explanation.  I now know what the center value represents.  I'm not sure how I would use this value though.  I guess it's just for informational purposes.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited February 2020
    It is used for operating digital modes, like PSK31 where you may want to know the center frequency offset for the receive filter if you are also using a narrow TX filter in the center of the RX passband.
  • K2KXK
    K2KXK Member ✭✭
    edited February 2020
    Makes sense Tim.  Thanks.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited February 2020
    You're welcome.

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