Loading ...
Sorry, an error occurred while loading the content.

Re: [perseus_SDR] Re: Perseus sensitivity

Expand Messages
  • Bjarne Mjelde
    Why should there be an error? bjarne
    Message 1 of 50 , Nov 30, 2007
    • 0 Attachment
      Why should there be an error?


      Nico iv3nwv skrev:
      > Maybe, Bjarne,
      > but if this is the case there should be an error somewhere, in my
      > calculus, in your instrument settings or measures, in the SDR-IQ
      > specification or in the book of Taub and Shilling.
      > Regards,
      > Nico
      > > Coincidental or not, I measured my SDR-IQs from RF Space to -102dBm
      > so as far as sensitivity goes they seem to go head to head.
    • pd0psb@hotmail.com
      Hi Leif, Thank you very much for your elaborate response. It is exactly the info I was looking for and quite a bit more as a bonus :-) As a rule of thumb I
      Message 50 of 50 , Nov 15, 2011
      • 0 Attachment
        Hi Leif,

        Thank you very much for your elaborate response.
        It is exactly the info I was looking for and quite a bit more as a bonus :-)

        As a rule of thumb I always try to achieve a rise in noise of around 6dB when plugging in an antenna for HF DX use, so that roughly corresponds to your presented calculations.

        Very clear explanation and very much appreciated!


        --- In perseus_SDR@yahoogroups.com, Leif Asbrink <leif@...> wrote:
        > Hello Paul,
        > > Regarding this subject I have a question for you which has been on my mind:
        > >
        > > Suppose.
        > > At the antenna I find a noisefloor of -115dBm and a weak signal
        > > of 3dB SNR just above the noisefloor.
        > >
        > > How far does the noisefloor+signal need to be lifted above
        > > the RX selfnoise so the original SNR at arrival does not deteriorate?
        > >
        > > IOW How far apart does the bandnoise at antenna and RX selfnoise
        > > need to be to make (partly) masking of the weak signal negligable?
        > Negligible depends on who you are. For casual DXing you might
        > say that 1 dB is negligible. In the EME community a signle dB is
        > a significant loss. When listening for weak signals bounced off
        > the moon one would not want to loose even 0.1 dB.
        > > I have always assumed that if antenna noise and RX selfnoise
        > > are equal, a weak signal still deteriorates by a certain amount
        > > by the adding of those "2 noise layers" , is this correct?
        > > and how much?
        > Yes. Absolutely.
        > The noise from the antenna and the RX selfnoise is uncorrelated.
        > They add linearly by power. When the noise floor of your antenna
        > equals the self noise of your RX the sum of both of them is
        > twice the power compared to the selfnoise alone. Twice the
        > power means 3 dB. That means that if your noise rises by 3 dB only
        > when the antenna is connected, 50% of the noise is the selfnoise
        > and your S/N is half as good as it whould have been with an appropriate
        > preamplifier. You would have a 3dB S/N loss.
        > The key factor is by what amount your noise floor rises when the
        > antenna is connected. Here are some examples where R=selfnoise and
        > A=antenna noise.
        > Rat=(R+A)/R is the amount by which the noise increases when the
        > antenna is connected. It is a power ratio and it can be expressed
        > in dB or as a factor in linear power scale.
        > S/(R+A) is the S/N ratio of the desired (weak) signal.
        > S/A is the S/N ratio we would have in a noise-free receiver.
        > Sens=[S/(R+A)]/[S/A] is the sensitivity we have vith respect to
        > the sensitivituy we would have had without selfnoise. It is a
        > power ratio. We can express it linearly or in dB.
        > Sens simplifies to A/(R+A) =1-R/(R+A)
        > The following table gives some numbers:
        > Rat Rat Sens Sens
        > (dB) (lin) (lin) (dB)
        > 3 2 1-0.5=0.5 -3.01
        > 6 4 1-0.25=0.75 -1.24
        > 10 10 1-0.1=0.9 -0.45
        > 16 40 1-0.025=0.975 -0.11
        > You should measure Rat with the best noise blanker enabled.
        > Pulses can be eliminated and should therefore not be included
        > in the noise floor summations.
        > A DXer should have at least 6 dB noise increase when
        > connecting the antenna while a moonbouncer should have a
        > noise increase of at least 15 dB when connecting the
        > antenna-mounted preamplifier.
        > With too much gain the receiver might saturate. There is no
        > reason to go above 18 dB or so. If you have saturation problems
        > it is reasonable to go as low as 6 dB. If it is necessary
        > to attenuate more one should use filters to filter out
        > the frequency of the local station(s) that cause saturation.
        > 73
        > Leif / SM5BSZ
      Your message has been successfully submitted and would be delivered to recipients shortly.