[TenTec] Questions about NR i.e. Orion

[geraldj]

On Sat, 2006-03-25 at 09:58 -0500, Bill Tippett wrote:
W9OY wrote:
> 
>  >I have been playing with a SDR-1k recently.  The
> filter scheme in that radio allows down to a 10hz
> bandwidth.  What I have discovered is there is no
> ringing in a filter this narrow generated by the
> algorithm used in the SDR, even if the signal being
> received has a bandwidth considerably wider than 10
> hz. (up to 200hz for example).  Precise tuning of the
> received signal is a requirement.
> 
>          Lee I have no doubt you are reporting
> exactly what you observed, but doesn't this seem to
> violate the laws of physics? (i.e. receiving a 200
> Hz signal through a 10 Hz filter).  Have you ever
> measured the actual BW of the "10 Hz" filter?  If
> not, I suspect there may be some optimism in the
> setting.  Many manufacturers including Ten-Tec
> do this.  For example Orion's "100 Hz" 6 dB BW
> is actually around 150 Hz.  Another issue is
> the shape factor of the filter.  If a "10 Hz"
> filter had an extremely wide shape factor, it
> would actually pass energy much wider than the
> stated 6 dB BW.  If I recall, Orion's "100 Hz"
> setting has a shape factor around 3:1.  This
> coupled with the actual 150 Hz measured BW
> accounts for the fact that Orion never rings
> even at its lowest DSP BW settings.
> 
>          If the SDR-1000 folks have discovered
> some way around Nyquist, Shannon and Hartley,
> I would love to know how they did it.
> 
>                                          73,  Bill  W4ZV
> 
> 
> A filter need not have square corners, nor a ringing response. A Butterworth filter
will take at least 5 times the speed of a square cornered filter and a Bessel will do
better than that. The DSP filters in common use do far better than similar analog
filters because the DSP filters have far higher orders allowing a better approximation
of the amplitude response desired along with a better time response. This can be
accomplished in analog filters too, but hardly ever is because of the significantly
greater filter complexity.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer