[Elecraft] RTTY FILter

[Joe Subich, W4TV]

> The Butterfly filter doesn’t introduce phase shifts or impulse
> response. It’s normalized for 170 Hz shift and for the most part,
> the filter bandwith is set by the Universal M8000 firmware.

Every filter has differential phase shift and and group delay (impulse
response) characteristics.  Cascading filters causes an interaction
between individual phase responses of the filters.

Have you measured the response of the Elecraft Dual PB filter without
the Universal Butterfly filter in line?  That is the only way you can
provide an clean evaluation of the Elecraft Dual PB filter.

> As I said “The bottom line is, If your using an external decoder, the
> AFSK single filter works the best”.

First, it's not the "AFSK single filter" as the filtering is identical
in both the AFSK A and FSK D data sub-modes.  Secondly, that is not
always the case ... if one uses the "AA6YQ FIR 512" profile in MMTTY
(another "butterfly" type filter) results are somewhat better with a
wider DSP setting and no Dual PB filter in the K3.  However, if one
uses a different "profile" (e.g., standard RTTY) or one of the optional
detectors (FFT or PLL) using a narrow (e.g., 400 Hz) DSP setting with
the Dual PB filter can provide better results that a simple wide DSP
setting.

None of that is surprising given the nature of cascaded narrow filters

73,

    ... Joe, W4TV


On 9/4/2011 2:23 PM, Radio Repair wrote:
> Hi Joe,
>
> The Butterfly filter doesn’t introduce phase shifts or impulse response.
> It’s normalized for 170 Hz shift and for the most part, the filter bandwith
> is set by the Universal M8000 firmware.   There are 6 bandwith settings
> from -3 to +3  that I can change on the M8000 front panel that will allow
> me to optimise the bandwidth.  If the filter is set too narrow the butterfly
> display will show the impulse response but the default setting “NOM” is
> optimal for all modes, shifts and speeds.  As I said “The bottom line is, If
> your using an external decoder, the AFSK single filter works the best”.
>
> I don’t know how the K3 handles the dual filtering internal to the DSP but
> I assume it’s optimized to the internal DATA decoder.   More than likely it’s
> OK to use the dual filter when your copying and sending RTTY on the K3
> without the use of an external decoder.
>
>
>