[Skimmertalk] Modus operandi?

[Pete Smith]

The secret is that Skimmer works with the digitized I/Q data.  Sound cards 
are available with reasonably flat response up to 192 KHz, and 96 KHz sound 
cards are dirt cheap.  So if you are using an SDR that relies on the 
computer sound card to produce the I/Q data stream, like a SoftRock, then 
your sound card determines the maximum bandwidth Skimmer can deal with - 
+/- 48 KHz for a 96 KHz sound card, for example - with the center frequency 
being set by the receiver.  Many modern SDRs have their own digitizing 
capability built in, and produce the necessary digital data stream for 
transmission via USB or a serial port to the computer.

The way to attach a Skimmer to the IF of any receiver is to take the RF out 
before the roofing filter, where the bandwidth is much larger, and then mix 
it down to a frequency that a simple SDR can handle.  Back in the neolithic 
era, radios like the TS-940 had this capability built in, for use with the 
Kenwood SM-220 monitor scope.

73, Pete N4ZR

At 07:11 AM 11/12/2008, Clive Whelan wrote:
>This will probably look like a pdq, but hey I'm just a CW operator  ;-)
>
>I can see how e.g Skimmer is working in a 3kHz b/w by looking
>sequentially ( or simultaneously?) at all audio tones in the passband
>and using its clever decoding algorithms.
>
>But how does it do this in say a 75kHz b/w ( assuming appropriate rx
>filters) when a sound card with response presumably limited to say 15kHz
>is being used to sample the output? Does it actually sample the passband
>in say 15kHz chunks and remember where it has been?
>
>As well as an academic interest, I am given to wonder whether modern
>receivers with only a 3kHz IF capability could achieve wider coverage (
>albeit not strictly in real time) by using their own internal scanning
>functions to cover the 75kHz capability of Skimmer?
>
>73
>
>Clive
>GW3NJW
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