[Elecraft] DSP
William Carver
bcarver at safelink.net
Fri Aug 7 15:19:25 EDT 2009
W7AAZ SAID:
> It's not true that once signals get through the xtal filter
> they cannot be removed. A DSP filter CAN remove them...after
> all, the FLEX radio works without ANY roofing filter!
W4TV SAID
That's not true. Even the Flex will not be able to remove a
-50 dBm signal (S9 +20) and allow you to copy a -135 dBm signal
(at the noise floor) 50 or 100 Hz away. That's a blocking issue
not an IMD issue. True IMD requires two or more interfering
signals with a specific relationship and with narrow filters
one of the two tones is almost never inside the filter.
W7AAZ now sez:
I agree, separating two signals isn't an IMD issue per se: third order
intercept or two tone dynamic range is only an indicator of linearity.
The difference between -135 dBm and -50 dBm is 85 dB. That's within the
envelope of DSP filter capability: it CAN separate two signals 85 dB
apart in amplitude if there is reasonable frequency spacing between
them. Choosing 50-100 Hz is NOT a reasonable criterion (there's no
ham-practical xtal filter than will do it, either!).
I have an RX with 3.1 KHz roofing filter and it can separate two CW
signals a few hundred cycles apart, both inside that filters passband. I
can easily copy a 0.1 microvolt signal with an S9+20 dB signal in the
passband of the filter. Its DSP is several generations old and less
capable than the K3.
The bottom line is if you can preserve second i.f linearity and make the
DSP better you can reduce dependence upon the roofing filter.
Alternatively if you can supply a narrow roofing filter you can relax
dependence upon strong second mixer and DSP performance. In fact, if you
have a just-wide-enough "roofing" filter, you don't NEED the second
conversion or DSP at all.
It's an engineering/economic tradeoff. Given the linearity of the K3
second i.f. the only option is more xtal filters.
Bill
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