Stephen,
There is a lot of ripple in the variable filter passband when it is set
to a wide filter value. Even with the ripple, the filter is quite
usable on SSB voice, but for data modes, it does have its shortcomings.
That is one good reason that I set up the RTTY filter set for use on
data modes. I typically set the FL1 position the same as SSB FL1 (with
the OP1 SSB filter), but I set relatively narrow widths for RTTY FL2, 3
and 4 - I like to use 1.00 for FL2, 0.70 for FL3 and 0.40 for FL4. I
center these filters at 1000 Hz (or 800 if the BFO range is not adequate).
In use, you can first tune in the desired signal using the FL1 position,
and then use the VFO to position that signal in the waterfall to 1000
Hz. You can operate with FL1 as long as no interfering signals come
along - should an interfering signal occur, you can lock the transmit
frequency in the software application and then use the XFIL button to
narrow your receive passband down to a point where the interference is
reduced. Locking the transmit frequency is required because of slight
differences in the BFO frequency as you shift between the filters - if
you need to retune the receiver frequency (in the waterfall), it will
not change the transmit frequency.
73,
Don W3FPR
Stephen Prior wrote:
I didn't build my K2 (I wish had!), so I have limited experience of its
alignment.
Having finally got around to getting back on psk31, I have noticed a banding
effect in the waterfall - both with mixw on the pc and cocoamodem on the
mac. Removing the audio feed from the K2 shows the usual digital artifacts
often observed with integrated soundcards in particular, but no evidence of
any ripple. I'm using the inbuilt sound on a new Intel mac mini running
both Leopard and XP. I don't believe the computer is to blame. Anyway, as
noted below, the OP1 filter shows none of this.
Running spectrogram with random noise at the antenna (not much else on 12m,
four hours after dark!), I can see a distinct ripple in the passband of all
filter widths, in every mode. Except, that the OP1 filter in the ssb board
gives a flat response in all modes. So it's something to do I guess with the
setup of the variable crystal filter. I am wondering how far back I will
need to go in the setting up of the K2 to find the adjustment which will
perhaps flatten the shape.
Using spectrogram (latest) with the xtal filter in usb at 2.1kHz, there is
about 10dB between peak and trough of the ripple. The peaks are roughly
600Hz apart. I'd never noticed this in casual listening, I doubt that I
could have done. Most of the time I am on cw and the 700Hz filter setting
shows no obvious ripple anyway. It does however certainly not look right on
the waterfall on psk31!
I'd be grateful for any advice!
Thanks,
Stephen G4SJP
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