[DSP-10] AA1YN bandpass flatness & birdies

Tue, 16 Mar 2004 21:40:12 -0000

Hi Bob,

Is this what u are looking for ??

73 de David GM6BIG

----- Original Message -----
From: "Bob Larkin" <[email protected]>
To: <[email protected]>
Sent: Tuesday, March 26, 2002 3:50 AM
Subject: [DSP-10] AA1YN bandpass flatness & birdies

> Hi Lee and others interested in these problems!
>
> As W7SZ has identified, the changing noise with tuning is associated with
> crystal filter flatness.
>
> First for the theory side of thing. The crystal filter, along with
virtually
> all modern filter designs, are "synchronously tuned." This means that each
> loop of the filter is tuned to the center frequency, with the bandwidth
and
> response coming from the interaction between the resonators. In our case,
> the synchronous tuning is pretty much set by the crystals and the coupling
> caps. The two tuning adjustment coils are really parts of an 'L' networks
> used to step up the impedance to that needed by the filter. But, they can
> present a reactance, and when all is tuned properly this reactance will be
> gone. Also, this will be the condition that will minimize the insertion
> loss. That is why the tuning for max signal will be very close.
>
> Fig 5 in Sept QST is an actual measured filter.
>
> Now for an alternate alignment procedure (this is from last Dec - I
thought
> I had posted it then. If so, I'm sorry for the duplication!):
>
> ---Procedure--------
>
> 1-First, the best test setup for playing with this is a noise input and
> watching the spectral response. The handiest noise source is a preamp or
> converter with a gain of 15 dB or more.  No antenna on the input. 50 Ohms
is OK.
>
> 2-Now what to watch. Put the DSP-10 in LSB (this can be done in USB, but
the
> frequencies get hard to follow. Activate LSB and use it). The hit Alt-A to
> see the frequencies. The DSP-10 IF#2 shown is the frequency corresponding
to
> zero-beat or the left edge of the spectral display. It will range from
10251
> Hz to 15250Hz as the radio is tuned.
>
> 3- Set the SA width to 4800 and the IF filter to wide (Alt-$). The
spectrum
> from about 500 to 3500 Hz will now be flat, except for the response of the
> crystal filter. Ignore everything outside of the 500 to 3500 range. Maybe
> put a piece of tape over it!  Set the SpecAve to 36 or so, to smooth the
> noise response out. Put the display in 2 dB/div.
>
> 4-Tune the DSP-10 to 146.159 700 and the IF#2 will be 10300. The region
from
> 500 to 3500 Hz corresponds to the IF#2 of 10800 to 13800. Then tune the
> DSP-10 up 100 Hz to 146.159800. The IF#2 is now 15200 ad you will see
15700
> to 18700 inside the 500 to 3500 range. These two frequencies are good
enough
> to see the response of the crystal filter.
>
> Concentrate on the lower response, from 10800 to 13800. It is much more
> sensitive to tuning than the upper one. First tune L12 and L13 for maximum
> response. You can use the Alt-2 and Alt-3 to keep the display on the
screen.
> Tune slowly because of the Spec-Ave of 36. Next, tune L12 and L13 in small
> steps to make the response as flat as possible. After the bottom part of
the
> filter  (10800 to 13800) is good and flat, look at the upper part (15700
to
> 18700). If it is not flat, try tuning it to correct this. On the ones I
> worked with, this was not needed.
>
> On mine, I was able to get the response flat to 1 or 1.5 dB peak to peak
> across any 3 kHz band.
>
> --------------end procedure -------------
>
> Lee, on the birdie, keep in mind that the 2nd I-F (10 to 20 kHz) is
> inverted. If you are in USB on 2-meters, you are in LSB at the 2ndd I-F so
> the birdie tunes backwards.
>
> There is a 5 kHz signal in U104, so this may be picked up by L32. Be sure
> that nothing in the way of wires is close to L32. Check the frequencies of
> the 2nd I-F DDS (variable BFO) by using an ALT-A followed by a frequency
> change. See if they make sense for being the harmonic of the 5 kHz signal.
> Also, if this fails, give us your settings and let me dig for a
> corresponding birdie.
>
> Good luck and thanks to Larry.
>
> 73, Bob  W7PUA
>
>
> >
> >At 09:51 PM 3/23/2002 -0500, you wrote:
> >>After much thinking about the wave of noise, I've come to a conclusion.
I
> >>may be so far off with my thoughts that I need someone like W7PUA to set
> >>me straight. - Anyway, here goes....
> >>
> >>The audio window is just under 5khz wide and we are moving that window
> >>5khz for a total of about 10khz.  We are asking the crystal filter to be
> >>flat for that 10khz.  If we adjust the filter coils L12 and L13 only for
> >>max signal, we may indeed be peaking that filter with a flat top width
of
> >>about 2khz.  If we start measuring the noise at a frequency just above
the
> >>5khz step and then slowly move up in frequency, you will see the peak of
> >>the noise at the high end of the audio window and it will look like a
wave
> >>as we move up in frequency until this wave is at the low end of the
audio
> >>window.  Once we reach that 5khz step point, the noise wave will then
move
> >>back up to the top of the window.
> >>
> >>Unless I am wrong, we really need a procedure for adjusting the crystal
> >>coils L12 and L13 so the crystal filter is as flat as possible for the
> >>full 10khz and not just centered in the 10khz window with it's 3db
points
> >>at +/- 5khz.  What I tried was to set the frequency to the low end of
the
> >>5khz step and adjust L12, set the frequency to the high end of the 5khz
> >>step and adjust L13.  By doing so, I was able to see 2 noise waves which
> >>kind of backs up what I've been describing. At least in my mind.
> >>
> >>If the local osc was continuously variable and we weren't moving an
audio
> >>window, you would never see this phenomenon and we could happily set up
> >>the crystal filter for a 3khz bandwidth and be pretty much flat across
it.
> >>
> >>Anyone else have any thoughts on this???
> >>
> >>As to the 5khz birdie I have, I have ran heavy grounding straps from the
> >>board to the chassis, from the board to the DSP box, I drilled 6 small
> >>holes in the top of the DSP box and soldered 14awg wire to tie the top
and
> >>bottom foils together next to each screw. I have double checked all
> >>grounding wires and touched up any which might be less than perfect.  I
> >>think I've reduced this birdie about 1db so it is now only 4db in
> >>height.  I did manage to lower the overall noise of the receiver from
> >>-149dbm to -152dbm with a 50 ohm termination on the antenna so there was
> >>an improvement there.  The birdie still moves up in the audio frequency
> >>when you move up in receiver frequency.  Since there are no actual
> >>frequency changes within the 5khz step, I still feel this has to be
> >>internal to the DSP program.
> >>
> >>One other ground point which eliminated my digital noise was to run a
> >>ground strap from pin 5 of the serial jack to its chassis mtg screw.  I
> >>had only grounded one end shields before at the DSP box.
> >>
> >>Hope my ramblings aren't too far afield.
> >>
> >>Lee Scott - AA1YN
> >>
> >>_______________________________________________
> >>DSP-10 mailing list
> >>[email protected]
> >>http://mailman.qth.net/mailman/listinfo/dsp-10
> >
> >
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> >
>
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