[DSP-10] Filter Sweeps - new results and questions from n5bf/6

[Bob Larkin]

Hi Courtney -

Great to hear from you.  Beb and i were just talking the other day about 
your apparent (but not true) absence!

Some while ago, I did an experiment to determine the response of my radio: 
I set my #2 DSP-10 to USB, Spec Width to 4800, IF to wide, scale to 1dB/div 
and SpecAve to 102. The antenna was terminated so that only noise was 
measured. I then took screen shots at 144.201, .203 and .205.  Cutting out 
pieces either 2 or 2.5 kHz wide and pasting them together shows the filter 
response from about i-f values of 18 to 12 kHz (Xtal filter from 19.662 to 
19.668).  In USB, you can see all the frequency relationships by doing an 
ALT-A.

On   http://www.proaxis.com/~boblark/p527xfil1.gif  I marked the i-f 
frequency at the bottom, and the frequency setting for each of the 3 
segments at the top.

The segments line up well, meaning that it is indeed the xtal filter 
response being seen.

Mine seems quite flat (about +/-0.5 dB) except at the right edge where it 
is dropping off by 2 dB or so.  This reflects the xtal filter topology that 
has the fast cutoff on the high frequency (19.668) side. I do not believe 
that any alignment will alter that very much.

Others may want to try this same experiment and report their results.

The tuning of the two coils does effect the response of the filter.  You 
might play with these. I thought we had some suggested procedure for this, 
but I can't find it right now. Maybe someone else can help us here.

Good luck!

73,  Bob

At 10:34 PM 8/20/2007 -0700, Courtney Duncan wrote:
>This year got a little busy.  In February my mother had both her knees 
>replaced and in May my first daughter graduated from college. Now I'm back 
>having a little DSP-10 fun again.  Recent results are posted at
>
>http://cbduncan.110mb.com/HamRadio/Dsp10/Dsp10.html
>
>(For those of you with old links, I've moved my website to this new location.)
>
>I have a couple of questions for the group about this filter sweep data:
>
>The first two plots cover 30 KHz, that is, six cycles of 5 KHz, 126 MHz 
>synthesizer switching in 100 Hz points.  I'm interpreting these data as 
>the ripple or some passband shape of the 19.665 MHz first IF filter.  (I 
>think hams call this the "roofing filter" in contest rigs, is that 
>correct?)  Peak to peak, the passband varies by about 2 dB which seems 
>about 1 dB more than it should be.  Have I installed something wrong or, 
>in any case, is there something I can/should do about this?  Or is it 
>about right for this case and I shouldn't worry?
>
>On the third plot, I've swept the whole band (10 KHz points) and it looks 
>about like you'd expect except that it seems to roll off excessively at 
>the bottom of the band where all the interesting activity is.  For 
>instance, I did most of my EME2 tests below 144.1 and was therefore losing 
>a dB or two just due to operating down there.
>
>Would it help to re-peak the filter chain at something like 146.5 or 146.0 
>rather than 147.0 or am I seeing some other effect here, like amplifier 
>device or filter rolloff?
>
>I picked the power setting so that all of this would be right around what 
>I think is the 1 dB compression point for the Brickette output (about 3 
>watts).  If anything, getting into compression should flatten a plot like 
>this shouldn't it?
>
>I guess most DSP-10 users are using the box as an IF to microwave 
>frequencies.  In that case the band-sweep issue might not be so important, 
>but it seems like the 19.665 MHz filter shape could be.
>
>As always, any insight would be greatly appreciated.
>
>73, Courtney, n5bf/6