[Elecraft] KDSP2 and Spectrogram

Thu Jul 17 07:58:00 2003

G'morning All:

At Eric's suggestion, I ran the same test as below but with the AGC OFF, RF
Gain at about 9 o'clock, and AF Gain sufficient to give a nice spike. The
results are:

 0 db (Peak) @ 610 Hz
-1 db               Width = ~31 Hz
-3 db               Width = ~61 Hz
-35 db             Width = ~225 Hz

Which IS a bit better than with the AGC turned ON... however, since I
operate with the AGC turned ON, I'm not sure what it all means to me<g>. The
other "however" is that I really don't think it matters that much. It is
pretty good no matter how you slice it!

73, Ken K3IU
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
----- Original Message -----
From: "Ken Wagner" <[email protected]>
To: "Lyle Johnson" <[email protected]>; "Dave" <[email protected]>;
<[email protected]>
Sent: Wednesday, July 16, 2003 18:48
Subject: Re: [Elecraft] KDSP2 and Spectrogram

| Hi Lyle:
|
| For your FWIW file<g>.
|
| Using the birdie at 4.0 as you suggested, I measured the following:
|
| CW Mode
| XFIL4 - 200 HZ
| DSP C4(default, not soft) - 50 Hz width at center freq of 600
| Noise Reduction OFF
| Both Gains set to 0.0
| Rcvr tuned to ~4.000+/-
| Output from External Speaker to 'puter
| Looking at it in Spectrogram
|
| 0 to -1 db (Peak) - Width = ~25 Hz
| -3 db               Width = ~85 Hz
| -35 db             Width = ~240 Hz
|
| So much to learn........
|
| 73, Ken K3IU
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ----- Original Message -----
| From: "Lyle Johnson" <[email protected]>
| To: "Dave" <[email protected]>; <[email protected]>
| Sent: Wednesday, July 16, 2003 17:51
| Subject: RE: [Elecraft] KDSP2 and Spectrogram
|
|
| | Hello Dave!
| |
| | > My KDSP2 arrived yesterday, took a couple of hours to build and
install.
| | > Works fine on air...
| |
| | This is what it was designed for, success on the air :-)
| |
| | > However, tonight I hooked up Spectrogram and a noise
| | > generator. The filter doesn't look as narrow as I would have expected
| and
| | > appears no narrower than about 200 Hz - even when set to "50 Hz".
| | > Settings, CW, 700 Hz centre frequency and de-noiser off.
| |
| | When you say no narrower than 200 Hz, do you mean at the top (3 to 6 dB
| | down), or at the "bottom" (30 to 60 dB down) of the Spectrogram plot?
| |
| | At the 30 to 50 dB down area, using the default (not "soft") filters, I
| | would expect 250 to 300 Hz to be displayed by Spectrogram.
| |
| | I wouldn't believe Spectrogram for anything below about -30 dB to -40dB
| from
| | its peak response (driven to the limit before distortion) without
actually
| | calibrating it.  It is a good relative tool, but not a lab instrument.
And
| | with a noise signal it may be hard to see the distortion. Try backing
down
| | the RF gain so the signal is about 20 dB weaker than you are currently
| using
| | it, and see what you get.
| |
| | With my unit and a noise generator, I see a peak of - 37 dB at 611 Hz,
| 280
| | Hz wide at -60 (or 23 dB down from the peak
| | ***
| |
| | A better test might be the following.
| |
| | 1) tune in the 4.000 MHz internal K2 calibration signal.
| | 2) disable AGC
| | 3) adjust RF gain so nothing is overdriven
| | 4) adjust AF gain for a reasonable indication on an AC voltmeter - or
| | Spectrogram if you wish, in which case go for a fairly high level
| | 5) tune the radio from the marker signal in 50 Hz steps (use the .1 kHz
| | tuning rate and note that there are two steps to change digits - the
radio
| | actually tunes in 50 Hz steps, or at least mine does :-)
| | 6) note the amplitude
| | 7) tune to the next step, etc.
| |
| | This may give you a better idea of what is happening.
| |
| | For example, in my unit I just measured the following:
| |
| | Freq Amp (dB)
| | 477  -60
| | 506  -40
| | 571  -32
| | 614  -32
| | 668  -38
| | 711  -54
| |
| | Note that since an FFT is being done in Spectrogram and the radio may
not
| be
| | tuned to exactly the center of a bin, the steps aren't exactly 50 Hz.
| |
| | What we see from this is the following
| |
| | Peak is about (614-571 =) 43 Hz wide.  OK for a 50 Hz filter with
limited
| | test gear applied.
| | 8 to 10 dB width is about (668 - 506 =) 162 Hz.  This is a reasonable
roll
| | off.
| | Approx 30 dB down is (711 - 477=) 234 Hz.
| |
| | The design is to have the filters be about 300 Hz wider at -40 to -60 dB
| | than the selected width.  At +/- 100 Hz, we're about 30dB down, so it is
| | working pretty close to spec.
| |
| | As always, the important thing is not what Spectrogram says, it is what
| your
| | ears tell you :-)
| |
|