[Elecraft] KDSP2 and Spectrogram

[Ken Wagner]

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" <wa7gxd@fidalgo.net>
To: "Dave" <dave@astromag.co.uk>; <Elecraft@mailman.qth.net>
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 :-)
|