Saturday, 9 January 2016

The things I have learned about SpectrumLab (2)

Adding a DSP Noise Blanker:

I have been using the Grabber on 40 metres recently, and had a terrible time with QRN crashes from lightning in thunderstorms around the Oceania Region.  The crashes are so bad they obliterate incoming QRSS signals completely at times.

SpecLab has a secret weapon to combat QRN!  I have learned that SpecLab has built in DSP Noise Blankers, which are completely configurable!  After reading the help file on the NB's and learned how to configure them correctly, I was utterly amazed at how effective they are.

How to configure a DSP Noise Blanker and get it up and running:-

You will need to reconfigure the default setting of Speclab in the **Components**Circuit Window** menu.  The default setting for the input to Spectrum Analyser1 is circuit point L1, this needs to be changed to circuit point L2 to include Blackbox1.  To do this click the box with "L1" in it shown wired to Spectrum Analyser1 (lower/centre).  In the pull down menu that appears click "L2=Left Blackbox1" then click OK.  You should have what is shown below on screen.

Spectrum analyser1 is now connected to the output of the left hand Blackbox1, this is where a Noise Blanker resides and can now be enabled.

Click left hand Blackbox1, and a pull down menu appears, select Noise Blanker, and enable it by clicking Noise Blanker and then Mode in the further pull down menu.  Blackbox1 between L1 and L2 should now turn green showing a NB is enabled.

You now need to configure the NB parameters to make it work effectively.

You need a receiver with an antenna connected to it, and setup on the band you want to use.  The noise spectrum from the receiver should fill the lower 5dB or so of the onscreen frequency spectrum.

Go back into the NB pull down and click "Trigger peak/avrg = xx" this will open a further pull down.  Now click "Connect to slider" then "OK".  In the 2 text boxes at lower right on the Components screen - the top one should now say "Noise Blanker Trigger Level L1" and beneath it you will see the level - xxdB.
On the far right of the components Screen there is a vertical slider, now slowly adjust the slider up and down until the yellow frequency Spectrum display just disappears.  Now readjust the slider until the Freq Spectrum reappears just back to it's normal level, now set the lower scale to read +1.0 dB above that normal level.  That is the most important parameter now set.

The rest of the parameters need to be set to the following list:-

Ramp Time = 0.01 sec
Pre-trigger blanking time = 0.0005 sec
Post-trigger blanking time =0.3 sec
Average detector RISE time const = 0.01 sec
Average detector DECAY time const = 0.01 sec

The figures below the dotted line in the NB menu are the current figures for the spectrum you are currently viewing and will change with time.

You can use the slider to adjust the values (don't forget to check the slider box to bring the slider up for that particular parameter) or just enter the figures given into the parameter box directly.  These figures certainly work on 40 Metres for me even with a full blown local thunderstorm raging around me!

The Post Trigger blanking time for me was the trickiest to set.  It needs to be long enough to just completely blank one lightning flash, and 0.3 seconds or slightly shorter seems to be best for me - too little time and you see the end of the noise pulse - too much time, left a black vertical line on the Spectrum screen.  Thicker vertical black lines on the waterfall are where the NB has blanked off multiple strikes, being retriggered several times during the Noise Spike so the shorter this can be, within reason, the better.  You will hardly notice the blanking caused by a single lightning strike - you will hear the spike in the receiver, but it will not show on the waterfall.

The lack of highly coloured vertical lines certainly makes better reading of the waterfall, but the NB will block only short rise time spikes - not QRM from CW, phone or telegraph transmissions.

Don't forget to do a save to your "Config" file, before you exit SpecLab!  Otherwise you will have to do this all over again....

10 Jan 2016

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