Re: Soundcard based Noise Figure Alignment aid


Ian White
 

Andy Talbot wrote:
If you've read December's RadCom (Design Notes) you'll have seen my
outline for a soundcard replacement for the G4COM box and request for
someone to write some code to do it.

Even before opening my copy of the magazine (having been away in
Birmingham for the RSGB EGM) Peter G3PLX had written some suitable
software and emailed it to me for testing.    Got back home, built up
a suitable driver, tested, and after a bit of tweaking it all works
FB.

Wrote it up,  and this plus the software can be found at
http://www.g4jnt.com/NoiseMeter.zip

The new design cantake in audio up to 22kHz, so by using both
sidebands from a direct downconverter you have 44kHz of noise
bandwidth to smooth out the reading better than an SSB filter
bandwidth allows.    Also, with a 16 bit soundcard means it can work
over 80dB or so of dynamic range
A very similar design with comprehensive software was published by Dave
Roberts G8KBB in Radcom January 2007. Details of the downconverter and
software are downloadable from
<http://g8kbb.roberts-family-home.co.uk/html/downloads.html> along with
some very detailed assessments of errors and performance.

Dave's software allows both manual control and automatic switching of
the noise source (and if needed, the DUT as well) through either a COM,
LPT or USB port. The automatic switching cycle includes a settling time
to allow the noise source and DUT to stabilise before the measurement
begins, and a delay after the end of each measuring period before the
system changes state. When the settling time is set correctly, this
should also take care of timing uncertainties in the PC ports.

If a 28V switched supply is provided for the noise head, that interface
needs to conform to industry standards: the instrument provides a
switched voltage source and the current regulator is in the noise head.
(I'm not sure what happens when two series-connected current sources are
switched on and off, but it probably isn't good.)

G(M)4PMK's RATS design in Radcom July-August 1995 includes a suitable
hardware interface, an LM317LZ voltage regulator with a MOSFET switch.



--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

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