Re: Soundcard based Noise Figure Alignment aid

Andy G4JNT

One thing RSGB need to do as part of their revamp  is to place past copies of RadCom on the website - and not just the previous year's.
They can't, surely,  expect us to pay for stuff we've already read once and 99/100 times have no further interest in.  
Needless to say I chucked out all magazines before 2008 :-)  Who wants paper copies of stuff now.
Hopefully my Rad Club library cupboard will go back that far
Tried to find that design on the website you list, but he only shows circuit diagrams in the native .SCH form - which is most definitely not readable by my EasyPC which what .SCH and .PCB files load into here.   
Why authors cannot give circuit diagrams on web sites as .GIF or whatever I just don't know.  They're just asking not to be found or looked at.
As for the two current sources in series by the way:  I did that deliberately. When feeding a separate I-source,  the PNP transistor one which is set to a higher current (20mA) just saturates and delivers an on-off switched 28V or whatever.  Teh main reason for doing it like that is, since teh PNP is needed anyway as a switch, all it wanted was another resistor and a zener to give a nice current protected output.
My MaCom noise diode does not have its own integral current source, but does have the value of 12.3mA engraved on it which is what I set the separate LM317 source to deliver.    Homebrew ones can probably work just with the PNP source adjusted appropriately.


On 25 November 2011 21:49, Ian White GM3SEK <gm3sek@...> wrote:

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
<> 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

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