[Test-Equipment] LF Ailtech 7514 and noise sorce

[Brooke Clarke]

Hi Ed:

In the mid 1960s we used the HP 340 (AFAMMG) and had it on a wheeled cart with 
a number of noise sources.  For a mixer we used a Narda coax coupler (blue with 
red metal label and came in octaves like 2-4 GHz, 4-8 Ghz, etc) and a tunnel 
diode detector as the mixer.  The coupler is connected in what might be called 
the backwards direction, i.e. the coupled arm feeding the TD detector.  Where 
you connect the LO and where the output of your DUT depends on the signal 
levels of each.

For most amplifiers what you'll get is called double sideband noise.  Say 
you're testing a 3.7 to 4.2 GHz amp with the LO at 4 GHz  The signal at 4 
GHz+30 MHz and the signal at 4 GHz-30 MHz both get converted to 30 MHz and so 
it's a DSB measurement.  BUT, if you're measuring a narrow band amp like 2.2 to 
2.3 GHz, like I was doing for some time, and the LO gets lower than 2.23 or 
higher than 2.27 then the NF drops as it transitions to a SSB measurement.  It 
takes a lot of hand waving to get the customer QC to accept it.

The other way of measuring noise is called the Y factor method.  I think it was 
an AIL box (brown color about 1 foot on a side) that has two 50 Ohm 
terminations, one in a LN dewar and the other in a heater.  You just measure 
the RMS voltage or power at the IF for each one and calculate the Noise Factor 
(different from Noise Figure).

A variation on that is called a Dicke Radiometer.  You point an antenna at 
outer space and switch the input to your LNA from the antenna to a termination 
that's at a known temperature.  When you switch you also switch the video 
output polarity.  If both signals are at the same power the video out will be 
zero, but if they are different then you get some DC out in a polarity that 
tells you which is larger.

Instead of using something pedestrian, like a square wave (which works fine), 
you can use a long PN code.  The later was built into one of our products.  On 
that one you could just hold the reference termination between your thumb and 
forefinger and after a while you knew your body temperature.


Have Fun,

Brooke Clarke, N6GCE
http://www.PRC68.com
http://www.precisionclock.com
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Edward J White wrote:
> Hi Gang: Anybody have a Noise system that works for sale Not looking for 
> something to expensive.
> Ed
> WA3BZY