On 14 Oct 2004 16:44:35 -0700, (Steve
Kavanagh) wrote:

I haven't really been following this thread but if you are looking for
noise diodes and associated stuff, you should take a look at:

http://www.noisecom.com/

Seems to me that at one time they would calibrate a homebrew source
for a modest fee. That may have been a ham that worked there paving
the way; not sure.




|"Jim" wrote in message ...
|
| This other method involves measuring the gain of the device under test and
| then measuring the noise power output with the input terminated
| properly
|
| ....any errors
| in the measurement can easily outnumber the actual noise figure
|
|Thanks for reminding me of that one, Jim. But I see your point about
|errors. For example, in measuring the gain one needs a standard. One
|of the few pieces of real test gear I have is a bolometer-type RF
|power meter which can measure about -13 dBm accurately. If the
|measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then
|thermal noise is -114 dBm. So I need about 100 dB gain for a very low
|noise figure DUT. To measure that I might need five 20 dB attenuators
|as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so
|there's +/-2.5 dB error (well, I suppose I could cross my fingers and
|RSS the numbers). Or I have a diode-type power meter that will
|measure lower power, which leads to issues of how the detector
|responds to noise. And then there's the problem of knowing the noise
|bandwidth precisely...
|
| is where half my gray hairs came from (the other half from being laid off).
|
|I think about half of mine come from the latter factor too !
|
| If you are measuring an entire receiver there are a few things you have to
| be careful with. The receiver must be a linear receiver (no FM, AM diode
| detector, etc.---basically just SSB). There should be a filter to pick just
| one sideband. Turn the AGC off. Make sure you measure the gain in the
| linear region, which also applies to a simple amplifier.
|
|Definitely. Though, with my method the input never gets more than 3
|dB above the receiver noise floor and in most cases a well designed
|receiver will have no AGC response at that level. But with a preamp
|in front it usually will activate the AGC, so AGC has to be switched
|off when comparing preamps - which of course is impossible to do in
|most ham rigs !
|
| If you have a DUT with a known noise figure, I think that this would be one
| way of calibrating a homebrew noise source.
|
|Hence the interest in NF repeatability of MMIC amps, since they are
|hard to build wrong, are well matched over a wide bandwidth and don't
|require tuning for best noise figure. The known DUT can also be the
|standard itself (in association with a receiver of only roughly known
|NF) to avoid issues of errors in calibrating the noise source ENR.
|
|73,
|Steve VE3SMA