I'll upload this SA 3002 software for you all so it will be there and
I will send out an announcement of it. Hopefully I will have some
online time I can use here tomorrow. {I have limited time in all
honesty, but hey its free internet that I use here. I don't pay
anything for my service or phone connection! If anyone would like to
have the ISP software for this I can get it to you! I can upload it.
Full time email but 24 to 48 hours a month online time.}
Amoung the numerous things that I felt the SA 3002 software could be
used for besides looking at receiver audio bandpass is to use it
perhaps to analyze oscillator phase noise?
How I am expecting to do this is to use a small preamp with a simple
AM diode detector (following the preamp) and then run the detected
audio to the sound card.
The software can plot a graph from 20 to 20 kHz and it contains its
own sweep generator. I expect to use a low noise Mosfet and the input
to the gate will have a 600 ohm gate bias resistor as a dB reference.
The input then has some attentuation resistors ahead of the gate and
600 ohm resistor. These should be fixed in value so there can be a
selectable range of fixed attentuation to calculate input levels to the
600 ohm reference.
Since the Mosfet has a high input gate impedance this 600 ohm
resistor then becomes the input load. The attenuators then are to
match the circuit to the oscillator under test. And act as voltage
dividers to the 600 ohm resistor. Then if you know your resistor
values, you can make some quick calculations. The software seems to be
fairly accurate on its dB scales. They appear to read the same as the
dB scales on Winrad.
{Come to think of it, Winrad might be used the same way as this
software idea!}
Actually it is not an elaborate circuit and who knows? It might work
out well for reading audio phase noise in oscillators within these
audio limits.
I expect to do some value tweaking of the source and drain resistors
so I can get a linear responce over the amplifier's bandpass. Once I
do come up with a working circuit then I will post that circuit.
Someone else might come up with it before me, who knows?
One might get a little diode noise added to the noise figure but that
can be calculated out once it is understood and if it does appear in
the output products?
The reason you do not want to use a rf mixer type of scheme for this,
is because the seperate oscillator will add its phase noise to the
output during mixing. So this simple AM detector seems the way to go.
ka9rza
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