Bob Camp wrote:
Hi
If you start with a mixer that runs 500 mV / radian (an RPD-1 at the typical 8
mV / degree + 10%) then -180 below that would be 0.5 nV. Since noise it
coherent close in, the DSB to SSB process nets you 1 nV out when you have -180
dbc noise.
With a capacitive IF port termination the mixer sensitivity increases
somewhat.
It increases more when using something like an HP10514 or 10534 than
with an RPD-1.
Such a termination isnt particularly useful for offsets much above
100kHz or so.
If one terminates all mixer ports in 50 ohms as some insist is the best
method, then the mixer phase sensitivity is much lower, in which case a
somewhat lower noise preamp may be required.
The posted plot does show (together with the noise plot for the HPS5.1
preamp) that the 2SK369 and the IF9030 have much lower flicker noise
than the BF862.
On Aug 22, 2010, at 10:51 AM, Bruce Griffiths wrote:
So everything above (and an AD797 and likely an OP-37) will do better than 2 nV / Hz into
1K Hz. That would let you check oscillators in the "below -170", but not below
-180 range. You might or might not find such an oscillator in your junk box. They
certainly do exist.
On the plot above, both devices would let you do the same thing at 10 Hz. Now you are into the
range of "highly unlikely to find". At reasonable frequencies, -135 is doing pretty well
at 10 Hz. Bragging rights start at about -140. "Highly unlikely" cuts in much past that
10 Hz offset. I'm not talking about a one of a kind piece of magic at NIST, but about what's in
your junk box.
The 2SK369 is still holding "ok for -170" at 1 Hz. Even for one of a kind
magic, that's pretty crazy. Unlikely to find (and really tough to measure) cuts in at
about -120 at 1 Hz.
At 0.1 Hz offset, you will need to run an instrument bandwidth below 0.01 Hz to
get anything close to a good approximation to the noise. Most lab analyzers
run> 100X t to get enough data. That puts you out around 10,000 seconds for
the run. That's a crazy long time. DC coupled offsets are likely to nuke that run
just about every time.
I'm by no means knocking the idea of having a good preamp. I'm only trying to
point out that the numbers above are *way* past what a reasonable person might
need to sort through their basement oscillator collection. 50 db is a lot of
margin.
Bob
For an AC coupled sound card based spectrum analyser dc frequencies much
below 2Hz or so are of little interest.
Being able to calibrate the preamp + sound card frequency response using
the thermal noise of a resistor is convenient.
This is more difficult to achieve with a bipolar input stage as the
amplifier input current noise is significant.
Bruce
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