Richard (Rick) Karlquist wrote:
Bruce Griffiths wrote:
Only if the noise figure of the following amplifier is 4dB or so.
With no extra amplification is used one only needs a signal level of
+1dBm to achieve a phase noise floor of -178dBc/Hz if the output is
extracted through the crystal in such a way that the thermal noise of
the load dominates.
Read US Patent 4283691, which explains how the 10811 works. The
situation is far more complicated than the simple analysis above.
If you play your cards right, you can get much better phase noise
than what you have indicated. The thermal noise of the load does
not enter into it. Unfortunately, the very low noise first stage
in the 10811 is degraded by the emitter follower after it. As I
previously stated, you can bypass these additional stages if you
want a lower phase noise floor.
Rick Karlquist N6RK
Eventually the buffer chain usually has to drive a resistive load such
as a cable terminated in its characteristic impedance.
Even with a noiseless source, the thermal noise of the load will affect
the apparent phase noise floor (unless the source impedance is much
lower than the load impedance).
Neither the 10811A nor a set of cascaded common base buffers have near
zero output impedance.
Of course one can correct for this but in a real world application the
thermal noise of the load is always present.
I should have said that the signal level in the load (not the crystal
dissipation or the input signal level at the input to the first common
base buffer as Ulrich Rohde would have us believe) needs to be at least
+1dBm when a high output impedance (eg a transistor collector) OCXO
output is driving a resistive load.
When a resistor is used to match the output impedance of the output
stage to the 50 ohm load then the thermal noise is that of a 25 ohm load
and the required signal level at the load for a -178dBc system noise
floor due load thermal noise alone is -2dBm.
Bruce
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