Hi,
It is well worth mentioning that a crystal filter on the output can
become a challenge, as the source impedance can be far from 50 Ohm, and
thus a bit of a challenge depending on how you measure.
Cheers,
Magnus
On 10/28/2015 06:11 AM, Bruce Griffiths wrote:
Ulrich
Surely you meant to write
PN(SSB) = -177 -Pout + NF
If we attempt to apply this equation to the 10811A for which you measured a PN
floor of -174dBc/Hz
this implies that
NF - Pout = 3dB
Best case (NF = 0dB - unlikely! Pout would need to be much higher for nonn zero
NF)
Pout =-3dBm or 500uW.
The question is identifying this power.
The crystal dissipation is 50uW (HP Journal March 1981 p24)
The signal power dissipated in the CB stage input R is around 10% of this or
about 5uW.
The answer to this conundrum is surely that the equation for PN doesn't apply
directly in this case
for offset frequencies outside the crystal bandwidth.
The Crystal actually bandpass filters the signal and PN noise generated by
oscillator.
For offset frequencies outside the crystal bandwidth the oscillator generated
PN is greatly attenuated
so that the noise of the buffer amplifier chain (CB stage plus output
amplifiers) dominates.
In calculating the noise floor of the buffer amplifier chain the fact that the
crystal has
a high impedance at these frequencies should be taken into account.
Bruce
On Wednesday, 28 October 2015 8:34 AM, "[email protected]" <[email protected]>
wrote:
I have bought and measured the hp10811 at about -174dBc/Hz. The interesting thing
is the feedback capacitor from collector to base which changes Rin=1/gm. Unless the
circuit has a hidden Q mulitplier the PN (SSB) can never be better then 177 (kT) in
dBm + Pout in dBm - NF of the oscillator transistor. Many of the GB stages are
potentially unstable , so the "hopeful' best PN (SSB) is 177dbm + Pout ! AT 100
Mhz the leaing values are -146/100Hz offset and - 183 far out and high crystal
dissipation, 2mW or so Ulrich In a message dated 10/27/2015 4:17:16 P.M. W. Europe
Standard Time, [email protected] writes:
As Rick has pointed out numerous times when the output signal is extracted via the
crystal by a CB stage (or cascade thereof) the PN floor is determined by the ratio of the
amplifier equivalent input noise current to the crystal current. That is the amplifier
equivalent input noise current at frequencies for which the crystal impedance is high. If
one neglects this crucial point one comes to the conclusion (e.g. see Eq 4.-1 page 274 of
Ulrich Rohde's: Microwave and Wireless Synthesisers Theory and Design.) that with a
crystal current of 1.4mA rms and a crystal esr of 50 ohms that the XO PN floor cannot be
lower than -154dBc/Hz. Even the XO circuit in the ARRL handbook (attributed to Ulrich)
using this method of signal extraction has a measured PN floor of -168dBc/Hz. Many other
XO's (including the 10811A which uses a crystal current of 1mA ) have an actual PN
significantly lower than this. One would have thought that this glaring discrepancy
between "theory" and practice would
have been noticed and corrected by now.
Bruce
On Tuesday, 27 October 2015 6:01 PM, Richard (Rick) Karlquist
<[email protected]> wrote:
The oscillator transistor and buffer amplifier are basically
the same as the HP 10811, except for the absence of mode
suppressors. The difference here is that the oscillator
self limits in the oscillator transistor, whereas the 10811
has ALC. The discontinuous operation of the transistor,
as explained by Driscoll some 45 years ago, is undesirable
because it increases the load resistance the crystal sees.
The 2 transistor "Driscoll oscillator" fixes this problem
by using an additional stage that limits instead of the
oscillator transistor. This has been widely used for
decades. It is interesting to note that the 10811 ALC
works by varying the DC bias current in the oscillator
transistor. This is in contrast to the elaborate DC
bias current stabilization here.
I have demonstrated that the close in phase noise in
the 10811 is entirely due to the flicker noise of the
crystal. The only place where the 10811 circuit comes
into play is beyond 1 kHz from the carrier, where the
Burgoon patent circuit (which apparently has prior art
from Ulrich Rhode) reduces the phase noise floor. I
have built two different oscillator circuits for 10811
crystals and have measured the flicker noise as being
the same as the intrinsic noise of the crystal.
Thus, obsessing over noise in oscillators circuits may
be overkill, unless you are planning to use a much
better crystal (BVA, etc). OTOH, it might be advantageous
to improve the reverse isolation by adding additional
grounded base buffer stages. There are various NBS/NIST
papers where several grounded base stages are cascaded.
I did this in the HP 10816 rubidium standard.
It is good to see time-nuts learning about oscillator
circuit by building them.
Rick Karlquist N6RK
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