dk...@arcor.de wrote:
----- Original Nachricht ----
Von: Bruce Griffiths<bruce.griffi...@xtra.co.nz>
An: Discussion of precise time and frequency
measurement<time-nuts@febo.com>
Datum: 28.12.2009 06:52
An inductor in series with the 220 ohm emitter resistor will improve the
phase noise floor.
In theory, yes. But already with only 220 Ohms, Q3 will oscillate wildly
at a few hundred MHz.
The mechanism is this: Somewhat hot RF transistor NE688, collector at
RF ground, emitter at high-ish impedance ---> When you measure into
the base, you see a negative resistance in series with a few pF.
Using a transistor with a higher ft than necessary in an oscillator
circuit isnt usually a good idea.
Add L6 = 82 nH with the other side at RF ground and you have built
the usual negative-impedance VCO for VHF/UHF.
The crystal and the intended feedback network just don't matter any more.
I should have re-read my own Dubus article on oscillator simulations from
6 years ago before I tried the Distaw. :-(
Other people have observed the wild oscillations, too.
Even without significant inductance between the oscillator transistor
base and ground the shunt capacitance of the crystal itself can cause
parasitic oscillations to occur.
A high frequencies the base is grounded via the tank capacitor, and the
emitter impedance exhibits a negative resistance in series with an
inductance.
Driscoll actually used ferrite beads on base and collector leads to
suppress such oscillations.
The location of these beads is clearly shown in the original paper.
Driscoll used a capacitively split tank so that there is a capacitor
from the oscillator transistor base to ground.
The MMIC output amplifier has a wider bandwidth than necessary and
doesn't have a particularly high reverse isolation.
Also, it has 20 dB gain, that alone guarantees a less than ideal
far-off noise level. The BAS70 clips at less than 1 V pp, this
should be more. Could be easily done in the Rohde style with a decoupled
DC divider and one Schottky that points from the divider to the
collector circuit.
A symmetric clipper (easily implemented by dc biasing an AC coupled 2
diode pp detector) has some advantages.
I have changed my own locked VHF crystal oscillator back to Butler - this time
single stage with 3* cheap NXP BF862 in parallel, common gate.
The gate can be grounded directly, needs no voltage divider and decoupling.
Input impedance of the 3 FETs is abt. 7 Ohms, which brings us close to
the point of diminishing returns for the usual 45 Ohm crystal.
The BF862 works to 700 MHz, so it is just fast enough and won't surprise
me at 3 GHz.
regards, Gerhard dk4xp
Some of Driscoll's later low phase noise OCXOs employ an MMIC as the
oscillator and use the crystal together with a diode limiter and
matching circuits to match the crystal to the 50 ohm input and output
impedance of the MMIC. A splitter at the MMIC output is used. One
splitter output drives the feedback loop whilst the other is used to
drive the ooutput buffer.
Bruce
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