Except that a quick look at the site suggests that this is likely to be the output from a "disciplined" oscillator.Well, nice simulation which also violates the laws of physic.
Ulrich
The best is to run the transistor at 15% of Icmax for best phase noise (flicker noise).
Be warned - these rules of thumb have limited validity, particularly if transistor flicker noise is significant...
For Colpitts designs, there are good reasons for the usual practice to set the base-emitter capacitors substantially larger than the emitter-collector capacitor (emitter-to-ground in Ulrich's ALC circuit).
If you take the output current from the collector, the signal swing at the collector should be smaller than at the base. If not, Miller effects will increase the effective feedback which is (often) equivalent to additional circuit loss. In this case it is advantageous to use a cascode for the output current.
I'm believe that JKAE would agree with all of this.
Follow-on crystal filters with Q substantially reduced by circuit loading are useful for achieving the lowest noise floors - though of course they have no effect on frequencies inside the resonator bandwidth.
---------- Post added at 17:53 ---------- Previous post was at 17:36 ----------
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"However, I need help on the amplitude noise, which is higher than the phase noise at some offsets. Especially the "hump" at 1M-10MHz is troublesome."
There is nothing apparent in your circuit to cause a 1-MHz to 10-MHz peak, though it is possible it is a resonance with a decoupling capacitor. But it is equally likely due to an integrated Voltage regulator somewhwere in the system; this could powering your oscillator or somewhwere in the measurement system. You could try an RC filter in the power line to see if it helps.
Stromer
So the goal of the feedback is to reduce the the DC fluctuation by a factor 10 000 (80dB), the flicker and related noise of the oscillator transistor under large signal condition and therefore reduce the modulation and up conversion within the bandwidth that makes the oscillator "noisy" .
This is standard feedback theory , Ulrich
"In this case an substitution of osc transistor noise by DC feedback noise occurs."
Thinking "out loud" to give more detail (not necessarily always entirely in agreement)...
N.B. the "mental reference circuit" for the non-ALC version is a common-emitter Colpitts design
The feedback is of the demodulated signal - so in a well-designed ALC the residual flicker noise should be dominated by the contribution from the amplitude detector.
Why, then is the demodulator less noisy than the oscillator?
. The situation is relatively clear if, in the absence of ALC-the limiting mechanism is collector-base conduction - small changes in amplitude cause large changes in circuit conditions (transistors with short reverse-transit times are less susceptible..).
. If the circuit is operated in current starvation the situation is less clear. Flicker noise will change the current through the bias resistor, and also the collector current, so:
. . If we use resistive feedback, the base current will change the DC collector Voltage. Clearly, this may also be avoided by using a damped inductor for DC feedback (would that this were practical in an IC - we have to use other techniques...).
. . Which potentially leaves the changes in collector current - which in current starvation is directly reflected as a proportionate change in output amplitude. ALC can reduce this variation, as only a small proportion of the signal has to pass through the detector transistors. However, there is another method available - buffer the base current so that your control current drives mainly the collector (you probably want the level shift anyway...).
All this ignores temporal changes in the crystal - but Q changes are at minimum accompanied by frequency changes equal to the change in 1/Q, and amplitude variation on this scale should not significantly impact frequency in a suitably-designed oscillator.
Provisional conclusion:
ALC can reduce both DC shift and amplitude variation, with direct impact on frequency , but
appropriate circuit techniques can apparently provide the same suppression of bipolar transistor flicker noise (and may be less subject to gain ringing - or even squegging)
Why, then is the demodulator less noisy than the oscillator?
"In simple terms, a transistor in an oscillator has energy going through it over and over and over."
??? Rich: That verbal argument can be taken two ways - the other being that each time the signal interacts with the maintaining amplifier the amplitude is corrected towards the available drive; this would suggest that the power dissipated in the resonator (crystal) should correspond precisely to the available drive. This is a in practice a reasonable assumption - the flicker-noise amplitude problem lies with the variation of the available drive, which is one of the two items I address in my post.
There is also an issue with flicker noise modulating base-emitter diffusion capacitance, but that is another story (and one of many reasons I use faster transistors than convention dictates, and suppress amplitude flicker noise in other ways).
Every time the signal goes thru the active device, the resonator phase shift interacts with the active device's phase shift, and where they cancel out conjugately--that is the frequency that you get the oscillation to occur at. Unfortunately, since the "gain" of the oscillator circuit is pretty flat (even for a narrowband resonator), and the phase shift is also fairly flat (when you think of a +/-1 Khz sort of range), that feedback mechanism is pretty weak!
The 1/f can disrupt the instantaneous phase of the active device slightly, so the resonator conspires to actually force the frequency momentarily off center--giving phase noise.
That is why feedback correction in an external circuit (such as a bias circcuit with 70 dB of active correctikng gain, or an external resonator or delay line discriminator feedback are so effective--you get far more stabilizing effect than you get from a passive resonator alone.)
70 db comes from either the rhode or wenzel circuit--I forget which one. Obviously, you only need enough feedback control loop gain to degernate the 1/f noise, and any extra just causes trouble.
As i have understood, stromer, ALC allow to supress an AM flicker-noise at amplifier output. This noise modulates an resonant frequency of quartz resonator which causes an flicker phase noise in output spectrum (AM-to-PM conversion?).
Another effect of flicker noise is an modulation of C-B capacitanse which tends to jitter of amplifier phase characteristic.
Supression of active device flicker noise allow to move flicker corner down. For this an current mirror can be used (schematic has been presented by Ulrich Rohde). But transistor in current mirror has it's own flicker noise. What flicker corner in case of real transistors in current mirror will be? Equal to flicker corner of these transistors?
I should clarify - I'm referring here to ALC that is driven from the collector current. ALC based on crystal (or "circulating") current gives advantage for variations both of crystal R1 (ESR) and of base input conductance.but (compared with current starvation in a Colpitts-family design) ALC does little or nothing to correct this, as the ALC acts on the collector current, not on the circulating current.
All resonators are in the end non-linear - for a CRO this might be signal loss -> heat -> thermal expansion.The language barrier affects...
Current mirror allow to reduce only amplitude variations at amplifier output. These variations make much difficulties in oscillators with drive-level-sensitive resonators. If I try to use it in CRO or similar, I will not see any effect in phase noise?
Spice models for NE688xx in Design Kit from CEL have non-zero Kf.
IMHO CRO's and conventional DRO's have very low Q to observe an AM-PM conversion because of drive level variations. Main effects of flicker noise are in jitter of phase of gain. I've observed that only for leucosapphire oscillators an necessity of ALC were considered.
Hi,
The crystals I have got from the Czech Republic, seem to be doing the job. I had aging issues with some local product, but this firm seems to supply "what one asks for".
Kevin
I can't answer for Kevin, but (in case you wish to start searches before he replies) I have heard good reports about KrystalyDo you have contact info on the Czech company which made SC cut crystals for you?
Thats
I am probably telling people what they already know, but...
When ordering crystals, specify what you want exactly, rather than just saying I want an crystal on XYZ frequency.
Also ask for preaged crystals, otherwise you might as well leave the oscillator running for a year (or more)
Specify the type of loading the crystal sees. Some VHF overtone circuits are used with overtone crystals in series resonance, but other circuits should be using crystals with capacitive loading
The circuit should have some amplitude limiting, outside the oscillator transistor. Otherwise the loaded Q (quality factor) of the crystal can be degraded further.
Kevin
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