Bernd,
Brilliant point. Easy to miss if one has a to simple model of the
oscillator at hand.
Since it is a single-side-band mode, it will show up both as AM and PM
with the same amplitude.
Cheers,
Magnus
On 09/10/2014 03:27 PM, Bernd Neubig wrote:
Hi Bob,
your description oft he spurious coming from higher overtone of low-frequency modes
is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT
and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of
spurs located slightly above above the main mode (fundamental or overtone mode).
"slightly means starting at about 50 kHz to 200 kHz above for fundamental mode
and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are
relaled to the length and width of the active area (electrode).
These spurious modes do not come only into play for wide-pull VCXO, but also in
the case that the EFC input is used for modulation with signals in the audio
frequency range.
Remember that a frequency modulated signal has side-lines which are N* the
audio frequency apart from the carrier. The amplitude of these side lines
follows the so-called Bessel functions and varies with the modulation index.
If it happens that such a "Bessel-line" for a particular modulation frequency
coincides with such a spur, it comes to an interference, This means the modulation
frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks
do even occur if the spurious is so weak that it can barely be seen on a network analyzer.
Regards
Bernd DK1AG
AXTAL GmbH & Co. KG
www.axtal.com
-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:[email protected]] Im Auftrag von Bob Camp
Gesendet: Sonntag, 7. September 2014 04:21
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi
Simple answer = crystals are never perfect.
Longer winded, but very incomplete answer =
A spurious response in a crystal normally refers to a mode that is not one of
the “identified” modes of the crystal. An AT has a set of identified modes, an
SC has a more complex set of modes. In the case of the AT it would be the
fundamental and the odd overtones. In the case of the SC you have the A, B, C
modes and their odd overtones. None of those are considered spurious.
A spur can come from a lot of different places. One common one is higher order
vibrations in a longer dimension face of the resonator. The 183rd overtone of
the width of the blank is still a legitimate resonant mode. Another source are
modes other than shear (like flex). Deriving a full catalog of all the modes of
an arbitrary blank design is a major project. There are only a handful of
people out there who are into that sort of thing (as opposed to simply cranking
through some formulas).
Practical answer = Don’t worry about it. Unless you are building a wide pull
VCXO or a wide deviation VCXO (often the same thing) you will never notice them.
Bob
On Sep 6, 2014, at 9:13 PM, Hal Murray <[email protected]> wrote:
[email protected] said:
The biggest problem comes from crystal spurs rather than crystal Q.
What's the mechanism for making spurs with a crystal?
--
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