[time-nuts] OCXO Voltage Input? (Bob Camp)
kb8tq at n1k.org
Wed Sep 10 18:18:11 EDT 2014
If you are modulating a normal OCXO EFC with audio, and the output frequency is not being multiplied up, the modulation index will be very low. Low modulation index means that the higher order FM sidebands will be quite far down.
If you take “audio” to be < 10 KHz, and a VHF OCXO to be 100 MHz: With a 10 ppm EFC range, you get 1.0 KHz of deviation. The modulation index is < 1 a decade below your upper modulation frequency. That’s already a pretty wide swing OCXO and a fairly high modulation frequency for an EFC line.
If you have a spur that is in the 50 to 150 KHz range, you are talking about the 5th to 15th sideband off of 10 KHz or the 50th to 150th sideband off of 1 KHz. At 50 sidebands out and an index of 1, you are in the “forget about it” region. Even at 10 KHz, the sideband is not likely to create much of an issue. The distortion from the non-linear EFC slope will be more of a problem in a practical sense.
Since the modulation is single sideband, yes it converts PM <-> AM. It also will be impacted by any limiters in the system and will not multiply the same way as a pure PM modulation. The phase of the sideband will change as you go through the resonance, further messing up the multiplication / limiter math.
On Sep 10, 2014, at 12:50 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 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.
> 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.
>> Bernd DK1AG
>> AXTAL GmbH & Co. KG
>> -----Ursprüngliche Nachricht-----
>> Von: time-nuts [mailto:time-nuts-bounces at febo.com] 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)
>> 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.
>> On Sep 6, 2014, at 9:13 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>> kb8tq at n1k.org said:
>>>> The biggest problem comes from crystal spurs rather than crystal Q.
>>> What's the mechanism for making spurs with a crystal?
>>> These are my opinions. I hate spam.
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>> and follow the instructions there.
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts