[time-nuts] OCXO Voltage Input? (Bob Camp)
EWKehren at aol.com
EWKehren at aol.com
Thu Sep 11 09:12:10 EDT 2014
I have on purpose stayed out of this discussions but I think it is time to
mention some benchmarks. Last year we did a work over of the Shera
controller when we where allowed to release assembly and hex code. In order to make
sure that the code was solid we ran extensive tests using a 1 pps from a
tbolt the original AD1861 which by no means is an ideal DAC and a Morion
MV89.
The DAC resolution is 1.7 E-13 per bit and after a couple of days warmup
the DAC did not change a single bit for 0.2 hrs and 1 bit over 0.4 hrs.
Over an 80 hr. period the total change was 120 bits mostly aging but you can
also see ambient temperature change.
Data and plots are available but can not be attached because of data
limitation
Bert Kehren
In a message dated 9/11/2014 7:37:03 A.M. Eastern Daylight Time,
kb8tq at n1k.org writes:
Hi
I understand that we are talking about a couple of different things. Since
we started out talking about OCXO’s I figured it was worth it to bring it
back to where we started.
Bob
On Sep 11, 2014, at 4:32 AM, Bernd Neubig <BNeubig at t-online.de> wrote:
> Hi Bob,
>
> your example is correct. However I was not talking about OCXO
specifically, but about crystal oscillators in general. And the effect I have
mentioned is not limited to wide-pull VCXO, but may occur at "normal" VCXO also. I
named the modulation "audio" for sake of simplicity of expression - it was
certainly not accurate enough. If you are modulating data (FSK) then such
interferences have a risk to occur even at moderate data rates.
> I do not talk about theorectical "can be's" but about practical
experience.
>
> Best regards
>
> Bernd
>
>
> -----Ursprüngliche Nachricht-----
> Von: time-nuts [mailto:time-nuts-bounces at febo.com] Im Auftrag von Bob
Camp
> Gesendet: Donnerstag, 11. September 2014 00:18
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
>
> Hi
>
> 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.
>
> Bob
>
> On Sep 10, 2014, at 12:50 PM, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
>
>> 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: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)
>>>
>>> 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 <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.
>>>>
>>>>
>>>>
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