Discussion:
Advice on sighting a roof mounted gps area please
(too old to reply)
swingbyte
2014-10-12 11:34:52 UTC
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Hi All,
I am building a house extension and part of the works involves adding a
new hip roof made of corrugated iron. I was thinking I would pass a
50mm pvc pipe through the roof with a tee and then mount two conical gps
timing antennas on top of it. I am in a low point and don't have
visibility of the horizons ( I'm not in the out-back).
My question is should I mount on the peak of the roof? How close can I
mount two antennas from each other? Can they interfere with each other?
I am also in the midst of some tall trees - although my new roof will be
pretty high it will still be below the tallest trees.
Of course the main reason for this is I want to do some accurate timing

ASCII art of proposed set-up

A A
| |
--------------------
|
|
^
/ \
/ \
/ roof \

Thanks for your advice

Tim
paul swed
2014-10-12 14:19:09 UTC
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Tim
The antennas should not interfere with each other due to rf leakage because
of the way the systems are designed. I will believe you are using 2 rf
feeds.
The more you can clear the trees the better. My very simple solution is a
90' tower.
A bit of humor it does have other uses.
Regards
Paul
WB8TSL
Post by swingbyte
Hi All,
I am building a house extension and part of the works involves adding a
new hip roof made of corrugated iron. I was thinking I would pass a 50mm
pvc pipe through the roof with a tee and then mount two conical gps timing
antennas on top of it. I am in a low point and don't have visibility of
the horizons ( I'm not in the out-back).
My question is should I mount on the peak of the roof? How close can I
mount two antennas from each other? Can they interfere with each other? I
am also in the midst of some tall trees - although my new roof will be
pretty high it will still be below the tallest trees.
Of course the main reason for this is I want to do some accurate timing
ASCII art of proposed set-up
A A
| |
--------------------
|
|
^
/ \
/ \
/ roof \
Thanks for your advice
Tim
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Bob Camp
2014-10-12 18:37:18 UTC
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Hi

If you are going to get any benefit from multiple antennas, you want to space them as far apart as possible. You are better off with one antenna and a splitter than with two close spaced antennas.

The cost of mucking around on the roof is non-trivial. The world is headed to L1/L2 operation on GPS and similar systems. Invest the money in one good antenna and mount rather than multiples.

Bob
Post by paul swed
Tim
The antennas should not interfere with each other due to rf leakage because
of the way the systems are designed. I will believe you are using 2 rf
feeds.
The more you can clear the trees the better. My very simple solution is a
90' tower.
A bit of humor it does have other uses.
Regards
Paul
WB8TSL
Post by swingbyte
Hi All,
I am building a house extension and part of the works involves adding a
new hip roof made of corrugated iron. I was thinking I would pass a 50mm
pvc pipe through the roof with a tee and then mount two conical gps timing
antennas on top of it. I am in a low point and don't have visibility of
the horizons ( I'm not in the out-back).
My question is should I mount on the peak of the roof? How close can I
mount two antennas from each other? Can they interfere with each other? I
am also in the midst of some tall trees - although my new roof will be
pretty high it will still be below the tallest trees.
Of course the main reason for this is I want to do some accurate timing
ASCII art of proposed set-up
A A
| |
--------------------
|
|
^
/ \
/ \
/ roof \
Thanks for your advice
Tim
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Chris Albertson
2014-10-12 15:17:45 UTC
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First off, why only 50mm and why plastic? The PVC will degrade in the
sunlight over the years. Use galvanized iron pipe. Make the mast as
tall as you can. It can extend sever feet below the roof and attach to
house structure using u-bolts. (Hight limited only by appearance from the
street.) Using iron pipe strength will not be an issue. Run the cable
inside the iron pipe to the attic space.

I would use two masts, one for each antenna. It will look better and be
easier to build and it will handle high winds better.

If you are worried about how this all looks use some spray paint to make it
either sky blue or light grey.

Do you need two antenna? You can feed multiple GPS receivers using a
splitter and amplifier from one antenna.

Be sure to follow the local rules for grounding antenna. In most places
you will need a heavy coper wire leading directly to a grounding rod. You
want to give lightening an easy path to ground that is not routed through
the interior of the house.
Post by swingbyte
Hi All,
I am building a house extension and part of the works involves adding a
new hip roof made of corrugated iron. I was thinking I would pass a 50mm
pvc pipe through the roof with a tee and then mount two conical gps timing
antennas on top of it. I am in a low point and don't have visibility of
the horizons ( I'm not in the out-back).
My question is should I mount on the peak of the roof? How close can I
mount two antennas from each other? Can they interfere with each other? I
am also in the midst of some tall trees - although my new roof will be
pretty high it will still be below the tallest trees.
Of course the main reason for this is I want to do some accurate timing
ASCII art of proposed set-up
A A
| |
--------------------
|
|
^
/ \
/ \
/ roof \
Thanks for your advice
Tim
_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
Hal Murray
2014-10-12 21:07:53 UTC
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Post by Bob Camp
If you are going to get any benefit from multiple antennas, you want to
space them as far apart as possible. You are better off with one antenna and
a splitter than with two close spaced antennas.
Does anybody have data? How would I measure it?

Where is the knee? I assume the distance is measured in wavelengths. Is it
1, 10, ...?


In a related area, does anybody have data that correlates with rain? (or
fog/mist)
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Bob Camp
2014-10-12 22:29:14 UTC
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HI

Many years ago, we got dinged on customer visit when they spotted our GPS antenna array on the roof. The claim made at the time was that anything under 20’ spacing was counterproductive. I’ve seen numbers like 5, 8, 10,15 and 25 feet mentioned by different people at different times.

The problems seem to be:

1) You have an amp in the antenna, like it or not, the antenna (and it’s coax) are not 100% shielded. They re-radiate.
2) The antenna structure (mast etc) is a reflector and you get multi-path.
3) The GPS solution does not vary enough over a short distance for a “second opinion” to be useful
4) Gear on the other end of the antenna could re-radiate. (number 4 on the list for an obvious reason … = I don’t believe it)

The first one on the list is what they dinged us on. Since it was their antenna, we sort of figured they knew something about what it did or did not do. The other three get mentioned here and there.

Bob
Post by Hal Murray
Post by Bob Camp
If you are going to get any benefit from multiple antennas, you want to
space them as far apart as possible. You are better off with one antenna and
a splitter than with two close spaced antennas.
Does anybody have data? How would I measure it?
Where is the knee? I assume the distance is measured in wavelengths. Is it
1, 10, ...?
In a related area, does anybody have data that correlates with rain? (or
fog/mist)
--
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Hal Murray
2014-10-16 02:56:48 UTC
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Is it silicon or is it something more exotic? In general, exotic is not good
for 1/F noise.
Data sheets say "submicron CMOS".
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Hal Murray
2014-10-20 17:15:28 UTC
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The combination of the constellation and the ionosphere are what I believe
give you the once a day (rather than once per 12 hours) bump.
There is another layer. In addition to the "normal" once-a-day type
differences, the pattern of satellites drifts slowly from day to day. So
there is another pattern with a period of something like a month. If you
have a marginal setup, for example an indoor antenna, you can see things like
the holdover times drifting both in time-of-day when they happen and in
length of holdover as the satellite pattern at dawn/dusk changes.

Of course, at that level of detail, there are lots of other contributions
like rain that will also show up and may be more important.
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Hal Murray
2014-09-07 01:13:43 UTC
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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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Andrew Rodland
2014-09-07 01:42:06 UTC
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Post by Hal Murray
What's the mechanism for making spurs with a crystal?
Get the corners nice and pointy and strap it to a boot.
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Bob Camp
2014-09-07 02:20:38 UTC
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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
Post by Hal Murray
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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and foll
Bernd Neubig
2014-09-10 13:27:41 UTC
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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-***@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
Post by Hal Murray
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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Magnus Danielson
2014-09-10 16:50:29 UTC
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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
Post by Bernd Neubig
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-----
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
Post by Hal Murray
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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Bob Camp
2014-09-10 22:18:11 UTC
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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
Post by Magnus Danielson
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
Post by Bernd Neubig
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-----
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
Post by Hal Murray
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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Bernd Neubig
2014-09-11 08:32:40 UTC
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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-***@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
Post by Magnus Danielson
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
Post by Bernd Neubig
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-----
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
Post by Hal Murray
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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Bob Camp
2014-09-11 11:12:03 UTC
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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
Post by Bernd Neubig
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-----
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
Post by Magnus Danielson
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
Post by Bernd Neubig
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-----
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
Post by Hal Murray
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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Hal Murray
2014-10-30 05:31:43 UTC
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It is not at all unusual for signals to be re-clocked when going into a
micro. Often the documentation on this process is somewhere between vague
and non-exsistant.
Reclocking is almost required if you want to avoid metastability issues.

There is often some "documentation" in the form of min high/low times.
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Hal Murray
2014-11-02 21:59:06 UTC
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The numbers quoted earlier (and they sound right) were 20 uA at 2.5V. That
would be well under your 100uA. My *guess* is that self discharge / aging on
a normal AA is going to limit things faster than a 20 uA drain.
20 uA would last 15 years. (assuming no self-discharge)

Self discharge is temperature dependent. Graph here:
http://data.energizer.com/PDFs/alkaline_appman.pdf
(poke the shelf-life button on the left)

At 20C, alkaline lose 20% in 10 years. Or roughly 50 years for the whole
thing. (assuming linear and waving my hands)
The UT+ data sheet from 1998 quotes an external backup supply of 2.5 to
5.35V with a drain of 5uA typical at 2.5 Volts.
Ahhh. That would be 60 years. (assuming no self-discharge) So it's roughly
matching the self discharge rate.
Your pair of AA’s will start off at 3.1V, but they will get to 2.5 long
before they are truly dead. Is the RAM gone at 2.5000 or 2.4 or “about 2
volts” 
.
The usual cutoff is 0.8 V. It falls off quickly at the end. It's still 50%
at 1.25 V. There is a graph at the above URL.
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Mark Sims
2014-11-02 22:11:37 UTC
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If you are going to back up the clock with AA cells, use lithium primary AA cells. They will last longer, plus they don't leak. EVERY alkaline cell will eventually leak... they don't call 'em Alkaleaks for nuthin'
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Hal Murray
2014-11-09 23:07:06 UTC
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What is going on is that people are confusing the estimation process that is
used by the selection process (which does look at a lot of stuff) and how
that is described. ...
In this context, it's important to remember that there are 2 parameters
associated with the goodness of clocks. The first is the error. The second
is the error bars. For NTP, the error bars are often huge and often differ
wildly between clocks.

I don't know how that translates into getting the best out of a batch of Rbs.

NTP has another trick worth adding to the collection. For things like PPS
processing, it collects a batch of samples, then discards roughly 1/3 of them
as outliers. That helps keep an occasional bad sample from biasing the
result.
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