Clock Driver Design
Hi
Two very conventional RF chokes (couple of uH each) and an NPO cap (couple hundred pf) are all you really need for the square to sine filter. It's probably a good idea to put a blocking cap on the thing as well. If you want to get fancy, put a three resistor 6 db pad on it as well. That way your cable will be rationally terminated over a fairly wide bandwidth.
Bob
On Sep 27, 2013, at 2:23 PM, Tom Minnis Tom_minnis@att.net wrote:
I haven't even begun to look for video amps yet. I may not need one if I filter an output of an high powered 5V buffer. What I hear is a simple passive low pass filter will do. That being the case, I may put them on all the outputs and make it a jumper option. The other project brewing here is developing a precision time stamp transceiver which needs the fast edges as opposed to the synthesizer reference which needs the accurate frequency aspect. Thanks again for all your helpful ideas.
Tom
On 9/27/2013 3:53 AM, Charles Steinmetz wrote:
Tom wrote:
One of my first applications is to use a 10MHz output to phaselock a VCXO master clock in a radio transceiver. * * * Next I went to IDT to find the best logic buffer I could find. I am looking at the IDT 74FCT38072 2 channel clock driver for PPS. It can drive about 50mA if needed with 1nS rise and fall times. The one I am looking at for 10MHz is the ICS553 4 channel clock driver. This one is good for 25mA drive and they actually give a typical output impedance spec of 20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a little faster with a 5V supply, 0.7nS and 35mA drive. To make a sine wave should I use one of the 4 ports on the 4 port driver to input to the filter or should I try to hook the filter input directly to the clock driver input?
Are there tried and true 10MHz filter circuits or is that a non issue?
After the filter would come the video amp set up for a 50 Ohm drive and into a splitter. That sound simple enough.
I strongly agree with Magnus that distributing square waves is asking for trouble and that converting to sine is preferable unless there is some very good reason not to.
IIRC, you said the source is CMOS. So you can do all of your fanout digitally, then filter each output (I believe that is what Bob had in mind). Or, as you appear to be contemplating based on your comments above, you could convert to sine immediately and then do the fanout in the analog domain with a video DA or whatever. One reasonable filter type to hang on a CMOS output is an L-C-L "tee" filter (there is really no reason not to add one more shunt C at the end, for L-C-L-C). This filter needs some termination at all times -- the open circuit output voltage can be pretty high. But you can usually get away with an internal termination of ~1k or so. If you need more current to get the output level you want, parallel several CMOS outputs (all on the same hex buffer chip, preferably). There is no need for very fast edges, particularly if you are filtering to sine wave. Nothing exotic is necessary.
The same is true even if you decide to distribute square waves. The fewer higher harmonics you have, the better off you will be.
Best regards,
Charles
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I checked Mini Circuits and they have 2 10MHz low Pass filters for
$8.95. I think I will try and lay out a discrete T band pass which can
easily be turned into a low pass if it gets too tweeky. This should be
less than $3. I think I can use a simple SPDT switch to option the
outputs for either sin or square wave output.
with regard to noise specs of oscillators, I was confused to see
Symmetricom spec. the CSAC module they supply at -70dBc at 10Hz and then
see Jackson implement a design using that part and put the 10Hz noise at
-90dBc!
Tom
On 9/27/2013 1:16 PM, Bob Camp wrote:
Hi
Two very conventional RF chokes (couple of uH each) and an NPO cap (couple hundred pf) are all you really need for the square to sine filter. It's probably a good idea to put a blocking cap on the thing as well. If you want to get fancy, put a three resistor 6 db pad on it as well. That way your cable will be rationally terminated over a fairly wide bandwidth.
Bob
On Sep 27, 2013, at 2:23 PM, Tom Minnis Tom_minnis@att.net wrote:
I haven't even begun to look for video amps yet. I may not need one if I filter an output of an high powered 5V buffer. What I hear is a simple passive low pass filter will do. That being the case, I may put them on all the outputs and make it a jumper option. The other project brewing here is developing a precision time stamp transceiver which needs the fast edges as opposed to the synthesizer reference which needs the accurate frequency aspect. Thanks again for all your helpful ideas.
Tom
On 9/27/2013 3:53 AM, Charles Steinmetz wrote:
Tom wrote:
One of my first applications is to use a 10MHz output to phaselock a VCXO master clock in a radio transceiver. * * * Next I went to IDT to find the best logic buffer I could find. I am looking at the IDT 74FCT38072 2 channel clock driver for PPS. It can drive about 50mA if needed with 1nS rise and fall times. The one I am looking at for 10MHz is the ICS553 4 channel clock driver. This one is good for 25mA drive and they actually give a typical output impedance spec of 20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a little faster with a 5V supply, 0.7nS and 35mA drive. To make a sine wave should I use one of the 4 ports on the 4 port driver to input to the filter or should I try to hook the filter input directly to the clock driver input?
Are there tried and true 10MHz filter circuits or is that a non issue?
After the filter would come the video amp set up for a 50 Ohm drive and into a splitter. That sound simple enough.
I strongly agree with Magnus that distributing square waves is asking for trouble and that converting to sine is preferable unless there is some very good reason not to.
IIRC, you said the source is CMOS. So you can do all of your fanout digitally, then filter each output (I believe that is what Bob had in mind). Or, as you appear to be contemplating based on your comments above, you could convert to sine immediately and then do the fanout in the analog domain with a video DA or whatever. One reasonable filter type to hang on a CMOS output is an L-C-L "tee" filter (there is really no reason not to add one more shunt C at the end, for L-C-L-C). This filter needs some termination at all times -- the open circuit output voltage can be pretty high. But you can usually get away with an internal termination of ~1k or so. If you need more current to get the output level you want, parallel several CMOS outputs (all on the same hex buffer chip, preferably). There is no need for very fast edges, particularly if you are filtering to sine wave. Nothing exotic is necessary.
The same is true even if you decide to distribute square waves. The fewer higher harmonics you have, the better off you will be.
Best regards,
Charles
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On 09/27/2013 12:53 PM, Charles Steinmetz wrote:
Tom wrote:
One of my first applications is to use a 10MHz output to phaselock a
VCXO master clock in a radio transceiver. * * * Next I went to
IDT to find the best logic buffer I could find. I am looking at the
IDT 74FCT38072 2 channel clock driver for PPS. It can drive about
50mA if needed with 1nS rise and fall times. The one I am looking at
for 10MHz is the ICS553 4 channel clock driver. This one is good for
25mA drive and they actually give a typical output impedance spec of
20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a
little faster with a 5V supply, 0.7nS and 35mA drive. To make a sine
wave should I use one of the 4 ports on the 4 port driver to input to
the filter or should I try to hook the filter input directly to the
clock driver input?
Are there tried and true 10MHz filter circuits or is that a non issue?
After the filter would come the video amp set up for a 50 Ohm drive
and into a splitter. That sound simple enough.
I strongly agree with Magnus that distributing square waves is asking
for trouble and that converting to sine is preferable unless there is
some very good reason not to.
That comment came out of another thread. Yes, there are issues with
square distribution. If you want to stick with it, then you need to take
care of that square. Square can have a benefit over sine thought,
smaller amounts of reflection can be ignored and not cause much shift as
they will be voltage-shifted out from the comparator
trigger-point/amplifier mid-point. This means that with some care, the
next step can remove it. A square signal is easy to diagnose with a
scope, just toss it in there and you have reflections showing up on the
scope. Sine require a little more subtle methods to figure out what is
wrong. So, it's a two-edged sword.
IIRC, you said the source is CMOS. So you can do all of your fanout
digitally, then filter each output (I believe that is what Bob had in
mind). Or, as you appear to be contemplating based on your comments
above, you could convert to sine immediately and then do the fanout in
the analog domain with a video DA or whatever. One reasonable filter
type to hang on a CMOS output is an L-C-L "tee" filter (there is
really no reason not to add one more shunt C at the end, for
L-C-L-C). This filter needs some termination at all times -- the open
circuit output voltage can be pretty high. But you can usually get
away with an internal termination of ~1k or so. If you need more
current to get the output level you want, parallel several CMOS
outputs (all on the same hex buffer chip, preferably). There is no
need for very fast edges, particularly if you are filtering to sine
wave. Nothing exotic is necessary.
The same is true even if you decide to distribute square waves. The
fewer higher harmonics you have, the better off you will be.
Yes and no, depending on what plagues your installation. Sine is the
perfect waveshape of converting additive noise (hum for instance) into
jitter if it meets a comparator. The low slewrate will be the problem
with the sine, so increasing the through-zero slew-rate reduces this
problem. The jitter conversion follows this formula:
t_rms = n_rms / S_signal
Where n_rms is the Noise (including hum and other unwanted signal) RMS
value (in V),
S_signal is the signal slew-rate (in V/s) and
t_rms is the jitter time (in s)
This have bitten me many times.
You can avoid having additive noise transform into jitter by not feeding
it into a comparator but rather linearly gaining yourself out of the
situation. Yes that gain-stage will saturate at the ends. See the TADD-2
input stage, as adapted from the Wenzel page on the subject. Such input
can also handle square signals.
Not all inputs is made to handle sine signals properly, so you can have
some additional noise on your reference as you lock up your instrument.
This also means that some receivers is best served by square, even if
square is tricker to handle to some degree. Others can be best served by
sine.
In the end, there isn't one way which is always right, rather you need
to adapt to the receivers needs.
It would be nice to have a little standard board with a good quality
sine squarer such that it can be put inline at or in the instrument.
Cheers,
Magnus
Hi Bob,
On 09/27/2013 01:44 PM, Bob Camp wrote:
Hi
Rise and fall times are not the thing to worry about on the gates. Look at the propagation delay. That's what will vary.
On Sep 27, 2013, at 2:11 AM, Tom Minnis Tom_minnis@att.net wrote:
Thanks for all your thoughts on the subject. Let me play back what I have learned and how it may apply to my challenge. One of my first applications is to use a 10MHz output to phaselock a VCXO master clock in a radio transceiver. The VCXO is the Christek CVHD-950 which has a noise floor of -164dBc and is -86dBc at 10Hz. The source I want to use is the Jackson Labs GPSTCXO which has a noise floor of -155dBc and is -73dBc at 1Hz and 103dBc at 10Hz. i did a quick survey of the phase noise specs on various Jackson products that claim to be ultra low phase noise and found similar numbers. One was -100dBc at 1Hz but only -145dBc at 100KHz. Another was down -90dBc at 1Hz and -160dBc at 100KHz. It would appear that even the best parts I could find quickly would not merit the fancy analog gizmo and that a good stiff logic buffer would work. Next I went to IDT to find the best logic buffer I could find.
The phase noise out of a TBolt is roughly -165 to -170 floor and -155 to -160 at 100 Hz. (plus spurs of course)
I am looking at the IDT 74FCT38072 2 channel clock driver for PPS. It can drive about 50mA if needed with 1nS rise and fall times. The one I am looking at for 10MHz is the ICS553 4 channel clock driver. This one is good for 25mA drive and they actually give a typical output impedance spec of 20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a little faster with a 5V supply, 0.7nS and 35mA drive.
Rise and fall times are not the thing to worry about on the gates. Look at the propagation delay. That's what will vary. If a 3 ns delay varies 1% (30 ps) over 1,000 seconds that's going to give you 3x10^-14 in your ADEV. Are your sources good to 3x10^-14 at 1,000 seconds? If not, don't worry about it.
Indeed. I would assume that temperature and power supply voltage be the
major contributors, and both can be handled if you care about it.
To make a sine wave should I use one of the 4 ports on the 4 port driver to input to the filter or should I try to hook the filter input directly to the clock driver input?
Are there tried and true 10MHz filter circuits or is that a non issue? After the filter would come the video amp set up for a 50 Ohm drive and into a splitter. That sound simple enough. What am I missing?
Simply use a three element Tee on the output of a logic gate. Run one per output. Don't split for multiple output. That way you will have much better isolation (which very much does matter).
Isolation is indeed something to care about. Just consider the effect of
someone connecting or disconnecting a cable. The unconnected output will
see the energy bounce back while the connected (and destination loaded)
output will transmit energy out. As you swap between these, the
isolation steers how well that is hidden from the other outputs. In a
precision environment, bad isolation essentially means you can't touch
it when it is operating. With isolation you can connect and unconnect
more freely even if the other outputs is operational.
A customer once asked if they could passively divide a signal (which is
wide-band) using a BNC-T. For the production environment they where
going to install, I more or less forbid them to do that. They where
thinking MTBF and cheap solution, but the lack of isolation means that
any fault will potentially kill both legs and there would be no
maintenance possible. Their counter-argument was that "well, if you cut
the cables to the wavelength" and I then pointed out that it only works
for sine and narrow-band signals, but this wide-band signal will not
handle it well. They took my advice to the best of my knowledge.
Cheers,
Magnus
Hi Tom,
The Jackson CSAC GPSDO solution has a vcxo-based noise filter pll on the pcb, improving the noise performance and removing spurs as well over just the CSAC by itself so the specs will be quite different.
In fact the LN CSAC version of that board achieves around -100dBc/Hz at one Hz, many orders of magnitude better than the CSAC itself. The noise of the CSAC is quite bad as you noted, and it has pretty bad spurs as well.
There was no space in the CSAC for the Symmetricom designers to put proper analog power conditioning and a high performance crystal into it, but thats something that can be rectified externally and should not detract from the amazing feat they accomplished and commercialized.
Bye,
Said
Sent From iPhone
On Sep 27, 2013, at 22:29, Tom Minnis Tom_minnis@att.net wrote:
I checked Mini Circuits and they have 2 10MHz low Pass filters for $8.95. I think I will try and lay out a discrete T band pass which can easily be turned into a low pass if it gets too tweeky. This should be less than $3. I think I can use a simple SPDT switch to option the outputs for either sin or square wave output.
with regard to noise specs of oscillators, I was confused to see Symmetricom spec. the CSAC module they supply at -70dBc at 10Hz and then see Jackson implement a design using that part and put the 10Hz noise at -90dBc!
Tom
On 9/27/2013 1:16 PM, Bob Camp wrote:
Hi
Two very conventional RF chokes (couple of uH each) and an NPO cap (couple hundred pf) are all you really need for the square to sine filter. It's probably a good idea to put a blocking cap on the thing as well. If you want to get fancy, put a three resistor 6 db pad on it as well. That way your cable will be rationally terminated over a fairly wide bandwidth.
Bob
On Sep 27, 2013, at 2:23 PM, Tom Minnis Tom_minnis@att.net wrote:
I haven't even begun to look for video amps yet. I may not need one if I filter an output of an high powered 5V buffer. What I hear is a simple passive low pass filter will do. That being the case, I may put them on all the outputs and make it a jumper option. The other project brewing here is developing a precision time stamp transceiver which needs the fast edges as opposed to the synthesizer reference which needs the accurate frequency aspect. Thanks again for all your helpful ideas.
Tom
On 9/27/2013 3:53 AM, Charles Steinmetz wrote:
Tom wrote:
One of my first applications is to use a 10MHz output to phaselock a VCXO master clock in a radio transceiver. * * * Next I went to IDT to find the best logic buffer I could find. I am looking at the IDT 74FCT38072 2 channel clock driver for PPS. It can drive about 50mA if needed with 1nS rise and fall times. The one I am looking at for 10MHz is the ICS553 4 channel clock driver. This one is good for 25mA drive and they actually give a typical output impedance spec of 20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a little faster with a 5V supply, 0.7nS and 35mA drive. To make a sine wave should I use one of the 4 ports on the 4 port driver to input to the filter or should I try to hook the filter input directly to the clock driver input?
Are there tried and true 10MHz filter circuits or is that a non issue?
After the filter would come the video amp set up for a 50 Ohm drive and into a splitter. That sound simple enough.
I strongly agree with Magnus that distributing square waves is asking for trouble and that converting to sine is preferable unless there is some very good reason not to.
IIRC, you said the source is CMOS. So you can do all of your fanout digitally, then filter each output (I believe that is what Bob had in mind). Or, as you appear to be contemplating based on your comments above, you could convert to sine immediately and then do the fanout in the analog domain with a video DA or whatever. One reasonable filter type to hang on a CMOS output is an L-C-L "tee" filter (there is really no reason not to add one more shunt C at the end, for L-C-L-C). This filter needs some termination at all times -- the open circuit output voltage can be pretty high. But you can usually get away with an internal termination of ~1k or so. If you need more current to get the output level you want, parallel several CMOS outputs (all on the same hex buffer chip, preferably). There is no need for very fast edges, particularly if you are filtering to sine wave. Nothing exotic is necessary.
The same is true even if you decide to distribute square waves. The fewer higher harmonics you have, the better off you will be.
Best regards,
Charles
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As always, be careful with the hard and fast rules. Most crystal oscillators generate sine waves first, but if the oscillator is part of a GPSDO, it will have to be converted to square to be processed by the logic within the GPSDO, so even if the device has a sine output, there will be a square wave version of the signal within the device. Eventually, the sine has to be converted into a square wave at the other end of the cable either to drive a mixer or a logic gate.
Where you do that conversion (before the cable or after the cable), or whether you need to go to sine altogether is something that has to be considered as part of the overall system design.
In many applications, there is absolutely no need to go to sine if you have a clean square wave to begin with, for instance if the cable is short or you connect with well matched devices.
For some info on what cable mismatch does to a square wave, look here: http://KO4BB.com/Test_Equipment/CoaxCableMatching.php
As Magnus pointed out, there are additional considerations such as ease of troubleshooting that come into play also.
Finally, if your primary concern is frequency distribution to several instruments as opposed to precision timing, sine will probably be easier because multiple reflections and mismatches that would affect time of arrival don't matter if you are only interested in a stable and precise frequency for your radios.
Didier KO4BB
Magnus Danielson magnus@rubidium.dyndns.org wrote:
On 09/27/2013 12:53 PM, Charles Steinmetz wrote:
Tom wrote:
One of my first applications is to use a 10MHz output to phaselock a
VCXO master clock in a radio transceiver. * * * Next I went
to
IDT to find the best logic buffer I could find. I am looking at the
IDT 74FCT38072 2 channel clock driver for PPS. It can drive about
50mA if needed with 1nS rise and fall times. The one I am looking
at
for 10MHz is the ICS553 4 channel clock driver. This one is good
for
25mA drive and they actually give a typical output impedance spec of
20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a
little faster with a 5V supply, 0.7nS and 35mA drive. To make a
sine
wave should I use one of the 4 ports on the 4 port driver to input
to
the filter or should I try to hook the filter input directly to the
clock driver input?
Are there tried and true 10MHz filter circuits or is that a non
issue?
After the filter would come the video amp set up for a 50 Ohm drive
and into a splitter. That sound simple enough.
I strongly agree with Magnus that distributing square waves is asking
for trouble and that converting to sine is preferable unless there is
some very good reason not to.
That comment came out of another thread. Yes, there are issues with
square distribution. If you want to stick with it, then you need to
take
care of that square. Square can have a benefit over sine thought,
smaller amounts of reflection can be ignored and not cause much shift
as
they will be voltage-shifted out from the comparator
trigger-point/amplifier mid-point. This means that with some care, the
next step can remove it. A square signal is easy to diagnose with a
scope, just toss it in there and you have reflections showing up on the
scope. Sine require a little more subtle methods to figure out what is
wrong. So, it's a two-edged sword.
IIRC, you said the source is CMOS. So you can do all of your fanout
digitally, then filter each output (I believe that is what Bob had in
mind). Or, as you appear to be contemplating based on your comments
above, you could convert to sine immediately and then do the fanout
in
the analog domain with a video DA or whatever. One reasonable filter
type to hang on a CMOS output is an L-C-L "tee" filter (there is
really no reason not to add one more shunt C at the end, for
L-C-L-C). This filter needs some termination at all times -- the
open
circuit output voltage can be pretty high. But you can usually get
away with an internal termination of ~1k or so. If you need more
current to get the output level you want, parallel several CMOS
outputs (all on the same hex buffer chip, preferably). There is no
need for very fast edges, particularly if you are filtering to sine
wave. Nothing exotic is necessary.
The same is true even if you decide to distribute square waves. The
fewer higher harmonics you have, the better off you will be.
Yes and no, depending on what plagues your installation. Sine is the
perfect waveshape of converting additive noise (hum for instance) into
jitter if it meets a comparator. The low slewrate will be the problem
with the sine, so increasing the through-zero slew-rate reduces this
problem. The jitter conversion follows this formula:
t_rms = n_rms / S_signal
Where n_rms is the Noise (including hum and other unwanted signal) RMS
value (in V),
S_signal is the signal slew-rate (in V/s) and
t_rms is the jitter time (in s)
This have bitten me many times.
You can avoid having additive noise transform into jitter by not
feeding
it into a comparator but rather linearly gaining yourself out of the
situation. Yes that gain-stage will saturate at the ends. See the
TADD-2
input stage, as adapted from the Wenzel page on the subject. Such input
can also handle square signals.
Not all inputs is made to handle sine signals properly, so you can have
some additional noise on your reference as you lock up your instrument.
This also means that some receivers is best served by square, even if
square is tricker to handle to some degree. Others can be best served
by
sine.
In the end, there isn't one way which is always right, rather you need
to adapt to the receivers needs.
It would be nice to have a little standard board with a good quality
sine squarer such that it can be put inline at or in the instrument.
Cheers,
Magnus
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https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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--
Sent from my Motorola Droid Razr 4G LTE wireless tracker while I do other things.
On 09/28/2013 02:24 PM, Didier Juges wrote:
As always, be careful with the hard and fast rules. Most crystal oscillators generate sine waves first, but if the oscillator is part of a GPSDO, it will have to be converted to square to be processed by the logic within the GPSDO, so even if the device has a sine output, there will be a square wave version of the signal within the device. Eventually, the sine has to be converted into a square wave at the other end of the cable either to drive a mixer or a logic gate.
Where you do that conversion (before the cable or after the cable), or whether you need to go to sine altogether is something that has to be considered as part of the overall system design.
In many applications, there is absolutely no need to go to sine if you have a clean square wave to begin with, for instance if the cable is short or you connect with well matched devices.
For some info on what cable mismatch does to a square wave, look here: http://KO4BB.com/Test_Equipment/CoaxCableMatching.php
As Magnus pointed out, there are additional considerations such as ease of troubleshooting that come into play also.
Finally, if your primary concern is frequency distribution to several instruments as opposed to precision timing, sine will probably be easier because multiple reflections and mismatches that would affect time of arrival don't matter if you are only interested in a stable and precise frequency for your radios.
Almost true. The missmatches depends on the coax electrical length, and
that vary with temperature, so if you are really deep down in stability,
it can affect you. Then again, most of the time the direct effect of
delay dominates.
So, there is no easy answer, there is a bunch of "it depends" and in the
process you learn that proper impedance match and termination is usually
a good thing. Choice of cable can alter your performance in many ways.
You signal and your input circuit needs to match for best performance.
Cheers,
Magnus
Hi
If we're talking about a basement TimeNut setup there are some things that matter and some things that really don't:
Matters:
Load isolation matters. You can get > 45 degrees (> 12 ns) quite easily with a load change
ADEV matters. Your frequency counter will not be happy with a 1 ppm ADEV at 1 second.
Spurs and phase noise matter close in. Properly designed gear will not depend on sideband phase noise on the reference
Doesn't matter:
Cable delay change. With < 20' of cable and a couple of degrees an hour, you won't see it.
Wideband phase noise - see above
Perfect match. You are unlikely to have APC-7 connectors on your setup. BNC's are only just so good.
Harmonics past some point. It's > 20 dbc, but certainly < 60. Your change in load / change in timing isn't that big on a short cable.
Yes, it you have a dozen hydrogen masers in your basement, these rules probably don't apply. If you are a "hey, I've got a TBolt" sort of member, they very much do apply.
A few cautions:
No, you don't drive your super duper phase noise mixer with the output of the distribution system. You use a dedicated source.
Same is true for your super duper ADEV at parts in 10 to the zillion setup. You *need* to be isolated from everything for this.
If your "lab" swings temperature by 100C per day, that's a problem. It needs to be fixed. That swing will impact a lot of other stuff well before it nukes the distribution setup.
If you daisy chain dozens of instruments with a forest of T connectors, you are headed for trouble. It's not the clock source that's the issue, it's the operator.
Your best setup is one output to one instrument.
With some common sense it all can work pretty well for very little money.
Bob
On Sep 28, 2013, at 5:21 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
On 09/28/2013 02:24 PM, Didier Juges wrote:
As always, be careful with the hard and fast rules. Most crystal oscillators generate sine waves first, but if the oscillator is part of a GPSDO, it will have to be converted to square to be processed by the logic within the GPSDO, so even if the device has a sine output, there will be a square wave version of the signal within the device. Eventually, the sine has to be converted into a square wave at the other end of the cable either to drive a mixer or a logic gate.
Where you do that conversion (before the cable or after the cable), or whether you need to go to sine altogether is something that has to be considered as part of the overall system design.
In many applications, there is absolutely no need to go to sine if you have a clean square wave to begin with, for instance if the cable is short or you connect with well matched devices.
For some info on what cable mismatch does to a square wave, look here: http://KO4BB.com/Test_Equipment/CoaxCableMatching.php
As Magnus pointed out, there are additional considerations such as ease of troubleshooting that come into play also.
Finally, if your primary concern is frequency distribution to several instruments as opposed to precision timing, sine will probably be easier because multiple reflections and mismatches that would affect time of arrival don't matter if you are only interested in a stable and precise frequency for your radios.
Almost true. The missmatches depends on the coax electrical length, and
that vary with temperature, so if you are really deep down in stability,
it can affect you. Then again, most of the time the direct effect of
delay dominates.
So, there is no easy answer, there is a bunch of "it depends" and in the
process you learn that proper impedance match and termination is usually
a good thing. Choice of cable can alter your performance in many ways.
You signal and your input circuit needs to match for best performance.
Cheers,
Magnus
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