Discussion:
Low phase noise affordable GPSDO
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Ethan Waldo
2018-06-20 14:27:35 UTC
Permalink
Hello time afficionados. I've tried my best to scan through the archives to see if this has already been answered, so apologies if I missed something relevant. I am going to be experimenting with >10Ghz microwave frequency grounded coplanar waveguide phase combining/cancellation/detection across a number of generated signals. That being said, I am in the market for a GPSDO that has the least amount of phase noise. A secondary factor in my choice is how many 10Mhz references the GPSDO can provide to sync all my signal generators; not as concerned about what amplitude is provided.

Most of the cheap GPSDOs on eBay seem to have only 1-2 sine outputs with no understanding of expected phase noise. A solid option seems to be the TrueTime XL-AK but something tells me there might be options out there better than 1x10-12. Price is the right ballpark though I could justify spending a little more for better phase noise specs. I have no need to run without GPS lock so Rubidium source doesn't seems as necessary compared to something with a high quality (D)OXCO, but it would be nice to add that as an external source at a later time. Build vs. buy is an option, but I would like to avoid making costly mistakes so would favor buy if specs are comperable.

I would really appreciate advice from folks around here who already have some hard-earned experience on what my best options are. Whatever I get I'm likely to be stuck with for a while so I would rather take the time and research than to do a knee-jerk buy. Thank you ahead of time for any replies.

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Attila Kinali
2018-06-20 21:26:46 UTC
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On Wed, 20 Jun 2018 14:27:35 +0000
Post by Ethan Waldo
I've tried my best to scan through the archives to see if this has already
been answered, so apologies if I missed something relevant. I am going to
be experimenting with >10Ghz microwave frequency grounded coplanar waveguide
phase combining/cancellation/detection across a number of generated
signals. That being said, I am in the market for a GPSDO that has the least
amount of phase noise.
Phase noise is usually not the strong traits of GPSDOs.
That said, I guess a Trimble Thunderbolt or Nick Sayer's GPSDO with
the OCXO option would probably fit your bill.

Alternatively, take any of those two GPSDO and replace the OCXO
by one that has as low phase noise as you need.
Post by Ethan Waldo
A secondary factor in my choice is how many 10Mhz references the GPSDO
can provide to sync all my signal generators; not as concerned about what
amplitude is provided.
You are looking a bit naively at this. The noise floor of modern,
low-noise OCXO is limited by the Johnson noise of the 50R impedance.
Ie the higher the output signal level, the lower the noise floor (in dBc).
Hence, just using some passive splitters will increase your noise
levels. The 1/f^a noise levels are unaffected by this, though.

I would recommend going for a seperate distribution amplifier in order
to keep the noise at a minimum. If you are fine with DIY, go for the
design by Anders Wallin.


Attila Kinali
--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.
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Bruce Griffiths
2018-06-20 22:32:02 UTC
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The PN of the output amplifier in the thunderbolt will limit the PN nois of the 10Mhz output to a value significantly higher than the actual PN at the output of a low PN OCXO.

Bruce
Post by Attila Kinali
On Wed, 20 Jun 2018 14:27:35 +0000
Post by Ethan Waldo
I've tried my best to scan through the archives to see if this has already
been answered, so apologies if I missed something relevant. I am going to
be experimenting with >10Ghz microwave frequency grounded coplanar waveguide
phase combining/cancellation/detection across a number of generated
signals. That being said, I am in the market for a GPSDO that has the least
amount of phase noise.
Phase noise is usually not the strong traits of GPSDOs.
That said, I guess a Trimble Thunderbolt or Nick Sayer's GPSDO with
the OCXO option would probably fit your bill.
Alternatively, take any of those two GPSDO and replace the OCXO
by one that has as low phase noise as you need.
Post by Ethan Waldo
A secondary factor in my choice is how many 10Mhz references the GPSDO
can provide to sync all my signal generators; not as concerned about what
amplitude is provided.
You are looking a bit naively at this. The noise floor of modern,
low-noise OCXO is limited by the Johnson noise of the 50R impedance.
Ie the higher the output signal level, the lower the noise floor (in dBc).
Hence, just using some passive splitters will increase your noise
levels. The 1/f^a noise levels are unaffected by this, though.
I would recommend going for a seperate distribution amplifier in order
to keep the noise at a minimum. If you are fine with DIY, go for the
design by Anders Wallin.
Attila Kinali
--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.
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Bob kb8tq
2018-06-20 21:32:06 UTC
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Hi
Post by Ethan Waldo
Hello time afficionados. I've tried my best to scan through the archives to see if this has already been answered, so apologies if I missed something relevant. I am going to be experimenting with >10Ghz microwave frequency grounded coplanar waveguide phase combining/cancellation/detection across a number of generated signals.
If you are multiplying up, even a very low phase noise 10 MHz will get you in trouble outside a pretty narrow region around carrier. It’s far better / easier to use a
100 MHz (or higher) cleanup VCXO ( or OCXO) than to try to find a -187 dbc/ Hz GPSDO.
Post by Ethan Waldo
That being said, I am in the market for a GPSDO that has the least amount of phase noise. A secondary factor in my choice is how many 10Mhz references the GPSDO can provide to sync all my signal generators; not as concerned about what amplitude is provided.
Signal generators / counters / test gear generally have pretty modest input requirements. They normally do an internal cleanup process on the reference input.
Put another way, they expect a dirty reference.
Post by Ethan Waldo
Most of the cheap GPSDOs on eBay seem to have only 1-2 sine outputs with no understanding of expected phase noise. A solid option seems to be the TrueTime XL-AK but something tells me there might be options out there better than 1x10-12.
Assuming you are talking now about ADEV, it is highly unlikely that you will find a 1x10^-12 at 1 second tau GPSDO. Yes it you go out far enough ( long tau )
they all will get there some day. Good ADEV is not an indication of good phase noise in the sense of doing things at microwaves.
Post by Ethan Waldo
Price is the right ballpark though I could justify spending a little more for better phase noise specs. I have no need to run without GPS lock so Rubidium source doesn't seems as necessary compared to something with a high quality (D)OXCO, but it would be nice to add that as an external source at a later time. Build vs. buy is an option, but I would like to avoid making costly mistakes so would favor buy if specs are comperable.
Simple answer is to get a reasonable GPSDO. Do something simple to distribute the output to your test gear. There are lots of ideas in the archives.
CMOS gate based buffers work perfectly well for a sub $20 sort of solution. Then take one output and run it through your cleanup chain to form
the base of your microwave stuff. That way each building block is easy to come by and none are ghastly expensive.

Bob
Post by Ethan Waldo
I would really appreciate advice from folks around here who already have some hard-earned experience on what my best options are. Whatever I get I'm likely to be stuck with for a while so I would rather take the time and research than to do a knee-jerk buy. Thank you ahead of time for any replies.
Sent from my Verizon Wireless 4G LTE Droid
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Ethan Waldo
2018-06-24 21:57:20 UTC
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This is a little bit of a delayed reply, but I want to thank everyone for their advise. I did end up deciding on a Trimble Thunderbolt that I can fiddle with the OCXO on later. I hope the one I got from China isn’t a ripoff. The advise to get a separate distribution system was well taken and I got the best thing I could tell that was available, a Precision Test Systems DA101010 http://www.ptsyst.com/DA101010-B.pdf. It will definitely be fun to play with whenever it shows up in July.



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