GPSDO Tutorial?

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Tony EI7BMB
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Wed Apr 25, 2018 9:08 am

Thanks for the info Scott . Showing as $89.99 here (plus vat) in EU which is a great price.
https://www.ebay.com/itm/NEW-10MHz-Sinwave-PLL-GPSDO-GPS-DISCIPLINED-OSCILLATOR-adapter-GPS-ANT-/262038924752?hash=item3d02bcadd0

I see Leo Bodnar has a single output for £100.00, I wonder which one is better bang for buck ?

http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301
JA2GXU
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Re: GPSDO Tutorial?

Postby JA2GXU » Mon Aug 27, 2018 5:37 am

Phase noise problem for use general kind of GPSDO.

Degrading phase noise occurs desired weak signal buried in increased noise floor
I recommend don't use general kind of GPSDO due to degrading phase noise.
Improved GPSDO includes VC_OCXO and PLL have more than 100 seconds loop time constant, thus don't occur degrading phase noise.
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w-u-2-o
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Re: GPSDO Tutorial?

Postby w-u-2-o » Fri Aug 31, 2018 12:33 pm

Cross posting this from the Yahoo group mailing list so that it does not get lost in the sands of time, as it is valuable discussion worthy of preserving:

John Ackermann N8UR jra@febo.com [apache-labs] <apache-labs@yahoogroups.com>
8:28 AM (3 minutes ago)
to apache-labs

Rob, you're correct that the ANAN and other HPSDR radios use a PLL to
lock their internal reference to an external source. More precisely,
there is a very low phase noise 122.88 MHz VCXO that is locked either to
an internal 10 MHz TCXO or external reference.

But an important note on external references with the HPSDR radios. A
few of us have tried to use Hermes-based radios for precise frequency
measurements and have discovered that there is an issue in the PLL
circuit that results in phase jumps at random intervals (usually in the
10s of seconds apart). These are severe enough to destroy sensitive
measurements. I think they are inherent to all the radios that derive
from the Hermes schematic and firmware.

After experimenting we've found one fairly simple fix that at some point
Phil will put into the production firmware. But even after that fix
there is a problem (though of much smaller magnitude) that I haven't
been able to track down yet. As I have time I'm continuing to work on that.

This problem doesn't affect phase noise or real-world frequency
stability, but does show up in sensitive measurements as random "ticks"
in the transmitted phase.

73,
John
----

On 08/31/2018 01:35 AM, Rob Sherwood. rob@nc0b.com [apache-labs] wrote:
> Hi Scott,
>
>
> I don’t know anything about the oscillators or GPSDOs you mentioned.
>
> My main Denver Rubidium is an Efratom M-100 military unit I purchased
> NOS at least 15 years ago that has been running 25/7 ever since. The
> backup Rb in Denver and my only Rb at Ault are Lucent branded with
> Efratom physics packages. My only GPSDO is a rack mount Trak Systems in
> Denver I use to verify my Rubidiums are happy. They all test very clean
> with noise about -135 dBm / Hz at 10 kHz. Not like my Wenzel, but
> reasonable house standards from a frequency standpoint. The Austron
> distribution amp feeds all the HP test equipment except the 8662As and
> the 8642As in Denver.
>
> I will be locking the 7000DLE to the Rb at Ault. I have two 8642As at
> Ault, so one of them will be my signal source. I have no idea which
> clock is in the 7000. Has that changed over time? Warren said an
> external reference locks to the external reference as opposed to
> replacing it.
>
> As a second test I’ll feed in the other 8642A in as the reference. The
> 8642A noise at 10 kHz offset is about -153 dBm/Hz.
>
> What level is optimum for feeding in the 10 MHz reference to the 7000DLE?
>
> Rob, NC0B
>
>
> Sent from my iPad
>
> On Aug 30, 2018, at 4:03 PM, Scott Traurig scott.traurig@gmail.com
> <mailto:scott.traurig@gmail.com> [apache-labs]
> <apache-labs@yahoogroups.com <mailto:apache-labs@yahoogroups.com>> wrote:
>
>> That will be very interesting, Rob. Do you have BG7TBL unit there and,
>> if so, which OCXO is it equipped with? That makes a big difference, of
>> course. The one I have here is equipped with the excellent Russian
>> Morian MV89 OCXO..
>>
>> 73!
>>
>> Scott/w-u-2-o
>>
>>
>> On August 30, 2018 5:51:28 PM EDT, "Rob Sherwood. rob@nc0b.com
>> <mailto:rob@nc0b.com> [apache-labs]" <apache-labs@yahoogroups..com>
>> wrote:
>>
>> I’ll test this, too, on a 7000DLE on Sunday or Monday. Rob, NC0B
>>
>> Sent from my iPad
>>
>> On Aug 30, 2018, at 2:56 PM, wa3jbt@usa.com
>> <mailto:wa3jbt@usa.com> [apache-labs] <apache-labs@yahoogroups.com
>> <mailto:apache-labs@yahoogroups.com>> wrote:
>>
>>> I have a GPS Disciplined Oscillator, Model BG7TBL that I got on
>>> Ebay from China that I used to use with my 200D. I now am using
>>> a 7000DLE and I'm wondering if this unit will degrade my receiver
>>> in regard to phase noise. I believe Scott you are using or were
>>> using this model of GPSDO
>>>
>>> Frank, W2SDR
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w-u-2-o
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Re: GPSDO Tutorial?

Postby w-u-2-o » Fri Aug 31, 2018 1:01 pm

Rob,

On the Orion MKII based hardware, I believe the 122.8MHz oscillator is a Crystek CVHD-950 (click the hot links in this post for the datasheets), with the following advertised phase noise performance:

Image

The on-board 10MHz TCXO I believe is a Connor-Winfield M100F with the following advertised phase noise performance:

Code: Select all

SSB Phase Noise for Fo=12.8 MHz
 @ 10 Hz offset -90 dBc/Hz
 @ 100 Hz offset -120 dBc/Hz
 @ 1 KHz offset -140 dBc/Hz
 @ 10 KHz offset -150 dBc/Hz
 @ 100 KHz offset -150 dBc/Hz
 @ 1 MHz offset -152 dBc/Hz

As you can see from those spec's, the out of the box performance should be quite good. The cheap eBay/Chinese GPSDOs will probably degrade that by 5dB, give or take, depending on the TCXO or OCXO in the GPSDO, but a more serious, more expensive GPSDO should be OK. As noted previously, I'm happy to give up a little phase noise performance for the convenience of cheap frequency accuracy and long term stability.

With respect to appropriate input levels for an external reference, I find that anything between +7 and +13dBm works. Below +7 I've observed inconsistent performance. Levels above +13dBm could put the input differential amplifier circuit at risk for damage.

Here is a snippet of the schematic showing the external clock input and automatic selection circuit.

The jumpers at the input provide options with respect to a differential or single ended input (J24) and a 50 ohm or high impedance termination (J22), whichever your external clock source requires. If you are having problems first ensure that these jumpers are configured properly for your external clock source.

Next up is a sine to square wave converter formed from a two-transistor differential amplifier. This allows the use of either sine or square wave external clock sources. The resulting square wave goes to IN0 on the 74LVC1G97 which acts as a switch.

The square wave also goes to a circuit that derives a logic level for IN2 on the 784LVC1G97. When an external clock is attached that level will be high and the output of the switch (Y) will be sourced from the external clock on IN0. When no external clock is present that level will be low and the output of the switch will be sourced from TCXO U26 on IN1.

The same logic level on IN2 also is applied to the gate of Q1, and when high due to the presence of an external clock, will drive the P-channel FET to cutoff and turn off power to the on-board TCXO.

Image

As for the firmware PLL problems that John mentions, I looked at the code and all I can tell you is that the code uses the standard Intel (nee Altera) "altpll" megafunction. You would think that a standard megafunction would be bug free?

I'll be very interested in whatever measurements you are going to make, Rob.

73!

Scott
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Re: GPSDO Tutorial?

Postby k6avp » Mon Nov 19, 2018 3:34 am

I just got one of these and it has a CTS 970-2178-46 oscillator in it with uBlox GPS. It seems to be an older CTS model 115 OCXO. 10MHz output is ~10Vrms unloaded.

I have an ANAN-100B (Hermes) so am going to add an auto-select/buffer board (w7ay/Kok Chen Hermes Reference Oscillator with no OCXO loaded) to my radio before attaching it.
*************

Tony EI7BMB wrote:Thanks for the info Scott . Showing as $89.99 here (plus vat) in EU which is a great price.
https://www.ebay.com/itm/NEW-10MHz-Sinwave-PLL-GPSDO-GPS-DISCIPLINED-OSCILLATOR-adapter-GPS-ANT-/262038924752?hash=item3d02bcadd0

I see Leo Bodnar has a single output for £100.00, I wonder which one is better bang for buck ?

http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301
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w-u-2-o
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Re: GPSDO Tutorial?

Postby w-u-2-o » Mon Nov 19, 2018 2:12 pm

Thanks for the info on the "new" BG7TBL unit :) CTS is a fine company and that OCXO probably has some good spec's. It's so old I couldn't find a data sheet online, but if you called CTS I bet they could dredge one up for you.
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Re: GPSDO Tutorial?

Postby k6avp » Tue Nov 20, 2018 9:54 pm

I did messed up on the output. It is ~1.5Vrms unloaded and ~1.2Vrms/15dbmW into 50 Ohms.
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Thu Jul 11, 2019 9:23 am

Here is another option for anyone who wants to make their own GPSDO. We currently use the RF zero board as the EI 6m beacon and I use the carrier to calibrate my TX in OpenHPSDR/Thetis , not 100% accurate of course because of doppler over the air but I've checked my TX against friends who are GPS locked and its right on the money.

From WA8TOD "I have breadboarded the RFzero together with an ADF4351 evaluation board. For frequencies up to 200 MHz I can use the RFzero directly. From 200 MHz to 4.4 GHz (yes, you read that correctly..... GHz!) the RFzero supplies 25 MHz, GPS-disciplined reference to the ADF4351. Everything in the picture, including the kitchen cutting board base, cost less than $100."


https://groups.io/g/RFzero/topic/2650_h ... 0,31676551

Image
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Re: GPSDO Tutorial?

Postby w-u-2-o » Thu Jul 11, 2019 10:28 am

Looks like fun for the experimenter. But quite a rat's nest! Once you add in real packaging the price is going to come up, and for $125 or thereabouts you can have a nice BG7TBL clone or even Leo Bodnar's entry level unit, and either will probably be more reliable because the parts count is so much lower.
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Thu Jul 11, 2019 10:42 am

True Scott but bear in mind its a lot simpler with less parts and cost for freqs less than 200 mhz , its basically the RF zero board @ $75 plus a case and display if needed.
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Re: GPSDO Tutorial?

Postby w-u-2-o » Thu Jul 11, 2019 1:07 pm

Tony EI7BMB wrote:True Scott but bear in mind its a lot simpler with less parts and cost for freqs less than 200 mhz , its basically the RF zero board @ $75 plus a case and display if needed.
So the output is not 10MHz?
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Thu Jul 11, 2019 1:37 pm

Yes it can be set to 10mhz or any freq required , more info here http://www.rfzero.net/
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Re: GPSDO Tutorial?

Postby w-u-2-o » Thu Jul 11, 2019 6:05 pm

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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Thu Aug 29, 2019 7:00 pm

With a 7000 mk 2 on the way I gave in to temptation and bought the BG7 based unit on aliexpress (item 33055900193) for $163 including shipping . I know its more expensive than the stock BG7 but I'm a sucker for good looking toys .

Image
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Re: GPSDO Tutorial?

Postby w-u-2-o » Thu Aug 29, 2019 11:26 pm

Nice! Did you get it yet? Curious to know what kind of OCXO it uses.
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Fri Aug 30, 2019 7:56 am

Thanks Scott, just ordered so 3/4 weeks I guess . I'll take a look and let you know when it gets here.
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Tue Sep 24, 2019 6:53 pm

Unit arrived today and works fine after I made up an external mount for the supplied GPS antenna. Have not had time to look for the OXCO type so far as I've been monitoring the EI0SIX beacon all day. Nice display too

gpsdisplay.png
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Re: GPSDO Tutorial?

Postby w-u-2-o » Tue Sep 24, 2019 10:31 pm

200 microHertz error. How on Earth do you find that acceptable? ;)
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Re: GPSDO Tutorial?

Postby Tony EI7BMB » Wed Sep 25, 2019 7:50 am

LOL guess I'll have to live with it :D

Edit: think I'm ok now
10mhz.png
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