Cheap GPSDO Buying Help 2024?

[NT0Z]

Hi, gang,

My "unusual and obscure" BG7TBL GPSDO variant (5-8 years old) has developed a heat-related intermittency issue, and if I can't fix it, I'm in the market for a cheap GPSDO. I am already in a bit over my head, so there's that...

Compared to many Super Metrologists, my needs are ridiculously minimal. I use the GPSDO to discipline a frequency counter that, to date, has never needed to count above 100 MHz. In fact, without a prescaler it CAN'T count above 100 MHz.   I simply need to corral that device to an accuracy level of about "1 Hz in 50 MHz," which isn't overly special. I also have a spectrum analyzer that has a 10-MHz ref input, so when I build a distribution amp, that thing can play along, too.

My now-ailing unit has two 10-MHz outputs, one sine, one square. When it was working I successfully connected it to my venerable 5328A counter using both outputs, but I'm not sure whether a sine wave reference is "better" than a square.

Does it matter to the general variety of home lab devices?

I ask because I have been seeing some inexpensive GPSDOs (and OCXOs) on eBay for $40-$80. They seem to put out square-wave signals at 10 MHz, so if I need sine -- or want sine -- that might be a problem.

I learned to use Lady Heather to check in on my now-ailing GPSDO, so I don't know whether these inexpensive units can "speak to Lady Heather." I don't use the GPS time-keeping function from the software, etc, I just use it to confirm that My unit is seeing a bunch of satellites and that the ref output is locked." For now, perhaps forever, the rest of the info is just noise.

Can these cheapie GPSDOs "indicate lock," etc, or would I be missing out on necessary functionality, even for my minimal needs, if I don't get a unit that can speak Heather?

Can anyone offer advice on which cheapie units they have had success with, etc?

I appreciate your input,

Kirk, NT0Z

P.S. I SHOULD just bite the bullet and buy a Bodnar GPSDO and be done with it, but I'm trying to buy other goodies, too, so saving money is key...

[EC8010]

I bought two GPSDO. The first was a cheapy 10MHz only from ebay; it works fine (for my needs, at any rate, which seem similar to yours).

https://www.ebay.co.uk/itm/175372737601?itmmeta=01HRN6BBY45ZGT320EXP7A8HTS&hash=item28d5079c41:g:OygAAOSwh6Vi6z5B&itmprp=enc%3AAQAIAAAA4M1Aia5X4rEdXaw7s6Vb30ruywauCAsV2eVjV0X9CH6pq2C9QE64e48y0%2BC3%2BJLICmp%2BfHtaJO7lbHcFXiHjnKbHi8QI4UP41ozQfjTxw9AY3JI%2BYnJ2NAQ36Kbo0HRSpMqw0%2BnV%2BT7vdIT50jjsGu6ppZIMYpQDjmvSGZ7HBO2KKfnvElld95N9jH5fWiNv0M2EiiopzTimat20yePubkWQ2ajUV5OemQ1nT0XevVR2j%2Bj0itTg1RzVIyKLDjbgQQe5aSwHUMuswz0HJxHDPp%2Fz7A6SgibR%2F8VTDCq5fdop%7Ctkp%3ABk9SR6a_rabFYw

Then I bought a Leo Bodnar that would do 10MHz and 96kHz on two separate outputs. A square wave output makes it easier for other equipment to lock accurately because most PLLs operate on a square wave internally. The cheapy GPSDO indicates when it has achieved lock (takes at least ten minutes), albeit not terribly intuitively. For best results, a GPSDO should be left permanently powered, so that is arguably unimportant. It is, after all, a crystal oscillator, and needs to settle. Something that did surprise me was that my oscilloscope initially refused to lock to the Leo Bodnar, yet locked happily to the cheapy. It turned out that the oscilloscope's Ref. In didn't terminate the cable and reflections were upsetting the square wave. A through termination solved that problem.

A distribution amplifier is the proper way to feed a number of destinations. But it's clear that some devices expect you to loop through. Like you, I have an RF oscillator that will lock to a 10MHz signal. Haven't yet tried it with a square wave.

No idea what Lady Heather is.

[GigaJoe]

for such application , I use  a well isolated OCXO in Styrofoam , and do adjust it against gpsdo once in a while ( usually twice per year , or less )
the cheap one  - https://www.ebay.com/itm/266588217507  (it AC CUT)
it constantly ON , no any movement or reposition,   (gravity affect oscillation freq)   the result after  6-8 month like:  9.99999999XX Mhz
sufficient for 8 digit counter .....

recently got this one - no clue how well it work
https://www.ebay.com/itm/333559659893

so maybe ocxo approach , with some periodic sync, will work for you as well  ...

[ddosegov]

Buildd one from here https://www.eevblog.com/forum/projects/budget-gpsdo-a-work-in-progress/

9A4BP

[GigaJoe]

for someone who not much familiar with  coding ..etc  . it a challenge

[trobbins]

for someone who not much familiar with  coding ..etc  . it a challenge

Which GPSDO is a challenge wrt coding?  I don't 'code', and prepared the GPSDO in post #8 link for about $30 worth of ebay purchasable parts.  You do have to install some free software, and use USB to RS232 like adaptors, so it helps if you aren't a PC or IT newb, but you have to do that anyway to monitor GPS connectivity with cheap GPS modules/antenna.

[vindoline]

If you are willing to DIY, this came out great:
https://www.eevblog.com/forum/projects/yet-another-diy-gpsdo-yes-another-one/msg5064550/#msg5064550

[NT0Z]

My GPSDO and my low-budget TCXO are always powered on.

Lady Heather is a PC program that monitors and controls a whole bunch of GPS/GPSDO/Freq Standard modules, mostly from 5-15 years ago (maybe newer?). It talks to my GPSDO via serial port to report on status, correction voltages, reference lock, etc. People way smarter than me wrote it and give it away for free. Yay!

I would be interested in building a GPSDO, but my time is limited and I'd rather spend it on the things that having a working GPSDO enables, like restoring vintage ham gear, building HF radios, etc.

If the supplied details were JUST A TITCH MORE COMPLETE I wouldn't hesitate, but I haven't programmed a PIC or an EPROM for 35 years, would prefer to work in an Arduino environment because I am a clueless digital newb, etc. My 45 years building circuits has all been analog/RF, so if I were to undertake the GPSDO build now it would take lots of time and resources, and chances are good that I'd end up, at least initially, with TWO non-working GPSDOs instead of just one (which I already have)   

A kit build would be much easier, but I'm not sure if any are available.

The DIY GPSDO referenced is a fantastic thing, but as is it's still a bridge too far to be practical for me. Unless there's a kit or semi-kit?

Regards,

Kirk, NT0Z

[GigaJoe]

right ....  sooo ...  hm ...

option 1 -
check this guy --- https://www.ebay.com/usr/macman1972a
the store has some  complete option based on DIY discussion ...  ( don't like price )

option 2 - ( my preference )
https://www.ebay.com/itm/133264349995
reclaimed board , no need any coding for a basic run ;  low jitter , quite precise , just need stable +5 2.5A
receiver not so sensitive and GPS only

[NT0Z]

Thanks for the specific links. Much appreciated!

--Kirk, NT0Z

[ddosegov]

Lady Heather is a PC program that monitors and controls a whole bunch of GPS/GPSDO/Freq Standard modules, mostly from 5-15 years ago (maybe newer?). It talks to my GPSDO via serial port to report on status, correction voltages, reference lock, etc. People way smarter than me wrote it and give it away for free. Yay!

Can it flash Trimble Lassen IQ GPS modules? Is there any firmware update for Trimble Lassen IQ? Few years ago there was a lot of very expensive  GPSDO/NTP equipment that had to be tossed away due to gps week rolover.

[NT0Z]

No idea about Lady Heather and flashing a Trimble module...but some folks on the "time nuts" list (google it) might. I think thats's where the software dev also hangs out.

Regards,

Kirk, NT0Z

[fourfathom]

If you want something that just works, not too expensive, and has reasonably good performance, it's hard to beat a Bodnar.  The hold-over during loss of GPS isn't as good as the fancy units, and there is some minor phase wander, but for my own needs (general measurement and frequency-stabilized transmitters and receivers used in ionospheric research) it's an excellent solution.

[Wallace Gasiewicz]

I built a GPS Disciplined OCXO out of stuff in my Junk Box. Including a Brick PS and an old GPS unit.  But I still had to buy some stuff.....     
It took quite a while and it works fine, the GPS signal disciplines the heater in the OCXO.   
After all the time and effort, I think I would have been better off buying a Bodnar....

[enut11]

My GPSDO and my low-budget TCXO are always powered on.

Lady Heather is a PC program that monitors and controls a whole bunch of GPS/GPSDO/Freq Standard modules, mostly from 5-15 years ago (maybe newer?). It talks to my GPSDO via serial port to report on status, correction voltages, reference lock, etc. People way smarter than me wrote it and give it away for free. Yay!

I would be interested in building a GPSDO, but my time is limited and I'd rather spend it on the things that having a working GPSDO enables, like restoring vintage ham gear, building HF radios, etc.

If the supplied details were JUST A TITCH MORE COMPLETE I wouldn't hesitate, but I haven't programmed a PIC or an EPROM for 35 years, would prefer to work in an Arduino environment because I am a clueless digital newb, etc. My 45 years building circuits has all been analog/RF, so if I were to undertake the GPSDO build now it would take lots of time and resources, and chances are good that I'd end up, at least initially, with TWO non-working GPSDOs instead of just one (which I already have)   

A kit build would be much easier, but I'm not sure if any are available.

The DIY GPSDO referenced is a fantastic thing, but as is it's still a bridge too far to be practical for me. Unless there's a kit or semi-kit?

Regards,

Kirk, NT0Z

I built this GPSDO from a partial kit and it works. No programming skills required.
https://www.eevblog.com/forum/projects/budget-gpsdo-a-work-in-progress/

[5U4GB]

option 1 -
check this guy --- https://www.ebay.com/usr/macman1972a
the store has some  complete option based on DIY discussion ...  ( don't like price )

option 2 - ( my preference )
https://www.ebay.com/itm/133264349995
reclaimed board , no need any coding for a basic run ;  low jitter , quite precise , just need stable +5 2.5A
receiver not so sensitive and GPS only

Nick Sayer on Tindie also has a bunch of GPS timing stuff, USB GPS timing module, full GPSDO, and an FE-5680A GPS discipline module which he's been updating continuously, just noticed that it's now quad-constellation.  He's also really good to deal with in terms of support, which you may not get from eBay sellers.

[NT0Z]

Nick appears to be taking a break until the middle of May.

--NT0Z

[NT0Z]

Okay. I did some poking, tapping, and freezing...

Lady Heather says the OCXO is GPS locked.

On pin 2 of the MAX4016 (inverting input 1) I have a 10-MHz signal that measures about 22 mV P-P with a 10X probe. So is that actually 220 mV? There is no output on pin 1, however (the corresponding op-amp output pin. This pin feeds the 74ACT244 chip, which feeds the output connectors via an SMD cap or two. There is "jittery stuff" on the output connectors, but that's it.

After applying freeze spray I can tap on either chip -- but mostly the larger 74244 -- and get short periods of BIG 10-MHz output.

In this area there are the two chips and a few SMD caps, but nothing underneath the PCB in that area.

There are two "larger" chip caps from the MAX4016 input pin that are paralleled to GND. I could solder a wire to that cap/rail and feed a MMIC amp to boost the signal, bypassing whatever is going on with the MAX4016 and the 74244?

I could apply a bit of liquid flux and heat the solder pads for the legs on the chips and the ends of the caps with my hot air tool in case one is intermittent?

If I need to replace either chip, should I start with the MAX4016? It has an input signal but no output. Vcc is 5-V dc, and Vee seems to be GND.

Replacing these parts isn't rocket science, but it's new to me, and my eyesight isn't helping...

Ideas?

Thank you,

Kirk, NT0Z

SORRY -- POSTED THIS TO WRONG MESSAGE, BUT YA NEVER KNOW!

[ddosegov]

Ideas?

Nice hi-res picture or schematic would be helpful, but amp giving nothing on output despite there is signal on input looks like troublemaker.

[SCSKITS]

When looking at a GPSDO, look for plots of frequency difference over time and Allan deviation at the frequency of interest.
In other words basic TimeLab measurements.

Look at the OCXO used. The larger double oven OCXOs are best but have a higher cost and require more power. The the mid size units (about 1.5" by 1") are single oven (a crystal heater), usually without insulation, but are good enough for most use in a GPSDO. The small (about 1" square) <$5 OCXOs are the worst of the lot but can still be disciplined by the GPS.

There are some GPSDO articles out there where they use measurement of the control voltage converted to frequency as their determination of stability. The DMM used can hide some variations as most meters have a long integration time that can smooth out some of the noise. This method also does not account for variation internal to the OCXO.

If Phase noise is important for your application, look for that also. Unfortunately I currently lack the equipment for phase noise measurement, working on it.

I recently made a couple of GPSDO boards using UBLOX M8 and M9 GPS receivers, an EPLD for the dividers and phase detector, an analog PLL filter, and a 5V OCXO.
The phase detector output to the filter is buffered by a CMOS gate powered from a precision reference. Both square wave and sine wave outputs are provided.
The output frequency difference is at the limit of the test equipment I have. Frequency difference and Allan Deviation plots are in the listing.
The GPSDO was tested using an Agilent 53210A frequency counter with an HP Z3801 GPSDO external reference.

Listing is here: https://www.ebay.com/itm/156042688976.

ed

[jjoonathan]

I'm in this market as well, looking to spend maybe a few hundred bucks but not much time, I'm just looking for something that works. Based on the many recommendations I'm sorta leaning towards the Bodnar GPSDO, but I have one big hesitation: does it really not have NMEA output? That's... sort of a big deal for me. I have major trust issues with "PLL Locked" LEDs. Maybe I've just spent too much time digging around in old broken equipment, but I've just seen far too many PLLs give an "all good  " when all was not in fact good. Seeing a bunch of satellite info and correct computation of time & location makes me much more comfortable. I wouldn't expect this to be a rare viewpoint, but it sure seems to be... does everyone here have 5 GPSDOs and equipment to compare them so missing NMEA on one of them is no big deal? Or does nobody else have trouble trusting the LED? Why is it so abnormal for me to want to see NMEA data?

[fourfathom]

I'm in this market as well, looking to spend maybe a few hundred bucks but not much time, I'm just looking for something that works. Based on the many recommendations I'm sorta leaning towards the Bodnar GPSDO, but I have one big hesitation: does it really not have NMEA output? That's... sort of a big deal for me. I have major trust issues with "PLL Locked" LEDs. Maybe I've just spent too much time digging around in old broken equipment, but I've just seen far too many PLLs give an "all good  " when all was not in fact good. Seeing a bunch of satellite info and correct computation of time & location makes me much more comfortable. I wouldn't expect this to be a rare viewpoint, but it sure seems to be... does everyone here have 5 GPSDOs and equipment to compare them so missing NMEA on one of them is no big deal? Or does nobody else have trouble trusting the LED? Why is it so abnormal for me to want to see NMEA data?

No, the Bodnar does not deliver NMEA data, and no, you are not alone on wishing it did.  In one of my applications I use a cheap GPS chip/module on my board to get NMEA as well as 1pps, and an external Bodnar for the reference clock.  I will eventually try to combine these functions, but for now using the Bodnar saves me the time and effort of implementing a GPDSO.

Is anyone aware of an inexpensive GPS chip that simultaneously does NMEA, pps, and high-frequency clock for a GPSDO reference?  This doesn't have to be time-nut quality, just good enough.

[PCB.Wiz]

Is anyone aware of an inexpensive GPS chip that simultaneously does NMEA, pps, and high-frequency clock for a GPSDO reference?  This doesn't have to be time-nut quality, just good enough.

Many allow the 1pps to change even up to some MHz on a NCO basis, but I've never seen one with two output pins.

[SCSKITS]

The LEA-M8F (if I recall correctly) has a 30.72MHz output and a programmable output.
Some of the others may also have two programmable outputs.
The 8, 9, and 10 series parts can save the settings in FLASH, the 5, 6, and 7 series need a battery to backup the settings.
The parts with USB are easy to configure using the UBLOX configuration program. I would assume the serial port on the parts is similar.

The BODNAR GPSDO (with two BNC outputs) is a UBLOX MAX-M8Q module and a SI5328C jitter correction chip controlled by a PIC with USB.
The SI5328C provides the two outputs, but the second output must be integer related to the first.
Of all the SI chips, the SI5328C has the lowest frequency loop filter for the PLL.

ed

[fourfathom]

Is anyone aware of an inexpensive GPS chip that simultaneously does NMEA, pps, and high-frequency clock for a GPSDO reference?  This doesn't have to be time-nut quality, just good enough.

Many allow the 1pps to change even up to some MHz on a NCO basis, but I've never seen one with two output pins.

Yes, I'm involved peripherally with some designs that run the clock out pin way up in frequency (to drive a GPSDO), but in addition to the frequency reference I also need the 1pps time reference.  Or *some* absolute time reference that's more precise than the arrival of an NMEA sentence.  Of course I also need lat/lon/day/hour/min/sec/status.  I can get this from my old thunderbolt box, but I want it on an inexpensive module I can have JLCPCB put on a board for me.  I'm afraid I will end up with two chips, one for time and the other set up for frequency.

[fourfathom]

The LEA-M8F (if I recall correctly) has a 30.72MHz output and a programmable output.
[...]

This could be useful info, thanks!  I've just begun to search for a solution.

[Leo Bodnar]

We have just introduced NMEA data output to LBE-1420 GPS locked clock source which is a revised version of mini GPS clock.

After I am happy that there are no side-effects to the main operation of the synthesizer we would introduce the same functionality to the mini GPS clock and the two port GPS clock.

Sorry, it took some time to implement it.

Leo

No, the Bodnar does not deliver NMEA data, and no, you are not alone on wishing it did.  In one of my applications I use a cheap GPS chip/module on my board to get NMEA as well as 1pps, and an external Bodnar for the reference clock.  I will eventually try to combine these functions, but for now using the Bodnar saves me the time and effort of implementing a GPDSO.

Is anyone aware of an inexpensive GPS chip that simultaneously does NMEA, pps, and high-frequency clock for a GPSDO reference?  This doesn't have to be time-nut quality, just good enough.

[jjoonathan]

Sweet! Order placed.

[PCB.Wiz]

Yes, I'm involved peripherally with some designs that run the clock out pin way up in frequency (to drive a GPSDO), but in addition to the frequency reference I also need the 1pps time reference.  Or *some* absolute time reference that's more precise than the arrival of an NMEA sentence. 

What frequencies do you need ?
The choice of CMOS counters/dividers is fading a bit over time, but you can still buy HC40103 /N  and even HC4059 dividers.
Those are simple to apply, with no coding, but the PCB area increases if you use multiple packages, and the prices of even moderate MSI parts are inching upwards.

Small MCUs are another option: their peripherals are getting smarter and not much code is needed to configure a 16-20 pin part as a multiple output hardware divider.

[fourfathom]

We have just introduced NMEA data output to LBE-1420 GPS locked clock source which is a revised version of mini GPS clock.

After I am happy that there are no side-effects to the main operation of the synthesizer we would introduce the same functionality to the mini GPS clock and the two port GPS clock.

Sorry, it took some time to implement it.

Leo

Very nice!  Do the NMEA sentences show up on the USB/serial port, and if so, how do you switch between configuration mode and NMEA mode?  Or are there two virtual ports?
When will this show up on the Mini and Dual units?  Is a firmware upgrade to existing mini's possible?

[Leo Bodnar]

Very nice!  Do the NMEA sentences show up on the USB/serial port, and if so, how do you switch between configuration mode and NMEA mode?  Or are there two virtual ports?
When will this show up on the Mini and Dual units?  Is a firmware upgrade to existing mini's possible?

Yes, the firmware upgrade adds CDC interface to the existing HID interface. NMEA messages flow through virtual COM port and HID configuration works like before, independently of NMEA.
I would say in a month or two both Mini and Dual will have the same functionality upgrade.
Thanks to all, Leo

[Leo Bodnar]

Small MCUs are another option: their peripherals are getting smarter and not much code is needed to configure a 16-20 pin part as a multiple output hardware divider.

tvb has been making his since the dawn of time http://www.leapsecond.com/pic/picdiv.htm

[NT0Z]

Update from OP:

I replaced the MAX4016 and the 74ACT244 -- reasonably monumental achievements for me as someone who has never done any SMD work previously -- and the GPSDO is behaving exactly as it did, save for the fact that freezing or tapping the 74ACT244 no longer introduces visible glitches on the scope or prompts brief periods of outputting a working 10-MHz ref signal.

Summary: My unusual BG7TBL-style GPSDO, which worked for 6 years, recently stopped outputting a 10-MHz ref signal, even though Lady Heather notes that the RX is working nicely, time functions, too, as well as OCXO lock/control, etc. Everything but the ONLY thing I require: a 10-MHz signal to discipline my freq counter.

The end of the ref output chain looks like this: MAX4016 op-amp, which feeds a 74ACT244 distribution or gating chip, which outputs 10-MHz ref signals, one sine, one square, through separate SMA connectors (just before the connectors there are a couple of SMD caps, probably some kind of low-pass filter?).

I have a 10-MHz "almost perfect sine wave" signal on the input pin of the MAX4016, but NO output to the 74ACT244. I replaced the MAX4016, no change, but when I tapped on the 74ACT244, I could "glitch" the 10-MHz signal. I replaced the 74ACT244. No change, but now tapping does nothing, which probably indicated a bad 74ACT244?

Lady Heather suggests that the rest of the unit is working fine. I have a 10-MHz signal on the input pin of the MAX4016, and on the "shoulders" of the 1k resistors there.

Thought 1: Acquire the signal through a small capacitor and feed it through a MMIC amplifier on a small PCB and, once buffered and isolated, pass it to the input of a distribution amp through a separate BNC connector for use in downstream devices (with controlled amplitude).

Thought 2: Acquire the existing signal as above, amplify it with a MMIC, and then reintroduce it to the caps in the output network, right near the sine wave SMA output connector, thus leapfrogging the goofy parts and supplying an amplified and buffered 10-MHz signal to the output. Looks like any dc present on the amplified signal would not be "fed back" into the 74ACT244 because there's a series, dc-blocking cap inline. This method might take advantage of the small network of caps just before the SMA output connector for harmonic filtering. ??

I could also buy a new cheap GPSDO unit as discussed above, but fixing and hacking this thing is now driving me to either fix it, or fix it to death.

Any thoughts?

BTW: I don't have a schematic for this unusual unit, but I will upload a pic or two later tonight.

As always, thanks,

--TG

[Leo Bodnar]

I have a 10-MHz "almost perfect sine wave" signal on the input pin of the MAX4016, but NO output to the 74ACT244. I replaced the MAX4016, no change, but when I tapped on the 74ACT244, I could "glitch" the 10-MHz signal. I replaced the 74ACT244. No change, but now tapping does nothing, which probably indicated a bad 74ACT244?

This sounds like a bad solder joint rather than a failed part.
Try resoldering all the connections downstream of the OCXO output — including SMT joints.
If this does not help, try replacing the passives — there is more chance that they have failed rather than ICs.
Leo

[NT0Z]

I can give this a try, but the risk of me killing the unit increases, as the system is comprised of three stacked PCBs held together by stand-offs and fragile ribbon cables. But I guess in for a penny, in for a pound...

I don't yet get how a signal can be present on the input pin of an op-amp without anything coming out of the same op-amp if that op-amp isn't defective. But apparently it can, cuz it's still that way even with the new chip.

Thanks,

Kirk, NT0Z

[NT0Z]

Pix promised by the OP:

PCB View: The SOIC8 at left if the MAX4016 opamp. The SOIC20 is the 74ACT244. Pin 2 of the opamp (top, second from right) is the input. Pin 1 is the output. The "hot" sides of the two 1k resistors just "north" of the opamp connect to the input pin. The 10-MHz waveform you see in the scope shot appears there.

Junk on scope: This janky, random waveform appears on the output pin of the opamp, which connects to the input of the 74ACT244 via a series cap. It also appears everywhere else downstream, right to the SMA output connectors.

Both chips have been replaced.

Thanks,

--NT0Z

[Leo Bodnar]

Check DC bias on pins 2 and 3 of the opamp.
It's possible that the trimpot or one of the 1k or 2k resistors are faulty.
Leo

[dietert1]

If your scope image can be trusted, you have about 25 mVss, which is a very small input signal. So the fault may be upstream. With that small input signal, output will appear only after readjustment of the potentiometer and it will be jittery.

Regards, Dieter

[NT0Z]

Hey, Leo and the gang,

Ironically, I'm trying to fix this GPSDO so I don't have to buy one of Leo's lovely units!

The pair of 1k resistors seems good. I measure 550 ohms between pin 2 and GND, and a pair of 1k resistors in parallel should be in that ballpark, right?

The 25 mV is while using a 10X probe, so 250 mV in actuality? My old Rigol scope doesn't seem to know what the probe setting is.

Is there a trimpot in there somewhere? Not the big blue precision pot that nets the OCXO, right?

I will measure/probe the other pins on that opamp.

Thanks.

--NT0Z

[dietert1]

250 mVss sounds better. The potentiometer sits next to the two 1 KOhm resistors. It's small.

[NT0Z]

Dang -- I didn't even know that was a potentiometer!

Any idea what it might do?

--NT0Z

[dietert1]

It is used to adjust the discrimination voltage of the amplifier. When it's off level, the 10 MHz outputs get stuck at logical 0 or 1.

[NT0Z]

Okay:

That mini trimpot is very twitchy and subject to being "bumped." I rotated it through its range, then gave it a drop of fader lube, then rotated it some more, and then I gave it a taste of the hot air station.

After cooling down I was able to find a fragile setting that results in this output from the square and sine wave ports. When doing so you can see what Lady Heather says.

For most of the pot's rotational range there is no coherent output on the scope. Shown here is the "best" setting, but the square wave isn't quite square, and the sine wave is a bit goofy, although the oscillator stops/locks at 10.0000 MHz (after drifting upward for a few hertz after every cold start or reset), which is the decimal precision limit of the counter in my old Rigol scope (never calibrated by me).

For various "nearby" pot settings the waveforms get weird double humps (lower side) and look really goofy, with the counter displaying freqs between 5 and 11 MHz (I assume because of the weird wave shapes or zero/trigger crossings?).

Left as shown, the waveforms have remained locked for a day or so.

On the MAX4016 opamp: When there's no coherent output on the scope, pin 8 (dc in) and pin 3 (non-inverting input) show 5 VDC. When the trim pot is rotated to produce the display as shown, and there is coherent output on pin 1, pin 3 shows 2.5 VDC -- half of the dc input. I think this is normal.

My problem all along probably was related to the trim pot, which seems to measure 500 ohms from top to bottom, although I don't know if any other resistances are involved. The coherent output is produced near the halfway point on the trimpot.

The pot is probably bad...and replacing it seems VERY difficult. I can barely see the three obscured solder pads. I could barely get the SOICs replaced.

What if I supply my own (separate) dc voltage to pin 3 with a real, large trim pot (through a 10k resistor to limit current)?

Can the existing waveforms be used to discipline my HP 5328A freq counter if I adjust the reference signal for proper input strength?

What if I connect the outputs to a distribution amp with adjustable gain?

I am learning a lot through this torturous process, which is good. But I'd really like to come through this with a working GPSDO.

Thanks again for your help,

--NT0Z

[dietert1]

The blue output waveform indicates a higher resistance termination, if you are using a 2 m cable between generator and scope. If you terminate the cable correctly at the scope end, the waveform will look clean. Amplitude will be about 2 V - half of the generator voltage.
If the HP 5328A external timebase input includes correct termination it will work.

Regards, Dieter

[NT0Z]

Dieter,

You are indeed an officer AND a gentleman! And perhaps a psychic!

New pic shows the dual-trace scope with 50-ohm terminating resistors "teed" onto each input with the requisite 2-meter-long BNC cables.

This looks usable! Sorry Mr. Bodnar -- no cash flow for you (just yet)!  Without replacing the SMD trimpot, I don't expect that this unit will work forever...

With help from many on this list I have learned a lot throughout this repair process. I learned how to replace SMD chips without killing the chips or the DUT. That's something that's been on my list for about 20 years. I learned that SMD trimpots exist -- who knew? I learned the basics of my DSO, and about potential termination issues. And more.

After learning about the discriminator level trimpot late in the game, it's my chips probably didn't need replacing, but it was for a good cause.

I will connect this to the HP 5328A later tonight, where I expect that things will work as normal.

If not, I'll be back. 

Thanks, everyone, for all the help. In the modern era most mentoring is distributed via the interwebs.

Regards,

--NT0Z

[EvgenyG]

Hi, Leo. Just received the new gen LBE-1420 unit, very happy customer. I've done some extensive research comparing the specs against the bigger A brand units and cheap no-name units and decided that it is absolutely worth extra $100. it is hard to beat that phase noise figure at that price point. I think most people don't understand the importance of phase noise. Good to know that there is NMEA support now, I was actually going to ask. Perhaps worth adding that to the product description page as well as brief information what different LED states mean.
Also wanted to note that the build quality is amazing.

[dietert1]

In order to get best results, the antenna makes a big difference independent of the GPSDO device. I mean a good antenna installed on the roof with a full free sky view can be difficult to obtain and will by far exceed the cost of the device. A geodesic antenna meant for measurements can be US$ 500 or more. One also needs to think about a good RF cable if it needs more than 5 or 10 m. Or place the GPS receiver next to or inside the antenna.
Once you use a patch antenna inside a building, the GPSDO device won't make a big difference.
Leo Bodnar's stuff is excellent but the LBE-1420 isn't exactly a GPSDO. Instead of adding some fanout for the 10 MHz it adds a wide band signal generator 1 Hz to 1.1 GHz. It can certainly be configured to output 10 MHz with low phase noise and one could try a splitter to sync two or three devices. Don't know if one can run two devices from one antenna, in case one needs 10 MHz and 1 MHz.

Regards, Dieter

Edit: Ours looks like this. Will replace it by a GNSS antenna.

[kevin.gibbs]

Does your frequency meter measure the period? If so, you can feed the 1PPS signal from the GPS receiver directly to the frequency meter input. You can also enable averaging (to filter receiver jitter). This should already be sufficient for your accuracy.

You can also take the signal from the MAX4016 input and use it.

[NT0Z]

Kevin -- thanks.

My HP counter can measure period, but I have never done so or made period measurements with any other devices.

After abusing the trimmer pot in the GPSDO (who knew that was a trimpot?) I have the sine and square wave output squared away as shown in the dual-trace photo. I am in the process of building a 10-MHz WWV receiver (TRF-style) so I can compare my GPSDO signals (on the scope) with those from WWV. After wrenching on the GPSDO and replacing two SOIC chips, I want to have another NIST-reasonable reference to make sure everything's okay.

I have a "TCXO in a box" that's been reading 1.2-Hz low at 10 MHz for several months. At least it's stable! In the interim I acquired a pair of inexpensive 10-MHz OCXOs, and if my counter is indeed disciplined correctly, each is reading 1.2 to 1.3 Hz high (no more adjustment range). One of the OCXOs will go into an insulated box and pretend to be a rubidium standard.

Using the WWV receiver I can feed a 10-MHz signal into the counter or into the scope that's 1 Hz or better at 10-MHz (avoiding dawn and dusk as appropriate) to at least verify that my GPSDO is locked onto the correct frequency...

I will also try the "zero beat" method over the weekend.

Regards,

--NT0Z

[SCSKITS]

Any of the inexpensive GPS modules should be able to produce better than 1Hz at 10MHz.
I ran a NEO-M9N on a MIKROE-3922 board (about $39 at Mouser) at 4MHz and got about +/-0.75E-8. At 10MHz, +/-1Hz would be +/-1E-7
A simple multiplier like the PT7C4511 set to 2.5 would give a 10MHz output.
The PTC4511 is in an 8 pin SOIC for easy soldering.
The frequency difference would be multiplied up also (by 2.5)  to less than +/-2E-8 assuming the PLL filter in the multiplier chip had little effect on the error. 
It looks like the 4MHz is getting better over time, I would have to run for a longer time to see just how stable it becomes.
If running the GPS module alone at 10MHz, the error will be greater than with the 4MHz output followed by the multiplier.
I have a test board that has the multiplier followed by a divide function that can be bypassed, I will try to get a run with that maybe over a longer time.

Another option is the LEA-M8F. It is more expensive but is one of the UBLOX timing modules and should do better than the NEO-M9N.
The 30.72MHz output is about +/-1E-9 right after startup, I did not run for a long time. I did not try the programmable output yet.
Starting with 30.72MHz output, you would need a more sophisticated multiplier divider such as the SI5328C which also requires a micro to initialize the part.
The SI-5328C is also difficult to solder for a DIY project.

ed