Using GPSDO or GPS for REF - Page 1
Forum section: electronics / projects

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 20, 2015, 02:55:28 pm
content: Hi all,How easy/difficult would be to lock an 10.480760MHz crystal to an GPSDO or GPS frequency standard of 10MHz.The schematic of the board with the crystal (10.480760) is below.Pin 1 - 9V inPin 2 - GND inPin 3 - Ref out to Local Unit

author: KJDS
author_rank: Super Contributor
timestamp: March 20, 2015, 03:23:40 pm
content: on a scale of 1 to 10 where 1 is very easy and 10 is almost impossible, it's about a 3

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 20, 2015, 03:37:58 pm
content: Hi KJDS,Ok where to start then?1 - Unplug the Molex J4207 and rout pin 1 through one switch back to J4207, that way I can feed the crystal and use internal reference, or cut the power to it and use the outside reference.2 - Plug the external reference to pin 3 of the Molex J4207 with RG-174.3 - Connect GND from RG-174 to GND of Pin 2.Next how to bring 10MHz to 10.480760?I think this is correct?

author: KJDS
author_rank: Super Contributor
timestamp: March 20, 2015, 05:10:29 pm
content: You need to make the oscillator tunable, which means adding a varactor.Feed the output of the oscillator into a fractional N PLL chip set to divide by 1.048076 and use the 10MHz from your GPSDO as the reference. Feed the PLL output through a suitable loop filter into the varactor and Robert is your mother's brother.

author: dannyf
author_rank: Super Contributor
timestamp: March 20, 2015, 05:49:13 pm
content: The answer depends on what access, if any, you have to the internals.The simplest would be to utilize that existing trimmer, on the far right of the schematic. Changing it will alter the output frequency.Another alternative is to alter the input voltage. The 68ohm resistor is likely a heater. It is wired in serial with that ptc to stablize temperature. Changing voltages will change the circuit parameters and will change the output frequency - limited in range.Or you can wire a diy trimmer - two isolated wires wound on each other - in parallel with the trimmer.Comparing two disparate frequencies can be difficult. One way is to reduce both to a common frequency, like 1pps. I just finished a prototype on that recently.10.480760Mhz however is a difficult frequency to divide.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 20, 2015, 05:58:28 pm
content: Hi guys,My aim was when the switch it's off, the crystal have no power so it's OFF, so the external reference is plug and ON.When the switch is ON the crystal is ON, and that reference is use, external reference will be unplugI've access to that board.My aim is to make thishttps://qlfecv.wordpress.com/2013/06/28/mod-ic-910-h-disciplinado/so I can loock my main reference.The frequency of the crystal is 10.480760MHz, I just want to make it steady on that frequency using an GPSDO or GPS.dannyf, that 68ohm and the NTC thermistor it's a mod done by another guy to try and stabilize the crystal on 10.480760, they ate not part of the original schematic.The NTC is solder to the crystal to heat it and stabilize it's temp, so he don run of frequency, and the 68ohm resistor is use from 9V to feed the NTC..

author: dannyf
author_rank: Super Contributor
timestamp: March 20, 2015, 07:02:01 pm
content: QuoteI'm sure that the above makes perfect sense to you but it may not be that clear to others: the above sounds like a switch to me, whatever "the external reference" is.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 20, 2015, 07:16:30 pm
content: dannyf, yes that is why I put the link to show better what I was asking, with the switch supplying or not the crystal I can use both references on the radio, when using it way from the house switch is ON, GPS unplug and internal crystal in use.When in the house switch OFF, GPS plug and external reference in use.From what I can understand from the above link, I'm at this point.Pin 1 of J4702 goes to the switch, and from the switch to pin 1 of J4702, this turn the crystal on or off.Pin 3 of J4702 goes to an SMA to the GPS through an RG-174.Pin 2 of J4702 goes to the GND of the RG-174.Now is just to multiply the 10.000.000.000 from the GPS by 1.0480760000, but guess it's not that simple.Hope this make sense.

author: dannyf
author_rank: Super Contributor
timestamp: March 20, 2015, 07:23:55 pm
content: QuoteYou probably want to think of breaking it into things that you can actually accompanish.On its surface, sounds like you want to sense if a house is powered (by electricity?). If it is, use the internal oscillator; If it is not, use the gps output.So you need some sort of a sensor that can detect "house switch" positions.Then you need to generate the 10.480760Mhz signal from the gps. Some GPS modules allow user-selectable output so that would be simple. If your gps can only output a fixed 10Mhz, you will need to use a multiplier. Try CDCE913 or similar parts from TI/OnSemi.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 20, 2015, 07:43:06 pm
content: dannyf,No "sensing" but an actual switch, SPDT ON/OFF, it's on the link I provide above.Next I need to generate the 10.480760000MHz from an GPS or GPSDO.Here the wires for the switch -https://qlfecv.files.wordpress.com/2013/06/ic-910h-4.jpgHere the GPSDO IN to the crystal -https://qlfecv.files.wordpress.com/2013/06/ic-910h-7.jpgHere you can see the switch on the lower left and the SMA -https://qlfecv.files.wordpress.com/2013/06/ic-910h-15.jpgSome photos of the mod in English-http://www.vk3um.com/Rubidium%20Pictures.htmland here an PDF -http://www.vk3um.com/Rubidium%20Standard.htmlHope this hel

author: dannyf
author_rank: Super Contributor
timestamp: March 20, 2015, 07:50:40 pm
content: QuoteIt can be as simple as just plug it in.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 20, 2015, 08:11:27 pm
content: The GPSDO that I've outputs 10.000.000.000.0MHz...If I get another one programmable could it give the 10.4, or from one with the 10MHz without putting the multiplier is possible?Does the rest of my thinking is ok, about the connections?

author: LaserSteve
author_rank: Super Contributor
timestamp: March 21, 2015, 01:22:48 pm
content: How precise do you need the reference to be? What is your intended goal?It's a shortwave receiver, not a narrowband microwave radio.Steve

author: bingo600
author_rank: Super Contributor
timestamp: March 21, 2015, 02:54:41 pm
content: Maybe one of those DDS'es that can multiply the 10MHz up , and then make it output the desired freq/Bingo

author: AF6LJ
author_rank: Unknown
timestamp: March 21, 2015, 03:21:34 pm
content: It would be nice if this was as simple as others here would have one to believe.First one needs to ask why the high stability is necessary?Just because wouldn't be a good answer.I am assuming you are talking about modifying an Icom IC-910 VHF / UHF transceiver?Look on Ebay and get the high stability oscillator unit. -/+ .5PPM is good enough for most applications.The problems with what you are trying to do are as follows;The phase noise introduced into the RX / TX signal path will be many times more than it already is. The end result of this will be degraded receiver performance and additional noise on the transmitted signal.The suggestion was made to use a PLL, this again will compound your phase noise issues.You could use a DDS, and one of those rubidium based frequency standards that are retired from Cell site service. After a day's warm up that should be more than stable enough for anything you may have intentions on using the IC-910 for (eg... 10GHZ communications)Even if cost is not an object you will need to decide if you can tolerate the added phase noise into the system. Can your applications stand another 6DB or so of phase noise in the system?

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 21, 2015, 04:27:53 pm
content: AF6LJ, this is for an FT-1000mp, I need to stabilize my crystal, temperature inside the radio room varies a lot, from few degrees on the final afternoon, to +40 on the morning and afternoon, so the crystal is allover the place, no TCXO since it+s been discontinued by Yaesu.The example I give of the 910H is working well since those guys work on microwave, and EME, so they would have noted the raising of the phase noise.To work JT65 you need stable signa.If it was something easy to do I could go forward, bbut it´s not easy maybe will not bother with it.

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2015, 05:18:36 pm
content: The particular frequency is difficult for a multiplier, because it contains a large prime number.I would use a tcxo/ocxo to drive a dds.

author: AF6LJ
author_rank: Unknown
timestamp: March 21, 2015, 05:40:21 pm
content: Quote from: Nuno_pt on March 21, 2015, 04:27:53 pmI am aware of the issues surrounding JT-65;Different radio; same set of problems.Just to bring others in the thread up to speed; JT-65 is a weak signal communications method that uses some information gathered from the Internet in order to recover signals that are many DB below the noise floor. Knowing when a station is transmitting and knowing a preamble used by the transmitter can be a great help in recovering the data transmitted. (JT-65 in forty words or less).Your best bet would be to work with the existing TCXO and work to improve its stability. Combinations of negative and positive thermistors. This is a lot of work, but may be the very best way to get the stability you need.You could search Ebay for a TCXO-6http://www.universal-radio.com/catalog/hamhf/ft1000mp.htmlwhich was the high-stab oscillator upgrade for the radio. Might be hard to find but well worth it since it is rated for .5PPM stability.I have experience with temperature compensation networks using thermistors.The data you collect regarding the drift you experience is the most important part of the process. It is not hard to build an inverse curve to match your drift curve but data must be the best you can collect and the most sensitive parts in the circuit must be identified and the thermistors must be sensing the temp in that aria.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 21, 2015, 05:57:55 pm
content: AF6LJ,I've measure it some times of the day with an HP 53132.Regarding the TCXO-6 and the 4 they are no longer made, and if some guys have them is not going to sell them, so no way to get them.

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2015, 06:14:28 pm
content: If you are good with programming, you can code a small mcu to measure the frequency of the oscillator - the mcu itself is driven by a tcxo/ocxo. The mcu can detect how far off the oscillator is and then apply a voltage on a varicap the oscillator to counter it.Minimum power consumption if you go down the path of a tcxo.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 21, 2015, 07:07:23 pm
content: no skills at programming.I'm reading about RDDS.

author: AF6LJ
author_rank: Unknown
timestamp: March 21, 2015, 08:55:47 pm
content: Quote from: Nuno_pt on March 21, 2015, 05:57:55 pmYou are quite correct they are not available.The circuit you have for that reference oscillator is interesting because of how the mod is drawn in, the mod as it is drawn will have a very small effect on the oscillator frequency. To actually alter the oscillator frequency there needs to be a varacap diode on the low end of the crystal, where the trimmer cap is, that is used to set the frequency of the reference oscillator.Let me draw out what I am talking about...I'll get back to you as soon as a finish a few chores I have to do.This is not complicated but trying to explain it all in words when a drawing would do is somewhat inefficient.Be back in an hour or so...

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2015, 08:56:44 pm
content: QuoteIn that event, get a gps module that can output your desired frequency.

author: AF6LJ
author_rank: Unknown
timestamp: March 21, 2015, 09:50:34 pm
content: A basic but usable circuit mod.The circuit uses positive temp coefficient thermistors.To use negative you simply reverse the position of the thermistor in the voltage divider.You notice the second cap on the low side of the crystal is removed. That cap most likely is responsible for temp comp in the first place.I decoupled the varicap using resistors instead of chokes for two good reasons.First the choke will have be chosen for high enough inductince without bringing the self-resonant frequency too close to the crystal frequency, that would introduce instability.Second the shunt reactance of a resistor is less than that of a choke.The diode is also DC isolated from the circuit.The back bias divider is there to provide a back bias for the varicap to overcome the peak to peak RF voltage on that end of the crystal. If you set the divider for 3V that should be good enough use resistors totaling around 10-20K or less but not too low. If the RF voltage exceeds the DC voltage the varicap will rectify part of the RF cycle and the diode will self bias. (instability and signal purity problems).The temp comp networks shows are dead simple, I am use to working circuits that are bridge liken in configuration and incorporate both negative and positive thermistors. And in stubborn cases a diode or two with additional passives to shape the curve over temperature.I think that should do the job, provided you have a reasonably linear drift over temperature.

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2015, 10:38:32 pm
content: Quoteto make it work, you would have to characterize the oscillator's tempco and then designed a temperature compensation scheme specific to that tempco curve -> I am going to say that it is quite difficult.In the simplest solution in the non-programming world would be to use a gps and output the desired frequency.If that doesn't work, I would ovenize the whole oscillator section -> not sure how big it is or how it is constructued, but I would put a constant-temperature heater (or a few of them if the oscillator is big) there.The simplest heater would be a ptc -> two wires and nothing else; The 2nd simplest would be a transistor + ntc or ptc.Keep the oscillator toasty and your problems are gone,

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 21, 2015, 10:49:25 pm
content: dannyf, the board is thishttp://www.k0to.us/HAM/FT1000MP/Temperature%20Stabilized%20FT1000MP%20REF%20oscillator_files/Temperature%20Stabilized%20FT1000MP%20REF%20oscillator.htmI think that I would go to the rout of the crystal heater, like the QH40 from herehttp://shop.kuhne-electronic.de/kuhne/en/shop/professionell/prof-zubehoer/Precision+crystal+heater+40%C2%B0+QH40A/?card=724And some foam around it to stabilize the temp.

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2015, 10:55:48 pm
content: You can find PTC heaters for a few dollars, shipping included. 40c is quite common. They are self-regulating, essentially the same as the resistor + ptc approach.

author: AF6LJ
author_rank: Unknown
timestamp: March 21, 2015, 11:47:33 pm
content: Quote from: dannyf on March 21, 2015, 10:38:32 pmBuilding a data set and a tc setup to compensate for it, is really easy, this is all Electronics 101 (basic DC circuits) stuff. A heater will work fine provided there is room for one. That board is about an inch by an inch and three quarters, give or take.Space permitting the heater would work.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 22, 2015, 11:03:54 am
content: Hi guys,Could this workhttp://www.qsl.net/ct1dmk/reflock.html

author: dannyf
author_rank: Super Contributor
timestamp: March 22, 2015, 11:25:58 am
content: You should probably go back and rethink what the problem is that you are trying to solve.I would venture to say that it has nothing to do with "locking" to anything. All you are trying to do is to generate a specific frequency over a wide temperature range, if I understand it correctly.Locking to a gps signal or not has nothing to do with that.ie. unless you can articulate otherwise, the link has no bearing on your end goal here.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 22, 2015, 12:00:31 pm
content: dannyf, let's if I can try to explain better what I'm trying to acomplish with this, like AF6LJ said Amateur TRX for certain modes and frequencies need to have a stable frequencyreference so each other can be heard, on a mode like JT65 or let's say on 50MHz 144MHz and the more up you go more stable the frequency have to be.My radio only has on Crystal can HC-xxx not sure of the model, that is the main reference for all the radio.Like AF6LJ said there was 2 options for that radio TCXO-4 and TCXO-6 with .5ppm drift to stabilize the crystal can, this are no longer made since Yaesu doesn't carry those parts long ago.There are some radios that use this reflock to stabilize the main reference of the radio, the Spanish link that I send use one PTS-60 (locked frequency synthesizer) to feed the signal to the radio bypassing the crystal can so now the reference of the radio is within .1Hz on 1.2GHz.Here you can see some versions for other radioshttp://www.vk3hz.net/XRef/XRef_Home.htmlOf curse the temp is a factor, but putting the temp aside, even let's say that with a TCXO with 25Hz drift for some modes that couldn't be stable enough, so the best way to keep the frequency stable enough is to bring it from a GPS or GPSDO.Hope that I could made myself clear, but AF6LJ probably understand better what the problem is, and the need for a more stable reference.Thanks

author: AF6LJ
author_rank: Unknown
timestamp: March 22, 2015, 12:14:30 pm
content: Quote from: dannyf on March 22, 2015, 11:25:58 amI would agree, adding locked oscillators to the system adds noise to the incoming signal, temperature stabilization is the most logical simplest and the answer that provides the least impact on the system in terms of noise.

author: dannyf
author_rank: Super Contributor
timestamp: March 22, 2015, 12:33:52 pm
content: QuoteNot sure of your particular crystal but those types are typically +/-30ppm over a 100c range, MAX.So unless you are doing some ultra-secrete super-narrow band transmission, what you are trying to do yields practically zero improvement.Plus, what does locking to a 10Mhz gps-based signal do for you when you need a 10.480760Mhz signal?The solutions to your problem are actually quite simple, as outlined earlier. You seem to be more interested in taking an exceedingly challenging route.

author: AF6LJ
author_rank: Unknown
timestamp: March 22, 2015, 12:48:18 pm
content: Quote from: Nuno_pt on March 22, 2015, 11:03:54 amThat might work, however you are back to having to program it and..1. you will have to modify your existing reference oscillator to make it into a VCXO, see my drawing...2. it will have to be programmed.The other option you have which has not been discussed is to pick up a signal generator that can use an external reference, or that out of the box is as stable as you need. This is a more costly way of getting what you want.Many of the more modern HF radios such as my Icom IC-756 use a 10MHZ master oscillator as the reference.In the end the only "Plug & Play" solutions to this issue involve large amounts of money. A wise man once said"The cost of a project is inversely proportional to the amount of knowledge possessed by the head engineer of that project".This is the roadblock many of us have run into when taking on some projects.There are also practical limitations to modifying equipment, and this radio may actually be unsuitable for JT-65 when used in an uncontrolled enviroment, and this may be the issue at hand. the radio was not intended to be used for modes of emission that require high stability. I certainly wouldn't consider using my Icom IC-745 for JT-65, although it is stable enough for PSK-31 and RTTY. When you get close to 100,000 hours of use you find that aging has effected the overall stability of most every piece of gear except for the very top end in terms of quality.(I have had my IC-745 since early 1985, every non-portable radio I own is old enough to vote or older).

author: AF6LJ
author_rank: Unknown
timestamp: March 22, 2015, 12:57:46 pm
content: Quote from: dannyf on March 22, 2015, 12:33:52 pmHere is some information about JT65,http://www.eham.net/articles/25604You really don't need frequency stability to a fraction of an HZ.1-2PPM will do below 30MHZ, the software can correct for some but not much drift. . This is also true with PSK31.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 22, 2015, 12:58:34 pm
content: AF6LJ well you're right, It doesn't seams to be worth the trouble.I'll add the QH40A crystal heater to the crystal can and be done with that.My radios too are old enough to vote.

author: AF6LJ
author_rank: Unknown
timestamp: March 22, 2015, 01:37:00 pm
content: Quote from: Nuno_pt on March 22, 2015, 12:58:34 pmSounds like your best alternative.Good luck.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: March 22, 2015, 01:48:03 pm
content: Thanks all.