Decent GPSDO to replace offair standard - Page 1
Forum section: electronics / general

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 12:01:58 am
content: Hi, with the imminent demise of Radio 4 on longwave on 198 kHz I'm looking for alternatives to my old homebrew 198 kHz offair standard. This outputs 10 MHz which I use to calibrate various OCXOs in my workroom about once or twice a year. It has served me well since I designed and built it in the early 1990s. So I'm going to be a bit sad when it becomes obsolete.One fairly desperate option is to convert it to 693 kHz as I think the local Droitwich transmitter may share the same reference for this station. However, I'm just guessing.The other obvious alternative is a GPSDO. I'm generally not so keen on these as they require an external antenna to work at their best and they can take a while to lock up. I don't want something that gets left on 24/7. However, I think I'm not going to have much choice but to opt for a GPSDO. Ideally, I'd want one with an OCXO but I'd consider a TCXO if they performance turns out to be OK. With all this in mind I've managed to beg/borrow three GPSDOs to play with over the weekend (and maybe a bit longer).I have a brand new in the box Leo Bodnar LBE-1421, an old Jackson Labs Firefly 1A (it has a DIL14 OCXO) and a Quartzlock E8-X. I already know the EX-8 doesn't work very well as I've borrowed it before. I have also borrowed the E8-Y version in the past but this isn't available to me anymore.The LBE-1421 is still wrapped up and sealed in the box so I'm not sure what to expect from it. It only has a TCXO but it offers two programmable outputs which is quite interesting to me.I'm going to be using an external active antenna on an external flat roof. I've got several GPS antennas like this including the ones bundled with the LBE-1421 and the E8-X. I also have a new Watson branded one (untested). I'm not a time nut by any means, what I really want from a GPSDO is reliable locking and a 10 MHz output that keeps within about 0.005 Hz of 10 MHz across several gate times of my reciprocal counter. That's all I need in order to check the various OCXOs in my workroom. I also want to be able to get BIT data from the GPSDO to see satellite info and lock status.Are there any other GPSDOs that are more suited to my needs? I would really prefer something with an OCXO rather than TCXO but maybe a TCXO will be adequate. Reasonably fast and reliable locking and stable performance (to within +/- 0.005 Hz) across maybe 30 seconds are more important to me than achieving crazy levels of accuracy across many minutes or hours. I want to be able to turn it on from cold and have adequate performance within about 30 minutes. An hour would be OK but I'd prefer something quicker.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 12:38:29 am
content: Although not really that relevant to my main requirement, I can also do some other tests on these three GPSDOs.I can compare the phase noise and I can probably set something up with my old HP 8405A vector voltmeter to see how the phase drifts over time.It might also be interesting to look at how clean the LBE-1421 is when configured for various frequencies across its range. I think it can tune up to 1.4 GHz which is quite impressive. I've got various analysers I can use to look at the output in terms of phase noise and jitter.I can also log the lock time performance from cold although I suspect that the Firefly 1A will have a degree of self learning and maybe the LBE-1421 will be the same. I might also be able to borrow a better Jackson Labs GPSDO than the Firefly 1A although this will probably be way too expensive for me to consider as an option.My old 198 kHz offair standard used to take less than two minutes to lock up but I upgraded it to a decent OCXO a while back. So it now takes about 600 seconds (ten minutes) to achieve good performance as in the image below. You can see it has a degree of residual 'wandering' even with the OCXO but this level of performance is fine for me. The scale is 0.01 Hz per vertical division.I'm going to assume the LBE-1421 will easily match this even if it takes a bit longer to lock. I'd be happy if it can match or get fairly close to this. If so I'll probably buy one unless there is something even better for a similar price.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 28, 2025, 08:15:33 am
content: Quote from: G0HZU on February 28, 2025, 12:38:29 amAnother options for you, would be to ask the GPS to output 198kHz, and use what you already have working.Typically they use NCO from 48MHz, so 198kHz will be 48M/198k = Divide by 242.4242424 or /(242+14/33) or about 0.411% of phase jitter, lowest component is 6kHzSee how it locks to that ? It might be that simple.Addit: another option if it does not like locking to that, could be a coded Si5351, taking 24 MHz to a VCO x33 and / 4000 for ps jitter.Or, if you prefer solder to programming, you could use hc4046+hc40103 x2, with /100 & /33, PFD= 600khz VCO = 19.8MHz and set the gps for 600khz

author: Gertjan
author_rank: Frequent Contributor
timestamp: February 28, 2025, 10:03:30 am
content: Hi G0HZU,First try the Leo Bodnar LBE-1421 you already have.Modern GPS receivers are much more sensitive, and an outdoor antenne is not a necessity anymore. My antenna is in the attic, but a southern facing window might do too.QuoteThe Bodnar will lock very fast, and accuracy will be good enough for your purpose. Probably better than your current 198kHz standard.Off course, when hunting for highest accuracy, lowest Allan Deviation etc., an outdoor antenna and OCXO will yield better results. But that is another ball game.Regards, Gertjan.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 10:42:21 am
content: Thanks for the replies and suggestions. I've unpacked the LBE-1421 and already tried it and it seemed to lock OK. I had to download the config program in order to program the outputs. This worked first time on my Win 11 desktop PC but my little EeePC running Win7 starter struggled to run it. It took a few fumblings with the Widows OS compatibility settings to get it to work. It seems to work OK on the EeePC now.See below for what is inside the box. It does seems to be nicely made and the cables look to be good quality.When first run from cold, the TCXO was quite a few Hz off frequency as would be expected. Sadly, I didn't log how long it took to to lock. Subsequent restarts begin with a frequency much closer to 10 MHz.I had a quick play with setting the frequencies and it is really easy to configure it for 1 PPS on one output and it states the other output can be set to anything from LF to 1.4 GHz.I've briefly tried it at 10 MHz, 144 MHz and 1GHz and it seemed to work fine.I'm not sure if there is a user manual for it. My first impressions are that it is very small and light and the front panel LEDs are very bright. I'm not sure if they change colour when fully locked. It would be nice if they changed to a dull green colour when locked. The LED next to the 1PPS output flashes at 1PPS.This happens straight away and this is different to other GPS receivers I've played with. Often there is a delay before the 1PPS signal appears as I think the system waits for a decent signal from several satellites. I'm relatively new to tinkering with 1PPS from GPS so I don't really know what is typical in this respect.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 12:47:17 pm
content: QuoteIt does look to be able to meet my requirements, but only just... I left the 1421 running through the morning and tested it for short term stability vs the OCXO reference in one of my spectrum analysers and against the offair standard.I have to confess I cheated a bit with the offair standard as I connected it to an external shielded loop antenna in the loft. I normally run it with an internal ferrite rod antenna tuned to 198 kHz.However, it sometimes struggles to lock with the ferrite antenna if I have lots of test gear running. The various displays in the test gear emit lots of local EMI in the LW band and this can upset the offair standard. So to rule this out I've given the offair standard the boost of having a very nice shielded loop antenna way up in the loft.I've read somewhere that ferrite rod antennas can cause subtle phase issues with an offair standard, but this has never really concerned me as long as I place the offair standard in a known dead (EMI) zone on a shelf in my workroom.So to give it the best performance I've kind of cheated and used a large (tuned) shielded loop antenna in the loft. I'm also only about 20 miles from the Droitwich transmitter which probably helps.My first few tests definitely put the offair standard ahead of the 1421 in terms of stability on the counter but not by much. Both are within my requirement of +/- 0.005 Hz across several (10 second) counts on my frequency counter. But the offair standard is best here. Across ten minutes it managed +/- 1 mHz (+/- 1LSD) where the 1421 wavered about +/- 3.5mHz. This is for a ten second gate time on the counter and so there were about 60 measurements made inside the ten minutes. I logged the worst +/- frequency deviation across all 60 counts.Some of this will be due to limitations in the counter so the offair standard may be more stable than this.I also tried measuring the OCXO in my best spectrum analyser and it also managed +/- 1mHz although it was nearly 1 Hz off frequency. It needs to be adjusted at some point.The counter is an old Anritsu MF1603A reciprocal counter with the top OCXO option 3 fitted. This is a huge OCXO. It is generally very stable across a ten minute timespan. I do have access to a more modern counter at work so maybe I could borrow one. However, it's probably no surprise that the 1421 can't compete here as it only has a TCXO inside.My offair standard has a fairly decent and quite large Quintenz OCXO in it and the time constant is very long. However, what I don't know is how accurate the BBC reference is at Droitwich or how much the propagation can cause subtle errors across seconds, minutes or hours. It's generally best to avoid using the offair standard at dawn or dusk as this is generally accepted as being the worst time for stable propagation. During the day it seems to give very repeatable performance.I'll try testing against the Firefly GPSDO next. It has a tiny OCXO in it.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 02:51:19 pm
content: The counter readings (taken every 10 seconds) from the Firefly 1A also managed to stay within +/- 1mHz across about 10 minutes. I did this test twice. I think this shows the benefit of the OCXO compared to a TCXO. However, I've borrowed one of these GPSDOs before and it was prone to giving sporadic unlock messages when monitored via RS-232. It did lose lock when this happened. So far this one has been OK. I think the other Firefly may be faulty in some way or maybe it has different firmware in it.My offair standard can also lose lock sometimes but when it does, it is really obvious as it goes about 150 Hz off frequency.I'll try the Quartzlock E8-X next although I already know it isn't a great performer.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 04:47:46 pm
content: I already knew what to expect from the E8-X in this test as I've done it before, but it only managed to stay within about +/- 20 mHz of 10 MHz when measured every 10 seconds across a 10 minute timespan.This is about the same level of performance as my offair standard when it still had my original homebrew VCXO I designed back in the early 1990s using a discrete 10 MHz crystal. To be fair, I had many years of good use from the offair standard when it had this basic VCXO fitted. I used to average several counter readings to try and get better performance from it. With this in mind all of these options (LBE-1421, Firefly 1A or E8-X) would be fine for me to use but the Firefly 1A would be the easiest to use and it would deliver the most confidence because it gives more consistent readings on my counter.I can compare the phase noise of each one although I'm not sure many people are concerned about this aspect of the performance unless they want to use the GPSDO as a reference for UHF or microwave RF test gear.The phase noise of the LBE-1421 looks good on the Leo Bodnar web page here:https://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=399&srsltid=AfmBOooQ9toYbLDnT-74mPOySejhzQlo5g78Xqu0ysC1a9_Po8vvmJkGThe phase noise for the Firefly 1A is given here:https://www.jackson-labs.com/assets/uploads/main/Firefly-1A_EXTENDED_TEMP_RANGE.pdfThis is quite a bit noisier than the LBE-1421 at offsets greater than 1 kHz.The phase noise spec for the Quartzlock E8-X is given here:https://www.testequipmenthq.com/datasheets/QUARTZLOCK-E8-X-Datasheet.pdfIt doesn't look very good at all when compared to the LBE-1421 as it appears to be about 15 dB noisier at offsets >= 1kHz.The phase noise plot for the LBE-1421 is shown below. This does look to be very good.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2025, 07:45:06 pm
content: Next, I logged the warmup response of both the LBE-1421 and the Firefly from a cold start. The aim here was to see how long it took to get within about +/- 2mHz of 10 MHz and keep it there.The LBE-1421 was easily the fastest and it took about 300 seconds (5 minutes). It showed a steady climb to 10 MHz with little or no overshoot.The Firefly 1A was much slower and had some overshoot in the warmup. It overshot by about 8 mHz after 500 seconds and it took a further 600 seconds to get back down within +/- 2 mHz which is about 18 minutes in total for the initial warmup time.My offair standard can do it in about 600 seconds or 10 minutes. All three lock times are OK with me as long as the lock is reliable. It's easy to do something else for 20 minutes whilst the system acquires lock.I'll dig out my old HP 8405A vector voltmeter next and do a few phase stability comparisons.

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2025, 01:16:06 pm
content: I've not had a chance to do the VVM testing although I'm not sure how much value it adds anyway. I just want something that behaves well on the old Anritsu frequency counter. By this I mean something that gives repeatable results within a few mHz each time the counter refreshes the display every 10 seconds.The clear winner (for me) in this respect is the Jackson Labs Firefly -1A but these are quite old now and probably very expensive. I've only got this one on loan so I'd have to buy one.The LBE-1421 is almost perfect, if it was just a bit more stable on the counter I'd buy one. I'm not sure what to expect for £200 but I'd be willing to trade the dual outputs and the fast lock time for something that was a bit more stable as long as it wasn't that much more expensive.I've seen the numerous GPSDOs on ebay for sub £70 but I'm not willing to gamble on these unless I see a decent review. I think I'll start a dedicated thread on the LBE-1421 to see what other people have found with this device. I haven't given up on it although I'd also consider the cheaper LBE-1420 with the single output.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 01, 2025, 10:11:25 pm
content: Quote from: G0HZU on March 01, 2025, 01:16:06 pmIf that is your yardstick, it is not surprising an OCXO system works better than a TXCO one.At 10 seconds, you are measuring pretty much entirely the oscillator, if you already have an aged OCXOin your existing 198KHz setup, why not use that?Digikey show OXCO from ECOC-7050-10.000-BN-TR XTAL OSC OCXO 10.000MHZ ±50ppb -40°C ~ 85°C LVCMOS 1: $32.37Epson have recently announced a new OCXO, that looks even lower power, with fast settling, but I've not seen price/product yet?https://corporate.epson/en/news/2024/241028.htmlJust noticed:Samples of the new OCXO will be available from April 2025

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2025, 11:39:30 pm
content: Quote from: PCB.Wiz on March 01, 2025, 10:11:25 pmSadly, I can't just use the old OCXO from the 198 kHz offair standard because I want to be able to check and adjust all the various OCXOs in my RF test gear. So I need something disciplined. I want to be able to adjust them across a few 10 second gate times. It's the way I've always done it.If I use the E8-X GPSDO it dances around too much on the counter because it has a really basic oscillator in it.The LBE-1421 is much better than I expected it to be despite the TCXO.As I mentioned earlier, I'd much rather have a GPSDO with an OCXO in it. I'd choose the Firefly-1A over the LBE-1421 partly because of the OCXO and partly because it is better documented and it has lots of (documented) diagnostic commands available.

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2025, 11:55:32 pm
content: To help put things in perspective, there not much point me adjusting the various OCXOs to be much better than +/- 0.01 Hz because over the course of a year or two they will typically age up to about +/- 0.05 Hz over the year. Some are better than this but none of them age more than about 0.1Hz per year.I don't want to pipe 10 MHz around my workbench from a 10 MHz GPSDO left running 24/7 in case anyone suggest this. There are various reasons why this is a bad idea in my case. I'm not a time nut so as long as the OCXOs remain within about +/- 0.2 Hz over a year or so I rarely see any issues. I could probably leave them several years and still be OK with the accuracy but I typically check them about once a year on average. Sometimes I use one OCXO as a master and link a few sig gens together but I don't do this very often. It lets me accurately adjust the phase between the outputs of the sig gens when I do this and this can very very useful sometimes. I also do this between a sig gen and an RTSA when capturing signals that then get post processed offline. I don't need absolute frequency accuracy when doing this

author: G0HZU
author_rank: Super Contributor
timestamp: March 02, 2025, 10:10:21 am
content: I had a go at measuring the phase noise of the Firefly-1A GPSDO with my old E5052A SSA.The datasheet states:10Hz -110dBc/Hz100Hz -135dBc/Hz1kHz -145dBc/Hz10kHz <-145dBc/HzMy results below agree very well with the datasheet. I've put the detection mode into 'omit' as the SSA has a couple of ultra low level internal spurious that are not due to the Firefly-1A. So the plot below just records noise and not discrete spurious terms. I don't think there are any discrete spurious terms from the Firefly-1A anyway. Just the usual internal spurious from the E5052A that are at a very low level.

author: FriedLogic
author_rank: Regular Contributor
timestamp: March 05, 2025, 10:13:59 pm
content: Did you try them with an inside or outside antenna?They may work inside, but be much more stable if outside.Although they need a good signal, the old Trimble UCCM GPSDOs settled quite fast. As far as I can remember, it was functional by 10 minutes and was about as good as it got by 30 minutes.

author: G0HZU
author_rank: Super Contributor
timestamp: March 05, 2025, 10:57:56 pm
content: I used an outside antenna for the tests. I used the Leo Bodnar antenna out on a flat roof. I also have a Watson GPS antenna and this works very well too.After doing a few more tests on the Firefly-1A I've realised that it doesn't fully settle for quite a long time. It performs at its best several hours after it has been turned on. It must need this much time to reach thermal equilibrium.The logging SW for it shows that the OCXO steering voltage doesn't settle to a shallow (ageing?) slope until it has been on for several hours.It then agrees very well with my offair standard when tested with the HP 8405A vector voltmeter across a 15 minute test for phase stability.I can't do any more testing on the LBE-1421 as I've returned it to its owner. I have to return the Firefly-1A tomorrow.There are some low cost GPSDOs on ebay that use an OCXO and I might buy one of these to play with as they are so cheap. Even if it ends up being worse than the LBE-1421 it will probably be good enough for my needs.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 06, 2025, 05:09:27 am
content: Quote from: G0HZU on March 05, 2025, 10:57:56 pmThere is another thread on firmware updates for one of those GPS+OCXO modules here.You might want to pick one of those ?https://www.eevblog.com/forum/testgear/10mhz-gpsdo-by-bh3sap/

author: G0HZU
author_rank: Super Contributor
timestamp: April 06, 2025, 09:19:34 pm
content: I managed to borrow another Quartzlock E8-X GPSDO to evaluate. This one looks the same as the other one and it says E8-X on the front but it is very different inside with a much more impressive PCB and oscillator.It also works a lot better, very similar to the Jackson Labs Firefly-1A GPSDO in terms of short term stability. I'm going to try and borrow it again and do some more tests on it. The PCB inside implies it is the E8-Y model but maybe the X and Y share the same PCB for this PCB version (dated 2013?).The PCB in the other E8-X looks much simpler (way fewer components) with a tiny and cheap looking oscillator that is probably only about 7mm x 5mm in size. So it's no surprise that the one pictured below works a lot betterDoes anyone know if this is really an E8-Y (OCXO) rather than an E8-X (TCXO) because the two TCXO locations on the PCB look to be unpopulated?