I just bought an Efratom 100318 - Page 1
Forum section: electronics / general

author: tkamiya
author_rank: Super Contributor
timestamp: January 15, 2019, 03:36:45 pm
content: Hi,I just bought an Efratom 100318-001. (Rubidium standard) Lamp voltage = 9.8VI see Efratom had a various models, including FRS-A, FRS-C, LPRO-101, and the last one I can't remember. 100318 is visually very close to FRS-C.Is 100318-001 same as FRS-C? How does it compare to other Rubidium standards? I did look over google results but this one appears to be least mentioned and I didn't gain any useful/tangible info.

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 15, 2019, 04:01:52 pm
content: There is a manual online for the FRS unit 100318-001.http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Efratom/Efratom_FRS-C_FRS_N_Rubidium_Oscillator_CO100138B.pdfExplains everything about this model including versions of 100318, FRS-C and FRS-NI think the 100318 is newer version of FRS-C.Output is 10MHz Sine wave.Very easy to connect to that 10-pin male connector.You can get Frequency,Lock Monitor which is TTL on that unit 0.06 = Locked 5V = Unlocked, Lamp Voltage and Ground, Power to Heater and + power is on one pin.

author: tkamiya
author_rank: Super Contributor
timestamp: January 15, 2019, 05:59:03 pm
content: Wow! Thank you!I went to BB's site but completely missed it. Wonderful resource with circuit diagram included....!

author: tkamiya
author_rank: Super Contributor
timestamp: January 17, 2019, 04:48:16 am
content: I ended up going directly to the D-sub connector. For whatever reason, the header type connector is missing few signals I wanted. Due to lack of heatsink, I could only test for a while but it locked quickly and output was remarkably on-the-spot.

author: tkamiya
author_rank: Super Contributor
timestamp: January 17, 2019, 07:41:27 pm
content: I have a heat sink mounted and RB running on lab power supply.I'm seeing 6x10^-11 with occasional jumps of magnitude 10. I guess when going for this kind of precision and accuracy, one can not go cheap. I am using a cheap 10 turn register for trimming. I am guessing it is affecting stability as it didn't move this much when trim was not present.I was trying to avoid spending 15 dollars (minimum) for name brand but oh well.Heatsink temperature has stabilized at 43.5C.I have a question for the group. My initial plan was to encase everything in a case including the sink. But if I do this, I'm sure temp will jump. What is the consensus on using a fan? All I need is a little bit of air movement. I wouldn't even consider this on OCXO but what about Rubidium? As I understand it, the absolute max is 65C at base plate, according to what I can dig up.I can also go larger on heatsink or mount it on thick case bottom to radiate via conduction. Manual doesn't specify what is and isn't recommended.

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 17, 2019, 11:35:22 pm
content: Bigger heatsink.Without a fan I get mine to 37 C after 6-8 hours with larger heatsink covering entire baseplate.I tried with fan and it stayed about 30 C.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 18, 2019, 01:03:21 am
content: Don't go crazy with the heatsink. A Rb standard has two or three ovens that have to stay hot. Crazy heatsinks just force these ovens to work harder. Most of the time, all you have to do is bolt the Rb standard to the outer case and make sure that air can flow around the case. Commercial units use external heatsinks so that they can run in high ambient temperatures. Does your workshop get up to 50C?Ed

author: tkamiya
author_rank: Super Contributor
timestamp: January 18, 2019, 01:12:05 am
content: I wish I could find out what the optimum temperature is.... So moving to Alaska is out? (I'm in Florida) I understand cooling is not for the oven'd part but for the electronics around it.My lab stays around 25 to 28C. (77F to 83F)... Oh I wish everyone standardizes on metric system!I guess I should be happy frequency is not based on presidents' whatever bodyparts....

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 18, 2019, 02:25:56 am
content: These 100318 units were mounted in a closed metal case.Labeled CTS-101With a buffer board providing a SMA connector out the side and a larger connector with 4 sine wave 10MHz outputs.I asked about the buffer board a few months back.Pictured below.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 18, 2019, 04:45:09 am
content: How was the Rb mounted inside that case? Was it screwed to one side? Did it use any form of thermal mounting material?Ed

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 18, 2019, 05:34:55 am
content: The box is about 3 times larger than the oscillator case.Just screwed to case.

author: Towger
author_rank: Super Contributor
timestamp: January 18, 2019, 02:03:34 pm
content: 15 x 6.5 x 2.5 cm is the size of the heatsink which came with my FRS-C. Which I have been told came from a decommissioned satellite ground station or uplink. It has a 10Mhz square wave output.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 18, 2019, 05:10:58 pm
content: I have a PRS10 Rb standard mounted in a box. It's bolted to the rear of the case. No thermal compound was used. The PRS10 is similar in size and form factor to the FRS-C. The PRS10 has a steady-state power drain of 14.4W @ 25C. They don't say whether that 25C is ambient or base plate temperature. The FRS-C is rated for 12W @ 25C base plate temperature.After running for a few hours in a 22C ambient, the base plate temperature of my PRS10 was about 45C. No further cooling is required. Even in summer, the base plate temperature won't come near the 65C limit. If I was concerned, the first thing I'd do is mount an external heatsink on the back where the PRS10 is bolted. Orient the fins vertically for best air flow. A fan isn't required. For comparison, The Efratom PRFS Rb frequency standard contains an FRS standard. It has no fan. On the rear, there's a heatsink with a small number of thin fins. The unit is rated for operation in a 50Cambient.Towger, are you sure that heatsink is original? The only way you'd need such a large heatsink is if you were running in a very high ambient temperature. The fins hanging out past the ends is also suspicious. In your pictures I can't see any holes, clips, etc. How would it be mounted originally?Ed

author: tkamiya
author_rank: Super Contributor
timestamp: January 18, 2019, 06:43:05 pm
content: Read the manual (for my Rb) again and concentrated over power supply requirement. It states 1mV ripple max is necessary, otherwise spectrum purity will suffer. It states "If non-harmonic at the supply ripple frequency (50, 60, or 400Hz) are not a consideration, the ripple can be higher"These days, I don't know anything that uses linear power supply. I am wondering now, how critical this is for my particular use.Also read into various technique where careful filtering of switching power supply can bring ripple (typically 100mv?) can be lowered to match that of linear supplies.FRS* has two input pins. One goes to electronics and the other goes to heater. Latter is not further filtered but former is regulated and filtered internally.I wonder if they came up with 1mV p-p as "matter of routine?"

author: Towger
author_rank: Super Contributor
timestamp: January 18, 2019, 06:43:12 pm
content: It came from eBay as spares or repairs, as the seller had not tested it. I was told it was part of a larger decommissioned satellite ground station system. I am assuming the heatsink was from its use in this system. The fins are professionaly cut out in the location of the factory mounting holes, you can still see pen marks at the side. The seller did not know the make and model, they were covered by the heatsink. He was worried it was near/at the end of its life as (he thinks) it was powered up for the life of the satellite. But when I tested it, it locked quickly and voltages were well in line with the manuals specs.It is another work in progress project to mount in a nice case etc...@tksmiya Thanks for that. I tested mine on a linear bench supply supply, but picked up a decent 24v SMPS for it. Even still i have my doubts about using a SMPS it. I'll keep a look out for a linear one.

author: tkamiya
author_rank: Super Contributor
timestamp: January 18, 2019, 09:12:43 pm
content: I guess we have to be careful in thinking if "some" is better, HUGE is more better....I found a statement in user manual that said bottom heat plate must be attached to a case better than 0.005". I think that's a bit thicker than 1mm? But didn't mention how big.I have a few switching power supplies that came out of HP equipment. I'm going to put some load on it and see how well it performs. I really don't want to use linear power supplies. Even high-end PLL signal generators today use switching power supplies.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 18, 2019, 11:22:38 pm
content: Quote from: tkamiya on January 18, 2019, 09:12:43 pmThat's not the thickness, it's flatness. And according to my calculator, 0.005" = 0.127 mm. That's closer to heavy duty aluminum foil than an aluminum case suitable for mounting anything. But you're right, they don't say anything about how big the surface should be. That's up to you to determine. The only thing they care about is that the temperature doesn't exceed 65C.QuoteIf you're not really careful, switching supply noise will leak into the output. It often shows up on phase noise plots. When manufacturers use switching supplies, they are very, very careful to make sure that doesn't happen. If your application isn't sensitive to phase noise, it might be okay.Linear supplies are bulky and inefficient, but very quiet. IMO, they're easier to repair than switchers, but I haven't had much experience repairing switchers so take that opinion with a few grains of salt.Ed

author: tkamiya
author_rank: Super Contributor
timestamp: January 18, 2019, 11:41:56 pm
content: I guess wrapping my Rb with aluminum foil is out then.

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 18, 2019, 11:51:52 pm
content: I really don't understand the disdain for linear power supplies.You can get such fantastic deals.Not Chinese manufacture but HP, Lambda, etc.Easy to use for hobbyist.I have a bunch of HP 6263A power supplies I paid 11 dollars each for !I am always repairing SMPS have never had to repair a linear.

author: tkamiya
author_rank: Super Contributor
timestamp: January 19, 2019, 12:25:52 am
content: Well, I have seven older rack mount HP power supplies. I probably paid $10 for each on average. I wanted to one light package. I know about those modules from old days. Well, I didn't want to get those again.Oh, why do I have so much STUFF that don't "just fit"!I am exactly the opposite. I had so many linear supplies to repair. I've just thrown away switchers when they fail.

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 19, 2019, 01:11:38 am
content: Off topic but the area we live in has such an abundance of surplus equipment.Both for price and ability the ability to pick heavy items up from sellers make this a hobbyist paradise.High tech to low tech within a fifty mile radius, surplus from all the space related companies.I love it !!

author: tkamiya
author_rank: Super Contributor
timestamp: January 19, 2019, 01:20:41 am
content: I'm near Orlando, Florida. It used to be much better but all the electronics surplus stores consolidated to just one. I also have other local sources.Yes, such a wonderful time we live in (as far as our hobby is concerned). A bit older QUALITY equipment are abundant available at such a bargain prices. Last year or so, I just went NUTS and went from just two multi-meters to lab full of equipment. Most of them purchased broken and repaired.If I were to do this during 80s, it would have cost me my life time worth of earnings and more.I also have a friendly local cal house which helps me out a lot.I am leaning more toward linear supplies, either internally or externally. I also came up with a neat "housing" idea. I have lots of non-working-and-hopeless HP sweep generators. I am thinking of putting in really nice power supply and distribution amp, etc into the mainframe part. Then use the plugin shell to have time standard. Perfect for experimenting. Just about everything already at home.With good cleaning and nice coat of paint, it's as good as new.

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 19, 2019, 02:50:23 am
content: If you want one of the boxes similar to the one I am showing with the output plug having a few 10Mhz outputs.Drop me a line.

author: FriedLogic
author_rank: Regular Contributor
timestamp: January 19, 2019, 11:58:30 pm
content: Quote from: tkamiya on January 17, 2019, 07:41:27 pmThe external frequency adjustment range is usually quite small, so you don't need anything special for the trimmer. A really cheap cermet would probably be fine, but a $1-$2 branded one might be more certain.In the 'Environmental Specifications' part of the LPRO Manual ( there is one athttp://www.ham-radio.com/sbms/LPRO-101.pdf) it gives the MTBF for 'ambient' temperatures from 20ºC (381,000 hours) to 60ºC (126,000 hours)There's normally fairly limited thermal coupling between anything that is meant to be hot and the base plate, so you're not likely to have an 'over-cooling' problem at anything close to normal room temperatures.

author: tkamiya
author_rank: Super Contributor
timestamp: January 20, 2019, 02:10:30 am
content: Wow.... I was *just* picking 15 dollars pieces from Digi-key's offerings..... Thanks!I am going to get some Burns kind for 2 dollars a piece. These are 20 turns, so tuning should be little easier. While using cheap ones, I was noticing changes in value weren't linear. At several spots, downward changes of frequency suddenly reverses course for a bit, then goes back to downward changes again. I hope better kind will have less of this type of issues.I am finding the module works from 18 volts and doing so will significantly reduce power consumption without anything negative. I would think heat generation will be less as well. (Yes, I know I'm just shifting where the heat will come from, power supply or rubidium module)It will be an interesting experiment.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 20, 2019, 04:51:09 am
content: Quote from: tkamiya on January 20, 2019, 02:10:30 amWhat you should see is that as you reduce the voltage, the current stays constant. Then you reach a point where the current starts to drop. This is the point where the internal voltage regulator is no longer able to regulate the voltage. Increase the voltage back to the stable point, and keep increasing to give the circuit at least a few volts of headroom. If you run too close to the dropout voltage, you may end up with lots of 120 Hz noise on the output.Ed

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 20, 2019, 12:38:08 pm
content: At 18 volts aren't you going to starve the circuit for power at start-up?Unit needs minimum 22 v at start-up through regulator.After lamp ignition unit self-regulates power to 17V.A couple of volts for regulator consumption thus minimum 24V as stated in specs.

author: tkamiya
author_rank: Super Contributor
timestamp: January 22, 2019, 10:22:41 pm
content: A very kind gentleman sent me a case that goes with CTS series and along it, came a PC board with buffers. Onboard this buffer board is a DC/DC converter and a circuit to make LED status output an open-collector. This thing is huge.... but quite useful.One thing I found out is, this case relaxes power source requirement from 1mv P-P noise to 100mv P-P. As a result, I was able to use a switching power supply I happened to have on-hand.Added a green LED for locking status.I changed my plan quite a bit from inception of my project. But for this and at this stage, an only thing I have left is calibration.Thanks an anonymous (well, I know who it is... but I don't have his OK to disclose his name) gentleman!

author: texaspyro
author_rank: Super Contributor
timestamp: January 23, 2019, 04:47:05 am
content: Quote from: Krytron on January 23, 2019, 03:36:29 amA lot of Rb lock monitor outputs are open-collector transistor outputs.

author: tkamiya
author_rank: Super Contributor
timestamp: January 23, 2019, 04:57:19 am
content: Mine is open-collector when combined with a factory buffer board. (as implemented)Without, it's a TTL output. (reverse logic)As to my heatsink, it has a lot to do with availability. Plus, factory manual says about maximum bottom plate temperature but not minimum. My understanding is, parts that needs to become hot is well insulated. Cooling is for electronics around these hot components. I just tweaked frequency using a trim pot accessible though base plate.Factory manual says maximum ripple is 100vm p-p. (with buffer board)Without, it's 1mv p-p. I covered this in my earlier post. I'll see what output looks like tomorrow via Spec Analyzer. If it is modulated, it should show in 10Hz bandwith view.

author: tkamiya
author_rank: Super Contributor
timestamp: January 23, 2019, 11:52:00 pm
content: I guess I'm not done, yet.Today, I decided stacking approach isn't going to work for me. So I arranged power supply and Rb with outer case and buffer board horizontally. It works fine on my bench. It doesn't work when I put it on shelf. Take it down to bench and it doesn't work now. Open it up and jiggle few things and enclose it. It works on bench but not on shelf. When it doesn't work, I get output but it never locks.I took it apart again and opened it up. Inside case is awfully hot on top side, while heatsink is relatively cool. Now I am keeping it outside the case, no buffer board, and wire directly to the weird coax in the middle pin outside connector. Powered from bench supply.NOW IT WORKS! I spent all day playing with this.Good thing is, the real module works but all that nicety the buffer board and outer (military spec) casing does not, or it appears to be. I'm awfully suspicious of 12 pin header connector that joins the module and the buffer board. Both are rigidly soldered, and alignment can't possibly be precise and accurate. In fact, I can see the outer case deform when bolting everything up.At this point, I don't need any advice. I'm just documenting this for sake of completing my project and sharing.

author: Johnny10
author_rank: Frequent Contributor
timestamp: January 24, 2019, 03:26:46 am
content: Another method of checking these units for correct lock signal is to open case and connect oscilloscope to TP1.This is signal on 7104 scope with Fast Acquisition enabled.

author: tkamiya
author_rank: Super Contributor
timestamp: January 24, 2019, 04:15:50 am
content: Nice Scope!

author: testpoint1
author_rank: Frequent Contributor
timestamp: February 23, 2019, 02:51:58 pm
content: the FRS is quite old clock, it uses dip component, LPRO-101 used SMT, and I tested more than 70 pcs LPRO, please keep the working temperature within 20-50C (manual said it is -25 ~+70) after one hour power on, I made a house for the LPRO by machine, I will release the cheapest and most accurate Rb clock (all calibrated, 1E-11 stability) then.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 25, 2019, 01:25:15 am
content: Hello everybody,Is there any one know what optimum temperature for Rb clock and where exactly to measure it ?

author: tkamiya
author_rank: Super Contributor
timestamp: February 25, 2019, 02:17:51 am
content: Which make and model do you have?I've been researching that very information for Efratom C and Stanford Research PRS-10. I am not having an easy time. There are two kinds of data. 1) What people say on Internet. 2) What manufacturers say on their manuals.As far as 1 is concerned, there are three groups of people.People who think lower is betterPeople who think higher is betterPeople who think there is a particular temp or range of tempAs far as 2 is concerned, it really depends on manufacturer.Efrratom (c) says base temperature needs to be in certain range. I don't recall the figure but it's clearly stated in user's manual. They don't seem to care about any other location. I have 3 commercially made frequency standard by Efratom/Bell. They all settle warm to touch.SRS for PRS-10 is more complicated. There is a lamp temperature, Xtal oven temperature, case temperature, and base templature. Case temperature is measured somewhere between the Rb lamp and the base plate. For many of them, they specify the set point. (a given value) and some are only decided after the unit goes through calibration at factory. What this value is, is stored in internal memory which you can read. Case temperature seems to be what they are mainly concerned with, and manual says it will be somewhere between ambient (room) and case temperature. It doesn't say what it should be though. I have a service request sent into SRS to find out the official data.My data is all over the place. Small heatsink, larger heatsink, and small heatsink with fan has been tried. It tends to settle somewhere between 60 degrees C and 70 something degrees C. It's quite hot to touch. Since SRS doesn't say what it should be, I have no idea if this is good or bad.Oh, one more thing. One of the manuals say life span of your Rb module depends on temperature. Higher the shorter, lower the longer.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 25, 2019, 03:39:49 am
content: Hello tkamiya,I have "bare" PRS-10, SR-625, LPRO 10, X72, FEI 5680A

author: tkamiya
author_rank: Super Contributor
timestamp: February 25, 2019, 03:48:31 am
content: Very nice collection! If I find out anything more about PRS-10, I'll post it here.SR625 is a complete product. I really don't think you need to worry about that one. For everything else, did you go through user manuals? I can say PRS-10 manual is incomplete. I don't have anything else you have.

author: testpoint1
author_rank: Frequent Contributor
timestamp: February 25, 2019, 03:47:13 pm
content: I think you need a Cesium standard to calibrate your Rubidium clock, since your collection seems is quite old, X72 also need new firmware then can be trained by 1pps signal.

author: tkamiya
author_rank: Super Contributor
timestamp: February 25, 2019, 08:12:25 pm
content: I agree.... I NEED a Cesium oscillator.

author: testpoint1
author_rank: Frequent Contributor
timestamp: December 06, 2019, 04:02:04 pm
content: Quote from: Theboel on February 25, 2019, 03:39:49 amHi, what about the internal oscillator of SR-625 ?

author: ArthurDent
author_rank: Super Contributor
timestamp: December 07, 2019, 01:21:52 am
content: My main Rb standard is a modified surplus unit and uses a Ball (Efratom) FRK-HLN Rb unit made in 1993 so it’s over a quarter of a century old. The ‘H’ stands for high stability and the ‘LN’ stands for low noise. I added a 10Mhz to 1Ghz multiplier that I found handy for checking the ‘C’ inputs on high frequency counters. I also added a small fan to move some heat out of the box and it is very quiet. The two sides of the case are also heatsinks to help dissipate the generated heat. The built-in distribution amp gives good clean multiple outputs. The internal supply is a SMPS that I’ve had no problems with.I have a second 10Mhz standard with 4 well filtered outputs. This was a quartz standard that I grafted a LPRO-101 into. The Rb unit was telco surplus with not much time on it, locks fast, and is quite stable. The adjustment I use on a lot of standards to get high resolution is to use a pot with a resistor in parallel that gives a much smoother and finer adjustment. I also use two stable resistors, one on each end of the pot to get even more resolution. The values aren’t that critical and depend on the oscillator you’re using. I set the 10-turn pot to half way then use the internal adjustment to adjust the warmed up standard to 10Mhz. I generally use 1% resistors I have several of. In the Quartzlock unit I used a 4.64K on each end of a 2K 10-turn pot with a 200 ohm resistor across the pot. The SMPS is external to this unit. The photos should pretty much illustrate what I have described.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 15, 2022, 08:08:34 pm
content: Hi Arthur,I've been pondering whether I should reply to what is now a 29 month old posting over the past two or three months since I revisited this topic thread for the lack of more current threads on the subject of rubidium frequency standards.I'd bought my own Efratom LPRO-101 the August before last and have been developing a thermally stabilised and barometrically compensated Rubidium standard ever since. I finally reached the stage where it is now housed in a custom built enclosure since about a month ago and have been fine tuning the base plate fan cooling based thermal control algorithms ever since.I've been able to stabilise the base plate to a 36.1*C set point temperature to within +/- 0.1*C but I'm still seeing what seems to be a diurnal effect which I'm beginning to think may be due to infrared heat leakage which seems to be more of an issue than I'd originally thought it might be. I'm now looking to purchasing a roll of IR heat shield tape and a tube of Autosol to polish the aluminium case panels to a bright mirror finish to address this "Gotcha".Anyway, all that aside, the thing that's been bugging me about your post is the bit about padding the ten turn adjustment pot using three 0.1% low tempco resistors. In this case, with the useful feature of these Efratom/Ball/Datum LPRO-101's internal trim pot adjustment and external EFC connection, the (only now!) blindingly obvious solution to getting very tine tuning control from a ten turn pot (+/- 1ppb in my case) is simply to connect the pot directly across your low tempco 5 v supply and wire the slider terminal to the EFC pin via a 3.3M resistor to take full advantage of that internal 10 turn trimpot to set the external pot midway and trim as close as possible to the 10MHz just as you described.You had all the pieces of this "simplification" puzzle to hand so it wouldn't surprise me if you'd already experienced this particular epiphany at some time over the past 29 months.I'd had mine only about 3 or 4 months ago.I've raised this point just in case you hadn't and also for the benefit of anyone interested in achieving such fine tuning control of an LPRO-101 or similar rubidium reference using nothing more than a 5 or 10K ten turn pot (or whatever ten turn pot you have to hand in the 1K to 100K range - exact value isn't critical) and a 3,3M or so resistor (also tolerance not critical).The beauty of this circuit being the elimination of expensive resistors and simplifying the desired amount of trimming range by changing the resistance value of only a single inexpensive resistor.I'm planning on creating a new topic thread describing my current RFS project once I have completed it (I still have to investigate, and hopefully fix, my suspected IR problem). In the meantime, I'm attaching four images to give a glimpse of the project. I've posted several photos in another rubidium oscillator related thread, linked to below.https://www.eevblog.com/forum/metrology/gnss-rb-(chinese)/msg3999667/#msg3999667

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 15, 2022, 08:16:28 pm
content: Wow! That was a bloody fast response to all your eevblog notification emails!At least four visits (aside from my own) in the space of just 5 minutes.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: July 03, 2023, 02:19:55 pm
content: Well, this time it's only been a mere 13 1/2 months since my last postI've not been idle during that time although the project did suffer a 5 month hiatus due to a computer hardware and OS upgrade exercise that had been bedeviled by several issues.I'd refined the temperature algorithm to the limits of the Arduino nano's 10 bit adc (and then some!), doubling the adc's sampling rate to 19 Ksps, oversampling that by 64 to gain an extra "3 bits" of resolution, increasing the SSD1306 display's I2C speed from 100K to 800K to increase the polling frequency from a lethargic 2.4Hz to a more responsive 12.5Hz before caving in to the need of a better adc almost a fortnight ago.Initial tests with the ADS1115 4ch 16 bit I2C module with my bread-boarded "Flight Spare" weren't very encouraging (11.6Hz cycle time and almost as 'noisy' a temperature display). However after closer inspection of its datasheet I realised that I'd chosen too high a sample rate (860sps) in a futile attempt to shorten the cycle time (the bottleneck on speed still being that damned SSD1306). Even 475sps was too noisy an option so I chose the 250sps for minimal acceptable noise without sacrificing too much by way of speed.That modification made a significant improvement in stability. Although the result looked noticeably calmer, I felt it could still do with further improvement so I tested the 2.048v FRS option over the 4.096v option I'd initially chosen to be safe with the use of a TL431 to provide a 2.5v Vinput to the thermistor/resistor circuit. This worked as expected, no change in base plate temperature readout yet an even more impressive calming effect on the stability of the readings.However, all is not quite as perfect as I'd like over my target ambient temperature range (15 to 30 deg C) and so I feel there is yet still room for further improvement.One obvious change I made, some 7 or 8 months back now, was to the display. Concerned about oled fade due to the 24/7 operation, I decided to invert it to black characters on a white background with the brightness setting reduced to its minimum usable level where the effect would become less obvious (and, incidentally, help even up the wear already incurred). I've attached a couple of photos, along with an earlier attached photo for comparison for review (and possible comments on the change).

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: February 03, 2024, 03:26:35 pm
content: I've made yet more progress with my rubidium frequency reference project over the past seven months. I really need to fabricate a barometric test chamber to fine tune the barometric compensation algorithm. One that can cover a modest +/- 25mBar range and maintain it to within 0.5mBar during settled weather conditions over time periods of a couple of hours or so. Any ideas on how to cheaply fabricate one?Anyway, I thought it might be of some interest to demonstrate the improvements reflected by the need to now display temperature readings to three decimal places (one milli Kelvin resolution). The attached images represent a short 25 seconds long sequence.You'll notice one other change, namely the renaming of the "Ambient temperature" readout label to "Plenum temperature". It's from the BMP280 sensor as before - I've simply moved its location from the mouth of the intake vent to in front of the fan intake within the induction plenum chamber. This now controls a set of servo driven flap diverter valves to transition between direct exhaust and total internal recirculation over a 1 degree range centered currently on 31 deg C. I may raise this another degree since once the flow diverter valves have "battened down the hatches" so to speak at an ambient around the 17 degrees mark, this gives me a more stable fan intake temperature which then falls at a steady 100mK per 1K drop in ambient temperature below that transition point.Trying to maintain temperature control with an 80mm CPU cooling fan being fed directly with raw ambient air below 18 deg is just not possible without resorting to shutting it off and then kick starting when it needs to provide a minimal level of cooling without introducing the Spectre of under and overshooting the target base plate temperature. This "Air Conditioning" of the fan intake plenum worked wonders for temperature control of the base plate over an ambient ranging from 10 to 28 degrees.I could go on at length about this project but I think it best to save it for a separate thread on the "completed' project at a later date. In the meantime, you can admiremy efforts thus far as demonstrated in the attached pictures.P.S I've added 4 screenshots of the Arduino serial plots of base plate temperature (a sequence of 6.4 second windows). The vertical scale is milli Kelvins referenced to a 36.000 degrees base line. There are 500 points per window. The control loop is now running at just over 78Hz. The "spikes" have been introduced every 498 th plot to suppress the counterproductive re-scaling that Arduino had cursed their serial plotter feature with (ver 1.8.19 - the ver 2.x.xx plotter is even more useless!).Without these "spikes" the vertical scaling would be dancing an epileptic jig, defeating the whole point of graphing data in the first placeOne good side effect of adding these pulse pairs (0 and 120) being that it allows a simple way to measure the control loop speed by timing each pass through a chosen X axis reference point with a stopwatch (6.4s per pass for a quick 'n dirty check or 64s for ten passes to obtain a more accurate measure).PPS added three DSO screenshots to illustrate the whole point of this rather protracted project. The first is from midnight the 2nd at 08:09 and the final taken a few minutes ago at 18:00. The DSO (an SDS1202X-E btw) was set to infinite persistence as it has been for the past few years I've had it running 24/7 to monitor the phase drift between the MK II gpsdo and the rubidium reference. CH1 (yellow trace) is triggered by the gpsdo with the ruby on CH2.The majority of the persistence is being painted by the 20ns diurnal wander of an M8T based G3RUH type gpsdo fed from a multi frequency GNSS/RTK antenna mounted with a clear all round view of the sky. This diurnal phase wander rather muddies the waters when it comes to syntonising the ruby to the atomic clocks world time reference/ frequency standard making it rather difficult to test the efficacy of my attempts to stabilise it against variations of both ambient temperature and barometric pressure. However, I do hope to reduce this effect by completing a MK III using a ZED9T gnss receiver module I'd purchased just over a year ago.Damn EEVBLOG's 10 file attachment limit ! I've had to drop the first photo of the ruby's oled display.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: July 01, 2025, 01:31:46 pm
content: Well, it's now well over a year since my last post to this moribund topic thread. but despite the 120 day warning, I thought I'd add a three minute video of my 1.54 inch oled upgrade displaying temperatures and absolute atmospheric pressure and my four latest SDS2104X-Plus screen captures to give you some measure of my progress thus far.I recently figured out a way to offload the display code onto a second nano2 via a one way master to slave data transfer. Currently, that's perched on top of the RFS awaiting the day when I can swap out the old lpro and control electronics package for a very much revamped MK II version that I'm in the middle of assembling.The 'scope screenshots show the ZED9FT MK III gpsdo reference on CH1, the RFS is on CH2 and CH3 (blue trace) shows the M8T based MK II gpsdo which suffers a diurnal drift anywhere from just 16ns to a high of ~ 28ns despite using the Galileo constellation to take advantage of its slightly more accurate (for the UK region) ionospheric correction data (GPS typically shows anywhere from 30 to 35 ns diurnal drift). This series of screen shots covers a time period of just ten minutes shy of the last 12 hours (date and time is in the bottom right hand corner of the display).

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: July 01, 2025, 02:02:50 pm
content: Oh, and for good measure, an update on the base plate temperature plots (4 minute movies this time). Please take note that the vertical scale in the previous screenshots covered a 120mK window above a 36 deg baseline whist these movies now cover a 20mK vertical range above a 37 deg baseline, centered on the new 37.0500 deg set point (effectively a sixfold zooming in on the previous vertical scale).tempBp plot 09 (4 minute run - hot reboot from upload).mp4.txt shows the typical recovery from the interruption due to reloading an updated sketch, showing the obvious effect of the 70ms pauses in the temperature sampling data stream at high rates of temperature change.tempBp plot 10 (4minute run - very hot reboot).mp4.txt was the result of reloading the sketch twice in succession to induce a larger overshoot to highlight the absence of this 70ms disruption after offloading the display code onto the 2nd nano3 via the one way SPI link.