Quartz oscillator accuracy vs. frequency - Page 1
Forum section: electronics / beginners

author: JeanF
author_rank: Regular Contributor
timestamp: October 15, 2020, 09:17:49 pm
content: Hello,There is something in an old issue of a French electronics magazine that didn't catch my attention back then, but now I'm a bit suspicious.It's about a clock made from CMOS ICs, without a microcontroller.The clock is mainly based on a CD4521. The author says: "In order to get a good accuracy, we are using a quartz resonator with a frequency of more than 4 Mhz". The crystal is indeed 4.194304 MHz, which is then divided by 2^22 to get one pulse per second.I looked online, but I couldn't find anything that may support the claim that higher frequency crystals are generally "more accurate". (Maybe I haven't looked hard enough...)Why not use a 32.768 kHz crystal instead ? If I understood correctly what I've been reading online, a watch crystal should be more stable than a 4 Mhz one because of its lower frequency. I didn't find much info about their absolute accuracy compared to other types, though. Probably because in commercial clocks, absolute accuracy of the crystal itself is not that important thanks to inhibition compensation.Even when looking at various random 32.768 kHz and 4.194304 MHz crystals on Digikey, the tolerances in ppm were often quite similar, nothing spectacularly different.So, am I missing something ? And then,hereis a nice photo of a French Navy marine chronometer which must have costed a fortune, and seems to be based on a 4.19 Mhz crystal, so such a frequency may have other advantages I'm not aware of!Thank you for any hints !

author: Wolfgang
author_rank: Super Contributor
timestamp: October 15, 2020, 09:22:28 pm
content: Well, not *all* higher frequency crystals are better. That depends on the cut.The best crystals you can buy today are SC cut at around 5MHz.Stabilities around 100ppt/day are possible.Drawback: They need a thermostat.

author: bob91343
author_rank: Super Contributor
timestamp: October 15, 2020, 10:06:50 pm
content: That small frequency difference isn't going to make much difference in stability. This subject has been worked over a lot over the years, particularly back when there were no synthesizers or even crystal based clocks (outside of the lab).There is more difference from one crystal to the next of the same frequency than there is by changing the frequency a small amount.If you are seeking the best accuracy you will need an oven or even a double oven. TCXOs just won't do it. Or use rubidium.1 ppm results in an error of about 30 seconds per year. If my calculation is right. If you want one second per year, assuming you can adjust that closely, you need 1 part in 30 billion, or 30 parts per trillion.So the first step is to decide how good you want it to be.

author: emece67
author_rank: Frequent Contributor
timestamp: October 15, 2020, 10:43:42 pm
content: .

author: Wolfgang
author_rank: Super Contributor
timestamp: October 15, 2020, 11:28:54 pm
content: Quote from: emece67 on October 15, 2020, 10:43:42 pmReason is the Vig Curve (google). The product Qf is a constant.

author: JeanF
author_rank: Regular Contributor
timestamp: October 16, 2020, 08:44:01 am
content: Thank you for your replies.So, it looks like in this case, the choice of a 4.194304 MHz by the author was a bit arbitrary. Maybe he had in mind that the SC cut ones in this range are good, as you say.But given that the BOM in the article just says "get a 4.194304 MHz crystal" without any part number or tolerance requirements, I guess the frequency is not that important and one could choose another clock frequency (and adjust the divider chain as required) and get a clock which would be anywhere between "more accurate" and "less accurate" than the original (not characterized) design, depending on the accuracy of the particular crystal that the builder may buy...As this was a hobby electronics magazine from 2004, maybe the author thought the average reader was going to buy the parts in a local brick-and-mortar store and get a quartz of the specified frequency, but without knowing any other characteristic. These stores (the majority of them went bankrupt over the years) often have only one particular model number for each part, and very little stock. And they may change supplier without notice.I wonder if in this context, of asking for "one quartz resonator, please" over the counter, without further details, there could be a probability of getting a 4.194304 MHz with tighter specs than a 32.768 kHz one.

author: emece67
author_rank: Frequent Contributor
timestamp: October 16, 2020, 09:14:07 am
content: .

author: HB9EVI
author_rank: Frequent Contributor
timestamp: October 16, 2020, 01:25:30 pm
content: if I need a 1sec tick on a microcontroller that doesn't have an onboard RTC, I use a 4.194304MHz crystal too; they are readily available for little money; of course we shouldn't go into deep with a time-nut attitude. In times of cheap GPS modules with 1PPS outputs, there's no reason to hunt for narrow specs crystals.

author: KMoffett
author_rank: Regular Contributor
timestamp: October 16, 2020, 03:39:12 pm
content: How about a gutted 1.5VDC battery desk or wall clock PCB? Are those accurate enough for you? Summing the 0.5 second alternating coil outputs gives you 1 second 30mS pulses. Some PCBs give you positive going pulses and some negative going pulses. There are all sorts of configurations, but they all work the same.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 16, 2020, 04:30:26 pm
content: Quote from: bob91343 on October 15, 2020, 10:06:50 pmThis is the whole answer in a nutshell. I recall early in my career when our counters were sent out for calibration they were returned with a warning. "Only calibrated to 1 part in 100,000,000". I was amused because the requirements for the job we were doing was several orders of magnitude lower than that.I would assume that the author of this hobby article really wasn't aware of or interested in the possibility of something that would achieve really high accuracy, but there is also the possibility that he was aiming at controlling jitter due to noise on the input gate to the counter chain. The faster rise time which might have been associated with the higher clock frequency may have been the source of his decision.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2020, 05:54:39 pm
content: Quote from: KMoffett on October 16, 2020, 03:39:12 pmIt's interesting how some ICs such as those found in clocks, only seem to be sold in bare die form and in bulk. I've never managed to find a datasheet for one.You might want to get rid of that sounder, just in case it goes off. It can be quite annoying.The usual hobbyist way to make 1Hz is to use the '4060 and '4013, with a 32768Hz crystal, but now I think it's more accurate to use 4.194304MHz and the HEF4521B. It's certainly simpler, as it's only one IC, rather than two. Note that it must be the HEF4521B, rather than CD4521B, because the latter is only guaranteed to work up to 2MHz, at 5V, where as the former can run at up to 5MHz, on 5V.

author: KMoffett
author_rank: Regular Contributor
timestamp: October 16, 2020, 11:25:02 pm
content: "It's interesting how some ICs such as those found in clocks, only seem to be sold in bare die form and in bulk. I've never managed to find a datasheet for one."No need for publicizing one. Clock manufacturers would get them though."You might want to get rid of that sounder, just in case it goes off. It can be quite annoying."I was just showing examples of different implementations.

author: JeanF
author_rank: Regular Contributor
timestamp: October 19, 2020, 10:34:47 am
content: Thanks for your comments. Yes, using a gutted 1.5 V clock PCB is an interesting idea, and would benefit from the calibration from the manufacturer. Trying to characterize several of them against a known reference (GPS PPS ?) could be a fun little project.Interesting thought about risetime and jitter, I'll have to read up on that.About making 1 Hz from 32.768 kHz with 1 chip only, obviously CD4521 doesn't work because its smallest division ratio is 2^18, but I just foundthis AHC divider series from NXP, in particular the74AHC1G4215that does just that, has a small package and is available for 50ct in qty. 1 from Digikey for example. For the moment these seem little known, maybe in the future we'll see more of them in DIY clock builds...

author: tkamiya
author_rank: Super Contributor
timestamp: October 22, 2020, 12:49:18 am
content: How about something like this?One of a well noted time-nuts coded PIC microcomputer to divide frequencies.http://www.leapsecond.com/pic/picdiv.htmIt's a one chip solution to divide whatever you have to 1Hz. Much easier and flexible than chaining flip-flops and decade counters.I don't know if it still exists. But EPSON used to make a hybrid 14 pin chip that included oscillator and divider. By pulling up certain legs, you could program various frequencies.Also, if you are reading old magazine, technology has changed quite a bit. There are way more than AT cut and ST cut. Stability has improved. Unfortunately, custom frequencies are almost impossible now.

author: JeanF
author_rank: Regular Contributor
timestamp: October 22, 2020, 07:39:46 am
content: I've heard of PICDIVs but haven't used them personally (yet). They look neat indeed.Anyway, maybe nostalgia or emotional reasons could lead someone to buildthe clock in questionbut from a technical point of view there is probably little point in using 12 ICs instead of one single microcontroller.I was just curious about the statement mentioned in the OP about quartz accuracy; there's certainly many ways to build a 7 segment clock (or to get 1 Hz from XX Mhz for any purpose)

author: Wolfgang
author_rank: Super Contributor
timestamp: October 22, 2020, 07:54:19 am
content: Good parts. I use them a lot in divider chains providing output with surprisingly low jitter.

author: tkamiya
author_rank: Super Contributor
timestamp: October 22, 2020, 10:51:31 am
content: I used to think 100KHz for AT cut is where the best stability is. I never found out the true answer.