Synchronising 32.768kHz oscillator - Page 1
Forum section: electronics / projects

author: npelov
author_rank: Frequent Contributor
timestamp: August 03, 2015, 12:01:45 pm
content: Hi,I want to synchronize 32.768kHz oscillator to a good 32.768kHx TCXO. InDS32kHz TCXO datasheeton page 7 they suggest how to connect it to existing oscillator with few passives. I've captured the signal on each pin of the quartz crystal (images attached). How do I know which one is the input? My wild guess would be that input is closer to pure sine because there is nothing to distort it (the input is high impedance. The output is probably an output of single logic element which tries to force it to square wave. Am I close to the truth?The device is a cheap chinese DIY clock from ebay - 32.768 kHz oscillator, 8Hz on the coil. Of course there is a chance the gear are not that accurate ("cheap" and "chinese"), but since I don't put my life on it, I'll take the chance. At least if it's not accurate it'll be the same deviations in all temperatures.

author: technix
author_rank: Super Contributor
timestamp: August 03, 2015, 12:10:11 pm
content: Quote from: npelov on August 03, 2015, 12:01:45 pmI think you should be able to bypass the entire oscillator module on the original board (cutting traces) and inject the clock signal from DS32KHz (or DS3231) directly.

author: npelov
author_rank: Frequent Contributor
timestamp: August 03, 2015, 12:28:25 pm
content: Quote from: technix on August 03, 2015, 12:10:11 pmFirst, there are no traces. There is a COB and everything enters the bare die And in theory if I know which one is input I could remove the quartz and pass the TCXO signal directly, but there is level difference - the clock is 1.5V powered, the TCXO - 3V. Also there are probably capacitors to ground inside the die which will require more power to drive. The circuit suggested in datasheet is to reduce power consumption. To me it sounds reasonable. All other options do not sound like a good idea. Divider to bring down the level - < 100k resistors consume too much power, > 100k resistors can't provide clean enough signal (tried that). The idea is that clock oscillator will provide clock and the TCXO will just slightly push it to the right direction with minimum power consumption. With 4uA max TCXO consumption I'll need one coin cell every 5 years and I'll only need to change the 1.5V clock battery.

author: dom0
author_rank: Super Contributor
timestamp: August 03, 2015, 01:40:43 pm
content: QuoteYes. Crystal oscillators for logic are usually Pierce oscillators with a simple CMOS inverter cell, so one side is driven and the other is only loaded by the load capacitance. The side with more distortion is the driven side.Just by the way, watch crystals aren't particularly low drift, but have near-zero tempco at room temperature.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 03, 2015, 04:30:33 pm
content: Watch crystals are amazingly sensitive to 32 Khz injection. Their Q factor is amazingly high and they will "ring" from any nearby 32 Khz source, even when out of circuit. Reduce the drive to the slave crystal and inject the signal from the master. It will most likely self lock. However have a buffer amp to prevent any slave oscillation feeding back into the master.*You can find a functioning watch crystal by hooking a leaded watch crystal can to a high impedance input on a oscilloscope. When you get within a meter or so, the free running crystal will vibrate in sync with the driven crystal. The phase will be different, but the sync is very close.Steve

author: npelov
author_rank: Frequent Contributor
timestamp: August 03, 2015, 09:57:50 pm
content: Quote from: dom0 on August 03, 2015, 01:40:43 pmThanks!Quote from: dom0 on August 03, 2015, 01:40:43 pmWell, room temperature is not always the same. Currently it's 29-30 degrees all the time - 24 hours a day. The winter it's 23-26 when we heat and it's 18-20 when we are away.But I admit that the one I'm testing is quite close - according to the scope it's 32.7677kHz and I believe the scope is not much out. The TXCO shows exactly 32.7680. But that's not going to be the case through the whole "room" temperature range.

author: npelov
author_rank: Frequent Contributor
timestamp: August 03, 2015, 10:10:59 pm
content: Quote from: LaserSteve on August 03, 2015, 04:30:33 pmI'm not sure I understand everything you said.Quote from: LaserSteve on August 03, 2015, 04:30:33 pmYou mean acoustic source? By air?Quote from: LaserSteve on August 03, 2015, 04:30:33 pmHow? Can I use a serial resistor on the output pin. Keep in mind that the load capacitors are hidden - probably in the potted chip. So if I put a serial resistor it'll be between the crystal leg and the output with the load capacitor.Quote from: LaserSteve on August 03, 2015, 04:30:33 pmIt's a rail to rail push-pull buffer - nothing will get back to master oscillator. At least not through 1MOhm resistor in series with 100pF cap.Quote from: LaserSteve on August 03, 2015, 04:30:33 pmAre you saying that it'll do that without any drive? Like piezo effect? I thought quartz resonates only when driven.

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2015, 10:19:41 pm
content: Try a capacitive divider with a 2:1 capacitor ratio (larger cap as the lower arm) to get 1/3 the TCXO output to inject at the waveform 1 terminal (0.9V pk-pk) with the crystal removed. I'd try 12pF and 22pF to start with. Be prepared to tweak the upper cap value to compensate for any internal caps in the COB and get 1V pk-pkA DC coupled divider isn't a good idea as it disturbs the oscillator amp's bias point.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2015, 11:17:58 pm
content: The OCXO has a digital logic output, yes?It's probably sufficient to hang a wire near the device, connected to said output pin. The impedance is quite high (probably, on one or the other pin, your scope probe dampened the waveform noticeably and shifted the frequency slightly -- but how would you know?), so this "gimmick" capacitance (on the order of 0.1pF) will probably couple enough in to achieve lock.If you have some means of testing jitter or amplitude modulation or frequency, you can probably observe the transition through states, from uncoupled, to superposition (both frequencies detectable yet independent), to pulling and mixing (phase mostly locked, but skipping from time to time), to full lock (little to no jitter, phase locked). A simple way to observe this is by scoping both signals simultaneously: the triggered sweep, and visual display, allows the measurement (or at least visualization) of frequency and amplitude modulation.Tim