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Thread: GPS Module VK2828U7G5 project98 days old

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    Default GPS Module VK2828U7G5 project

    I'm going to start with a project using the GPS Module VK2828U7G5. For my use it is quite cheap. Has anyone used it? Does this GPS have any special difficulties?
    Thanks for any info.

    Alberto
    [U]73's de Alberto ea3agv[/U]

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    Default Re: GPS Module VK2828U7G5 project

    I am going tonuse the UBLIX M8N GPS with Glonass. Seems a good device as it is used in drone and autonomous flight.
    Regards

    Joseph

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    Default Re: GPS Module VK2828U7G5 project

    Joseph, does the Ublix provide a non jittery 1PPS. Some of my hobby projects involve GPS controlled VCOs in amateur transmitters/receivers. I'm thinking about David's comments about jitter.
    John

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    Default Re: GPS Module VK2828U7G5 project

    Hello John,
    I think you will find that all the more recent uBlox receivers will provide a good 1pps output. Jitter will likely be around 20-50nS but is somewhat better if you use a dedicated timing engine. These are much the same hardware but have different firmware that assumes the receiver is stationary and that all Doppler is due to SV movement rather than station movement. The timing engines are less available from the usual hobby/geek outlets but are still out there.
    Jitter is partly due to micro granularity but a fair chunk is due to atmospheric delay variables. Any disciplined oscillator approach will need some degree of integration of the 1 pps pulses to average the jitter over a longer period. Averaging 100 pulses reduces your jitter to 2-5nS since it's non-accumulative. You also need a low noise oscillator with low aging and a small VCO range. Some of the second hand OCXOs are good but may have aged beyond their control range.
    Some of the PICs have nice high speed (64MHz) 16 bit PWMs which make a good poor mans DAC when integrated. PWM carrier is around 1kHz so not too bad to filter. Provided the PIC's supply is very stable the precision is down to the duty cycle of the PWM which is governed by timers - PWM frequency doesn't come in to the equation.

    The uBlox M8N runs simultaneously 3 different constellations (US, China, Russia) and it's common to see them tracking 15 SVs indoors. These are highly favoured by the drone industry and a uBlox based module with magnetometer (compass) is about AUS$24.

    I'm sure you'll have fun.

    Cheers,
    David

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    Default Re: GPS Module VK2828U7G5 project

    Thanks David, that's a very comprehensive answer. I don't fully understand but gives me some leads for more googling. I have much to learn. At present my projects don't need GPS but a couple of my ham friends use GPS as a standard frequency control with some averaging I think. They will appreciate your information and I'll try to help them. There was an article in Silicon Chip magazine a year or two ago. I'll have a look in the town library but I think they were just using one of the NMEA strings.
    John

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    Default Re: GPS Module VK2828U7G5 project

    Yes. And you can customize it how you want.
    Regards

    Joseph

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    Default Re: GPS Module VK2828U7G5 project

    Hi John, Joseph,
    I'm surprised a NMEA string would have the required jitter since serial output often has a low interrupt priority (especially Windows), there may also be 1 or 2 characters of buffering and the NMEA fields can vary in length depending on conditions. Long term accuracy will be good but jitter, I believe, would be quite large so considerable averaging or filtering is required to avoid imposing that on the control signal.

    I know a lot of ham projects were based around the old Jupiter 12 engine which had 1pps and also 10kHz. At 10kHz it was feasible to do a PLL but at 1Hz I think it would be problematic and a FLL or disciplined approach is easier to achieve. These engines are now starting to get rare and were based on an old Conexant chipset.

    You may find this link interesting but there is a variety of different approaches used.
    http://ve2zaz.net/GPS_Std/GPS_Std.htm

    With a good OCXO it should be possible to get below 1E-11 and you should be able to get to <1ppb within about 10 minutes. It also helps if you can periodically store the tuning value (DAC or PWM) so that following a power down or outage you can restore it and only have to wait for the OCXO to warm up (~5-10 min) rather than wait 6-12 hours for a low bandwidth tracking loop to pull in.

    Probably for your radio applications it is best to see what others are using for the same application you intend to use it for. This provides you with a low risk approach with plenty of backup support. Cost and complexity are another concern. The key component is the oscillator and the budget allocated for it should reflect this.

    Cheers,
    David

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    Default Re: GPS Module VK2828U7G5 project

    Hi David,
    The NMEA string was just used to extract the time not to stabilise an oscillator.
    Yes, I remember that the Jupiter engine was used by a number of people and I believe gave quite reasonable results. There are quite a few fairly cheap Caesium standards around the place and these are being pressed into amateur service (I believe they came out of mobile phone base stations). My rigs don't require that degree of stability but some of the new digital modes benefit from the stability offered. Mostly my interests are elsewhere but I'm happy to help out people in the local radio club with the things we've been discussing.
    I'm in danger of getting off topic, but it's all very interesting and challenging.
    I've put your posts in a safe place where I can think about them more when the need arises. Thanks for your comments David.
    Best wishes
    John

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    Default Re: GPS Module VK2828U7G5 project

    Hi John,
    Thanks for clearing up my misunderstanding there. Yes if you just want time readout to the nearest half second it's ideal. Just don't get confused around Daylight Saving Time.

    As you say the Cesium standards are most likely from decommissioned cellular networks. Good stability but only average phase noise and rather power hungry. They have an adjustment to shift frequency so may need calibration again.
    There's a group called Time Nuts who really get excited about this sort of stuff.

    Good luck with your projects.

    Cheers,
    David

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    Default Re: GPS Module VK2828U7G5 project

    Thanks for your interest and help David.
    John

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