[time-nuts] Re: Realtime comparing PPS of 3 GPS
Carsten, around the edges of the patch are all fringing fields, so the required coupling to the plane may be quite low, depending on the design. The compromised ground plane may also be permitting some rear size pickup - multipath ! I'll do some EM modelling later to try and put some numbers on the requirements. Those numbers to date were for the ideal scenario. If there are rear side pickup issues, a screen / plane at least a few wavelengths (19cm) square would stop the patch seeing the reflections / rear sensitivity without being tightly coupled or connected to the patch ground. Maybe that is what you are seeing when you mention that you've experienced poor performance without an additional plane. I don't know for sure. I've only built patches that were high performance, no compromises- It's likely the mfrs are intentionally throwing away low angle sensitivity and axial ratio equality , and rear pickup rejection to get the size down as it still suits the majority of users. What you can try is turn the patch upside down...and under the patch at least 10cm away, use a sea of steel wool (maybe 50x50cm ) to act as an effective absorber (so the sky wont reflect back underneath) . -glen On 31/05/2022 20:23, Carsten Andrich via time-nuts wrote: On 31.05.22 01:10, glen english LIST via time-nuts wrote: Be aware not to confuse the antenna ground plane (the patch will always have its own plane because the top metalization must be fed against a plane or counterpoise - and a ground plane behind the antenna. I can see the usefulness of the larger ground plane for any purchased patch antenna to reduce the likelihood of interference underneath (if the feed coax has a good RF contact with the plane), and if the plane is coupled well, it may improve the low angle response . The supplementary ground plane doesnt have to have a galvanic connection if the gap between the underside of the patch is low- IE use purely a capacitive coupling to tie the patch antenna ground to the large ground sheet- [...] ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
On 5/31/22 3:23 AM, Carsten Andrich via time-nuts wrote: On 31.05.22 01:10, glen english LIST via time-nuts wrote: Be aware not to confuse the antenna ground plane (the patch will always have its own plane because the top metalization must be fed against a plane or counterpoise - and a ground plane behind the antenna. I can see the usefulness of the larger ground plane for any purchased patch antenna to reduce the likelihood of interference underneath (if the feed coax has a good RF contact with the plane), and if the plane is coupled well, it may improve the low angle response . The supplementary ground plane doesnt have to have a galvanic connection if the gap between the underside of the patch is low- IE use purely a capacitive coupling to tie the patch antenna ground to the large ground sheet- [...] That means reducing the gap to about 0.05mm OR increasing the area- probably means using a bigger patch. Hi Glen, thank you for the insight. I was referring to a ground plane behind the antenna. Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with IP67 housing and an external ground plane seem practically challenging to me. When it comes to stacked patch multi-band antennas like u-blox' ANN-MB [1], the gap between the top patch and the external ground plane is probably significantly higher. Yet, u-blox generally recommends the use of a symmetric ground plane for the RTK applications [1,2]. From my experience, the M8P and F9P RTK fix barely works without a ground plane under the u-blox antennas. While it's just an empirically educated guess, I'd assume that what is required for RTK will not hurt for timing. Could you share your expert opinion on this? My antenna expertise is admittedly limited to reading data sheets and picking the right one for the particular RF measurement requirements. Thanks and best regards, Carsten I would think that the large grounded sheet below the antenna helps more for making the pattern uniform, and, to a certain extent, suppressing some multipath coming from "below" the plane of the sheet. - not as good as a choke ring(s), but not bad. That is, the sheet is not intended to couple to the antenna's ground plane, but is there as a predictable surface (and, probably, to provide a magnetic material for a puck to stick to). As such, the distance from the antenna's ground plane is not particularly critical. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
On 31.05.22 01:10, glen english LIST via time-nuts wrote: Be aware not to confuse the antenna ground plane (the patch will always have its own plane because the top metalization must be fed against a plane or counterpoise - and a ground plane behind the antenna. I can see the usefulness of the larger ground plane for any purchased patch antenna to reduce the likelihood of interference underneath (if the feed coax has a good RF contact with the plane), and if the plane is coupled well, it may improve the low angle response . The supplementary ground plane doesnt have to have a galvanic connection if the gap between the underside of the patch is low- IE use purely a capacitive coupling to tie the patch antenna ground to the large ground sheet- [...] That means reducing the gap to about 0.05mm OR increasing the area- probably means using a bigger patch. Hi Glen, thank you for the insight. I was referring to a ground plane behind the antenna. Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with IP67 housing and an external ground plane seem practically challenging to me. When it comes to stacked patch multi-band antennas like u-blox' ANN-MB [1], the gap between the top patch and the external ground plane is probably significantly higher. Yet, u-blox generally recommends the use of a symmetric ground plane for the RTK applications [1,2]. From my experience, the M8P and F9P RTK fix barely works without a ground plane under the u-blox antennas. While it's just an empirically educated guess, I'd assume that what is required for RTK will not hurt for timing. Could you share your expert opinion on this? My antenna expertise is admittedly limited to reading data sheets and picking the right one for the particular RF measurement requirements. Thanks and best regards, Carsten [1] https://content.u-blox.com/sites/default/files/ZED-F9P_IntegrationManual_UBX-18010802.pdf#page=114 [2] https://content.u-blox.com/sites/default/files/ZED-F9P-MovingBase_AppNote_(UBX-19009093).pdf#page=8 ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
Be aware not to confuse the antenna ground plane (the patch will always have its own plane because the top metalization must be fed against a plane or counterpoise - and a ground plane behind the antenna. I can see the usefulness of the larger ground plane for any purchased patch antenna to reduce the likelihood of interference underneath (if the feed coax has a good RF contact with the plane), and if the plane is coupled well, it may improve the low angle response . The supplementary ground plane doesnt have to have a galvanic connection if the gap between the underside of the patch is low- IE use purely a capacitive coupling to tie the patch antenna ground to the large ground sheet- If we consider the patch area to be 10x10mm = 100uM^2, and the gap being air (for simplicity sake) of 0.25mm, the capacitance is Epsilon-nought times area, all divided by the distance between the two plates For the above example this is about 3.5pF or (-)j30. Really needs to be < j5 . That means reducing the gap to about 0.05mm OR increasing the area- probably means using a bigger patch. You might be able to sweat solder the patch antenna (bottom) to a sheet of FR4- that would be my approach. -glen On 31/05/2022 1:24 am, Carsten Andrich via time-nuts wrote: Hi Erik, have you tried running all receivers off the same antenna via a power splitter (make sure to dc block all but one receiver)? That should remove the uncertainty due to antenna differences (location, RF characteristics, etc.). Also, are you using ground planes for your puck antennas? These types of antennas typically require a ground plane for optimal performance [1]. Best regards, Carsten [1] https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16 On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote: Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables. After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference. The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position. It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna. Erik. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com -- Glen English RF Communications and Electronics Engineer CORTEX RF Pacific Media Technologies Pty Ltd trading as Cortex RF ABN 40 075 532 008 PO Box 5231 Lyneham ACT 2602, Australia. au mobile : +61 (0)418 975077 ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
Erik, The NEO-7M may have sawtooth correction output, have you checked that and made compensations? Since the oscillator is not steered and free-floating, the cycle-assignment of the PPS may be less than optimal so just measuring that without the compensation can cause a wider range of PPS than the actual receiver time stability represents. In particular, check chapter 12 and the TIM-TP message of [1]. [1] https://content.u-blox.com/sites/default/files/products/documents/u-blox7-V14_ReceiverDescriptionProtocolSpec_%28GPS.G7-SW-12001%29_Public.pdf Do notice that the TIM-TP message is documented to be issued before the (PPS) pulse it report on. The variations you report is consistent with what the datasheet report for the pulse assignment, which may not be representative of the receivers performance. Cheers, Magnus On 2022-05-30 13:00, Erik Kaashoek via time-nuts wrote: Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables. After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference. The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position. It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna. Erik. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
Hi The variation you see is dependent on a number of things. One of them is space weather. If you do your run during a very active period ( typically peak sun spots) you may see some very dramatic swings on a single band device. Bob > On May 30, 2022, at 3:00 AM, Erik Kaashoek via time-nuts > wrote: > > Further evaluation did shown the time differences between the 3 GPS modules > was due to difference in the trigger level setting of the timer/counter and > difference in length of GPS antenna cables. > After removal of the phase drift due to Rb frequency offset the attached > image shows the phase differences of the 3 modules versus a Rb reference. > The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M > varies wildly with phase errors above 100 ns. Possibly due to a somewhat > less optimal antenna position. > It seems phase variations over time in the order of 10-20 ns are indeed > unavoidable, even with a good antenna. > Erik. > <3GPS_phase_difference.png>___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
Hi Erik, have you tried running all receivers off the same antenna via a power splitter (make sure to dc block all but one receiver)? That should remove the uncertainty due to antenna differences (location, RF characteristics, etc.). Also, are you using ground planes for your puck antennas? These types of antennas typically require a ground plane for optimal performance [1]. Best regards, Carsten [1] https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16 On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote: Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables. After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference. The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position. It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna. Erik. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables. After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference. The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position. It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna. Erik. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
