Re: [time-nuts] Surface Mount OCXO Questions
Hi The control voltage on the 12 V OCXO is likely 0-10V or 0-5V. The tune on the 3.3V part isn’t going to be above 3.3V and it may be 0-2.5V. The 3.3V part is going to be at least 8X more sensitive to grounding issues. To put this in perspective, you can see a change on a normal 12V part grounding it on the top side of a PC board vs grounding it on the bottom side of the board. The 0.032” of solid ground lead has enough drop to be noticeable. Bob On Oct 30, 2013, at 9:47 PM, Graham / KE9H time...@austin.rr.com wrote: John: Look at the ppm (or however they express it) as to the sensitivity of the frequency stability of the OCXO relative to Voltage input. Say the oven power drops from 3 watts to 1 Watt as the oven comes up to temperature. At 3 Volts, relative to 12 Volts, for a given resistance, it is four time the Voltage change due to the higher currents, and an additional four times the percentage of the operating Voltage as a ratio. So additional design consideration for Voltage control/stabilization is needed. If you have a solid (wide, thick, multi-layer) ground, then that can work. To reduce the voltage drop feeding the OCXO, you might consider putting a dedicated LDO regulator, right at the OCXO, that shares the ground reference with the OCXO, so any voltage drop in the feed side is removed, as well as any Voltage variability with current in the ground system. As to why they are selling the 3.3V part, they probably started selling it before they had some customers get into performance issues per the above. But once offered, they have to continue to support their customers. I think they are just telling you that it is somewhere between 4 and 16 times easier to get the full performance out of the part with a 12 Volt power feed than a 3 Volt power feed, not that you can't get full performance with a 3.3V feed. I am sure their parts meet specs, you just need to understand them. P.S. - I would stick with linear regulators feeding the OCXO, not a switcher. --- Graham == On 10/30/2013 7:37 PM, John C. Westmoreland, P.E. wrote: Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.comwrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
On 10/31/13 4:02 AM, Bob Camp wrote: Hi The control voltage on the 12 V OCXO is likely 0-10V or 0-5V. The tune on the 3.3V part isn’t going to be above 3.3V and it may be 0-2.5V. The 3.3V part is going to be at least 8X more sensitive to grounding issues. I've got a BUNCH of VCOs that are 3.3V or 5V, and have 15V tuning ranges. It's a real pain when you're looking to swap VCOs in a PLL to change the tuning range in a breadboard. Minicircuits ROS-3710 is a fine example.. 5V operation, tuning range 0.5-13V ROS-3388-219 is the same. It's because the tune port goes right to the varactor, which is, not so oddly, DC blocked from the amplifier circuit running on 5V To put this in perspective, you can see a change on a normal 12V part grounding it on the top side of a PC board vs grounding it on the bottom side of the board. The 0.032” of solid ground lead has enough drop to be noticeable. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
A couple tricks I've learned along the way: 1) If using a switching supply is required to get a higher voltage, follow it with a good LDO to reduce the noise level. I've done this successfully for powering handheld radio microphones with built-in amplification, video amplifiers, and for operating a GPS receiver and antenna. 2) If there is a possible ground current path problem, break the ground path by using a unity gain difference amplifier like the AD8276, or a difference amplifier with gain like the INA145. The input is differential to the source signal, and the output has a moderately high impedance reference terminal that can be tied directly to the load device ground pin. I use this technique all the time to keep down the noise when driving remote analog loads that have large ground currents themselves. (Picture a 110W 40MHz transmitter running off 12V, at the bottom of a stack of other transceivers in the trunk of a police car - with 40A of lights flashing, etc.) Bob LaJeunesse From: John C. Westmoreland, P.E. j...@westmorelandengineering.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, October 30, 2013 10:54 PM Subject: Re: [time-nuts] Surface Mount OCXO Questions Graham, Good points - yes, I have this part currently in the design: TPS75833KTTT (LDO from TI) - putting another one down (just) for the OCXO isn't a problem. And a nice 12V rail isn't a problem either since this is for a radio with a nice 12V source. Could I boost the 3.3V rail to 12V or maybe 5V to 12V - sure - but your point about the switcher is well taken and I agree. Having a nice, fat, analog ground plane isn't a problem either. And, this is just a 'dev' board so we can do what we need to make the OCXO work as good as possible. From their spec sheet: '... the supply voltage sensitivity and load sensitivity is 5E-11 for a 5 % change in voltage or load impedance.' Thanks, John ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Bob, These are good suggestions. I will probably end up doing 1) and 2) you have outlined above. Thanks! John W. On Thu, Oct 31, 2013 at 7:51 AM, Robert LaJeunesse rlajeune...@sbcglobal.net wrote: A couple tricks I've learned along the way: 1) If using a switching supply is required to get a higher voltage, follow it with a good LDO to reduce the noise level. I've done this successfully for powering handheld radio microphones with built-in amplification, video amplifiers, and for operating a GPS receiver and antenna. 2) If there is a possible ground current path problem, break the ground path by using a unity gain difference amplifier like the AD8276, or a difference amplifier with gain like the INA145. The input is differential to the source signal, and the output has a moderately high impedance reference terminal that can be tied directly to the load device ground pin. I use this technique all the time to keep down the noise when driving remote analog loads that have large ground currents themselves. (Picture a 110W 40MHz transmitter running off 12V, at the bottom of a stack of other transceivers in the trunk of a police car - with 40A of lights flashing, etc.) Bob LaJeunesse From: John C. Westmoreland, P.E. j...@westmorelandengineering.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, October 30, 2013 10:54 PM Subject: Re: [time-nuts] Surface Mount OCXO Questions Graham, Good points - yes, I have this part currently in the design: TPS75833KTTT (LDO from TI) - putting another one down (just) for the OCXO isn't a problem. And a nice 12V rail isn't a problem either since this is for a radio with a nice 12V source. Could I boost the 3.3V rail to 12V or maybe 5V to 12V - sure - but your point about the switcher is well taken and I agree. Having a nice, fat, analog ground plane isn't a problem either. And, this is just a 'dev' board so we can do what we need to make the OCXO work as good as possible. From their spec sheet: '... the supply voltage sensitivity and load sensitivity is 5E-11 for a 5 % change in voltage or load impedance.' Thanks, John ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Hi There are a *lot* of SMT OCXO’s out there. A J lead part is SMT, but identical to it’s through hole counterpart. It will mount pretty much same / same…. Bob On Oct 29, 2013, at 10:18 PM, John C. Westmoreland, P.E. j...@westmorelandengineering.com wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.comwrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Hi Consider that microvolts matter on the EFC. Unless you have a separate return for the oven current it’s going to be tough to keep everything separate. One might ask “why no separate return”. Well when you design one in, and then go look at people’s layouts - you might as well not have designed it in ….. Bob On Oct 30, 2013, at 8:37 PM, John C. Westmoreland, P.E. j...@westmorelandengineering.com wrote: Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.comwrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Bob, OK - that makes sense. If you follow good analog/digital layout rules then this may not be a problem. But your point about the EFC sensitivity is well taken. But, that is always a problem. Yes, do a lot of people violate good analog/digital layout rules, especially on the ground planes - yep. This is exactly why I am asking these questions. Even in manufacturer's recommended layout instructions you can find mistakes. Unless you have a dev board that you have used and have the gerbers from that board so you know exactly how that part behaves with that layout - you cannot know for sure you have a sound layout for your design. You also have to be careful with board material, dielectrics, and copper weight, not to mention controlled impedances. Remember the early days of DC/DC converters? Thanks! John / AJ6BC On Wed, Oct 30, 2013 at 5:52 PM, Bob Camp li...@rtty.us wrote: Hi Consider that microvolts matter on the EFC. Unless you have a separate return for the oven current it’s going to be tough to keep everything separate. One might ask “why no separate return”. Well when you design one in, and then go look at people’s layouts - you might as well not have designed it in ….. Bob On Oct 30, 2013, at 8:37 PM, John C. Westmoreland, P.E. j...@westmorelandengineering.com wrote: Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.com wrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nuts https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nuts https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Hi You may have a good layout on your board, but they have to get the current to that pin / pad somehow. In all likelihood the amps of current through the pad it’s self are going to cause issues. Also remember that the ground is likely an RF return as well. Multiple rules and layout issues all collide at that pin….. Bob On Oct 30, 2013, at 9:02 PM, John C. Westmoreland, P.E. j...@westmorelandengineering.com wrote: Bob, OK - that makes sense. If you follow good analog/digital layout rules then this may not be a problem. But your point about the EFC sensitivity is well taken. But, that is always a problem. Yes, do a lot of people violate good analog/digital layout rules, especially on the ground planes - yep. This is exactly why I am asking these questions. Even in manufacturer's recommended layout instructions you can find mistakes. Unless you have a dev board that you have used and have the gerbers from that board so you know exactly how that part behaves with that layout - you cannot know for sure you have a sound layout for your design. You also have to be careful with board material, dielectrics, and copper weight, not to mention controlled impedances. Remember the early days of DC/DC converters? Thanks! John / AJ6BC On Wed, Oct 30, 2013 at 5:52 PM, Bob Camp li...@rtty.us wrote: Hi Consider that microvolts matter on the EFC. Unless you have a separate return for the oven current it’s going to be tough to keep everything separate. One might ask “why no separate return”. Well when you design one in, and then go look at people’s layouts - you might as well not have designed it in ….. Bob On Oct 30, 2013, at 8:37 PM, John C. Westmoreland, P.E. j...@westmorelandengineering.com wrote: Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.com wrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nuts https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nuts https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
John: Look at the ppm (or however they express it) as to the sensitivity of the frequency stability of the OCXO relative to Voltage input. Say the oven power drops from 3 watts to 1 Watt as the oven comes up to temperature. At 3 Volts, relative to 12 Volts, for a given resistance, it is four time the Voltage change due to the higher currents, and an additional four times the percentage of the operating Voltage as a ratio. So additional design consideration for Voltage control/stabilization is needed. If you have a solid (wide, thick, multi-layer) ground, then that can work. To reduce the voltage drop feeding the OCXO, you might consider putting a dedicated LDO regulator, right at the OCXO, that shares the ground reference with the OCXO, so any voltage drop in the feed side is removed, as well as any Voltage variability with current in the ground system. As to why they are selling the 3.3V part, they probably started selling it before they had some customers get into performance issues per the above. But once offered, they have to continue to support their customers. I think they are just telling you that it is somewhere between 4 and 16 times easier to get the full performance out of the part with a 12 Volt power feed than a 3 Volt power feed, not that you can't get full performance with a 3.3V feed. I am sure their parts meet specs, you just need to understand them. P.S. - I would stick with linear regulators feeding the OCXO, not a switcher. --- Graham == On 10/30/2013 7:37 PM, John C. Westmoreland, P.E. wrote: Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.comwrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Surface Mount OCXO Questions
Graham, Good points - yes, I have this part currently in the design: TPS75833KTTT (LDO from TI) - putting another one down (just) for the OCXO isn't a problem. And a nice 12V rail isn't a problem either since this is for a radio with a nice 12V source. Could I boost the 3.3V rail to 12V or maybe 5V to 12V - sure - but your point about the switcher is well taken and I agree. Having a nice, fat, analog ground plane isn't a problem either. And, this is just a 'dev' board so we can do what we need to make the OCXO work as good as possible. From their spec sheet: '... the supply voltage sensitivity and load sensitivity is 5E-11 for a 5 % change in voltage or load impedance.' Thanks, John On Wed, Oct 30, 2013 at 6:47 PM, Graham / KE9H time...@austin.rr.comwrote: John: Look at the ppm (or however they express it) as to the sensitivity of the frequency stability of the OCXO relative to Voltage input. Say the oven power drops from 3 watts to 1 Watt as the oven comes up to temperature. At 3 Volts, relative to 12 Volts, for a given resistance, it is four time the Voltage change due to the higher currents, and an additional four times the percentage of the operating Voltage as a ratio. So additional design consideration for Voltage control/stabilization is needed. If you have a solid (wide, thick, multi-layer) ground, then that can work. To reduce the voltage drop feeding the OCXO, you might consider putting a dedicated LDO regulator, right at the OCXO, that shares the ground reference with the OCXO, so any voltage drop in the feed side is removed, as well as any Voltage variability with current in the ground system. As to why they are selling the 3.3V part, they probably started selling it before they had some customers get into performance issues per the above. But once offered, they have to continue to support their customers. I think they are just telling you that it is somewhere between 4 and 16 times easier to get the full performance out of the part with a 12 Volt power feed than a 3 Volt power feed, not that you can't get full performance with a 3.3V feed. I am sure their parts meet specs, you just need to understand them. P.S. - I would stick with linear regulators feeding the OCXO, not a switcher. --- Graham == On 10/30/2013 7:37 PM, John C. Westmoreland, P.E. wrote: Graham and Time Nuts, (thanks for the answers.) I have another question - I am looking at a part from MTI. I wanted to use one of their 3.3V parts. They are telling me to use the 12V part because the 3.3V part can have an issue with ground loops due to the higher current requirements at that voltage for the oven. Have any of you experienced this? Makes me wonder a little why they offer the 3.3V part. It would seem good layout can control any possibility of ground loops becoming a problem. Thanks and Regards, John W./AJ6BC On Wed, Oct 30, 2013 at 6:54 AM, Graham / KE9H time...@austin.rr.com wrote: John: All SMT OCXO's will either have a recommended PCB footprint in the spec sheet or will refer you to a recommended footprint in another document. Some don't care about a ground plane under the part, some require it with no crossing signals, some require an open thermal hole underneath the oven. I have seen all three cases. As usual, it is suggested that you read the [] manual. Best regards, --- Graham / KE9H == On 10/29/2013 9:18 PM, John C. Westmoreland, P.E. wrote: Hello, I was wondering if I could get some recommendations on surface mount OCXO's vs. the traditional through hole. I was also wondering on the board layout - if you found it necessary to leave a thermal moat so to speak - and what worked best. Maybe the OCXO has an internal air barrier that maybe would make this unnecessary - not sure. Your input and experience appreciated. Thanks, John Westmoreland __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
