[TangerineSDR] [HamSCI] RE: Notes from PSWS / TangerineSDR call of 07-26-2021
Julius Madey
hillfox at fairpoint.net
Wed Jul 28 16:31:49 EDT 2021
On that note, I wonder if it would be a good idea to put a common mode
choke (multiple turns of the power feed line through an appropriate
ferrite mix) in the line ? K9YC's audio RFI paper is a good reference.
73,
Jules-K2KGJ
On 7/28/2021 4:02 PM, Ward Silver wrote:
> A quick note - rectifiers, LEDs, or any non-linear devices connected
> to unshielded cables of any significant length will happily act as
> mixers or harmonic generators. Whenever one of these is used, put a
> 0.01uF disc ceramic across it to bypass it at RF.
>
> 73, Ward N0AX
>
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>
>
> On Wed, Jul 28, 2021 at 11:41 AM Jonathan <emuman100 at gmail.com
> <mailto:emuman100 at gmail.com>> wrote:
>
> Hi Tom,
>
> That makes a lot of sense. I’ll try fit a 0.1uF and a 22-47uF
> electrolytic on the feedline loop.
>
> Thanks again!
>
> Jonathan
> KC3EEY
>
> On Jul 28, 2021, at 10:55 AM, Tom McDermott <tom.n5eg at gmail.com
> <mailto:tom.n5eg at gmail.com>> wrote:
>
>> Hi Jonathan - a concern is that the DC-DC converter input stage
>> would become unstable and oscillate
>> with the cable inductance. If that happens then there could be
>> high frequency AC voltages present at the
>> input of the converter. Would those over-voltage the input?
>> Possibly. Would it radiate RFI from the cable? Likely.
>>
>> Good design dictates sufficiently low source impedance to the
>> DC-DC converter. Almost all linear regulators
>> require it. An electrolytic and ceramic bypass pair right at the
>> DC-DC input would be considered good design by many.
>>
>> -- Tom, N5EG
>>
>>
>>
>>
>>
>>
>> On Wed, Jul 28, 2021 at 7:30 AM Jonathan <emuman100 at gmail.com
>> <mailto:emuman100 at gmail.com>> wrote:
>>
>> John,
>>
>> Thanks for the suggestion! I think I’ll start with the
>> overvoltage age spike of 50V for 100ms or less based on the
>> datasheet spec of the DC-DC converter. I was thinking of some
>> sort of fusable circuit protection now that I’ll be using a
>> TVS diode to mitigate stress from the follow current.
>>
>>
>> Tom,
>>
>> The datasheet wasn’t clear on the need for input filtering.
>> On the output of both the IP2415S, I have 0.1uF ceramic
>> capacitors. In the datasheet, it only mentions a spec of
>> “Input Reflected Rated Current” with a series inductor and
>> shunt capacitor. I don’t really understand exactly what this
>> is, but it lists “20mAp-p through a 12uH inductor and 47uF
>> capacitor”. Nothing else is mentioned about input protection
>> or filtering.
>>
>> In both situations that the DC-DC failed, the cable length
>> was only 4’ of cat 5. This circuit that was proved and tested
>> by Paul, including the receiver and schematic I attached, did
>> not have such an issue, but the only difference is that the
>> voltage was lower than 30V. That’s why I was thinking it was
>> a turn-on/off spike. I think that in a lot of typical
>> applications, the input voltage isn’t that high, so spikes
>> don’t normally present a problem. The DC-DC that feeds the
>> receiver DC-DC has never failed and it was always fed with
>> 18V or less.
>>
>>
>> Jules,
>>
>> You recommend the Schottky diode in parallel with the TVS,
>> then a series PTC, so during a spike, the TVS will conduct
>> and the PTC would fuse, and during a reverse polarity
>> condition, the Schottky diode will conduct with limited
>> current due to the PTC fusing? I can see the benefit that the
>> series PTC in normal a normal circuit condition would drop
>> much less voltage that the Schottky diode.
>>
>>
>> All in all, I agree with adding some capacitance to filter
>> voltage spikes and reduce or eliminate them. I’ll need to be
>> sure not to exceed the DC-DC max load capacitance, which is
>> 47uF in this case.
>>
>> But, to me, the most plausible cause for the failures are
>> turn on/off voltage spikes, even with a short length of
>> cable. Adding electrolytics is a bit difficult due to the
>> tight space constraint as seen in my previous email, so I’ll
>> try out the TVS and a PTC. Other than voltage spikes, does
>> anything else come to mind?
>>
>>
>> Thanks guys.
>>
>> Jonathan
>>
>> KC3EEY
>>
>>
>> On Tue, Jul 27, 2021 at 9:34 AM Jonathan <emuman100 at gmail.com
>> <mailto:emuman100 at gmail.com>> wrote:
>>
>> Hi Everyone,
>>
>> Before the meeting ended last night, I wanted to discuss
>> the issues I’ve experienced with my VLF active antenna.
>> As I mentioned, the DC-DC converter (XP Power IP2415S)
>> failed after I applied power. I turned the volume up
>> because I wasn’t hearing sferics, and shortly after, I
>> heard the squeal of the DC-DC converter. When I pulled
>> out the smaller foam core partially, I heard a squeal
>> coming from the DC-DC converter itself.
>> <image3.jpeg>
>>
>> I had a failure of the DC-DC converter when I first built
>> the VLF preamp. It was being powered by the Raspberry Pi
>> box (same power circuit as my text box). I had a power
>> failure and when the power was restored, the DC-DC
>> converter failed. In both situations, it was being
>> supplied with 30VDC, with the limit is 36V. The reason
>> for using 30V is to overcome the voltage drop from long
>> cable lengths.
>>
>> After both failures, there was a low impedance across the
>> input of the DC-DC converter causing the supply to be
>> pulled down. Because the input and output are isolated, I
>> figured the fault had to have been caused by a voltage
>> spike on the input due to power being turned on and off.
>> The datasheet mentions that it can withstand 50V spikes
>> at 100ms, but I can’t think of anything else that could
>> cause the DC-DC to fail, especially that 30V is used to
>> power it and any voltage spike would ride on top of that
>> 30V. I contacted XP Power about the issue.
>>
>> Here is the schematic of the power path. The blocks are
>> the IP2415S DC-DC converter.
>> image1.jpeg
>> I’m powering the DC-DC converter with 30V from another
>> DC-DC converter in the test box and Raspberry Pi box
>> (Raspberry Pi box not shown, but same circuit). Keep in
>> mind, the length is still relatively short in my testing,
>> so cable inductance isn’t the primary reason for the
>> spike. Plus, at long lengths, the voltage would be much
>> lower due to voltage drop across the length, and I think
>> this was why Paul might not have seen this issue when he
>> originally tried and built this design.
>>
>> To mitigate this problem with the supply voltage at 30V,
>> I decided to use a TVS diode across the input of the
>> DC-DC converter. Here is the schematic: (my apologies for
>> the TVS diode being backwards)
>> image2.jpeg
>> I chose a unidirectional TVS so there is also protection
>> if the voltage spikes have any negative components. I
>> picked a TVS with a working voltage of 33V, a Littel Fuse
>> P4KE39A. According to the datasheet, it looks like this
>> would be adequate. TVS diodes have a PN junction that is
>> more rugged, with a larger surface area for greater
>> current density. One thing I overlooked with this type of
>> circuit protection is that once the TVS conducts, current
>> from the power supply will be shunted too, known as the
>> follow current, and in some situations, this may cause
>> issues requiring a fuse. In this case, since the spikes
>> are probably short duration, and since the DC-DC
>> converter in the text/Pi box has short circuit
>> protection, this should not be an issue.
>>
>> So the questions I have are: do you also think it could
>> be voltage spikes that caused the failures, and if so,
>> was my choice of TVS diode adequate. Also, one important
>> question, if I connect the anode of the TVS diode to the
>> receiver side ground, will I lose isolation? I know that
>> during spike current conduction, I will. I’m sure I can
>> still get adequate protection with the TVS diode anode
>> connected to the negative of the isolated power loop too,
>> but I just wanted to be sure.
>>
>> Lastly, since the magnetometer board will undergo another
>> revision, I feel it may be worth it to add some sort or
>> adjustable regulator or use of an external power source
>> (not connected to the 5V pin of the Pi header) because
>> even with paralleling wires or pairs, some installations
>> will require very long lengths (especially since
>> interference-free locations are often far away from the
>> shack) and the voltage may drop to where the LDO cant
>> regulate. Using a higher voltage at the Pi end (like I do
>> with the VLF preamp) may be required for some
>> installations. I think this should be considered if it
>> already wasn’t addressed.
>>
>> Jonathan
>> KC3EEY
>>
>>
>>
>> On Jul 27, 2021, at 7:21 AM, Dr. Nathaniel A. Frissell
>> Ph.D. <nathaniel.frissell at scranton.edu
>> <mailto:nathaniel.frissell at scranton.edu>> wrote:
>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Thank you, Tom.
>>>
>>>
>>>
>>>
>>>
>>> The recording will be available later today at
>>>
>>> https://youtu.be/AammohuygMw
>>> <https://youtu.be/AammohuygMw> and hamsci.org/telecons
>>> <http://hamsci.org/telecons>.
>>>
>>>
>>>
>>>
>>>
>>> 73 de Nathaniel W2NAF
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *From:* TangerineSDR
>>> <tangerinesdr-bounces at lists.tapr.org
>>> <mailto:tangerinesdr-bounces at lists.tapr.org>>
>>>
>>> *On Behalf Of *Tom McDermott via TangerineSDR
>>>
>>>
>>> *Sent:* Monday, July 26, 2021 10:02 PM
>>>
>>>
>>> *To:* TAPR TangerineSDR Modular Software Defined Radio
>>> <tangerinesdr at lists.tapr.org
>>> <mailto:tangerinesdr at lists.tapr.org>>
>>>
>>>
>>> *Cc:* Tom McDermott <tom.n5eg at gmail.com
>>> <mailto:tom.n5eg at gmail.com>>
>>>
>>>
>>> *Subject:* [TangerineSDR] Notes from PSWS / TangerineSDR
>>> call of 07-26-2021
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Notes from PSWS / TangerineSDR call of 07-26-2021
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> 1. Bill is using chart.js for magnetometer charting. He
>>> is setting up a database using Django web and database
>>> framework for Python.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> 2. Scotty is looking at the Intel (Altera) Arria 10 GX
>>> FPGA 10GX270 for the version 2 Data Engine (supporting
>>> 10GE). These FPGAs appear to be more available than the
>>> MAX10 FPGAs. The intention is to develop DE Ver 1 and DE
>>> Ver 2 in parallel
>>>
>>> while awaiting FPGA component availability. The 10 GX
>>> development boards are pretty expensive.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> -- Tom, N5EG
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
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>>>
>>>
>>>
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>>>
>>>
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>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>>
>>>
>>> Please follow the HamSCI Community Participation
>>> Guidelines at
>>> http://hamsci.org/hamsci-community-participation-guidelines
>>> <http://hamsci.org/hamsci-community-participation-guidelines>.
>>>
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