[TangerineSDR] [HamSCI] RE: Notes from PSWS / TangerineSDR call of 07-26-2021

Julius Madey hillfox at fairpoint.net
Wed Jul 28 16:12:00 EDT 2021


Jonathan,
You can use a standard silicon diode like the 1N4000 series (1A rating) 
for the polarity protection diode unless you want to clamp the reverse 
voltage at less than 0.7-0.8 volts, in which case a Schottky will 
provide a lower clamping voltage.  Since the reverse protection diode 
will clamp reverse transients, you don't need a bidirectional TVS.
Jules


On 7/28/2021 3:16 PM, Jonathan wrote:
> Phil,
>
> This design (Integrated VLF antenna/preamp in PVC pipe with Raspberry 
> Pi box) was used for an MIT study and the author of vlfrx-tools built 
> a bunch of these antennas and boxes for those conducting the study. 
> This was in 2018. Since this might have fallen under your umbrella, 
> did you hear of it or know who the PI might be? The DC-DC converter 
> has an operating frequency of 100-600 kHz, so it doesn’t really affect 
> anything below that. Paul really liked the performance of his design. 
> He even wrote a guide on how to set up the Radpberry Pi as a VLF SDR 
> using his vlfrx-tools software.
>
> Jules,
>
> I really appreciate the supplied schematic. Should I use the Schottky 
> diode where you drew the regular diode, in parallel like you said? I 
> could also probably solder the MLCC across the leads of the 
> electrolytic and install it. Also, that TVS is bidirectional. Should a 
> unidirectional TVS be used so it clips any negative spikes closer to 0V?
>
> David,
>
> Tom mentioned the same. I’ll be working on trying to fit an 
> electrolytic and a ceramic capacitor.
>
> Khan,
>
> This is good news. Email me if you have issues.
>
> Jonathan
> KC3EEY
>
>
>
> On Jul 28, 2021, at 2:27 PM, Julius Madey <hillfox at fairpoint.net 
> <mailto:hillfox at fairpoint.net>> wrote:
>
>> Jonathan,
>> I still think you need to get rid of the series polarity protection 
>> diode and suggest the following schematic (with 22uf 50 volt Aluminum 
>> electrolytic)
>>
>>
>> <eaeficdplljflikc.png>
>>
>> The PTC poly fuse will open either on overload, sustained voltage 
>> above TVS rating or reverse polarity.  This will also snub transients 
>> on the line the series diode may have permitted.  There might be some 
>> virtue in adding an MLCC cap of ~ 0.47uf in parallel with the 
>> electrolytic for lower overall ESR.
>>
>> Regards,
>> Jules-K2KGJ
>>
>>
>>
>>
>>
>>
>> On 7/28/2021 12:41 PM, Jonathan 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
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>         -- 
>>>>>
>>>>>
>>>>>         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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>>>>>         <https://groups.google.com/d/msgid/hamsci/SA0PR03MB5547CBF733EFEB4D829E659FF2E99%40SA0PR03MB5547.namprd03.prod.outlook.com?utm_medium=email&utm_source=footer>.
>>>>>
>>>>>
>>> -- 
>>> Please follow the HamSCI Community Participation Guidelines at 
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>>

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