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

Ward Silver hwardsil at gmail.com
Wed Jul 28 16:46:45 EDT 2021


Blocking common-mode RF current using ferrite chokes is a good technique.
So is using shielded cable for everything and metal enclosures bonded
together.  Jim has several tutorials about this available on his web page (
k9yc.com/publish.htm) - see the slide show about reducing received noise.
This was an evolution of a presentation on common-mode chokes (
http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf) by
Chuck Counselman, W1HIS in the context of reduced received noise. (The
presentation was created before Type 31 ferrite became available which is a
much better material for EMI suppression.)

73, Ward N0AX

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On Wed, Jul 28, 2021 at 3:32 PM Julius Madey <hillfox at fairpoint.net> wrote:

> 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> 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> 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> 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> 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.
>>>> [image: 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)
>>>> [image: 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> wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Thank you, Tom.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> The recording will be available later today at
>>>>
>>>> https://youtu.be/AammohuygMw and hamsci.org/telecons.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> 73 de Nathaniel W2NAF
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> *From:* TangerineSDR <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>
>>>>
>>>>
>>>> *Cc:* Tom McDermott <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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>>>>
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>>
>>>> Please follow the HamSCI Community Participation Guidelines at
>>>> http://hamsci.org/hamsci-community-participation-guidelines.
>>>>
>>>>
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