RF Quiet Wall warts

Piotr Wyderski <peter.pan@neverland.mil> wrote:
> Rob wrote:
>
>> I know, I have clocks like that and I am seeing all the time how they
>> have more and more difficulty receiving the signal.
>
> But they are just consumer products, they can fail without many
> serious implications. Remote meter switching for the purpose of
> proper bill calculation is another story, can end with a lawsuit.

But is this system still in active use and planned to remain so?
Over here, meter switching traditionally was done using audo frequency
switching (superimposed on the mains itself), but this is being phased
out and replaced by systems for remote meter readout over GPRS.
The support for day/night switching on traditional meters will end in
about a year, from then on passive meters will only offer single-tariff.

>> So, I now use GPS as a reference.
>
> GPS requires direct sky visibility, which sometimes is a serious
> problem. The VLF signals can be received easily deep in the concrete
> buildings, it's just the noise that makes it hard to decode.

It is not so bad as it used to be.  GPS receivers can operate in
a building, certainly when only used for network-grade time referencing.

VLF in concrete buildings never worked great, it sort of worked but
with limitations.  And interference from SMPS is not the first problem,
in the past interference from CRT monitors was also a big problem.

With a suitably placed outdoor antenna it will likely still work fine.
(stronger signal and weaker interference)

On 27 Aug 2017 08:38:51 GMT, Rob <nomail@example.com> wrote:

>So, I now use GPS as a reference.

And what do you do when the GPS band noise level increases, swamping
the GPS signal. After all, the despread SNR is quite low to begin
with.

Just what is going to happen on the ISM bands, such as 2.45 GHz when
more and more devices are installed in that band. A specific WLAN link
might work today at the desired distance, but what happens next week
when one new devices are introduced ? The link range may drop
significantly and there is really much you can do about it.

On Sat, 26 Aug 2017 22:50:24 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>Rob wrote:
>
>> This is also the reason why nobody cares about the interference from
>> wall warts.  The AM (HF) bands are no longer used by the public.
>
>Germany and a fair fraction of Europe work thanks to the AM
>transmissions (DCF77@77.5kHz for time broadcast, DCF49@129kHz
>for remote control of power meters etc.). Unfortunately, the
>lower LF band is where the EMI is highest. There are PM signals
>too, but no consumer-grade devices use that due to the involved 
>complexity. There is also LORAN, but it is not very useful on land.
>
>	Best regards, Piotr

In the US, the problem is that WWVH at 60.0kHz also transmits BPSK
data, the technology is tied up by patents.  The patent holder,
Xtendwave (Everset Technologies), was suppose to provide a suitable
chip but hasn't done anything since it was introduced.  Cut-n-pasted
from something I posted to S.E.D. last year on the topic with a few
additions and corrections:

(...)
<http://www.xtendwave.com>
<http://everset-tech.com>
The mythical ES1000 chip:
<http://everset-tech.com/wp-content/uploads/2014/11/ES100DataSheetver0p97.pdf>
<http://news.thomasnet.com/companystory/Xtendwave-Selects-EnSilica-s-eSi-3200-Processor-for-the-Receiver-IC-Implementing-NIST-s-Next-Generation-WWVB-Atomic-Timekeeping-Signal-616104>
Patents and applications:
<https://www.google.com/patents/US20130121397>
<https://www.google.com/patents/US8270465>
<https://www.google.com/patents/US20120082008>
<https://www.google.com/patents/WO2013074789A3>
and a whole bunch more patents and applications:
<https://www.google.com/search?tbm=pts&q=xtendwave>
Patent ownership is now in the hands of Grindstone Capital.  See
"Legal Events" at the bottom of the page on most of the patents.
<http://www.plainsite.org/profiles/grindstone-capital-llc/>
(click on "patent assignments" tab).
Unless something has changed in the last year, at the current rate of
progress, I don't know if we'll ever see PM chips.  If anyone has some
up to date info on the topic, I would be very interested.

Switching to BPSK offers a 10dB processing gain.  Accumulating an
hours worth of data will add maybe another 18dB of processing gain.
Since most of the switching power supply noise and interference
appears as amplitude modulation, switching to a phase modulated system
will also add quite a bit of noise immunity.  I recall seeing some
info on the topic in the Time-Nuts mailing list, but I can't seem to
find it.  

The basic "interference" problem is that it takes a full minute to
obtain one full frame, and two or three frames to determine that the
received data is valid.  Three seconds is a very long time to expect
noise free reception.  Simply flipping a nearby light switch will
produce a small arc, which will disrupt reception.

A rather complex do it thyself WWVB BPSK receiver:
<http://www.arrl.org/files/file/QEX_Next_Issue/2015/Nov-Dec_2015/Magliacane.pdf>

Some of my playing around with WWVB antenna polarization and
orientation showing the effects of cross polarization:
<http://www.learnbydestroying.com/jeffl/WWVB%20test/>
My comments on the photos:
<https://groups.google.com/d/msg/sci.electronics.repair/MqjLM2aTNs8/5_kBf3NKNTMJ>

The LORAN C system was turned off in the US in 2014 and in most of
Europe in 2015:
<https://en.wikipedia.org/wiki/Loran-C#Loran-C_in_the_21st_century>


-- 
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

upsidedown@downunder.com <upsidedown@downunder.com> wrote:
> On 27 Aug 2017 08:38:51 GMT, Rob <nomail@example.com> wrote:
>
>>So, I now use GPS as a reference.
>
> And what do you do when the GPS band noise level increases, swamping
> the GPS signal. After all, the despread SNR is quite low to begin
> with.

It isn't.  You must be referring to the SNR before despreading.
GPS is at 1575 MHz.  It will not be affected soon by the kind of
interference we are talking about.

> Just what is going to happen on the ISM bands, such as 2.45 GHz when
> more and more devices are installed in that band. A specific WLAN link
> might work today at the desired distance, but what happens next week
> when one new devices are introduced ? The link range may drop
> significantly and there is really much you can do about it.

That is what you get on ISM bands.  Free spectrum but no allocation
policy and no guarantee of interference-free use.

The part of the ISM band that overlapped with the 13cm HAM band has
been declared receive-and-satellite-uplink-only here.  (2400-2450)
(to reduce the risk of interference from HAMs to WiFi)

On Sun, 27 Aug 2017 18:20:19 +0300, upsidedown@downunder.com wrote:

>On 27 Aug 2017 08:38:51 GMT, Rob <nomail@example.com> wrote:
>
>>So, I now use GPS as a reference.
>
>And what do you do when the GPS band noise level increases, swamping
>the GPS signal. After all, the despread SNR is quite low to begin
>with.

You won't have long to wait for that to happen in the US.  A few years
ago, Lightsquared donated enough money to the Democrats to convince
the FCC to allocate the spectrum they had won at auction to
terrestrial cellular service.  Unfortunately, the channels were
adjacent to the GPS frequencies.  Also unfortunately, GPS receivers
have rather wide receiver front ends[1] making them susceptible to
interference from nearby services.  The outcry by GPS users was
sufficient for the FCC to table the matter and for Lightsquared to
back off, for a while.

Well, they're back as Ligado Networks:
<http://ligado.com>
<https://www.washingtonpost.com/news/the-switch/wp/2017/05/05/this-company-wants-to-build-a-wireless-network-for-drones-trains-and-automobiles/>
They've spent the last 5 years doing "research" and are offering a new
grand plan to any company or organization that had previously
protested their scheme.  They've also continued to "support" the
proper political party and office holders.  Since the FCC is no longer
interested in what the public needs, I predict that Ligado will get
approval to use their frequencies.  Since the FCC is no longer in the
enforcement business, I can also predict that any interference reports
will be ignored.  Say goodbye to GPS in metro areas.

My guess(tm) is that they will use terrestrial repeaters to extend
coverage, exactly like Sirus/XM does.  These are NOT low power devices
and are intended to fill in coverage holes that are inaccessible by
satellite.  The problem is that most of metro coverage for Sirus/XM
comes from these repeaters, not satellite.  I expect much the same
from Ligado where the satellite system is simply an excuse to build a
terrestrial network.

>Just what is going to happen on the ISM bands, such as 2.45 GHz when
>more and more devices are installed in that band. A specific WLAN link
>might work today at the desired distance, but what happens next week
>when one new devices are introduced ? The link range may drop
>significantly and there is really much you can do about it.

Yep, it's a problem.  The surest sign of success is pollution and WiFi
is certainly successful.  The unlicensed outdoor long range links are
the most susceptible.  It's impossible to get any kind of long range
link on 2.4GHz.  5GHz is still possible, but only with high gain
antennas to provide the best possible SNR and to reduce interference
from the sides and back.  I don't know how long that will last.  There
are a few such links that have already moved up to the next available
band at 24GHz.
<https://www.ubnt.com/airfiber/airfiber24/>


-- 
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Jeff Liebermann <jeffl@cruzio.com> wrote:
> The basic "interference" problem is that it takes a full minute to
> obtain one full frame, and two or three frames to determine that the
> received data is valid.  Three seconds is a very long time to expect
> noise free reception.  Simply flipping a nearby light switch will
> produce a small arc, which will disrupt reception.

You mean 3 minutes.  With DCF77 there is more possibility for checking
as there are parity bits and also the values are in BCD and have a limited
value range within that (e.g. minutes field can only be 00..59)

Usually a receiving system has "some idea of the correct time" and can
determine with reasonable certainty if a received datagram is correct.
Receiving multiple datagrams of course can improve that certainty, but
they do not have to be consecutive for that.

Still, it can be tricky to obtain a clean datagram under conditions of
interference.

Jeff Liebermann <jeffl@cruzio.com> wrote:
> Yep, it's a problem.  The surest sign of success is pollution and WiFi
> is certainly successful.  The unlicensed outdoor long range links are
> the most susceptible.  It's impossible to get any kind of long range
> link on 2.4GHz.  5GHz is still possible, but only with high gain
> antennas to provide the best possible SNR and to reduce interference
> from the sides and back.

But that is as it should be!  With the limited number of channels
available, it should be considered criminal to try to setup an outdoor
long range link without using high gain antennas with small sidelobes.

On 27 Aug 2017 16:32:07 GMT, Rob <nomail@example.com> wrote:

>Jeff Liebermann <jeffl@cruzio.com> wrote:
>> The basic "interference" problem is that it takes a full minute to
>> obtain one full frame, and two or three frames to determine that the
>> received data is valid.  Three seconds is a very long time to expect
>> noise free reception.  Simply flipping a nearby light switch will
>> produce a small arc, which will disrupt reception.

>You mean 3 minutes.

Oops.  Side effect of talking on the phone while writing all that.

>With DCF77 there is more possibility for checking
>as there are parity bits and also the values are in BCD and have a limited
>value range within that (e.g. minutes field can only be 00..59)

The current AM WWVB coding does not include any parity bits:
<https://en.wikipedia.org/wiki/WWVB#Amplitude-modulated_time_code>
The new PM WWVB encoding includes 5 parity bits and repeats one LSB:
<https://en.wikipedia.org/wiki/WWVB#Phase_modulated_time_code>
More than you probably wanted to know about the protocol used:
<https://www.nist.gov/sites/default/files/documents/2017/05/09/NIST-Enhanced-WWVB-Broadcast-Format-2012-09-26-2.pdf>

The most common AM time receiver chips by C-Max use an algorithm that
requires 2 or 3 sequential successful frames before the clock will be
updated:
<http://www.c-max-time.com>
See the "compare both telegrams" box in the flow chart:
<http://www.c-max-time.com/tech/software6005.php>

>Usually a receiving system has "some idea of the correct time" and can
>determine with reasonable certainty if a received datagram is correct.
>Receiving multiple datagrams of course can improve that certainty, but
>they do not have to be consecutive for that.

There is no storage in the C-Max chipset.  There are receivers with
storage capabilities, but those use the storage for error correction
and accuracy improvement by accumulating previous frames.

>Still, it can be tricky to obtain a clean datagram under conditions of
>interference.

Tricky? no.  Expensive? maybe.  Messy? probably.

For example, how hard would it be to throw together a diversity
reception system using two DCF77 receivers that are geographically
separated?  With data arriving at the screaming rate of 1 bit/second,
there's plenty of time to perform computation on the probability that
the bit is a one or a zero.  The disk drive manufacturers do that
because the data from the head looks more like garbage than data. Lots
of other error correction tricks can be used, all of which require
storage of previous data and adequate computation time.

Incidentally, a problem with using WWVB as a reference is that it
doesn't really make a good stabilized clock oscillator.  There were
WWVB disciplined oscillators available:
<http://www.ebay.com/itm/Spectracom-8164-NBS-Receiver-Disciplined-Oscillator-/172838067755>
but they were replaced by GPSDO's long ago.


-- 
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

On 27 Aug 2017 16:27:06 GMT, Rob <nomail@example.com> wrote:

>upsidedown@downunder.com <upsidedown@downunder.com> wrote:
>> On 27 Aug 2017 08:38:51 GMT, Rob <nomail@example.com> wrote:
>>
>>>So, I now use GPS as a reference.
>>
>> And what do you do when the GPS band noise level increases, swamping
>> the GPS signal. After all, the despread SNR is quite low to begin
>> with.
>
>It isn't.  You must be referring to the SNR before despreading.

I am referring to the despread SNR. Going from the 1.023 MHz chip rate
to 1 kHz carrier gives about 30 dB processing gain. The GPS data is
transmitted at only 50 bit/s, with a further processing gain of 13 dB.

Detecting this 50 bit/s will require a few dB positive SNR. 

Counting backwards, the on-air SNR in 1.023 MHz bandwidth must be
better than -40 dB. 
 
>GPS is at 1575 MHz.  It will not be affected soon by the kind of
>interference we are talking about.

Just add a large number UWB sources and the noise level will increase
all over.

On 27 Aug 2017 17:02:44 GMT, Rob <nomail@example.com> wrote:

>Jeff Liebermann <jeffl@cruzio.com> wrote:
>> Yep, it's a problem.  The surest sign of success is pollution and WiFi
>> is certainly successful.  The unlicensed outdoor long range links are
>> the most susceptible.  It's impossible to get any kind of long range
>> link on 2.4GHz.  5GHz is still possible, but only with high gain
>> antennas to provide the best possible SNR and to reduce interference
>> from the sides and back.

>But that is as it should be!  With the limited number of channels
>available, it should be considered criminal to try to setup an outdoor
>long range link without using high gain antennas with small sidelobes.

Criminal?  Are you planning to prosecute someone for failing to use
good engineering practices?  I wouldn't mind seeing that because it
would put most of my competitors in jail.

Actually, the FCC "encourages" users of point to point links to use
big high gain antennas.  Part 15.247 describes how the gain of a
directional antenna can be increased by 3dB for every 1dB drop in
transmit power.  That grinds out to a 24 dBm transmitter and a 24 dBi
antenna, which produces 48 dBm EIRP or 62 watts EIRP.

Note that the interference problem with high gain antennas and point
to point links is along the line of sight of the antenna, not to the
sides or back.  Along the line of sight, the gain of the antenna is
highest.  It's very likely that some other wi-fi transmitter is
located along this line of sight, usually behind the intended other
end of the wireless link.  I have a 400ft link between my house and
one of the neighbors.  When I first set it up, I failed to notice that
a local mountain top radio site was directly in line with my antenna.
The mountain top is about 8 miles away and is full of interference
sources.  Nothing would work until I reduced my antenna gain and moved
the link.

Incidentally, for point to point wireless bridges, I like to use
Ubiquiti PowerBeam PBE-M5-400 radios at about $100/end.
<https://dl.ubnt.com/datasheets/powerbeam/PowerBeam_DS.pdf>
There are 3 different sizes of antennas and some options to deal with
the side lobes.  As long as I pick my channels carefully, I don't have
much trouble with interference from other users of the 5GHz band.  The
built in spectrum analyzer software is a big help.

Fun and games:
<https://en.wikipedia.org/wiki/Long-range_Wi-Fi>


-- 
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558