<div dir="ltr"><div>If you are using an RTL-SDR + RPi then that is a reasonable approach. <br></div><div><br></div><div>Snapshot 1 sec of data then go from there.</div><div><br></div><div>Yup, like GPS you're probably shooting for a 2 step approach:</div><div>Acquisition: finding the signal frequency offset (similar to GPS doppler determination) + beep repeat rate (similar to GPS code).</div><div>Tracking: keeping synced to these.</div><div><br></div><div>Uncle Google should help you find some good texts.</div><div><br></div><div><a href="https://berdiansklib.blogspot.com/2011/07/the-hobbyist-guide-to-rtl-sdr-really.html">https://berdiansklib.blogspot.com/2011/07/the-hobbyist-guide-to-rtl-sdr-really.html</a></div><div><a href="https://www.rtl-sdr.com/gnss-sdrlib-real-time-gps-positioning-rtl-sdr/">https://www.rtl-sdr.com/gnss-sdrlib-real-time-gps-positioning-rtl-sdr/</a></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, Mar 22, 2020 at 6:34 PM Trevor Wignall <<a href="mailto:zl3adz@gmail.com">zl3adz@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">Yes, some form of iterative or parallel technique sounds like a workable approach.<div>I was thinking of interfacing the SDR USB dongle to a Raspberry Pi and remoting into it from a laptop to do the display. Once a signal has been acquired and locked, I am guessing the processor load can be reduced, so the processor can move on to other transmitter frequencies and keep a lock on multiple transmitters. Useful to help find people that you learn are missing some time later...</div><div>Trevor</div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, Mar 22, 2020 at 6:01 PM Charles Manning <<a href="mailto:cdhmanning@gmail.com" target="_blank">cdhmanning@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Hi Trevor</div><div><br></div><div>If you don't know the carrier frequency accurately enough then it sounds like one of those cases where you want to store the signal, then iterate over it beating with various carrier frequencies until you find one that is close to zero.</div><div><br></div><div>Similar techniques are commonly used in GNSS/GPS processing and I've played with techniques that can find satellites in 10milliseconds of snapshotted data.<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, Mar 22, 2020 at 2:09 PM Mark Atherton <<a href="mailto:markaren1@xtra.co.nz" target="_blank">markaren1@xtra.co.nz</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<div>I would start off with a PC and a
dongle capable of a waterfall display centred on the nominal
carrier, with scan width set to maybe 3kHz, then you may be able
to find the carrier.</div>
<div><br>
</div>
<div>Then you can improve centre frequency
so you can drop scan width, and IF bandwidth.<br>
</div>
<div><br>
</div>
<div>Repeat until carrier is within a couple
of Hz, and IF is maybe 20Hz wide. You now have the possibility of
a very sensitive receiver.</div>
<div><br>
</div>
<div>If you can get an SDR handheld with an
IF bandwidth of 20Hz, all you have to do is transfer the exact
frequency of the carrier, and you have a very sensitive portable
device.</div>
<div><br>
</div>
<div>In terms of automation, initially
don't. Get the hang of a process that works, and equipment that
will do the job. Once you have ironed out the fiddling to make the
system work, then you can transfer your knowledge into an
algorithm to automate the process.</div>
<div><br>
</div>
<div>Required laptop and SDR sounds easy to
set up. Handheld SDR sounds expensive / difficult.</div>
<div><br>
</div>
<div>Having said that, you could set up a
network of laptops + SDR dongles at fixed locations and manage
some kind of tri/quad/hex-angulation. Each dongle must have an
accurate and stable timebase. Laptops must be connected to the
same network.</div>
<div><br>
</div>
<div>Even a 50Hz wide IF for a CW receiver
might do the job, but you will need around 10Hz accuracy at 300Mhz
which 33 x 10-9 which sounds like a Rubidium reference standard.</div>
<div><br>
</div>
<div>The other option is to load a
calibration number into the handheld each day to adjust it's
reference which will hopefully stay put for 24 hours.</div>
<div><br>
</div>
Just wondering about a homebrew Zero-IF RX, you can easily crank
down IF bandwidth on such a device...<br>
<div>
<div><br>
</div>
<div>Hopefully some of that makes sense.<br>
</div>
<div><br>
</div>
<div>-mark<br>
</div>
</div>
<div> </div>
<div><br>
</div>
<div><br>
</div>
<div><br>
</div>
<div>On 22/03/2020 1:22 PM, Trevor Wignall
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">The frequency is around 300MHz and is known to
within a couple of kHz - later models are a bit more stable, so
perhaps within +/1 1kHz. Repetition is 1-2 seconds. So
essentially a CW (morse) transmitter sending the word "e".
Transmit power and aerial are not relevant to trying to improve
the receiver - I'll take any improvement I can get! There is a
test transmitter operating with a coverage of most of
Christchurch - clearly heard at the Idris Rd clubrooms.
<div>I don't want to put too many specifics in an open thread -
anyone seriously interested can contact me and I will give
them more specifics (after I have cleared it with the
Wandersearch Trustees).</div>
<div>Trevor</div>
<div><br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sun, Mar 22, 2020 at 12:59
PM Mark Atherton <<a href="mailto:markaren1@xtra.co.nz" target="_blank">markaren1@xtra.co.nz</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<div>Some parameters please</div>
- max and min limit of carrier frequency, or Fnom <span style="color:rgb(60,64,67);font-family:arial,sans-serif;font-size:14px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:left;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">±
Ftolerance<br>
</span>
<div>- repetition rate of the 100ms bursts ?
<div>- transmit power</div>
<div>- transmit antenna</div>
<div><br>
</div>
<div>Do you have a pendant in hand to take apart ?<br>
</div>
<div><br>
</div>
<div>So unmodulated carrier, and no ident modulation ?</div>
<div><br>
</div>
<div>- Mark<br>
</div>
</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
</div>
<div>On 22/03/2020 12:44 PM, Trevor Wignall wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi all</div>
<div>As a Wandersearch responder (i.e. someone who goes
out with a directional receiver and tries to locate
the small transmitter pendants worn by people who tend
to go wandering e.g. dementia patients), I am looking
at using a Software Defined Radio (SDR) to receive
these signals. However as the signals are weak, I am
looking for ways of extracting the short transmitted
pulses (around 100-200 milliseconds of unmodulated
carrier) from the background noise using narrow band
filtering. The problem is that the exact transmitter
frequency is not known due to tolerances, temperature
changes etc.</div>
<div>If anyone familiar with SDRs would like a challenge
to combat boredom, any help would be appreciated.</div>
<div>Trevor</div>
</div>
</blockquote>
</div>
</blockquote>
</div>
</blockquote>
<br>
</div>
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