[chbot] Planning for the next few weeks - SDR Experts?

Tue Mar 24 15:56:40 GMT 2020

Hi Trevor,
  Have you had a look at GNU-radio? It allows for easy experimenting and
visulation of signal-flows.
I'd be happy to help and give it a try if you could give me a few more
details of the test transmitter.
/synco

On Mon., 23 Mar. 2020, 08:09 Charles Manning, <cdhmanning at gmail.com> wrote:

> If you are using an RTL-SDR + RPi then that is a reasonable approach.
>
> Snapshot 1 sec of data then go from there.
>
> Yup, like GPS you're probably shooting for a 2 step approach:
> Acquisition: finding the signal frequency offset (similar to GPS doppler
> determination) + beep repeat rate (similar to GPS code).
> Tracking: keeping synced to these.
>
> Uncle Google should help you find some good texts.
>
>
> https://berdiansklib.blogspot.com/2011/07/the-hobbyist-guide-to-rtl-sdr-really.html
> https://www.rtl-sdr.com/gnss-sdrlib-real-time-gps-positioning-rtl-sdr/
>
>
> On Sun, Mar 22, 2020 at 6:34 PM Trevor Wignall <zl3adz at gmail.com> wrote:
>
>> Yes, some form of iterative or parallel technique sounds like a workable
>> approach.
>> 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...
>> Trevor
>>
>>
>>
>> On Sun, Mar 22, 2020 at 6:01 PM Charles Manning <cdhmanning at gmail.com>
>> wrote:
>>
>>> Hi Trevor
>>>
>>> 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.
>>>
>>> Similar techniques are commonly used in GNSS/GPS processing and I've
>>> played with techniques that can find satellites in 10milliseconds of
>>> snapshotted data.
>>>
>>> On Sun, Mar 22, 2020 at 2:09 PM Mark Atherton <markaren1 at xtra.co.nz>
>>> wrote:
>>>
>>>> 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.
>>>>
>>>> Then you can improve centre frequency so you can drop scan width, and
>>>> IF bandwidth.
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> Required laptop and SDR sounds easy to set up. Handheld SDR sounds
>>>> expensive / difficult.
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> Just wondering about a homebrew Zero-IF RX, you can easily crank down
>>>> IF bandwidth on such a device...
>>>>
>>>> Hopefully some of that makes sense.
>>>>
>>>> -mark
>>>>
>>>>
>>>>
>>>> On 22/03/2020 1:22 PM, Trevor Wignall wrote:
>>>>
>>>> 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.
>>>> 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).
>>>> Trevor
>>>>
>>>>
>>>> On Sun, Mar 22, 2020 at 12:59 PM Mark Atherton <markaren1 at xtra.co.nz>
>>>> wrote:
>>>>
>>>>> Some parameters please
>>>>> - max and min limit of carrier frequency, or Fnom ± Ftolerance
>>>>> - repetition rate of the 100ms bursts ?
>>>>> - transmit power
>>>>> - transmit antenna
>>>>>
>>>>> Do you have a pendant in hand to take apart ?
>>>>>
>>>>> So unmodulated carrier, and no ident modulation ?
>>>>>
>>>>> - Mark
>>>>>
>>>>>
>>>>>
>>>>> On 22/03/2020 12:44 PM, Trevor Wignall wrote:
>>>>>
>>>>> Hi all
>>>>> 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.
>>>>> If anyone familiar with SDRs would like a challenge to combat boredom,
>>>>> any help would be appreciated.
>>>>> Trevor
>>>>>
>>>>>
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