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

[Trevor Wignall]

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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