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

[Charles Manning]

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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