[OZAPRS] Brisbane APRS

Tue Oct 31 21:06:37 AEDT 2017

Hi Glen,

Yes, modern (reputable) radios tend to exhibit a very little deviation
at the beginning of the transmission. However, at the end of the day
you are after some sort of "uniqueness" and you don't need to know if
that little artifact was due to the synthesizer or due to some
transient in the audio chain.

And this is not unique to analog radios... as the OP mentioned, radios
doing AFSK in APRS can be uniquely identified. I also noticed that one
of my DMR radios (of a certain brand, probably due to the chipset they
use) transmits symbols out of the 4FSK constellation every now and
then, like if it had some internal overflow (observed in the eye
pattern as deviation well below and above the upper and lower symbol).
And some Chinese radios transmit a short subaudible tone at the end of
the transmission, like a 55 Hz roger beep (the same Chinese radios
will mute the received audio if someone transmits that sub-audible
tone)

And this is obvious but... if you tune to 145.175 you can find
straight away who is doing pre-emphasis and who is not, and who is
overdriving/underdriving the audio input, who has a faulty power
supply...

This SDR thing is great !

On 10/31/17, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
> actually Carlos, I wonder how useful the frequency chirp is now with the
> advent of fractional-N synthesisers with their MHz wide loop bandwidth,
> the radios are generally well and truly settled by the time the TX is
> turned on....
>
>
> On 31/10/2017 3:48 PM, Andrew Rich wrote:
>> I used to check RADAR encoders with a logic analyser
>>
>> And change valves
>>
>> I used to work MIR on C64 and a Baycom modem
>>
>> Those were the days before SMART phones
>>
>> Andrew
>>
>>
>>> On 31 Oct 2017, at 2:09 pm, Glen English VK1XX
>>> <glenlist at pacificmedia.com.au> wrote:
>>>
>>> yeah, I  used to do TX key-up freq chirp characteristic /amplitude
>>> envelope fingerprinting in the mid 1980s with a microbee and parallel
>>> port ADC...
>>>
>>> cheers
>>>
>>>
>>> On 31/10/2017 2:55 PM, Carlos Peco-Berrocal wrote:
>>>> Hi Glen,
>>>>
>>>> The transient at the beginning/end of the transmission is unique. I
>>>> even compared two portable radios and differences in battery level and
>>>> temperature made them drift differently during the transmission
>>>> period.
>>>>
>>>> Those transients can be recorded and then new transmissions are
>>>> correlated with the entries in your database. There are standards for
>>>> that database so you can share fingerprints with "your mates".
>>>>
>>>> I found a couple of photos, I don't know if the link to imgur will work:
>>>>
>>>> https://imgur.com/a/kTJ4S
>>>>
>>>> That repeater was off frequency few hertzs, you can see the vertical
>>>> pink lines (markers).
>>>>
>>>> The last image was HF, there are lots of interesting details in the
>>>> waterfalls.
>>>>
>>>>
>>>> If you monitor Canberra Approach you'll see the air traffic controller
>>>> in the center (GPS disciplined radio from some Germans you know) with
>>>> the aircraft appearing at the sides. The automatic simultaneous
>>>> transmission detection available in the current generation of ATC
>>>> radios from the same German brand exploits that: if two simultaneous
>>>> transmissions occur, it is highly likely that their instantaneous
>>>> frequencies will be few hertz apart so they will appear as two peaks
>>>> in the FFT of the complex envelope.
>>>>
>>>> https://www.youtube.com/watch?v=Y983aZelEkM
>>>>
>>>> Back in the days of CB27 in AM you could hear a beating tone when two
>>>> transmissions happened at the same time and had power levels in the
>>>> same range (if one transmission is MUCH stronger than the other,
>>>> you'll hear nothing but the strongest transmission).
>>>>
>>>> Sorry for the long post, I hope you find it interesting. Happy to take
>>>> some toys to your place so you can tinker a bit with this.
>>>>
>>>> cheers
>>>> Carlos VK1EA
>>>>
>>>>
>>>>
>>>> On 10/29/17, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
>>>>> hmm
>>>>>
>>>>> how much sag is that  in kHz, or even better Hz/sec ?
>>>>>
>>>>>
>>>>> On 29/10/2017 6:35 PM, vk4tec at tech-software.net wrote:
>>>>>> http://59.167.159.165/aprs.jpg
>>>>>>
>>>>>> -----Original Message-----
>>>>>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen
>>>>>> English
>>>>>> VK1XX
>>>>>> Sent: Sunday, 29 October 2017 5:00 PM
>>>>>> To: ozaprs at aprs.net.au
>>>>>> Subject: Re: [OZAPRS] Brisbane APRS
>>>>>>
>>>>>> Andrew, there are a few checkpoints to this
>>>>>>
>>>>>> 1) I gather you know precisely what frequency error your spectrum
>>>>>> analyser
>>>>>> has.
>>>>>>
>>>>>> 2) I gather your spectrum analyser , if FFT / digital  type, has
>>>>>> sufficient resolution bandwidth to tell a non aliased story.
>>>>>>
>>>>>> 3) It is not usual for station's TX  PLL /synthesisers to not be fully
>>>>>> settled during the keyup phase, and take 30- 100mS to establish fine
>>>>>> frequency lock .
>>>>>>
>>>>>> 4) I would expect +/- 5ppm for most radios, that being about +/- (5 x
>>>>>> 145.175 Hz)
>>>>>>
>>>>>> 5) It's probably not going to hurt much until +/- 1kHz, and not really
>>>>>> hurt until +/- 2kHz, depending on the TX deviation. (IE how far
>>>>>> outside
>>>>>> the Receiver IF filter the modulation protrudes, as the received
>>>>>> signal
>>>>>> will be asymmetrical in the receiver's IF passband, the higher the
>>>>>> deviation, the greater the extremes)
>>>>>>
>>>>>> How far out are they ?
>>>>>>
>>>>>> -glen
>>>>>>
>>>>>>
>>>>>> On 29/10/2017 5:51 PM, vk4tec at tech-software.net wrote:
>>>>>>> Hello
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I just had a quick look on 145.175 MHz on a spectrum analyser.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I can see stations off frequency
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I can see transmitters shifting frequency on keyup
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Andrew
>>>>>>>
>>>>>>>
>>>>>>>
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