[OZAPRS] Brisbane APRS

Tue Oct 31 21:07:49 AEDT 2017

by "analog radios" I mean "voice traffic"

On 10/31/17, Carlos Peco-Berrocal <carlos.peco at gmail.com> wrote:
> 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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