Short 14 millisecond TETRA pulses not consistently detected in SDR Sharp

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ramusters

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Hi everyone,

I am using SDRSharp with an Airspy Mini running at 10 MSPS and I am using 64x decimation. I am tuned to 384.250 mHz and am expecting to receive a pulse from a portofone (transceiver) every 4 seconds. I am using an appropriate dipole TETRA antenna (for the frequency of 380.25 mHz). However I am not always seeing a pulse in the waterfall graph. Sometimes it shows a couple of pulses 4 seconds apart and other times it doesnt show a pulse for like 20 seconds

I don't understand why I am not seeing every pulse. Is it bad reception from the antenna? Is it something to do with the ADC? Is 10 MSPS sampling rate too low for the 380.25 mHz frequency? Or are the 14ms pulses to short to detect consistently?
 

SCPD

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are the 14ms pulses to short to detect consistently?
Likely so.

That's 1 / 71th of a second. The lines in the waterfall are a running average of the FFT bins. On top of that, the scroll rate may only update 10 or 20 times a second - further diluting the short spike you expect to see.

Try this: set up AM or FM demodulation at 25 khz with a squelch level that is full quite when there is no spike. Instead of looking for the spike, you may hear it as a brief pop.
 

ramusters

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Hi,

I just tested this. I tuned to 384.250 mHz and set squelch to be just silent without a signal. But I did not receive audio signals every 4 seconds when the pulses would show almost every 4 seconds.
I did not get any audio signals. Even when the yellow lines every 4 seconds in the FFT display would show, indicating a pulse, I would not get an audio signal. Tried bandwidth of 25kHz (same width as pulse) 32kHz (wider) and 2kHz (smaller). Even tried squelch to zero and wait for a pulse to hear if the white noise would change in a different sound

Attached a screenshot of the pulses:
 
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ramusters

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I am trying to detect the presence of a transceiver that transmits this TETRA 'synchronization burst' . Ithen want to program a script to light up up to 8 leds depending on the strength of the signal

right now I have a 31% succes rate on the periodic 4-second signal but i want to receive the pulse every 4 seconds
 

slicerwizard

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I am trying to detect the presence of a transceiver that transmits this TETRA 'synchronization burst' . Ithen want to program a script to light up up to 8 leds depending on the strength of the signal

right now I have a 31% succes rate on the periodic 4-second signal but i want to receive the pulse every 4 seconds
You should be processing the actual I/Q stream. I/Q data tells you if a signal is actually present and how strong it is.
 

ramusters

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for?
Tetra is Encrypt,
Greetings from Vienna
Thank you. I don't need to decrypt it, I just want to detect it

Slicerwizard, thank you for the tip. I will look into that. proabably with another program like GNURadio I can export just the output from IQ stream to detect pulses above the noise floor threshold (?)
 

ramusters

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actually dont need the bursts specifically. Want to detect the presence of tetra signals (sync pulses + communication) and the strength of these signals as a proxy for the distance. So that if an ambulance or police vehicle nears you and wants to overtake, you get a signal in advance
 

GTO_04

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actually dont need the bursts specifically. Want to detect the presence of tetra signals (sync pulses + communication) and the strength of these signals as a proxy for the distance. So that if an ambulance or police vehicle nears you and wants to overtake, you get a signal in advance
Would it be possible to do the same with a P25 system? Does it have these same sync pulses?

GTO_04
 

Your_account

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Does Puls are generated by the "Cell Tower" ?!
Why not search on the Upstream Channels? (w/o decode)
 

morfis

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actually dont need the bursts specifically. Want to detect the presence of tetra signals (sync pulses + communication) and the strength of these signals as a proxy for the distance. So that if an ambulance or police vehicle nears you and wants to overtake, you get a signal in advance
Isn't that why they use bright blue flashing lights and noisy sirens? Seems these days people don't notice either of those so I can't see why they'd bother going to the trouble of installing some extra equipment into their vehicle to be able to detect an EMS unit they should already be seeing and hearing.
 

Your_account

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Nope the dont until your got cadged.
The Police here in Europe use unmarked Cars to.
 

ramusters

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Hi everyone,

I found out why I am not receiving a pulse every 4 seconds. It is because a pulse is not transmitted every 4 seconds. Turns out the hardware used by police does not transmit every four seconds. Fire department trucks do transmit every four seconds and I was able to record pulses (nearly) every four seconds here.

Thanks for the replies
 
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