[Hackrf-dev] Narrowband fm - what is the secret?

[Cinaed Simson]

On 09/15/2016 08:03 AM, Kevin Reid wrote:
> On Sep 14, 2016, at 19:55, Cinaed Simson <cinaed.simson at gmail.com> wrote:
>> Hi - neither APRS or AFSK are FM modulated.
>>
>> AFSK is digital and uses 2 frequencies - FM is analog and uses a wide
>> range of frequencies.
>>
>> APRS is packet based and you need to decode it - which is what your
>> handset is doing.
> 
> This is not correct.
> 
> “A”FSK (as opposed to FSK) refers to FSK whose two frequencies are in the audio range, and in this context specifically placed into the audio input/output of an FM transmitter/receiver.

2 audio frequencies are 2 frequencies.
> 
> APRS consists of digital data on a specific AFSK modulation. Gavin Jacobs is talking about _listening_ for that audible AFSK signal -- it has an easily recognizable sound and should not resemble silence with a narrow-band FM demodulator. From everything I read, they are doing everything right.

You can demod the APRS signal as AM, FM, LSB, USB, CW-L or CW-U - they
will all make sounds when the APRS signal is present.

> 
>> On 09/14/2016 09:03 AM, Gavin Jacobs wrote:
>>> Yesterday (thanks to help here on this mail list) I got my hackrf One
>>> working as an FM receiver. I have SDR# v1.0.0.1483 running on Windows
>>> 10. So, today I want to move on to narrowband fm. I picked APRS channel
>>> because there is steady traffic. I tuned SDR# to 144.39 MHz, NFM,
>>> bandwidth 8k, LNA gain 32, VGA gain 24, amp off (also tried on), squelch
>>> off, antenna adjusted to 52 cm.
>>>
>>> If I key my handheld, I see a big spike on the frequency; and if I zoom
>>> in, it appears centered on 144.39.  Meanwhile, on my handheld, I can
>>> hear the AFSK tones, so I know there are signals coming in, but the
>>> hackrf/SDR# doesn't change (neither frequency nor audio).
> 
> If I read your description correctly, then you're saying that
> * you can hear APRS packets on your handheld, and
> * you can see a signal from your handheld on your HackRF One, but
> * you cannot see or hear APRS packets using the HackRF One.
> 
> This suggests that the signal is sufficiently weak that it is below the HackRF One's receive noise floor, but not for your handheld. Then, some possibilities:

But he indicated there was a "big spike" in the frequency - by big I
presume it was well above the noise floor. In fact, it was probably well
above the HackRF's DC offset.

It's unlikely it's an antenna problem - which doesn't mean it can't be
improved.

Also, is turning off the squelch equivalent to setting it to 0 dB?

Try a little squelch and see what happens.

If that doesn't work, try the same FM station as before again and see if
you can hear anything.

And as a last resort, given it's a Windows 10 machine, try rebooting it.

> 
> 1. Your HackRF One does not have a good enough antenna (for the frequency) attached to it.
> 
> 1a. Your handheld is more sensitive/more selective/... and so you will need a _better_ antenna to get the same performance.
> 
> 2. Your gain settings are too low (signals below inherent noise) _or_ too high (overload creates more noise). Find some other signal in the band (e.g. a repeater) and adjust for maximum SNR (vertical distance between the signal peak and the noise floor).
> 
> 3. Your HackRF One is damaged. Damage can be caused by overly-strong nearby signals such as transmitting with your handheld, but IIRC the most likely failure is the 14dB amplifier and that can be bypassed, so that's not the problem here given that you have the amp off. But as a general principle, don't do that unless you have an attenuator or dummy load on one or the other device.
> 
> 
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