Noise from speaker

[Deker]

I think the interference is definitely from the PAW, as the noise is perfectly in tune with the flashing LEDs on startup.  It then settles down to some static, plus a 5 hz square-wave pulsation.

I think that Roger is saying there is also intermittent 'click / buzz' at 5 x per sec (5Hz) as well as the 50hz noise??

Deker

[rogellis]

50hz to me says 'earth loop'

Sorry, the mention of 50 hz was just a guess. 
I found this noise generator online and was able to precisely duplicate the buzz, and it is actually a square waveform 5 hz. 
http://onlinetonegenerator.com/

This might suggest the PAW transmitter, which I understand works at about 5 hz.  There is also a background static hiss.
Someone said a PAW pulse every 1.8 seconds, but I was told that PAW transmits 6 times a second.


As a reminder, when the amp is working on battery, there is no noise at all. 
And when I switch to working from the Raspberry USB 5v output, the noise is halved.
And the other PAW unit has the same noise, but much quieter.
I tried using a different dc converter, but the noise remained.


Will a suppressor (capacitor) on th 5v input work?  - Like we used to place on car ignition-coil systems? 
It was a simple solution to a buzzing radio.
If so, what size capacitor should I look for?

Thanks,
R

[Paul_Sengupta]

A clicking sound rather than a buzz?

[Ian Melville]

As a reminder, when the amp is working on battery, there is no noise at all. 

is that with a running PAW connected?

[rogellis]

A clicking sound rather than a buzz?

A 5 hz click, just like that online tone generator on square wave mode.
Also a background hiss, but this is not as prominent and annoying as the click.
That is ONLY with the PAW connected and running.
With either non amp/speaker connection (battery supply) or no PAW running, there is perfect silence.


On startup, the speaker gives various bleeps and clicks, perfectly in tune with the PAW LED flashes.
When up and running, it gives the 5 hz clicks and background buzz.


R

[JCurtis]

If you leave the GPS and WiFi dongles off do you still have the click? Could easily be conducted emission from one of the other devices plugged into the Pi

[rogellis]

If you leave the GPS and WiFi dongles off do you still have the click?
Could easily be conducted emission from one of the other devices plugged into the Pi

Ok, got around to some more testing at last.

The primary noise is coming from the ADSB aerial, or rather from the dongle.  If I disconnect the ADSB aerial, the noise remains, but if I disconnect the ADSB dongle the vast majority of the noise instantly goes away. 

So I am presuming that different ADSB would solve the problem.  The other PAW unit makes a similar noise, but much quieter, as if there is some variability in these units.

As an aside, the ADSB connector really need a locking mechanism for aircraft usage, as the aerial is forever falling out of the dongle.


(On its own, the PAW unit has a soft 0.5 hz heartbeat, which is oddly comforting....)


Thanks,
R

[rogellis]

How does one buy a new ADSB aerial?
I cannot see one on PAW hardware, Amazon, or Ebay.

Is it the Noor Nano-2?
https://www.amazon.co.uk/dp/B018PUYPCA

Cheers,
R

[Ian Melville]

I would have said it was the ADSB dongle that is generating the noise, the aerial is just broadcasting it. Best to replace both dongle and aerial, which will come with the dongle anyway.

There seems to be a lot of variability in these units, evidenced by some running hotter than others.

[Paul_Sengupta]

The ADS-B dongles are these:

https://www.ebay.co.uk/itm/Mini-USB-DVB-T-RTL-SDR-Realtek-RTL2832U-R820T-Stick-Receiver-Dongle-MCX-Input-PK-/222637370515

They come with an aerial. If you just want the aerial:

https://www.ebay.co.uk/itm/High-Gain-Signal-5dBi-Antenna-Aerial-Digital-Freeview-For-DVB-T-TV-HDTV-Black/112435558439

The antenna plug should be a fairly snug fit. They don't take too kindly to frequent plugging and unplugging though. If it's was loose when it arrived from PilotAwareHardware, get Dave to swap it.

[rogellis]

I would have said it was the ADSB dongle that is generating the noise, the aerial is just broadcasting it. Best to replace both dongle and aerial, which will come with the dongle anyway.

There seems to be a lot of variability in these units, evidenced by some running hotter than others.

I thought the ADS-B aerial was receive only? 
Why does it seem to be working much harder than the PAW aerial, which is transmitting? 
A portable FM radio does not work very hard, and does not use so much power....

Cheers,
R

[JCurtis]

These dongles are Software Defined Radios, and are pretty power hungry and inefficient, however they get the job done and are as cheap as anything as they are churned out in huge numbers.  Unlike an "normal" radio they do a lot of signal processing to extract the signal of interest and the design of these things is based on cost rather than efficiency or adherence to emissions standards (regardless of what the label says).  You could buy a box of 500 from a Chinese supplier and get 10 different versions in the pack, cost is king for these things.

The "ideal" way to receive ADSB (and all such similar signals) is to have an RF receiver tuned for those frequencies and then stick a small(ish) FPGA behind it to extract the data from the signals received.  But you won't get one of them for $5 delivered from china.  It's a reasonable amount of work to make one of them, but the performance would be orders of magnitude better than an SDR dongle.  If you did this, you would also take quite a load off the Pi too as it spends lots of time running software to decode the ADSB and similar signals from the stream of data from the SDR dongle, which opens all sorts of possibilities for Lee's PAW code to be ported to a more elegant bespoke bit of hardware - but the end result would be more expensive and there would be no BYO option available.

[exfirepro]

I thought the ADS-B aerial was receive only? 

Cheers,
R

Rog,

A great explanation from Jeremy C as to the limitations of commercial SDR dongles.

WRT the 'normally receive' ADSB antenna 'broadcasting' the noise, its simply that if the SDR dongle is generating 'Radio Frequency Interference' (RFI) internally, the attached (normally receive) antenna, or its cable (or any other piece of connected wire come to that) can then act as an antenna and radiate that interference locally, where it is probably being picked up by the additional wiring and amplified through your amplifier / speaker system.

In the case of the PAW 'Bridge', which is a much more expensive piece of kit, the transceiver is screened inside a grounded metal case with the input and output filtered to prevent the transmission of any off frequency signals. To do this with standard SDR Dongles would significantly increase production cost, weight, size and complexity and has never been found necessary for 'normal' PAW use.

Hopefully swapping your dongle and taking care with component selection and placement will solve your interference problems, however as space seems to be less of an issue than the interference, you could try one of the 'better quality' SDR dongles available, such as the NooElec Temperature Controlled (TCXO) dongles which we use in the PAW OGN-R Rebroadcast stations (primarily because they are less subject to frequency drift - which is not an issue for normal PAW 1090 MHz use, but critical when looking for very low power glider signals on 868 MHz). See...

https://www.amazon.co.uk/NooElec-NESDR-SMArt-Enclosure-R820T2-Based/dp/B01HA642SW/ref=sr_1_2?ie=UTF8&qid=1515074676&sr=8-2&keywords=nooelec

The NooElec SDR can be fitted as a direct swap replacement for the normal PAW SDR and has a metal case and standard SMA antenna connection, so perhaps worth a try.

Best Regards

Peter

[rogellis]

Thank you both for the answers x it is beginning to make sense.

How about this even smaller NooElectric dongle, would this work too?
There is a Nano-2
https://www.amazon.co.uk/NooElec-Compatible-Foreflight-FlightAware-Applications/dp/B01K5K3858
And a Nano-3
https://www.amazon.co.uk/NooElec-NESDR-Nano-Bundle-Stratux/dp/B076GWF6FF

It is tuned to the required frequencies, and appears to have a screw-in fixing.  The cable dropping out of the other version is a continual problem.

Would this NooElectric have a lower energy consumption than the standard one?  We are working on batteries, so any reduction in power consumption would be appreciated.

Thanks,
R

(PS - just noticed the optional heat sink, so perhaps it is not so efficient....)

.

[JCurtis]

There won't be much, if any, power usage difference between them - it's a "feature" of using this type of dongle.  Double whammy really, using an SDR means you need take quite a chunk of the Pi CPU horsepower so both end up needing more current and become voltage sensitive too, the RF noise reduces sensitivity of the system as a whole.  It's a cost/benefit thing and give a bit of insight into where there is such a price gap between a PAW and other solutions available.