ALFAT SDR Board and GPS Interference

Hi all,

Hope someone can help me out here.

I am trialing an ALFAT SDR/F40 board with a PJRC Teensy 3.2 micro controller and a GPS.

Both the SDR and ALFAT SDR are connected via separate UART ports.

Both boards work well in isolation of each other.

The GPS works perfectly with NMEA sentences coming out at the correct 9600 baud rate into either Serial1, 2, or 3 (depending on how their wired and read from within code). I’ve tried all 3 with the same effect.

The ALFAT SDR board also works perfectly on either of the serial ports.

Now this is where is gets interesting.

If the ALFAT SDR board is powered, then the GPS module cannot see any satellites and loses its fix. That is, if the GPS module is running prior to powering up the ALFAT SDR and receiving satellites, as soon as the ALFAT SDR board is powered on, whether or not it is connected to any serial ports on the Teensy, the GPS receiver loses all signals.

The GPS module still continues to transmit its 9600 baud NMEA sentences, but cannot receive any satellite signals. I can see the sentences work perfectly receiving satellites and a fix, and as soon as the ALFAT SDR is powered up the sentences lose their GPS data. All NMEA sentences from the GPS unit are being transmitted and received fine.

I’ve even tried the ALFAT SDR board on different power supplies with the same effect. As soon as the ALFAT SDR board is powered up - it kills the GPS’s ability to receive satellite signals.

All power supplies have been grounded - thereby eliminating possible ground issues.

I have tried 3 different GPS modules, 2 of the same NEO UBLOX 6 and 7 type and even a SkyMaster GPS module. The ALFAT SDR module has the same effect on all three.

So, all I can assume now is that the ALFAT SDR board is emitting so much that it is interfering with any type of GPS module to receive signals.

Has anyone had this issue before?

If so, how did you resolve the issue?

Does GHI know about this issue?

If so, is there any workaround?

I really like your product for its plug and play features, but unfortunately, its not workable around GPS receivers in its current format.

Any help is greatly appreciated.

1 Like

One other thing to consider is power sources. With both a GPS and F40 board running, you may be overwhelming your power supply circuit. What are you powering the entire thing by?

How are each of the devices connected? Short cables, long cables, any attention to not making them antennas?

@ Brett -

Thanks Brett for your response.

The ALFAT SDR Board has been connected into the circuit via a PCB, so there are no leads.

Any more ideas?

How about GHI themselves?

We are not aware of such noise, which sounds unlikely butt I do not have an explanation.

Add a crazy experiment, ground an aluminum foil and put around the board. Does the GPS work then?

@ Gus -

Hi Gus,

Thanks for your prompt response.

I’ve created the Faraday’s cage using grounded aluminium around the outside of the ALFAT SDR module.

Unfortunately it has exactly the same effect on the GPS unit.

Cage on and ALFAT SDR not powered: the GPS unit is emitting nice clean NMEA sentences with 8 or more satellites in view - giving a 3d fix.

Cage on and ALFAT SDR powered: the GPS unit is emitting nice clean NMEA sentences with 0 or more satellites in view. No fix. That is the SDR unit is killing the GPS signal to the receiver. Not a good outcome.

The next step would be a complete led casing - but given the type of application we are aiming for, this will be unrealistic.

Can you think of anything else?

Any thoughts as the product is exactly what we want.

Kind regards

Gavin Brown

Ferrite bead on the signal lines maybe

seperate powe supplies?

He already tried that.

ooops :-[

The interference exists with or without signal lines from the board.

Just powering up the ALFAT SDR board kills the GPS signal.

I’ve tried different power supplies on both the GPS and ALFAT SDR boards - still the same effect. Power up the ALFAT SDR and a GPS module anywhere near it loses its GPS signal.

Is it possible for you to power up the ALFAT once you have reliable signal lock on the GPS to show that?

I was thinking that the next step might be to send a sample board to GHI, but I wondered if you had someone local that has RFI measurement. I’d be still looking to power circuit behaviours as I always think of GPSes as things that you really need a rock-solid stable power circuit to run; more caps scattered around the power lines so you minimise power spikes and dips as well as induced frequencies.

OK, got a little further with this one.

The ALFAT UART TX and RX using fly leads resonate enough RF to kill a GPS signal from less than 30 cm’s.

The ALFAT UART TX and RX with 5cm PCB traces emit enough RF RF to kill a GPS signal from less than 5 cm’s.

Test setup:

ALFAT SDR powered by separate source at 5V. TX and RX fly leads connected one end to the ALFAT SDR and the other to nothing. Power is off. Distance 40cm from GPS.

Result: GPS module TX, RX, 3.3V and GND connected to the Teensy. GPS and Teensy working fine. GPS signals received - NMEA all good. Good fixes.

Power on ALFAT SDR with TX and RX leads flying free. At 40cms no issue. Move to 30cms from the GPS module and GPS signal dies.

Remove TX and RX fly leads from ALFAT SDR and power up at 30 cm’s from the GPS module. GPS signal received ok until ALFAT SDR board moved 1 cm away from the GPS.

Next check is to shield the TX and RX lines and see if that stops the ALFAT SDR TX and RX lines becoming antennas.

OK, gotten further again with this little issue:

Any pin, including GND, VCC, D+, D-, TX, RX, etc… from the ALFAT SDR board emits unwanted EMI that will kill a nearby (<40 cms) GPS unit.

The only way to get around this issue is to shield all traces/connections from the ALFAT unit. Bit of a difficult task when you want to PCB mount this board with other RF based modules/components.

Anyone got any ideas?

How are you measuring the EMI?

Are you powering this all through one power source? I don’t really understand how you say that GND or VCC can be causing issues… Have you searched for power supply influence, or AC coupled noise?

I’d also suggest you engage GHI directly - the forum here isn’t really a commercial support channel, make sure you point out this thread when you are discussing this.