Performance of R820T v3 dongle

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Kitski
Posts: 2
Joined: Fri Apr 03, 2020 3:29 am

Performance of R820T v3 dongle

Post by Kitski » Fri Apr 03, 2020 4:36 am

Hi Team,
I'm a newbie to the SDR world - amazing performance from a $30 device !

To complete a project, I need to characterise the RTL-SDR.COM v3 dongle along similar lines as 'Simon' undertook here:
http://www.simonsdialogs.com/2014/09/r8 ... mic-range/

Using an HP signal generator locked to GPS, the frequency set at 150MHz, 2uV RF output, AM modulation, the RTL was set to a centre frequency of exactly 150MHz on the FFT screen, max gain, AM mode (3kHz BW) and the frequ offset set to -1 ppm. sample rate set at 1.024 MSPS.

Using Airspy as the GUI, the 'classic' AM signal displayed on the RF FFT graph should be 2 equal sidebands either side of the carrier (150MHz). When the signal generator was set exactly (within 1Hz or so) of the centre frequency, the carrier would disappear(null). If the signal generator was set 100Hz higher (or lower) than 150MHz, the signal was displayed correctly.

Changing signal generators or GUI's (to CubicSDR) made no difference. The dongle otherwise works perfectly. Changing the sample rate didn't help. This website notes that this dongle isn't crash-hot when receiving narrow-band signals.

My thoughts are that this is a (digital) sampling artefact when the SDR FFT display is set exactly at it's mid-point and receiving a signal exactly on that frequency. Then, for unknown reasons, the sampled signal digital value is (I suspect) zero - hence no carrier displayed on the FFT screen. This occurs for any input frequency - there's nothing special about "150MHz" or AM (FM or CW signals do the same thing).

Can any DSP guru offer a logical (or even sensible) explanation ?


Many thanks, Kitski

alanzfq
Posts: 112
Joined: Thu Oct 04, 2018 11:18 am

Re: Performance of R820T v3 dongle

Post by alanzfq » Fri Apr 03, 2020 7:18 am

Many SDRs work like this.
In the exact centre they receive nothing.
The reason is that they act as a direct conversion receiver, the oscillator is in the centre. You do not see this. Signals are received both sides. The IQ processing determines which side a signal really is.
Always use an offset, do not include the exact centre within the bandwidth of the signal you are receiving.
Alan

Kitski
Posts: 2
Joined: Fri Apr 03, 2020 3:29 am

Re: Performance of R820T v3 dongle

Post by Kitski » Sat Apr 04, 2020 6:39 am

Alan,
Many thanks.
Other tests I made (but didn't report in my original post) showed that if the FFT display was offset by a nominal value from the desired signal, the centre-frequency 'disappearing trick' didn't occur.
Your reply - with 100% hindsight - makes perfect sense ! Another Simpson's 'Duhh' moment.
Cheers, Kitski

hotpaw2
Posts: 135
Joined: Sat Jan 14, 2017 11:07 pm
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Re: Performance of R820T v3 dongle

Post by hotpaw2 » Sun Apr 05, 2020 9:55 pm

For a pure sinusoidal signal at the exact center frequency, an IQ demodulator produces a constant output. This constant in the FFT result at frequency zero looks almost exactly the same as an unwanted DC bias or IQ imbalance in the IQ demodulator, so is sometimes removed (“compensated” or zero-ed out by rebalancing the input) somewhere in the hardware or software signal chain. Check to see if your SDR software is doing this.

alanzfq
Posts: 112
Joined: Thu Oct 04, 2018 11:18 am

Re: Performance of R820T v3 dongle

Post by alanzfq » Mon Apr 06, 2020 8:09 am

It is my understanding that it is a balance defect in the ADC that produces the centre spike. It is at zero frequency nothing can be received.
Some are better than others, some give a big spike, others much smaller. As you say, software usually greatly reduces it and it's effects.
Alan

radiolistener
Posts: 75
Joined: Thu Jan 25, 2018 10:48 pm

Re: Performance of R820T v3 dongle

Post by radiolistener » Tue Apr 07, 2020 4:21 am

It is disappears, because you're using SDR software with enabled DC filter.
Just disable DC filter and it will not disappears.

If you don't want to disable DC filter, you can change DC filter bandwidth. HDSDR is configured for 25 Hz bandwidth by default, so all carriers which falls into +-25 Hz will be removed by DC filter.

alanzfq
Posts: 112
Joined: Thu Oct 04, 2018 11:18 am

Re: Performance of R820T v3 dongle

Post by alanzfq » Tue Apr 07, 2020 7:26 am

I'm well aware of the way HDSDR and other software hides the centre spike.
But how many SDRs have you tried? Some of them have very low spikes. I'm not talking RTL exclusively.
Alan

radiolistener
Posts: 75
Joined: Thu Jan 25, 2018 10:48 pm

Re: Performance of R820T v3 dongle

Post by radiolistener » Wed Apr 08, 2020 12:18 am

I tried a lot of different SDR software, they all works almost the same. The most flexible and best quality is HDSDR.

Some of software hiding filter and other setting, so you will be unable to configure it as you wish. But HDSDR allows you to configure almost everything.

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