RTL-SDR Dongle V3: Frontend Effecting Filter Response, Manufacturing Fault?

[kevin29]

Good day,

I have just received my RTL-SDR Dongle V3 and the RTL-SDR FM rejection filter that I purchased from RTL-SDR.com today. Having previous experience using Nooelec's RTL SDR dongle I am well familiar with setting up the dongle for SDR reception and spectrum analysis.

As a first exercise, I decided to measure the new RTL-SDR FM Bandstop filter's frequency response using "Spektrum" spectrum analyzer software with the new SDR dongle along with a white noise generator. However the filter response looked nothing like what it was expected to look like.

RTL-SDR Dongle Filter Measurement

Faulty SDR Band Filter Measure.JPG (94.26 KiB) Viewed 3323 times

My setup was as follows:

RTL-SDR V3 Setup

RTL-SDR Dongle V3 Setup.JPG (92.41 KiB) Viewed 3323 times

Next I did the same measurement using the Nooelec's RTL SDR Dongle and got the correct result.

Nooelect Filter Measurement

Non Faulty SDR Measurement.JPG (94.02 KiB) Viewed 3323 times

From the results it seems that something on the RTL-SDR Dongle V3's frontend is effecting the response of the filter when attached. I have tried the same test with another filter that I have made and characterized with a spectrum analyzer at work and it showed the same changes when attached to the new dongle.

An inspection of the dongle's internal shows that D12 which I presume is the ESD diode has been replaced by a capacitor, which may be the culprit of the issue? I will post image of my board in next post.

[kevin29]

Here is the image of my SDR dongle internals with D12 replaced with what looks to be a capacitor.

SDR Internals.JPG (155.06 KiB) Viewed 3322 times

[rtlsdrblog]

D12 is the ESD diode, some components in use are just white.

The cause of the different response is probably just the gain settings. On the V3 the signal strength is probably higher, and it's overloading. Try turning down the gain a bit.

[kevin29]

Looks like that was the case, the noise source signal was saturating the SDR input. Made myself an attenuator in series with the noise generator and the SDR. After that the response becomes what was expected.

Thanks for the hint.

[radiolistener]

I have similar noise gen: https://www.aliexpress.com/item/32845101232.html

it has about 6-10 dBm RMS output measured for DC...500 MHz bandwidth and about 6 Vpp peak pulses on the output.

So, I think it will be not safe to connect such noise gen output directly to SDR receiver.

You're needs to use 20-30 dB attenuator, otherwise you may burn out your SDR receiver.

[AcidJazz]

Or another way, connect noice source to 5V instead 12V.

[kevin29]

Thanks for the advice with Noise Gen, I agreed that it would be wise to put an attenuator in series to prevent overloading/damaging the SDR.

An attenuator set would be my next in the RF shopping list.

I have read a while back that the higher the supply voltage the better linearity of the noise bandwidth? I will have to check this up again. I'm sure that powering it at lower voltages would reduce the heat generated by that board.

[AcidJazz]

Add to your shopping list NanoVNA. Measuring filters with noice source was justified when VNAs were expensive, but not now.