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

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kevin29
Posts: 4
Joined: Wed Nov 06, 2019 7:42 am

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

Post by kevin29 » Wed Nov 06, 2019 8:17 am

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.
Faulty SDR Band Filter Measure.JPG
RTL-SDR Dongle Filter Measurement
Faulty SDR Band Filter Measure.JPG (94.26 KiB) Viewed 472 times
My setup was as follows:
RTL-SDR Dongle V3 Setup.JPG
RTL-SDR V3 Setup
RTL-SDR Dongle V3 Setup.JPG (92.41 KiB) Viewed 472 times
Next I did the same measurement using the Nooelec's RTL SDR Dongle and got the correct result.
Non Faulty SDR Measurement.JPG
Nooelect Filter Measurement
Non Faulty SDR Measurement.JPG (94.02 KiB) Viewed 472 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An inspection of the dongle's internal shows that D12 which I presume is the ESD diode has been replaced by a capacitor, which may be the culprit of the issue?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
Posts: 4
Joined: Wed Nov 06, 2019 7:42 am

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

Post by kevin29 » Wed Nov 06, 2019 8:19 am

Here is the image of my SDR dongle internals with D12 replaced with what looks to be a capacitor.
SDR Internals.JPG
SDR Internals.JPG (155.06 KiB) Viewed 471 times

rtlsdrblog
Site Admin
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Re: RTL-SDR Dongle V3: Frontend Effecting Filter Response, Manufacturing Fault?

Post by rtlsdrblog » Fri Nov 08, 2019 4:28 am

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
Posts: 4
Joined: Wed Nov 06, 2019 7:42 am

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

Post by kevin29 » Fri Nov 08, 2019 4:56 pm

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. :D

radiolistener
Posts: 54
Joined: Thu Jan 25, 2018 10:48 pm

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

Post by radiolistener » Sun Nov 10, 2019 9:09 am

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
Posts: 6
Joined: Sat Nov 25, 2017 3:29 pm

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

Post by AcidJazz » Sun Nov 10, 2019 11:47 am

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

kevin29
Posts: 4
Joined: Wed Nov 06, 2019 7:42 am

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

Post by kevin29 » Tue Nov 12, 2019 9:25 am

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
Posts: 6
Joined: Sat Nov 25, 2017 3:29 pm

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

Post by AcidJazz » Wed Nov 13, 2019 12:05 am

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

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