Low Pass Filter for RTL-SDR Direct Sampling Mode

Over on his blog (in Japanese) Nobu has been working on prototyping a 14 MHz low pass filter (LPF) product for direct sampling modified RTL-SDR dongles (in Japanese, use Google Translate). Direct sampling mode is a hardware modification that allows the tuner chip in RTL-SDR dongles to be bypassed, allowing reception of signals between 0 – 14 MHz. However, after performing this mod there is no filtering and images from higher frequencies such as broadcast FM can be problematic. To fix these problems a low pass filter is required.

Another product Nobu is working on is an isolation transformer (aka Galvanic Isolator) which can be used together with an upconverter to help reduce noise generated from common ground sources such as the PC. The isolation transformer is inserted between an upconverter and antenna.

Low Pass Filter (Top), Isolation Transformer (Bottom)
Low Pass Filter (Top), Isolation Transformer (Bottom)

In the image below Nobu shows the effect of inserting the LPF . An interfering FM broadcast band signal is removed after inserting the LPF.

Effect of inserting the Low Pass Filter
Effect of inserting the Low Pass Filter

The image below shows the effect of the isolation transformer showing a clear decrease in noise floor and increase in signal strength.

Effect of an Isolation Transformer when used with an Upconverter
Effect of an Isolation Transformer when used with an Upconverter
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William Curlew

I wanted to use the SDR with my dx-80 OCF antenna to use as a waterfall display on the HF bands, especially 80 and 40 meters. The broadcast radio interference is not being mitigated at all as far as I can tell from the sounds or the displays. Do I have to do anything special to use the low pass filer? My SDR is a RTL2832U R820T2 TXCO BIAS T HF.

William Curlew

I purchased the low pass filter and it had absolutely no effect. I assume I’m doing something wrong.


What are L/C values?


With only a 8 bit ADC, these RTL dongles have very poor dynamic range (e.g. about 50dB). Much of the resulting ill effects (e.g. overload, IMD etc.) can be greatly reduced with still more filtering on the input. This would apply to direct sampling or using an upconverter to receive HF. Some examples.

Adding a high pass filter will eliminate overload from strong AM broadcasters. Putting high pass and low pass in tandem between the antenna and dongle would provide a wide HF only band pass filter, eliminate the ill effects from both FM and AM broadcasters (e.g. 1.6 – 30 MHz w/ upconverters or 1.6 – 14.4 MHz w/ direct sampling) .

HF reception could be further improved, from strong in band HF signals, by using a even narrower band pass filters.

Alternatively, using a small tunable magnetic loop antenna will do wonders as well.


A FM trap also works very good also , I must admit I do like the low pass filter idea .