RTL SDR device notes

Problems with Elonics E4000 tuners

RTL-SDR devices come in many sizes and shapes. One thing that will vary is the type of tuner chip that has been chosen by the manufacturer. As far as we are aware, all of them work, including the E4000 tuner. There has been many reports of issues with devices containing E4000 tuners though since they only accept a limited set of gain values. The full list of possible values is available from the rtl_test command line tool.

One other very common complaint with the E4000 tuners is that they show a very prominent DC offset, which shows as a very strong "line" going right down the middle of the waterfall, sometimes even flanking out to the sides. This cannot be fixed by OpenWebRX since it is contained in the IQ data that is sent by the device, or in other words: What you see is what OpenWebRX receives from the hardware. There are algorithms to compensate for this issue, they are however unable to recover any signals that have been compromised. OpenWebRX does not currently implement any such algorithm.

Direct sampling mode

In their original design, RTL-SDR devices have a frequency range that starts around 20-25 MHz (depending on the exact model), and are therefore unsuitable for the reception of shortwave frequencies. A hardware modification, which bypasses the tuner chip, can effectively turn the device into a direct sampling receiver, and allow it to receive shortwave bands.

Some RTL-SDR devices include this modification from the factory, the most prominent example being the RTL-SDR blog v3 devices.

The direct sampling mode can be enabled in the configuration, both I and Q branch can be used. The correct choice depends on the way the hardware is connected, the most common form uses the setting Direct Sampling (Q branch).

There is a few caveats concerning this modification:

• Bypassing the tuner also bypasses all available gain stages on the chip, so the "Device gain" setting will not have any effect in direct sampling mode.
• Since there are no gain stages available, the device will not be very sensitive in direct sampling mode.
• The modification also bypasses the RF filtering on the tuner chip, so an external lowpass filter is recommended.