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update rtl-sdr section; add direct sampling paragraph; remove sample 

config
Jakob Ketterl 2021-04-22 00:11:47 +02:00
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RTL-SDR-device-notes.md

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# 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 do not initially work due to the value `"rf_gain": 30` that is contained in the default configuration. The E4000 tuners do not support this value, so the simple solution for these issues is to change this to a supported value (the closest nearby values are `29` and `34`; the full list of possible values is available from the `rtl_test` command line tool).
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 is 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.
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.
# Sample configuration
# Direct sampling mode
This config shows some more advanced use of an RTL-SDR device, including the `direct_sampling` mode which allows to tune to shortwave frequencies. Please note that direct sampling is not available on al RTL-SDR devices.
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.
```
sdrs = {
"rtlsdr": {
"name": "RTL-SDR",
"type": "rtl_sdr",
"ppm": 0,
"direct_sampling": 0,
"profiles": {
"NOAA": {
"name": "NOAA Weather",
"center_freq": 162475000,
"rf_gain": 30,
"samp_rate": 2000000,
"start_freq": 162475000,
"start_mod": "nfm",
},
"2m": {
"name": "2m Packet",
"center_freq": 144390000,
"rf_gain": 30,
"samp_rate": 2000000,
"start_freq": 144390000,
"start_mod": "nfm",
},
"70cm": {
"name": "70cm 440",
"center_freq": 438800000,
"rf_gain": 30,
"samp_rate": 2000000,
"start_freq": 439275000,
"start_mod": "nfm",
},
"FM1": {
"name": "FM1",
"center_freq": 9000000,
"rf_gain": 10,
"samp_rate": 2000000,
"start_freq": 9000000,
"start_mod": "wfm",
},
"20m": {
"name": "20m",
"center_freq": 14150000,
"rf_gain": 10,
"samp_rate": 2000000,
"start_freq": 14070000,
"start_mod": "usb",
"direct_sampling": 2,
},
"30m": {
"name": "30m",
"center_freq": 10125000,
"rf_gain": 10,
"samp_rate": 2000000,
"start_freq": 10142000,
"start_mod": "usb",
"direct_sampling": 2,
},
"40m": {
"name": "40m",
"center_freq": 7100000,
"rf_gain": 10,
"samp_rate": 2000000,
"start_freq": 7070000,
"start_mod": "usb",
"direct_sampling": 2,
},
},
},
}
```
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, the most common form uses the "Q branch" setting.
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.