The SDRplay RSP1a receiver is able to support a much larger bandwidth needed for the 80m, 40m and 20m band. The disadvantage is that we have to use the 8-bit dongle interface to the websdr software. The websdr does (not yet) support 16-bits. This limits the dynamic range!
We only need to pass the ham bands, so a sample rate of 512kHz is chosen. A higher sample rate needs significantly more processing power.
IF band pass filtering is used to minimize ADC overload by strong out of band signals. On 80m and 20m it is a 300kHz band pass filter. On 40m a 200kHz band pass filter.
A basic noise shaping is used to increase the dynamic range with 3-4dB.
The noise is shaped outside the ham bands. It is visible as an increased noise level at the left and right hand side of the waterfall.
In a 2800Hz bandwidth the overall dynamic range is > 76.5 dB:
- 8-bits: 49.9dB
- Processing gain: 22.6dB
- Noise shaping: >4dB
The sensitivity is set to receive a few dB band noise over the receiver noise.
Practically that results in a dynamic range of about >73 dB.
Note: the websdr S-meter is not an RMS meter. For RMS noise level indication subtract 5 dB.
In case very strong signals overload the ADC, a 6dB attenuation is switched on until signals are again below a certain level. This is at the cost of some sensitivity for weak signals.
An exceptional strong ham radio station or non-ham station, like Over The Horizon Radar (OTHR), can cause overloads.
We also investigated GNUradio as a interface between the RSP1a receivers and WebSDR.
It supported 16-bit making the full dynamic range of the RSP1a receivers available for the websdr. At the same time GNUradio provides the tooling to developing and incorporate an effective noise blanker for the noise peaks coming from several electric fence wires.
However we were not able to implement a stable operation with GNUradio.
- A quiet location needs a higher dynamic range.
- The omnidirectional radiation pattern of the loop however limits the needed dynamic range.
- By filtering the ham bands in the RSP1a the out of band very high power broadcast stations cannot overload the ADC.
- Not only the noise level makes a difference. Received signals can be attenuated in a residential area by surrounding buildings negatively affecting the signal to noise rate.