On the 80m, 40m, 20m, 15m, 12m and 10m we use the SDRplay RSP1a SDR receiver.
For best large signal behavior and minimizing ADC overloading, band pass filters are applied in the IF. On 80m the bandwidth is 300kHz, on 40m and 12m 200kHz and on 20m, 15m and 10m 600kHz. On 20m, 15m and 10m the effective bandwidth is limited by the noise shaping to 460kHz.
The FiFi SDR and sound card are used on 60m and 30m and 17m band. The effective bandwidth is about 130kHz. That is the result of the frequency behavior of both the FiFi and sound card. In addition the noise level in the sound card increases for frequencies at the band edges (noise shaping).
The WebSDR supports an 8-bit interface for RTL dongles and RSP1a receivers. It not yet supports 16-bits. This limits the dynamic range for quantization noise and ADC overloading by strong signals. The Maasbree WebSDR applies noise shaping on 80m/40m/20m to increase this dynamic range by lowering the quantization noise. On 80m by 11dB. On 40m and 20m 18dB**. This allows signals to be correspondingly stronger before ADC overload occurs. Noise shaping shows up as an increased noise level outside the amateur band at the band edges in the waterfall. This noise is masked (attenuated) in the WebSDR. Because of this masking the waterfall brightness is equalized over all bands (no change in S-meter reading).
** translated in effective bits:
- 11dB adds 1.83 bit ==> 9.83 bit on 80m (@512kHz)
- 18dB adds 3 bit ==> 11 bit on 40m (@512kHz) and 20m (@1024kHz)
Note: the ENOB of the RSP1a is higher than 10.4 for these sample rates, rather 12 bits. In a 2800Hz bandwidth the dynamic range (for quantization noise and ADC overloading):
80m 40m/12m 20m/15m/10m
8 bit 49.9 49.9 49.9 dB
processing gain 22.6 22.6 25.6 dB
noise shaping 11 18 18 dB
Total 83.5 90.5 93.5 dB
The sensitivity is set to receive a few dB band noise over the quantization noise.
Note: the websdr S-meter is not an accurate RMS meter. For RMS noise level indication subtract about 5 dB.
Attenuation of 6dB
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.