Gain settings


Question is how to set the gains in the RX-888 MKII and KA9Q software to have just enough sensitivity  without overloading by the strongest signals?


We have to set three gains in this order:

1) the 1st front end FE gain in the RX888 to set the level on the ADC input

2) the 2nd KA9Q gain for the 16-bits multicast streams to the WebSDR and to wsprdaemon

3) WebSDR gain (S-meter and waterfall)


The 1st FE gain sets the receiver noise level (sensitivity) and maximum signal level (overload) on the ADC input over 0-30MHz.

In order to set it to a correct value we have to know the:

  • received lowest noise level on each band
  • strongest signal possible in 0-30MHz
  • shelving filter behavior between antenna and receiver

Received noise levels on each band depend on location and antenna factor of the antenna!


Even the RX888 has a limited dynamic range (distance between quantization noise level and overload level). The front end gain in the RX888 has to be set to receive just enough (6-10dB) band noise on all bands. The front end gain is set for the entire shortwave.


Band noise levels however decrease with frequency. As a result the noise levels on the lower bands are higher. To minimize the chance of overloads, we have to adjust the signal levels for each band/frequency so the receiver receives just enough band noise on each band. A higher level noise level only increases the noise level at the cost of potential overloading problems.


We use an active small broadband loop antenna with an almost constant antenna factor (about -6dB) over frequency. The Turn Island Systems shelfing filter attenuates the lower bands and equalizes the noise level over all bands as much as is practically possible.


The advantage of our active small loop antenna is that we know the lowest possible noise level on each band. Which is in fact the output noise of the antenna amplifier without the loop connected (no signals).
So we connect the amplifier without loop to the RX-888 and measure the noise level increase using KA9Q "control" command. This window shows all relevant info:


For the measurements the Signal box is most relevant:

  • FE gain (1e gain) dB: gain before the input signal is applied to the ADC
  • Baseband dB: (noise) level in the full bandwidth of that channel
  • Gain dB: the gain for the 16 bit output multicast signal level
  • Output dBFS: is the Baseband (noise) level amplified with the Gain

Note: the KA9Q software Output dBFS levels are already corrected for the setting of the 1st FE gain!

Output dBFS is the 16 bit multicast stream.


When only a noise source is connected, Baseband and Output represent the noise level in the full bandwidth. Bandwidth setting can be found in the Tuning Hz box: Filter high – Filter low

The output sample frequency is in the Filtering box: Fs out


The 1st FE gain is set to a value resulting in a noise increase of about 6-10dB on all bands.

You can use an adjustable noise source also if you know the lowest noise level received on your antenna terminals on all bands. On the band to measure the noise source is set to the same level as you expect on that band.


Note: a higher 1st FE gain reduces the dynamic range (distance noise level and overload) at the ADC input. The receiver noise level on the ADC increases with gain (about half the increase in gain) while the 0dBFS voltage stays the same.

So use the lowest gain setting possible as needed to receiving band noise for maximum dynamic range.

ADC overload level is (0dBm – 1e gain) at the input of the receiver.




When the 1st gain is correctly set, the 2nd gain can be set.

The needed 2nd gain scales with the processing gain:


BW is the bandwidth of the output multicast stream. 30MHz is the input bandwidth to the ADC.

A smaller bandwidth BW needs a higher 2nd gain.

However the signal (and noise) level is scaled with the 1st gain before converting to 16-bits.

And the noise level on the input of the ADC increases with 1st gain. For instance a gain increase from 0dB to 20dB results in a 9dB increase in noise level on the ADC input.

This complicates things.

An easier way to set the minimum needed 2nd gain, is to apply the lowest noise level on that band and tune the output level (Output dBFS) to -84dBFS. That is 12dB higher than the -96dBFS quantization noise level. If expected in band maximum signal levels allow, the 2nd gain can be set higher.


The gain for the WebSDR (in websdr.cfg) scales with the sample rate. This gain sets the waterfall brightness. We use separate corrections for the S-meter readout so the indication is as if the antenna has an antenna factor of -9dB.


Note: if your band noise is very low (quiet rural) may be you need an extra Low Noise preamp.

In the overpopulated Netherlands there are practically no locations anymore with that low noise levels.

So we don’t have that “problem”.


go to Buffer settings


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