Feeling lost - no documentation or tutorial

[jschall]

I've been struggling with my RSP for about a month now, and I still don't know how to use its settings.

The settings consist of a cute block diagram with sliders and drop-down selectors, but I have no idea what they do or what the acronyms mean.

For example, what is meant by "Low IF" and "Zero IF"? "LNA Trip Threshold"?

And what the heck is "Gain Reduction"?

I have found no documentation other than the HDSDR-specific how-to's. Those are well done, but they only help to de-mystify that very obtuse program.

SDR Sharp is a much easier program to use, and with the RTL-SDR dongle I have no problem with its settings.

But as soon as I switch to the SDRPlay, I'm lost.

Are there any tutorials or documents available that explain the RSP Device Controller?

[Tech_Support]

Dear Jschall,
The level of flexibility offered by the RSP can appear daunting and confusing, but there are only a few basic concepts that you need to understand and many of the features offered my be of little interest to you.

The first point to understand is the basic architecture of the receiver. The RSP simply selects a segment of spectrum and pipes it through the USB interface to where it is further processed in software. The spectrum selected can be presented in one of two forms:

1. As a Real intermediate Frequency ( Low IF), where the spectrum is centred on an intermediate frequency (2.048 MHz)
2. As Zero IF, where the spectrum is represented in its baseband Cartesian form of I + j.Q

All SDR programmes process I and Q data (the baseband Cartesian representation given by a zero IF) and so even when you select Low IF, this has to be converted to a zero IF in software before it is processed any further.

There are plusses and minuses to using Zero IF or Low IF in the tuner, but as a starting point, I would simply stick with Zero IF.

The next key point is how gain control works. Because signal strengths presented by the antenna vary tremendously, it is necessary to adjust the gain of the receiver to get the optimum performance. All receivers have a certain maximum level of gain in a given band and the RSP uses a series of digital attenuators to reduce the gain from its maximum value. Do for example a Gain Reduction of 40 dB is simply that the gain is set to being 40 dB below its maximum value.

The LNA (Low noise Amplifier) on the RSP also has a programmable gain step. In the HF and VHF bands, this is a 24 dB gain difference between the LNA being 'on' and 'off'. If you have a series of very strong signals, you may wish to turn the LNA off and reduce the receiver gain by the amount of this step. This may be necessary to prevent overload of your receiver, but if you do not have very strong signals, it is better to leave the LNA on. The point at which this gain step is applied is controlled by the LNA trip threshold. So for example, if you select 59 dB for this threshold, then the LNA will remain on until the receiver gain has been turned down by 59 dB, thereafter, it will be turned off.

The gain, can only be adjusted manually if the tuner AGC is turned off. The tuner AGC simply automatically prevents the signals from overloading the ADCs. This is done by measuring the signal power and the ADC inputs, and automatically adjusting the receiver gain until the power level is a certain amount below the full scale range of the ADCs. This point is determined by the ADC set point (the point at which the signal level is set at the ADC inputs).

To get started, I would do the following:
1. Enable AGC
2. Set the ADC set point to -40dB
3. Set the LNA trip threshold to 59 dB
4. Use zero IF
5. Select whatever bandwidth you want (1.536 MHz is a good start)

and enjoy.

Once you have got a certain sense for how the receiver works under these conditions, you might want to experiment somewhat to see if you can improve your reception.

The first thing to try would be to disable AGC and adjust the gain manually aiming to adjust for the best SNR for the signal that you are monitoring. You may wish to increase the receiver bandwidth so that you can monitor a larger slice of spectrum at any one time, but be aware that if you widen the bandwidth, you increase the possibility that a strong signal may fall within the receiver bandwidth, necessitating a change in the gain setting.

The key thing is not to try to change too many things at once and you will soon find a series of settings that work best for your particular circumstances.

Sincerely

SDRplay Tech_Support

[jschall]

Thanks for that tutorial, @SDRPlay Tech Support. I feel less lost, now. I might even dare to press the Advanced Settings button and play with the Block Upconverter!

I suggest you publish a White Paper with more of the details of the RSP architecture.