If you take a close look, you may find that each of your SDR receiver front end designs is quite different. You need to learn the differences to get best performance from your radios.
While digging into wideband receiver performance with wideband active loops, I realized that I needed a better understanding of SDR receiver front end designs. In particular, I wanted to know the best ways to mitigate ADC overload from strong, local medium wave signals. Turns out that all four of my SDR handled this issue quite differently.
Shown above are the SDR receiver front end designs in all four of my regularly used radios. I have omitted the requisite anti-alias filter, and also the frequency translation in the RSPduo which is not direct sampling. Every ADC (analog to digital converter) has an overload or clipping point that needs to be respected. Nearby AM broadcasters generate signals greater than -20 dBm which can overload ADC.
So, let’s look at SDR receiver front end design first. I will cover performance in subsequent articles. With its strong 16 bit ADC, my Flex 6300 (green) is pretty simple. Your front end is a combination passive attenuator and low noise amplifier with a range of 35 dB. You can add attenuation at low frequencies and gain at higher ones, especially above 14 MHz. This receiver is wideband direct sampling all the way. And even with my wideband loop pointed at the strongest local AM broadcaster (- 5 dBm) overload is not an issue.
Next up, my Perseus (orange) with its 14 bit ADC is much more complicated but works almost as well. Key adjustments come from the use of multiple, fairly narrow bandpass filters (lowpass below 2 MHz) and a 10-20-30 dB step attenuator. The LNA is always on, but the “preamp” raises or lowers the clipping point of the ADC as required.
SDR Receiver Front End – Dual Channel Gear
My Afedri AFE822x Dual Channel (blue) contains no bandpass filtering ahead of its 12 bit ADC. Instead, it relies primarily on a 45 dB digitally controlled variable gain amplifier at the front end. Gain or attenuation ranges from -10 dB to +35 dB. You control this performance either directly or using AGC. In addition, you can increase ADC headroom by an additional 6 dB with a programmable gain amplifier at the ADC input.
Finally, my RSPduo (red) operates its ADC at 12 or 14 bits and provides some front end filtering. These are broad bandpass and lowpass filters just ahead of the ADC, and a switchable 40 dB MW/FM bandstop filter right after the antenna input. But mainly, overload is controlled through manual LNA gain reduction of up to 60 dB. Unlike the Afedri, RSPduo AGC is used only to optimize signal levels for the ADC input. Most users just leave the AGC on and make overload adjustments with Gain Reduction.
So, each of my SDR receiver front end designs is quite different, yet all are quite effective. My word of warning is to get rid of all your assumptions about how things work, and take a detailed look at the actual signal chain and ADC capabilities.