Your SDR receiver RF Front End may not require as much filtering as legacy receivers, as long as it’s using direct sampling.
Super heterodyne receivers arrived 100 years ago. Engineers discovered that it was easier to amplify and filter HF signals at a lower Intermediate Frequency. Much later, with the arrival of frequency synthesizers, designers added up-conversion to a high first IF which made general coverage easier – no more band knobs or fine tuning dials. However, the major issue with heterodyne or multiple-conversion receivers is suppressing unwanted images. Every time you mix two frequencies together you get images. As a result, receivers adopted elaborate selection of oscillator frequencies, together with extensive and complex filtering.
Front end filtering accomplishes two goals. The first goal is suppressing images created by frequency conversion. The second goal is preventing or limiting unwanted signals from reaching the IF stage. We call this bandpass or preselection filtering.
So today, in the new world of direct sampling, how have SDR receiver RF front end requirements changed? Quite a lot, actually. Typically, your receiver will have preselection and amplification before the analog-to-digital converter.
Let’s start with gain and sensitivity. In most cases, the ADC has the highest Noise Figure of all the parts in the signal chain. Since overall system NF is mainly determined at the front end, we typically place a Low Noise Amplifier ahead of the ADC. In an SDR, automatic gain control is used to ensure the best input signal level to the ADC. This AGC controls gain and attenuation to prevent overload and make the best use of ADC dynamic range.
A designer must try to optimize all three of strong, weak and normal signal performance. Some radios have two types of AGC – one to optimize the low noise amplifier, and the other to optimize ADC input levels.
Antennas provide a spectrum of radio signals at hundreds frequencies and widely varying amplitudes. The LNA and ADC must accommodate these. Although mixing images are no longer an issue, there may still be strong interference outside the listening band. Preselection or bandpass filtering removes or reduces these unwanted signals. That is good. On the other hand, analog filtering is complex, adds distortion and is really expensive. Not so good.
SDR Receiver RF Front End – How Much Filtering?
Minimal front end filtering for direct sampling on HF comes with two parts. First there must be a low pass filter to eliminate signals in the alias bands above the Nyquist sampling range. This is mandatory, otherwise local FM stations will alias in. Second, there should probably be a high pass filter to reduce strong signals from nearby Medium Wave stations.
Beyond this, filtering somewhat optional. If you live in a geography with many strong HF signals (like Europe), have a nearby ham running high power a block away, or operate a multi-transmitter contest station, you can benefit from preselection. If not, you can probably get by with a wideband front end, i.e. no additional preselection.
The odds that your ADC will overload due to strong signals are low. As long as a whole bunch of interfering signals are not correlated, they will tend to offset each other. Signal voltages only add destructively when they are correlated. This means aligned in amplitude and phase. As long as signals are quite different from each other, the sum of their voltages is not a sum of individual peak voltages. It will actually just be close to the voltage of the strongest signal. Check out this one-page tutorial on adding amplitudes and levels.
Lastly, even if there is an overload on a high speed ADC, it is typically only microseconds in duration and goes by unnoticed. So, the statistics of signal spectrum comes to the rescue. Steve Hicks of Flex Radio provides a pretty good article about ADC overload on direct sampling receivers.
Having said this, most new direct sampling SDR receiver RF front end design still includes bandpass filters. Some higher end rigs like the new ICOM 7610 even add a tracking preselector.
But my new Flex Radio 6300 is wideband. And my Perseus has a wideband option. In my location I have never experienced any overload problems without preselection.