How an SDR receiver works

Key takeaways An SDR receiver is a short chain: front-end filter + low-noise amplifier (LNA) boost the faint antenna signal; a mixer driven by a local oscillator (LO) shifts your chosen band down to a low frequency; and an analog-to-digital converter (ADC) samples it into numbers — emitting IQ samples. Tuning is just changing the LO. Most VHF/UHF SDRs use quadrature sampling (mix to baseband, then digitise), while some use direct sampling (digitise the RF straight away).

Last lesson said the SDR’s job is to turn a slice of spectrum into samples. Now let’s open the box and see the stages that do it — because knowing them makes gain, sample rate, and troubleshooting far more intuitive.

The receive chain at a glance

The receive chain. Each stage has a job — and each is a place a signal can be lost or distorted, which is why this map helps troubleshooting.

Front-end filtering and the low-noise amplifier

The antenna delivers a faint, messy mix of everything in the band. A front-end filter trims the worst out-of-band energy so it can’t overload later stages. Then a low-noise amplifier (LNA) boosts the wanted signal early — this matters because every later stage adds noise, and you want the signal strong before that happens. The LNA’s quality (its noise figure) largely sets how weak a signal the whole receiver can detect — its sensitivity.

Mixing down with a local oscillator

The ADC can’t directly digitise, say, an 850 MHz signal cheaply. So a mixer combines the incoming signal with a pure tone from a local oscillator (LO), shifting your chosen band down to a low frequency the ADC can handle (baseband or a low intermediate frequency).

The beautiful part: tuning the radio is just changing the LO frequency. Want a different part of the spectrum? Move the LO so that part lands in the ADC’s window. There are no mechanical dials — just a number.

Worked example. To receive an 851.0 MHz control channel, the tuner sets its LO so that 851.0 MHz mixes down to baseband (0 Hz at the centre of the capture). To jump to 770.0 MHz instead, it simply moves the LO down by 81 MHz — the same hardware, a different number. Whatever sits within ±half the sample rate of the LO appears in your captured band, ready for software to channelise.

The analog-to-digital converter (ADC)

The ADC measures the down-shifted signal millions of times a second, turning the continuous wave into a stream of numbers. Two ADC properties shape everything downstream:

Sample rate — how fast it measures, which sets how much bandwidth you capture.
Bit depth / range — how finely and over what span it can measure, which is why gain must be set so strong signals don’t exceed the ADC’s ceiling (clipping at 0 dBFS).

Direct vs. quadrature sampling

Two ways to get the signal into the ADC:

Approach	How	Trade-off
Quadrature sampling	Mix down to baseband as two channels — I and Q — then digitise	Works at any tuned frequency with a modest ADC; the norm for VHF/UHF SDRs
Direct sampling	Digitise the RF immediately with a very fast ADC, no mixer	Simple, but needs a fast/expensive ADC and is usually limited to lower bands

Quadrature sampling is why SDR output comes as pairs of numbers — the I and Q that the next lesson is all about.

How this maps to real hardware

An RTL-SDR uses a tuner chip (the mixer + LO) feeding the RTL2832U’s ADC, outputting IQ — classic quadrature sampling across ~24 MHz–1.7 GHz. (Some support a direct-sampling mode for HF.)
Airspy radios use higher-quality front ends and faster ADCs for better sensitivity and bandwidth; the Airspy HF+ is optimised for the lower bands.
HackRF is a wideband quadrature transceiver (it can transmit too).

The Hardware guide and the SDR hardware lesson cover which to choose; here, the point is that they all implement this same chain.

Quick check: what actually changes when you "tune" an SDR to a new frequency?

Recap

The chain is filter → LNA → mixer (LO) → ADC → IQ samples.
The LNA sets sensitivity; tuning = changing the LO.
The ADC sets capture bandwidth (sample rate) and the clipping ceiling.
Quadrature sampling (mix to I/Q) is standard; direct sampling digitises RF directly.

Next: those pairs of numbers — what I and Q actually mean.

Frequently asked questions

What are the main parts of an SDR receiver?

An SDR receiver chains a few stages — front-end filtering and a low-noise amplifier (LNA) to boost the faint antenna signal, a mixer driven by a local oscillator that shifts the band of interest down to a lower frequency, and an analog-to-digital converter (ADC) that samples it into numbers. The output is a stream of IQ samples for software to process.

What does the mixer and local oscillator do?

The mixer combines the incoming signal with a pure tone from the local oscillator (LO) to shift the frequency you care about down to a low “intermediate” frequency or to baseband, where the ADC can handle it. Tuning the SDR is really just changing the LO frequency so a different part of the spectrum lands in the ADC’s range.

What is the difference between direct sampling and quadrature sampling?

Direct sampling digitises the radio signal straight away with a fast ADC, no mixer — simple but it needs a very fast converter and is usually limited to lower frequencies. Quadrature sampling first mixes the signal down to baseband as two channels (I and Q), so a slower ADC can capture a chosen slice of spectrum at any tuned frequency. Most VHF/UHF SDRs use quadrature sampling.

Why does the front end need a low-noise amplifier?

Signals at the antenna can be extremely weak, and every stage adds some noise. A low-noise amplifier boosts the signal early, before later stages can swamp it with their own noise, setting the receiver’s sensitivity. Its noise figure largely determines how weak a signal the whole receiver can usefully detect.