Checking Receiver AGC Settling Within a Burst Preamble

A receiver’s automatic gain control can settle eventually yet still miss the useful acquisition window of a short packet. The relevant requirement is when the gain becomes suitable relative to preamble and payload timing. Test that relationship with realistic initial conditions rather than only a continuous input signal.

Define the available acquisition interval

The Analog Devices AD936x AGC tuning study evaluates fast attack behavior for waveforms with a preamble followed by payload. It demonstrates why the timing of gain acquisition and subsequent payload processing must be considered together.

Record preamble duration, packet detection delay, receiver enable timing, and any period reserved for synchronization or equalization. The entire preamble is not necessarily available to the gain control loop.

Exercise different initial states

Start bursts from the gain states the receiver can actually retain after silence or a previous packet. Include a strong packet after a weak one and a weak packet after a strong one. Gain reduction and gain increase may follow different paths and take different times.

Vary input level across the required range and include realistic blockers where relevant. A setting optimized for one amplitude step may not handle another without clipping or excessive noise.

Capture gain and signal quality together

Record gain index or supported AGC status with the received sample stream. Mark preamble and payload boundaries. Check clipping, amplitude settling, and the selected demodulation metric over time.

Do not use a final gain value alone as evidence of timely acquisition. The correct value reached after the payload begins may still leave too little usable information for decoding.

Verify the chosen control policy

If gain is held during payload, confirm when the hold occurs and what happens if the signal changes afterward. If gain remains active, verify that changes do not invalidate channel estimation or other receiver assumptions.

Tune supported thresholds, wait intervals, and step sizes with the actual sample rate and waveform. Register values that represent clock cycles change their time meaning when the receive rate changes.

Repeat the burst after both short and long idle intervals, because retained gain state or reset behavior can change the next acquisition trajectory.

Save initial state, burst levels, timing markers, gain trajectory, and packet or modulation performance. Repeat the required combinations after configuration changes. A useful AGC result establishes acquisition within the available interval and acceptable payload behavior. A short settling time measured from an already favorable gain setting, or a successful long packet, does not establish reliable reception of the shortest supported bursts.