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DIRECT_SAMPLING.md

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Direct Sampling Mode

Objective

Enable direct sampling mode for receiving HF frequencies (below 30 MHz) without an upconverter.

Scenario

A user wants to receive shortwave radio, amateur radio, or other HF signals directly using the RTL-SDR's direct sampling capability, bypassing the tuner chip.

Prerequisites

  • RTL-SDR device with direct sampling support
  • Appropriate antenna for HF frequencies
  • Understanding of I-ADC vs Q-ADC sampling modes

Implementation

Using I-ADC Direct Sampling

using RtlSdrManager;

var manager = RtlSdrDeviceManager.Instance;
manager.OpenManagedDevice(0, "hf-receiver");

// Enable direct sampling on I-ADC input
manager["hf-receiver"].DirectSamplingMode = DirectSamplingModes.I_ADC;

// Configure for HF reception
manager["hf-receiver"].CenterFrequency = Frequency.FromMHz(14.2); // 20m amateur band
manager["hf-receiver"].SampleRate = Frequency.FromMHz(2.4);
manager["hf-receiver"].TunerGainMode = TunerGainModes.AGC;
manager["hf-receiver"].AGCMode = AGCModes.Enabled;

// Start receiving
manager["hf-receiver"].ResetDeviceBuffer();
manager["hf-receiver"].StartReadSamplesAsync();

Console.WriteLine("Direct sampling enabled for HF reception");

Using Q-ADC Direct Sampling

// Alternative: Use Q-ADC input
manager["hf-receiver"].DirectSamplingMode = DirectSamplingModes.Q_ADC;

// Q-ADC typically provides better performance on some devices
Console.WriteLine("Using Q-ADC direct sampling");

Switching Between Modes

// Disable direct sampling to return to normal tuner mode
manager["hf-receiver"].DirectSamplingMode = DirectSamplingModes.Disabled;
Console.WriteLine("Direct sampling disabled - using normal tuner");

// Now can tune VHF/UHF frequencies normally
manager["hf-receiver"].CenterFrequency = Frequency.FromMHz(145); // 2m band

Frequency Coverage

// Direct sampling mode typically covers:
// - Long Wave: 30 kHz - 300 kHz
// - Medium Wave: 300 kHz - 3 MHz
// - Short Wave: 3 MHz - 30 MHz

// Example: Receiving AM broadcast band
manager["hf-receiver"].DirectSamplingMode = DirectSamplingModes.Q_ADC;
manager["hf-receiver"].CenterFrequency = Frequency.FromKHz(1000); // 1 MHz MW
manager["hf-receiver"].SampleRate = Frequency.FromMHz(2);

Expected Results

  • Device receives HF frequencies without an external upconverter.
  • Frequency range extends down to DC (0 Hz).
  • Tuner chip is bypassed.
  • Sampling uses the ADC directly from the antenna input.

Notes

  • Direct sampling bypasses the tuner chip entirely.
  • I-ADC and Q-ADC inputs may have different performance characteristics depending on the device.
  • Sample rate and gain settings still apply.
  • Not all RTL-SDR devices support direct sampling equally well.
  • Hardware modifications may improve HF reception (bias tee removal, better filtering).
  • Antenna design is critical for HF reception quality.

See Also