Block 6 — RF frontend workflow

Block 6 — RF frontend workflow

This block moves the student from digital DSP/FPGA processing to a real RF bench: frequency plan, levels, bandwidth, gain, safe connection and external signal observation.

Main engineering chain

flowchart LR
    BB[Baseband signal] --> DAC[DAC / TX digital path]
    DAC --> TXRF[AD9363 TX RF chain]
    TXRF --> PATH[Coax / attenuator / antenna]
    PATH --> RXRF[Receiver RF chain]
    RXRF --> ADC[ADC / IQ capture]
    ADC --> ANALYSIS[FFT, levels, SNR, overload check]
    ANALYSIS --> REPORT[RF report]

Block 6 inputs

Artifact from earlier blocks	How it is used in the RF experiment
Sample rate	defines digital bandwidth and FFT interpretation
Digital mixer / NCO	defines signal offset in baseband
FIR/decimator	defines useful bandwidth and out-of-band rejection
IQ metadata	records capture parameters for reproducibility
HDL/AXIS interface	connects FPGA streaming path to the RF chain

Minimum RF discipline

Before enabling transmission, record:

Parameter	Example	Why it matters
TX center frequency	915 MHz	transmitter LO frequency
RX center frequency	915 MHz	receiver LO frequency
Sample rate	2.4 MS/s	digital observation bandwidth
RF bandwidth	2 MHz	analog chain bandwidth
TX gain / attenuation	-20 dB	protects receiver from overload
External attenuation	20–60 dB	safe cabled connection
Expected tone offset	100 kHz	frequency-plan verification

Safe first-bench connection

flowchart LR
    TX[AD9363 TX] --> ATT[30-60 dB attenuator]
    ATT --> RX[RTL-SDR / receiver]
    RX --> PC[HDSDR / Python / MATLAB]

RF safety

Do not connect TX directly to a sensitive receiver without attenuation. Start with minimum TX gain, external attenuation and overload monitoring in the spectrum.

Normal operating signs

Sign	Meaning
One stable peak	frequency plan is correct
No broad flat top	no obvious ADC/RF overload
Stable noise floor	gain is reasonable
Side spurs below the useful signal	NCO/LO/quantization do not dominate
Gain changes level predictably	chain is not saturated

Overload signs

Symptom	Possible cause	Action
Wide flat spectral top	ADC/RF overload	reduce gain / add attenuation
Many harmonics	amplifier saturation	reduce TX power
Noise floor rises with signal	nonlinearity or AGC	disable AGC, reduce level
Peak does not change with gain	clipping/limiting	check levels and cables
Signal drifts in frequency	LO offset / drift	measure frequency error

Basic RF report

Every Block 6 experiment should contain:

1. RF experiment goal;
2. connection diagram;
3. frequency plan;
4. gain/bandwidth settings table;
5. screenshot or FFT plot of normal operation;
6. overload signs or their absence;
7. IQ metadata file;
8. engineering conclusion.

Connection to later blocks

Block 6 prepares the real RF bench for:

• TX/RX chain experiments;
• modulation and synchronization;
• recording and analysis tools;
• integrated SDR project;
• final report with measured IQ data.