Лабораторная 6.1 — RF frequency plan

Lab 6.1 — RF Frequency Plan

Goal

Build a clear frequency plan for the first SDR RF experiment using a Zynq/AD9363 transmitter and an external receiver such as RTL-SDR or another SDR receiver.

The lab answers the practical question:

Where should the useful signal appear in the receiver spectrum, and how do sample rate, LO frequency and digital offset interact?

Basic frequency model

flowchart LR
    BB[Baseband tone offset] --> TXLO[TX LO]
    TXLO --> RF[RF signal]
    RF --> RXLO[RX LO]
    RXLO --> IF[Observed baseband offset]

If the transmitter uses center frequency f_tx and generates a digital tone at offset f_bb, the RF tone appears at:

f_rf = f_tx + f_bb

If the receiver center frequency is f_rx, the observed baseband offset is approximately:

f_observed = f_rf - f_rx

Therefore:

f_observed = f_tx + f_bb - f_rx

Example plan

Parameter	Value	Comment
TX center frequency	915 MHz	AD9363 TX LO
RX center frequency	915 MHz	RTL-SDR/HDSDR center
Digital tone offset	+100 kHz	generated by FPGA/DSP
Expected observed offset	+100 kHz	peak should appear at +100 kHz
Sample rate	2.4 MS/s	observation bandwidth
RF bandwidth	2 MHz	analog chain bandwidth

Frequency plan variants

Case	TX LO	Digital offset	RX LO	Expected observed offset
Same LO	915 MHz	+100 kHz	915 MHz	+100 kHz
RX shifted up	915 MHz	+100 kHz	915.05 MHz	+50 kHz
RX shifted down	915 MHz	+100 kHz	914.95 MHz	+150 kHz
Negative tone	915 MHz	-100 kHz	915 MHz	-100 kHz

Aliasing and bandwidth check

The expected observed offset must be inside the receiver Nyquist range:

|f_observed| < Fs_rx / 2

For Fs_rx = 2.4 MS/s:

|f_observed| < 1.2 MHz

Add margin for analog filters and receiver imperfections. Do not place the test tone too close to the band edge.

Practical procedure

1. Select a legal and safe test frequency for your lab environment.
2. Select TX LO and RX LO.
3. Select a small digital tone offset, for example 50–200 kHz.
4. Check that the expected offset is inside the receiver bandwidth.
5. Start with low TX gain and external attenuation.
6. Observe the spectrum.
7. Record actual peak frequency.
8. Estimate frequency error.

Frequency error

frequency_error = measured_peak - expected_peak

Possible contributors:

• LO frequency error;
• receiver oscillator offset;
• sample-rate error;
• FFT bin resolution;
• sign convention mistakes;
• wrong IQ swap or conjugation.

Report checklist

• [ ] State TX center frequency.
• [ ] State RX center frequency.
• [ ] State digital tone offset.
• [ ] Compute expected observed offset.
• [ ] State sample rate and RF bandwidth.
• [ ] Verify Nyquist margin.
• [ ] Record measured peak frequency.
• [ ] Estimate frequency error.
• [ ] Explain possible error sources.

Engineering conclusion template

The planned RF tone is at ____ MHz and should appear at ____ kHz in the receiver baseband.
The measured peak is ____ kHz, giving a frequency error of ____ Hz.
The result confirms / does not confirm the frequency plan because ______.