Unit 1 — Theory of Operation
TM-TOOL-009 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the SDR INTERFACES AND PRESELECTOR; identify key specifications Estimated time: 20 minutes
Step 1: Read the TM
Open TM-TOOL-009. Read Chapter 2 — Theory of Operation completely.
Then come back here.
Chapter 2 Content
2-1 HF Upconverter
An SA612A double-balanced mixer converts HF input (0.1–30 MHz) to IF (125.1–155 MHz) by mixing with a 125.000 MHz TCXO local oscillator. The RTL-SDR is then tuned to 125–155 MHz to receive HF signals without needing direct-sampling mode. An SPDT relay bypasses the upconverter for 6m and above (where RTL-SDR tunes directly). Optional SPF5189Z LNA (+19 dB, NF = 0.6 dB) precedes the mixer.
| Parameter | Value |
|---|---|
| Input frequency | 0.1–30 MHz |
| LO frequency | 125.000 MHz (±2.5 ppm TCXO) |
| IF output | 125.1–155 MHz |
| Conversion gain (passive) | −2 dB |
| Conversion gain (with LNA) | +17 dB |
| Image frequency | >155 MHz (suppressed >45 dB by IF BPF) |
| LO leakage to RF port | 2-2 16-Band Switchable Preselector |
Sixteen bandpass filters (one per amateur band, 160m through 23cm plus one bypass) select with an SP4T×4 relay tree driven by a 4-to-16 decoder (74HC154) from ESP32 GPIO[3:0]. Filter topology: 3-pole Chebyshev BPF for 160m–10m; interdigital for 2m/1.25m; stripline/SAW for 70cm and above. Rejection of out-of-band signals: ≥50 dB adjacent band, ≥60 dB for FM broadcast (88–108 MHz) when tuned to HF.
2-3 Bias-T
A bias-T injects DC power onto coax to supply a remote LNA or active antenna. An RF choke (L, >10µH SRF) passes DC while blocking RF; a DC-blocking capacitor (C) passes RF while blocking DC. The HF version (Version A) covers 0.1–50 MHz; the VHF/UHF version (Version B) covers 10 MHz–3 GHz.
2-4 PE4302 Digital Step Attenuator
The PE4302 provides 0–31.5 dB attenuation in 0.5 dB steps via 6-bit SPI control. Frequency range DC–3 GHz, IP3 = +40 dBm, NF = attenuation setting + 0.5 dB. Combined with an SPF5189Z LNA (+19 dB gain, NF = 0.6 dB), the combination provides adjustable gain from −31.5 dB (full atten) to +18.5 dB (no atten, LNA on).
Why Theory Matters
You cannot use a measurement tool correctly without understanding how it works. Theory tells you: - What the tool measures and how it converts the quantity to a readable output - What the sources of error are — so you can recognize and minimize them - What the valid operating range is — so you stay within its specifications - How to interpret results that don't match expectations
If a measurement looks wrong, theory is where you look first.
Self-Check Questions
SC1-1. In one sentence, state the operating principle of the SDR INTERFACES AND PRESELECTOR as described in Chapter 2.
SC1-2. What does Chapter 2 identify as the primary source(s) of measurement error or uncertainty?
SC1-3. What key specification(s) (accuracy, range, frequency coverage) does the TM state?
SC1-4. What does Chapter 2 say the SDR INTERFACES AND PRESELECTOR cannot do — what are its limitations?
SC1-5. List two formulas or relationships from Chapter 2 that govern the tool's operation.
Answer Key
SC1-1. See TM §2-1. Compare your sentence to the first substantive paragraph of Chapter 2.
SC1-2. See Chapter 2. Look for language about error sources, accuracy limits, parasitic effects, or frequency dependence.
SC1-3. See Chapter 2. Look for numbers with units: %, ppm, Hz, Ω, dB, W.
SC1-4. See Chapter 2 and Chapter 1. Limitations are often stated as frequency range, power limits, or accuracy bounds.
SC1-5. See Chapter 2. Equations or proportionality statements are the relationships that govern the tool.
Checkpoint
Before proceeding, state without looking: - The operating principle of the SDR INTERFACES AND PRESELECTOR - The primary error source(s) - At least one key specification with its value
→ Proceed to Unit 2