E4D

Dynamic range, intermodulation, link budget

Explain dynamic range and intermodulation, find third-order products, read intercept points, and work a dB link budget.

14 pool questions1 on the exam

Syllabus: Receiver performance characteristics: dynamic range; intermodulation and cross-modulation interference; third-order intercept; desensitization; preselector; sensitivity; link margin

Dynamic range is the window between the noise floor and trouble

Example numbers. Signals inside the window are received cleanly; below it they vanish in noise, above it the receiver distorts.

Blocking dynamic range = dB from the noise floor up to the level that causes 1 dB of gain compression. Poor dynamic range means cross modulation and desensitization (reduced sensitivity) from strong adjacent signals.

Fix	Why
Attenuation before the first RF stage	lowers the strong signal, so less desensitization
Preselector	rejects signals outside the band being received

RememberStrong neighbor in, sensitivity out. Cut it before the first stage.

Nonlinear stages mix signals into new ones, close to the originals

Third-order products sit the same spacing beyond each signal: 2f₁ − f₂ and 2f₂ − f₁.

Second transmitter146.34 MHz

Receiver at 146.70interference

Intermodulation is caused by nonlinear circuits or devices. Two signals f₁ and f₂ create third-order products at 2f₁ − f₂ and 2f₂ − f₁. Odd-order products land near the originals, so two in-band signals make products that are also in the band.

146.52 MHz neighbor, receiver on 146.702 × 146.52 − 146.34 = 146.70 and 2 × 146.61 − 146.52 = 146.70

Two nearby repeaters: their outputs mix in the final amplifier of a transmitter. A properly terminated circulator at the transmitter output blocks the other signal from getting in.

RememberOdd-order products (2f₁ − f₂) fall in your band.

Third-order products rise 3 dB per dB, so the intercept sets the limit

Example receiver. Both axes in dBm. The products rise three times as fast as the signals, so they climb out of the noise quickly.

Level of each tone−30 dBm

3rd-order product−110 dBm

Raise two tones by 1 dB and the third-order products rise 3 dB. Extended, the two lines meet at the third-order intercept: a pair of 40 dBm signals would theoretically produce a third-order product at the same amplitude as either input. A higher intercept point is a better receiver.

RememberIntercept = where the product would equal the tones.

A link budget is dB arithmetic: add gains, subtract losses

Gains add, losses subtract. Start at the transmitter's dBm, end at the level reaching the receiver.

Margin = received level − (MDS + required S/N). The MDS is a power: 0 dBm = 1 mW, and each −10 dB divides by 10.

Level	Power
0 dBm	1 mW
−30 dBm	1 µW
−60 dBm	1 nW
−90 dBm	1 pW
−100 dBm	0.1 pW

RememberdBm: gains add, losses subtract. Margin = what you get − what you need.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 14 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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