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Link Budget Analysis: Making the Numbers Close

A link budget is an accounting exercise. Every dB of gain and every dB of loss, tracked from the DAC output to the antenna port. If the numbers don't add up on paper, they won't add up on the bench.

Link budget analysis is the system-level check that determines whether your transmitter — with all its individually specified components — actually delivers the required output power with the required spectral quality. It's where component datasheets meet system reality.

What a transmitter link budget contains

A transmitter link budget tracks signal power through every stage, from the digital-to-analog converter output to the antenna connector. At each stage, you add gains and subtract losses:

Starting point: DAC output power. Typically -10 to 0 dBm full-scale, depending on the DAC and the digital back-off applied. A 14-bit DAC might deliver -6 dBm at the analog output with 3 dB of digital back-off applied to prevent clipping.

Baseband filter/amplifier. Anti-aliasing or reconstruction filter after the DAC. Typically 1-3 dB of insertion loss, possibly with a variable-gain amplifier (VGA) providing 0-30 dB of adjustable gain for power control.

Mixer. Conversion gain (active mixer, +5 to +15 dB) or conversion loss (passive mixer, -5 to -8 dB). The mixer's gain/loss is one of the largest single contributors to the budget. An 8 dB difference between an active mixer at +8 dB gain and a passive mixer at -7 dB loss means 15 dB of difference in downstream gain requirements.

Channel filter. Insertion loss of 1-3 dB. This is pure loss — no way to avoid it, and the filter technology choice directly affects how much loss you take here.

Driver amplifier. Gain of 15-25 dB per stage. If two stages are cascaded, total driver gain can reach 30-40 dB. The driver's output power must be sufficient to drive the PA to its required input level.

Interstage matching loss. Matching networks between the driver and PA introduce 0.5-1.5 dB of loss. An attenuator pad adds 3-6 dB intentionally. These losses are easy to overlook in early budget estimates.

Power amplifier. Gain of 10-30 dB depending on the device. The PA's gain varies with output power level — typically 2-3 dB of gain compression occurs at the rated output power, meaning the PA delivers less gain at full power than at small-signal conditions.

Output matching and harmonic filter. Combined loss of 0.5-2 dB. The output matching network transforms the PA's low output impedance to 50 ohms, and the harmonic filter suppresses second/third harmonics. Both are passive networks with finite Q-factors.

Isolator (if used). Insertion loss of 0.3-0.8 dB in the forward direction. A small but non-negligible loss, especially in systems where every dB matters.

Cable/connector losses. From the PCB to the antenna connector. 0.1-0.5 dB per connector, plus cable loss that depends on frequency and length. At 28 GHz, a 10 cm cable can add 1-2 dB of loss.

A worked example

Consider a 5G sub-6 GHz transmitter targeting +27 dBm at the antenna port at 3.5 GHz:

DAC output: -3 dBm Baseband filter: -2 dB → -5 dBm Active mixer: +8 dB → +3 dBm Channel filter (BAW): -2 dB → +1 dBm Driver amplifier: +18 dB → +19 dBm Interstage pad: -3 dB → +16 dBm PA gain: +15 dB → +31 dBm Output match + harmonic filter: -1.5 dB → +29.5 dBm Isolator: -0.5 dB → +29 dBm Connector: -0.2 dB → +28.8 dBm

Result: +28.8 dBm at the antenna port, providing 1.8 dB of margin over the +27 dBm requirement.

That 1.8 dB of margin is deliberate. It accounts for component tolerances, temperature variation, and aging. A budget with zero margin is a budget that fails in production.

Gain budgeting across temperature and frequency

Datasheet specs are typically at 25C and center frequency. Reality includes both extremes.

Temperature effects. Most amplifiers lose 0.01-0.02 dB/C of gain as temperature increases. Over a -40 to +85C range, that's 1.25-2.5 dB of gain variation per stage. A two-stage driver plus PA can vary by 3-5 dB across temperature. This variation must be accounted for — either by designing margin into the budget or by using automatic gain control (AGC) to compensate.

Frequency effects. Gain, insertion loss, and matching all vary across the operating bandwidth. A driver specified at 18 dB gain at 3.5 GHz might provide 16.5 dB at 3.3 GHz and 17 dB at 3.7 GHz. If the link budget is calculated at center frequency, the band edges may be 1-2 dB short. Always evaluate the budget at band edges and center — the worst case is the actual budget.

Production spread. Components have tolerances. A PA specified at +15 dB gain might vary from +13.5 to +16.5 dB across the production population. Using typical values in the budget is optimistic. Using worst-case values for every component simultaneously is overly pessimistic (it's unlikely all parts will be at their worst simultaneously). A root-sum-square (RSS) analysis of tolerances gives a more realistic estimate.

Noise floor analysis

In a transmitter, the noise concern is the broadband noise power emitted outside the intended channel. This noise can violate adjacent-channel emission limits or desensitize nearby receivers.

Noise cascading. Each amplifier stage adds noise. The total output noise power is the sum of noise contributions from every stage, each amplified by the gain of all subsequent stages. The PA, being the last stage, adds its own noise floor — but the noise from earlier stages, amplified through the driver and PA, often dominates.

Quantifying transmitter noise. Start with the DAC's noise floor (typically -150 to -160 dBm/Hz). Add the noise figure of each stage. Multiply (in dB, add) the total chain gain. Compare the result to the emission mask requirements at various frequency offsets.

For example: DAC noise floor of -155 dBm/Hz, amplified by 50 dB of total chain gain, produces an output noise floor of -105 dBm/Hz. If the emission mask requires -120 dBm/Hz at 10 MHz offset, you have a 15 dB shortfall — meaning filtering must provide at least 15 dB of attenuation at that offset, or the upstream noise needs to be reduced.

Power consumption analysis

The link budget isn't just about RF power — it also determines DC power consumption and battery life.

PA efficiency dominates. In a transmitter delivering +27 dBm (500 mW RF), the PA at 30% PAE consumes 1.67 W of DC power. The driver at 20% efficiency consumes perhaps 250 mW. The mixer and VCO together add 200-400 mW. Total DC consumption: roughly 2.1-2.3 W, with the PA accounting for 70-80% of the total.

Back-off efficiency. For signals with high PAPR, the average operating point is well below the peak. A PA with 40% PAE at peak power might average only 15% PAE with a 5G NR waveform. This means average DC consumption is much higher than the peak-power efficiency would suggest. Using average efficiency, not peak efficiency, for battery life calculations is critical.

Power control range. Many transmitters need to adjust output power over a 30-60 dB range. This is typically achieved through a combination of digital back-off (in the DAC), VGA adjustment (in the baseband), and PA bias control. The link budget must close at every power control step — not just at maximum power.

When the budget doesn't close

If the numbers don't add up, you have limited options:

  1. Increase PA gain or output power. Usually means selecting a different, more capable (and more expensive, higher-power) PA.
  2. Add a driver stage. Increases gain budget by 15-25 dB but adds power consumption, board space, and cost.
  3. Reduce filter insertion loss. Move to a higher-Q (more expensive, larger) filter technology.
  4. Reduce interstage losses. Optimize matching networks. Remove the attenuator pad between driver and PA (accepting the matching and stability risks).
  5. Relax the system requirement. Negotiate lower output power, reduced range, or relaxed spectral requirements. Sometimes the physics wins and the spec has to flex.

Option 5 is the one that nobody wants to exercise but is sometimes the right answer.

Margin allocation

A well-designed link budget includes explicit margin allocation:

  • Component tolerance margin: 2-3 dB (RSS of individual component tolerances)
  • Temperature margin: 1-2 dB (gain variation across operating temperature)
  • Aging margin: 0.5-1 dB (performance degradation over product lifetime)
  • Design margin: 1-2 dB (unknowns, measurement uncertainty, PCB effects)
  • Total typical margin: 3-5 dB above the minimum required output power

Under-margined designs work in the lab at 25C and fail in the field at 50C. Over-margined designs waste DC power and may not be cost-competitive. Finding the right balance requires experience and knowledge of which uncertainties are correlated.

How much margin do you build into your transmitter link budgets? I've seen teams work with as little as 2 dB total and as much as 8 dB — and both have been burned at different times.

This is Part 7 of an 8-part series on RF transmitter design. Previously: "Power Amplifier Selection." Next: "Integration: From Block Diagram to Working Hardware."