dBc
Understanding dBc
The dBc unit answers a simple but critical question: how clean is your signal? Every real RF signal has imperfections: phase noise sidebands around the carrier, harmonic content at integer multiples of the frequency, spurious signals from PLL reference leakage, and intermodulation products from nonlinearity. All of these unwanted components are measured relative to the desired carrier power in dBc.
The beauty of dBc is its universality: whether your carrier is at −10 dBm or +30 dBm, a −60 dBc spurious signal is always 60 dB below whatever the carrier happens to be. This makes dBc the natural unit for spectral purity specifications that must hold across a range of output power levels.
dBc Equations
Pspur(dBc) = Pspur(dBm) − Pcarrier(dBm)
Carrier +10 dBm, spur −50 dBm:
Spur = −50−10 = −60 dBc
Phase noise integration:
Ntotal(dBc) = L(f) + 10log(BW)
−100 dBc/Hz, 10 kHz BW:
N = −100+40 = −60 dBc
Absolute power from dBc:
Pspur(dBm) = Pcarrier(dBm) + P(dBc)
+20 dBm carrier, −60 dBc spur:
Pspur = 20−60 = −40 dBm = 0.1 μW
Common dBc Specifications
| Parameter | Typical Spec | Unit | Component | Standard |
|---|---|---|---|---|
| Phase noise | −100 to −160 | dBc/Hz @ 10 kHz | Oscillator | IEEE 1139 |
| 2nd harmonic | −20 to −30 | dBc | PA | 3GPP/FCC |
| Reference spur | −60 to −90 | dBc | PLL synth | Datasheet |
| LO leakage | −25 to −40 | dBc | Modulator | Datasheet |
| OOB spurious | −30 to −50 | dBc | Transmitter | 3GPP 36.101 |
Frequently Asked Questions
What does dBc measure?
Power relative to carrier. P_spur(dBc) = P_spur(dBm) − P_carrier(dBm). Always negative for sub-carrier components. Independent of absolute power: −60 dBc is always 60 dB below carrier regardless of carrier level. Used for all spectral purity specs: phase noise, harmonics, spurs, LO leakage.
dBc for phase noise?
L(f) in dBc/Hz: noise power in 1 Hz bandwidth at offset f from carrier, relative to carrier. −100 dBc/Hz @ 10 kHz: noise 100 dB below carrier per Hz. Total noise in BW: N = L(f)+10log(BW). −100 dBc/Hz over 10 kHz = −60 dBc total. Degrades EVM, BER, adjacent channel performance.
Typical specs?
Phase noise: TCXO −140, OCXO −160 dBc/Hz @ 10 kHz. PLL: −90 to −110. PA harmonics: H2 −20 to −30, H3 −30 to −40. LO leakage: −25 to −40 dBc. PLL ref spurs: −60 to −90. 3GPP LTE UE: in-band −30, OOB −41 dBc. FCC Part 15: −41.2 dBc OOB.