Field Guide · term

Also known as: C/N, CNR, carrier-to-noise ratio

Carrier-to-noise ratio (C/N or CNR) is the ratio, in decibels, of the received modulated carrier power to the noise power within the receiver’s bandwidth — the signal-to-noise ratio measured before demodulation.1 Where plain SNR is often quoted at baseband after detection, C/N is a pre-detection figure taken at RF or IF, on the carrier as it arrives. It is the natural link-quality metric for systems where the carrier is measured directly, such as satellite and digital-broadcast receivers.

RF / IF C/N (pre-detect) demod SNR (post)
C/N is measured on the modulated carrier before the demodulator; post-detection SNR is measured on the recovered baseband afterward.

How it works

C/N = carrier power − noise power (both in the same units) over the noise-equivalent bandwidth of the receiver. Because it is defined over the carrier’s occupied bandwidth, C/N depends explicitly on that bandwidth: widen the receiver and you admit more noise, lowering C/N for the same carrier. This bandwidth dependence is exactly why C/N is not directly comparable between systems — and why engineers convert to Eb/N0, which normalizes out bandwidth and bit rate, when comparing modems.

The two are related by C/N = (Eb/N0) × (Rb / B), where Rb is the bit rate and B the noise bandwidth — the ratio Rb/B being the system’s spectral efficiency. A closely related figure, C/N0 (carrier to noise density, in dB-Hz), divides carrier power by noise power per hertz rather than over the full bandwidth; it is common in GNSS and satellite work because it separates the carrier strength from the choice of receiver bandwidth.

For analog FM, C/N above the detector’s threshold produces a much higher post-detection SNR thanks to FM’s processing gain; below threshold the familiar “picket-fencing” breakup sets in. For digital systems, C/N (via Eb/N0) maps onto a bit error rate through the modulation’s waterfall curve.

In practice

  • Satellite TV and DVB-S receivers report C/N (or the related MER/link margin) as the headline link-health indicator, since the carrier is measured directly off the transponder.
  • GNSS receivers report per-satellite C/N0 in dB-Hz — typically 35–50 dB-Hz for a healthy fix — because bandwidth-independent carrier density is the meaningful quantity there.
  • A “threshold C/N” is specified for each system: the minimum below which the demodulator can no longer maintain lock.

Relevance to SDR

C/N is the pre-detection language of satellite, digital-TV, and GNSS links, and it is the quantity a spectrum analyzer or waterfall display most directly shows — a carrier rising above the surrounding noise floor. For a trunking decoder like GopherTrunk, the carrier’s C/N as it arrives at the front end sets the ceiling on everything downstream: the demodulator can only recover a post-detection SNR and BER as good as the incoming C/N and the mode’s processing gain allow. When GT reports a demod SNR for a P25 or DMR channel, it is effectively translating the arriving carrier-to-noise ratio into a post-detection quality figure — and no processing can conjure margin the carrier never had.

Sources

  1. Carrier-to-noise ratio — Wikipedia, definition, bandwidth dependence, and relationship to SNR and Eb/N0. ↩

See also