A just barely understandable SSB signal (recoginize your call and pull in a signal report) would be 0 db SNR. This is what we show in Figure 2.įigure 2: SNR performance referenced to casual copy SSB signal in 2500 HzĪs shown in Figure 2 a 6dB SNR is required for casual SSB communications. It is common in the amateur radio literature to express SNR performance, ie the ability to hear and decode weak signals, in dB releative to noise in a 2500 Hz bandwidth. Let's look at some common modes used for DXing. Different modulation modes inherently have different performance on weak signals. The scope of encoding the signal is beyond this discussion.Ĭoncentrating on HF amateur communications a DXer is focused on long distance, usually weak, signals. Encoding means they are putting in redundant information and/or reducing the alphabet, or known outcomes *1, of the decoded signal.Įncoding improves the data recovery of a signal at a given SNR. They are doing DSP to narrow the noise bandwidth as tight to the signal as possible. With DSP (Digital Signal Processing) you can even go narrower.Įssentially this is what all of the new DSP modulation schemes are doing. Common filters used include 2400Hz, 500Hz, 250Hz, and 100Hz. Bandwidth becomes theĬW is a much narrower signal.
This loss of signal will quickly make the SSB signal unintelligible. In smaller filters you start to loose signal energy in addition to lowering the noise energy. For instance SSB is a signal that is 2300 Hz wide. There are pros and cons to the use of each mode. By mode I am referring to SSB, CW, RTTY, JT65, etc. Across the amateur radio community 2500 Hz has been adopted as a figure of merit for comparing the receivingĬapability of a mode. SSB signals are normally received through either a 2600 Hz or 2400 Hz filter. Signal power (red) is compared to noise power in a received bandwidth (blue) It can be dominated by external noise received by the antenna (the usual case for RF signals 30MHz, VHF, UHF).įrom Figure 1 you can see that noise is a broadband signal and to measure its power we must specify the bandwidth it is measured in, known as the Noise Bandwidth (NBW).įigure 1: Comparison of different SNRs. Noise is throughout the radio frequency spectrum. Passing an SSB signal that is nominally 2300Hz wide through a 500 Hz filter would make the signal unintelligible. Signal of interest must still fully pass through the filter. The narrow bandwidth filters out more noise so your SNR improves. The noise reading should go down to just under Let's say this signal is received through a wide receiver filter bandwidth of 2400 Hz. If a received CW signal is S9 and the meter reads S7 without a signal, the SNR is 12dB (normally each S-unit is 6dB). SNR(dB) = 10*LOGįor loud signals you can roughly calculate SNR off the S-meter. It is calculated as the ratio of signal power to the noise power. Signal to Noise Ratio (SNR) is a figure of merit that compares the level of a desired signal to the level of background noise. The signal is your radio signal and the noise is atmospheric, electrical, or other interfering radio signals. This is identical to what happens in the Radio Frequency (RF) world where You had to compensate by speaking louder (more signal power) to maintain a conversational voice level against the increased noise level (more noise level power). What has happened is that the background (ambient) noise level has risen, and in order to maintain communications You find yourself shouting to your partner in order to be understood. By the time dessert comes the restaurant and bar area is nearly full. Conversation is easy between the both of you. You and your partner go to an early dinner. Contact me through: to Noise Radio, definition and application to Radio Communications