The receiver threshold of reception is.. well, just that. It’s the input to a receiver required to just produce the required communication result.
For most radio amateurs, the threshold they’re most used to is that for voice communications using SSB modulation. At VHF, that might be a voltage level of 0.25mV PD for 10dB SINAD.
Definition
That’s a quarter of a microvolt (potential difference, PD, as opposed to electromotive force) at the receiver input to cause a stated signal plus noise plus distortion to noise plus distortion. 10dB SINAD is about the minimum signal to noise figure that would be intelligible to the average operator. Potential difference (PD) is half electromotive force (EMF). PD is when the voltage is measured across a 50W load impedance. EMF is the same but across an open circuit – hence twice PD. Academic – but important.
So, a properly quoted threshold in this case would be 0.25mV PD for 10dB SINAD for an SSB communication.
(measured in voltage or power considering the receiver input impedance)
The receiver threshold is always quoted for a given communications system as an input that will cause the required output from the receiver.
In digital systems, it might be -119dBm for a bit error rate of 10-4. That’s a signal power in a 50W system of -119dBm to cause the required bitstream output with one error in 10,000. If we wanted better BER, we’d likely have to put more signal in. Likewise, in the analogue system, we’d need more signal in for 20dB SINAD. A signal power of -119dBm converts to about 0.25mV PD.
Receiver threshold as reference
The threshold definition is incomplete without a definition of the communications system. For example, so far, we’ve just discussed threshold for a voice SSB system. This is typically the reference when radio amateurs discuss modulating a sub-carrier to send data as used in the FT8 system. The threshold improvement achieved by reducing the bandwidth and introducing coding can be stated with reference to the SSB value. I use this approach when discussing EME and other systems.
The receiver threshold is the reference which is modified by noise, low noise amplifier noise figure (and gain), feeder losses, and antenna gain when calculating the ability of a radio amateur radio station to communicate with others.
Receiver threshold is always an input level quoted for a response in the receiver for a given communications system.
The receiver threshold forms one of the two parameters to calculate the System Value. The other is the power output from the corresponding transmitter. The System Value is the total loss permissible between transmitter output and receiver input.