Why does the K9KAO SCRAP repeater hear weak signals so well?
Why can I access the SCRAP repeater with my HT and not others that are much closer in distance?
I get asked this question all the time.
This is a brief explanation of noise figure and its importance to repeater system performance.
I have also included some real measurement data of hardware that is installed on the SCRAP repeater system.
Modern receiving systems must often process very weak signals (like HT’s), but the noise added by the system components tends to obscure those very weak signals.
Sensitivity and noise figure are system parameters that characterize the ability to process low-level signals.
Of these parameters, noise figure is unique in that it is suitable not only for characterizing the entire system but also the system components such as the pre-amplifier, receivers, feed lines, cavities, duplexers, and antennas that make up the system.
By controlling the noise figure and gain of system components, the designer directly controls the noise figure of the overall system.
Once the noise figure is known, system sensitivity can be easily estimated from system bandwidth.
Noise figure is often the key parameter that differentiates one system from another, one amplifier from another, and one transistor from another.
The reason for measuring noise properties of networks is to minimize the problem of noise generated in receiving systems.
One approach to overcome noise is to make the weak signal stronger.
This can be accomplished by raising the signal power transmitted in the direction of the receiver, or by increasing the amount of power the receiving antenna intercepts, for example, by increasing the aperture of the receiving antenna.
Raising antenna gain, which usually means a larger antenna, and raising the transmitter power (not possible with HT), are eventually limited by site restrictions, cost, or practicality.
The other approach is to minimize the noise generated within receiver components.
Noise measurements are key to assuring that the added noise is minimal.
Once noise joins the signals, receiver components can no longer distinguish noise in the signal frequency band from legitimate signal fluctuations.
The signal and noise get processed together.
Subsequent raising of the signal level with gain, for example, will raise the noise level an equal amount.
The K9KAO SCRAP repeater uses a Motorola Quantar and its measured noise figure from the RX connector to 1st stage mixer is 3.7 dB.
Good but not great.
Overall system performance could be enhanced by adding a very low noise preamplifier before the Quantar repeater.
Most of the system noise figure performance is dictated by the components before 1st stage mixer and primarily the 1st stage of amplification.
By adding this preamplifier, we reduced the system noise figure at the 1st stage mixer to .60 dB!
A HUGE improvement of system noise figure.
See Friis Formula for noise: Friis formulas for noise - Wikipedia, the free encyclopedia
Noise figure plots of the new pre-amplifier: http://n6dto.com/pre_amplifiers/Noise_Figure_21223.png
But at a cost, now the big concern is overdrive of the Quantar’s own internal preamplifier and creating our own mixing properties and nonlinearities.
I have to admit. I did not measure the Quantar’s third-order intercept point when I was setting up the repeater.
Stupid DTO. But I will in the future.
Third Order Intercept plots of new pre-amplifier: http://n6dto.com/pre_amplifiers/IP3@-20dBm_21223.png
So what’s all this third-order intercept point stuff?
Think of it like this.
If you turn on the stereo in your car with the windows rolled down and try to listen to your favorite crappy DXN country radio station, you might understand the words over the noise (volume set at 3).
If you then roll up the windows the words become more legible due to less noise (volume still set at 3).
You can also turn up the volume and then clearly hear all the instruments (volume now increased to 7).
But if you continue to increase the volume, distortion occurs and the words and instruments are no longer intelligible (volume now at 9). Or you have bumped the tuning to a RAP channel.
The added pre-amplifier does both: it’s like rolling up the windows (lowering the noise floor) AND lowering the point of distortion (volume can now only go to 6 before distortion occurs).
So in the RF world we use terms like third order intercept to describe the point of distortion and saturation of active components.
I will follow up more on this in another post here.
I am too dam drunk right now to have an in-depth discussion about IP3.
N6DTO
Why can I access the SCRAP repeater with my HT and not others that are much closer in distance?
I get asked this question all the time.
This is a brief explanation of noise figure and its importance to repeater system performance.
I have also included some real measurement data of hardware that is installed on the SCRAP repeater system.
Modern receiving systems must often process very weak signals (like HT’s), but the noise added by the system components tends to obscure those very weak signals.
Sensitivity and noise figure are system parameters that characterize the ability to process low-level signals.
Of these parameters, noise figure is unique in that it is suitable not only for characterizing the entire system but also the system components such as the pre-amplifier, receivers, feed lines, cavities, duplexers, and antennas that make up the system.
By controlling the noise figure and gain of system components, the designer directly controls the noise figure of the overall system.
Once the noise figure is known, system sensitivity can be easily estimated from system bandwidth.
Noise figure is often the key parameter that differentiates one system from another, one amplifier from another, and one transistor from another.
The reason for measuring noise properties of networks is to minimize the problem of noise generated in receiving systems.
One approach to overcome noise is to make the weak signal stronger.
This can be accomplished by raising the signal power transmitted in the direction of the receiver, or by increasing the amount of power the receiving antenna intercepts, for example, by increasing the aperture of the receiving antenna.
Raising antenna gain, which usually means a larger antenna, and raising the transmitter power (not possible with HT), are eventually limited by site restrictions, cost, or practicality.
The other approach is to minimize the noise generated within receiver components.
Noise measurements are key to assuring that the added noise is minimal.
Once noise joins the signals, receiver components can no longer distinguish noise in the signal frequency band from legitimate signal fluctuations.
The signal and noise get processed together.
Subsequent raising of the signal level with gain, for example, will raise the noise level an equal amount.
The K9KAO SCRAP repeater uses a Motorola Quantar and its measured noise figure from the RX connector to 1st stage mixer is 3.7 dB.
Good but not great.
Overall system performance could be enhanced by adding a very low noise preamplifier before the Quantar repeater.
Most of the system noise figure performance is dictated by the components before 1st stage mixer and primarily the 1st stage of amplification.
By adding this preamplifier, we reduced the system noise figure at the 1st stage mixer to .60 dB!
A HUGE improvement of system noise figure.
See Friis Formula for noise: Friis formulas for noise - Wikipedia, the free encyclopedia
Noise figure plots of the new pre-amplifier: http://n6dto.com/pre_amplifiers/Noise_Figure_21223.png
But at a cost, now the big concern is overdrive of the Quantar’s own internal preamplifier and creating our own mixing properties and nonlinearities.
I have to admit. I did not measure the Quantar’s third-order intercept point when I was setting up the repeater.
Stupid DTO. But I will in the future.
Third Order Intercept plots of new pre-amplifier: http://n6dto.com/pre_amplifiers/IP3@-20dBm_21223.png
So what’s all this third-order intercept point stuff?
Think of it like this.
If you turn on the stereo in your car with the windows rolled down and try to listen to your favorite crappy DXN country radio station, you might understand the words over the noise (volume set at 3).
If you then roll up the windows the words become more legible due to less noise (volume still set at 3).
You can also turn up the volume and then clearly hear all the instruments (volume now increased to 7).
But if you continue to increase the volume, distortion occurs and the words and instruments are no longer intelligible (volume now at 9). Or you have bumped the tuning to a RAP channel.
The added pre-amplifier does both: it’s like rolling up the windows (lowering the noise floor) AND lowering the point of distortion (volume can now only go to 6 before distortion occurs).
So in the RF world we use terms like third order intercept to describe the point of distortion and saturation of active components.
I will follow up more on this in another post here.
I am too dam drunk right now to have an in-depth discussion about IP3.
N6DTO
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