Among the many performances of radio receivers, "sensitivity" is undoubtedly the most important one, and at the same time, it may be the one that has encountered the most misunderstandings.
I have heard that an OM tried to add a high-gain preamplifier between the antenna and the input of the receiver to increase the sensitivity. Whether this approach is correct remains to be discussed.
The noise-to-signal-to-noise ratio is literally meaningful. We understand that sensitivity is the ability to receive weak signals. To receive weak signals, the general idea is to try to amplify the signal reserves, that is, to increase the gain (Gain) to receive weaker signals, so the receiver with high gain must have a higher sensitivity.
In this paragraph, the first half of the definition of sensitivity is basically correct; but in the second half, the deduction of the relationship between gain and sensitivity differs from the actual situation by 180,000 miles. This characteristic is the most misunderstood.
Before entering the topic, let us talk about the problem of noise.
Turn on the receiver, and when there is no signal coming in, you can usually hear a small "rustle" sound, which is the noise. When a signal comes in, if the intensity is enough, this kind of rustling sound can hardly be heard. However, if the signal is weak, we will turn up the volume of the receiver and want to hear the signal more clearly. In this way, the rustle will become relatively loud. If the signal is weaker, even if the receiver's volume is turned up to the maximum, it is only in vain to raise the sound of "spit", the signal is still unclear.
It can be seen that to receive the weak signal clearly, the problem is not how much to turn up the volume (increasing the gain). If you just want to increase the gain, it is really too simple, just add another amplifier. The key is the relative strength of the signal and noise, whether the signal has sufficient strength, not covered by noise.
This comparison of signal strength and noise strength is called "signal-to-noise ratio" (Signal to Noise Rate) or S / N ratio for short; of course, the S / N ratio is also often expressed in dB.
Noise heard from the audio output of the receiver (such as a speaker). Can be divided into two categories. The first type is the external noise received from the antenna end along with the signal. For this "sky" electrical noise (or background noise), it is difficult for us to make a difference, so we have to listen to fate. The second type is internal noise that has nothing to do with the external environment. Even if the input signal is reduced to zero, the noise can still be heard. This is completely internal noise generated by the receiver itself.
For the second type of internal noise, you who are smart should have noticed that there must be a close relationship with the sensitivity of the receiver.
Noise index and noise coefficient
Describe the internal noise size of a system (such as a receiver), which can be expressed by the noise factor (Noise Factor) F, or take its logarithmic value and become the noise index (Noise Figure) NF.
F = (Si / Ni) / (So / No)
NF = 10 logF = 10 logSi / Ni-10 logSo / No
The addition of noise in the receiver will reduce the signal-to-noise ratio at the output, so the noise factor F must be greater than 1, and the noise index NF value is greater than 0dB.
For a system that can be divided into several stages in series (as shown in Figure 1), the overall noise factor F can be calculated from the gain and noise factor of each stage.
Since the noise of the first stage will be amplified by each stage, the noise factor F that affects the whole is most significant. The second level does not need to go through the amplification of the first level, and the influence is second, ...
The more it reaches the later level, the less significant the impact is, as in formula (1). In the usual calculation, it is sufficient to only consider the second level, and the subsequent levels are negligible.
For common receivers, the input of the antenna is followed by the radio frequency (RF) amplifier stage, and then enters the mixing (MIXER) stage, intermediate frequency (IF) amplifier stage ... as shown in Figure 2.
The mixing stage in Figure 2 is a balanced mixer, so instead of gain, there is a loss, but no noise. Noise factor of the entire receiver:
F = 1.585 + [(1.778-1) / 8 * 0.398] = 1.829
NF = 2.62 dB
Because the gain of the RF amplifier stage is 12 dB, the overall noise index is only increased by 0.62 dB. Of course, increasing the gain of the RF amplifier stage can reduce the noise factor of the overall receiver, but after increasing to a certain level, the effect is not obvious.
Conversely, too high gain of the RF amplifier stage will also cause problems with the mixing stage. Therefore, the gain purpose of the RF amplifier stage, in establishing the overall noise factor, does not need to be too high, about 10dB.
Instead, the RF amplifier stage dominates the overall noise factor. How to select active components with low noise, how to design a bias circuit, and minimize the noise of the RF amplifier stage is the most important issue. According to general HF receivers for special communications, the noise index is about 5-10dB.
Definition and Measurement of Sensitivity In fact, the sensitivity indicated by the receiver manufacturer and the noise factor discussed above, although closely related, are different. The sensitivity usually marked is such as: input impedance 50Q, frequency range 1.8 ~ 30MHz, for 10dB S / N ratio, the sensitivity of 0.2μV. This notation can be used to understand the meaning of the actual measurement method, as shown in Figure 3:
Connect a real rms meter to the audio output of the receiver, and connect a signal generator to the input (must pay attention to impedance matching). First set the signal generator and receiver to a specific measurement frequency, and adjust the signal generator so that its output is zero. At this time, the reading on the rms meter is the internal noise power generated by the receiver itself. Then slowly increase the output of the signal generator until the reading of the rms meter is increased by 10 dB from the original, that is, when the S / N ratio is 10 dB, when reading the output voltage level of the signal generator, if it is 0.2 μV , The sensitivity of this receiver is 0.2μV.
For different measurement frequencies, the receiver will have different noise coefficients. Therefore, to compare the sensitivity of the receiver, it is necessary to specify the measurement frequency and the size of the S / N ratio to make sense. In addition, the different modulation modes need different S / N ratios to clearly copy and receive signals. For example, CW mode requires 3dB, SSB mode requires 10dB, and AM mode requires 17dB.
Therefore, we know that in the same situation, the CW mode can copy weak signals, followed by the SSB mode.
in conclusion
After the discussion, I believe you should have a clear and correct understanding of the close relationship between the receiver's sensitivity and noise. It is the biggest purpose of this article to clarify the misunderstanding of ordinary people on sensitivity and thus establish a correct concept.
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