Test the high and low local oscillator of DVB-S2 sensitivity

顾知之

= =A bit worried that the description is not clear

High LO: 10700MHz, Low LO: 9750MHz (can be set, such as 5150 for single LO)

The intermediate frequency is 950~2150MHz.

I use an instrument (SFC) to test the sensitivity. The code stream is available. The frequency points that need to be tested are 950/ 1000 /1200 /1400 /1450/ 1500/ 1550/ 1650/ 1850/ 2000/ 2150MHz. I would like to ask:

1. For the Ku-band, the frequency of the satellite transponder = local oscillator + intermediate frequency. Can we consider 9750+950~9750+2150, that is, 10700~11900MHz as the uplink frequency; similarly, 11550~12750MHz as the downlink frequency?

2. The tested frequency is in the range of 950 to 2150, and the intermediate frequency is also in this range. So when the TV tuner receives it, does the signal contain the intermediate frequency?

3. When testing, for example, for the frequency point 950, I input the frequency: 950 + 9750 = 10700, but I don’t know why. Is it because 10700 corresponds to the frequency range calculated by the low local oscillator mentioned in “Question 1” ? I asked someone, and he said that it would be OK to use the corresponding local oscillator to calculate…

But for dual local oscillators, it is not clear which one is the local oscillator.

The above seems even more confusing and messy. I hope to get an answer. Thank you.

chunyang

The concept of "superheterodyne" is involved here. For a received signal of a specific frequency, a local oscillator signal of a suitable frequency is selected, and then the difference between the two is obtained through mixing, i.e., the intermediate frequency signal, and then further processed. This is the typical structure of a superheterodyne receiver. The high local oscillator, i.e., the frequency of the local oscillator is higher than that of the received signal, and the low local oscillator is lower than that of the received signal, but both of them differ from the received signal by an intermediate frequency. The difference frequency is divided into the upper difference frequency and the lower difference frequency, that is, the question of who subtracts from whom, and the final result is the same intermediate frequency signal.

顾知之

chunyang posted on 2019-8-26 14:58 The concept of "superheterodyne" is involved here. For a received signal of a specific frequency, a local oscillator signal of a suitable frequency is selected, and then mixed to obtain...

Thank you for your answer. I still have a limited understanding of the concepts of "superheterodyne", "LNB", "tuner", etc. However, how should I understand the phrase "received signal of a specific frequency"? I am a bit stuck.

顾知之

Supplement 1: Due to the limitation of the input frequency band of the satellite TV receiver, in order to achieve full-band reception, two local oscillator frequencies, 9.75 GHz (low local oscillator) and 10.7 GHz (high local oscillator), are used simultaneously to mix the input RF signals of 10.7 to 11.9 GHz and 11.6 to 12.75 GHz respectively.

——Satellite downlink frequency refers to the frequency that the satellite transmits to the ground. The first question in the main post is wrong.

顾知之

The RF signal is received by the antenna, enters the high-frequency head, is received by the probe, amplified by the low-noise amplifier (LNA), selected by the image rejection filter, and mixed with the local oscillator to obtain the intermediate frequency signal IF.

Among them, the low noise amplifier is mainly used to amplify the satellite signal, while also meeting the low noise requirement; the local oscillator LO is used to generate a stable oscillation signal to mix with the received RF signal; the image suppression filter is mainly used to reduce the impact of image noise on the noise of the entire system. In addition, at the output end of the mixer, there will be one or two intermediate frequency amplifiers to further amplify the system power gain.

Supplement 2: Since the satellite signal is already very weak before reaching the parabolic antenna (= = how weak?) , and the coaxial cable has a large loss when transmitting high-frequency signals, an LNB is needed to improve the received satellite TV signal.

The working process of LNB (low noise down conversion module) is to first amplify the satellite high-frequency signal to hundreds of thousands times (how large is it at this time? I want to know an approximate order of magnitude ), and then use the local oscillation circuit to convert the high-frequency signal to an intermediate frequency (the intermediate frequency range is determined by the type of LNB) to facilitate the coaxial cable transmission of the signal and the demodulation of the satellite receiver's TV signal. Its core function is to amplify and down-convert the satellite TV signal received by the antenna, and output the intermediate frequency signal IF that the receiver can receive and process.

——I connected the instrument directly to the high frequency head of the board, and then set and searched for channels to conduct the test. What signal did the TV receive at this time? Is it the intermediate frequency signal?

chunyang

Gu Zhizhi posted on 2019-8-26 15:51 Thank you for your answer. I still have only a limited understanding of the terms "superheterodyne", "LNB", "high-frequency head" ...

For example, the frequency of an FM radio station is 100MHz, and you want to listen to the program of this station. The signal of this station is the "received signal of a specific frequency" mentioned above for the radio. For a superheterodyne FM radio, you have to adjust the local oscillator frequency to 89.3MHz to receive the radio station (the "low local oscillator" used by the radio). At this time, the difference between the local oscillator frequency of 89.3MHz and the signal frequency of the radio station of 100MHz is 10.7MHz. Then, after secondary frequency conversion and frequency discrimination, the audio signal is obtained, and the speaker is driven by the audio power amplifier, so you can hear the radio program of the radio station.

chunyang

Gu Zhizhi published on 2019-8-26 16:04 The RF signal is received by the antenna, enters the high-frequency head, and is amplified by the low-noise amplifier (LNA) after being received by the probe. The image rejection filter is selected...

What signal the TV receives depends on the output of the satellite receiver. The output of traditional satellite TV receivers has two types: RF and AV. RF output converts satellite TV signals into VHF signals that ordinary TVs can receive. This output was common in satellite TV receivers in the last century. AV output directly outputs baseband audio and video signals, which requires the TV to have an AV interface. It is only since this century that TVs have generally had AV interfaces. In addition to AV interfaces, S terminals and color difference interfaces are also used in satellite TV receivers. These are all analog video interfaces. In recent years, with the gradual elimination of traditional analog TVs, video interfaces have gradually become digital. Digital interfaces such as VGA, DVI, HDMI, and DP are now mainstream. In terms of home appliances, HDMI interfaces are the most widely used.

chunyang

Gu Zhizhi published on 2019-8-26 15:58 Supplement 1: Due to the input frequency band limitation of satellite TV receiver, in order to achieve full-band reception, 9.75 GHz (low local oscillator) and 10.7 GH ...

It is recommended to clarify the basic concepts first, otherwise the questions you ask are invalid. Not only question 1, but all your questions are problems in themselves.

顾知之

chunyang posted on 2019-8-26 21:02 For example, the frequency of an FM radio station is 100MHz. You want to listen to the program of this station. The signal of this station is what the radio said before...

Thank you for your example. Let me repeat it. If the frequency of a radio station is 100MHz, to listen to the program, the whole working process of the radio is as follows: adjust the local oscillator frequency to 89.3MHz (Excuse me, how do you know it will choose a low local oscillator? I just don't understand this ==), then the local oscillator frequency is mixed with the signal frequency of the program (ie 100MHz) to generate a 10.7MHz intermediate frequency signal, which is processed to listen to the program.

...I can't help but think of the process of watching TV programs (DVB-S). The signal frequency of the TV program I want to watch is 1400MHz, and the LNB has dual local oscillators (low local oscillator 9750MHz, high local oscillator 10700MHz) (when to choose to use the high local oscillator and when to choose to use the low local oscillator).

The program frequency is mixed with the local oscillator to produce an intermediate frequency signal (it can be understood that the TV receives this intermediate frequency signal and then performs other processing)

(I forgot to explain why the problem of "selecting high or low local oscillator" arises. Sorry, when the program signal frequency is 950/1000/1200/1400/1450/1500/1550/1650/1850, the channel search is successful using the frequency mixed with the low local oscillator and the corresponding signal frequency). However, when the program signal frequency is 2000/2150MHz, the frequency calculated by the low local oscillator cannot search for the channel, only the high local oscillator can be used, so I have this problem.

Sorry for so many words again

顾知之

chunyang posted on 2019-8-26 21:16 It is recommended to clarify the basic concepts first, otherwise the questions raised are not valid. Not only question 1, all your questions are problems in themselves.

Thank you for your advice. I was indeed a little too irresponsible to rely on others.

chunyang

Gu Zhizhi posted on 2019-8-27 09:52 Thank you for your example. Let me repeat it. If the frequency of a radio station is 100MHz, to listen to the program, the entire working process of the radio is as follows: ...

Generally speaking, the lower the frequency, the lower the requirements for the process and materials of device manufacturing. Correspondingly, the lower the cost, and products always pursue cost-effectiveness. When the frequency is high enough, such as satellite receivers, the cost difference between high local oscillators and low local oscillators is almost negligible. At this time, factors such as intermediate frequency interference must be considered, so there is the option of high local oscillators. Most superheterodyne radio receivers use low local oscillators.

顾知之

chunyang published on 2019-8-27 10:39 Generally speaking, the lower the frequency, the lower the requirements for the process and materials of device manufacturing. Correspondingly, the cost is naturally lower. Products always pursue cost-effectiveness...

Hmm, this is another level of analysis, thank you for your answer (〃'▽'〃)