The I and Q signals frequency near the local oscillator frequency after mixing are output from the secondary of the transformer, and then pass the 3dB directional coupling phase shifter, making the mixed I and Q signals phase-shifted by 90 degrees and added together.
Finally, the modulated signal with suppressed sidebands is output at the RF end of the modulator. If the baseband I and Q signals are single tone signals with the same phase quadrature amplitude, the spectral characteristics of the output end are as follows:.
Figure 2: spectral characteristics of radio frequency output. Fc is the carrier center frequency and fi is the baseband tone signal frequency. The difference between them relative to the power of the enhanced signal in power dBm is the carrier rejection and sideband rejection respectively. In addition, due to the nonlinearity of the mixer, the output frequency also contains the second, third, fourth, and fifth harmonics of the baseband signal and frequency components modulated by the carrier signal.
Take EKIN as an example. Under the action of the local oscillator signal, the two mixer diode stacks inside the EKIN are in a repeated switching state. In order to make the impedance of the local oscillator port close to 50 ohms, the on-resistance of the diode is generally close to ohms. The signals of I and Q ports are low-frequency baseband signals, and the port impedance is equal to the parallel connection of the on-resistance of two diodes the DC resistance of the diodes , which is about ohms.
The coupler is a capacitive transformer component designed according to the port impedance of 50 ohms, so the characteristic impedance of the radio frequency output port is 50 ohms. According to the above port characteristics, the local oscillator and the RF port are designed to match 50 ohms. The signal power of the baseband port is based on the amplitude of the baseband signal, considering the internal resistance of the signal source and the impedance of the baseband port of the modulator as the load to calculate the baseband input power value.
The reason why it is called Gilbert integrated modulator is that it is mainly composed of a local oscillator power division phase shifter, two Gilbert mixers, and an output power synthesis amplifier. Figure 3: Internal structure diagram of Gilbert integrated modulator. As shown in the figure, LOIN and LOIP are the differential input terminals of the local oscillator, internally connected with a two-pole amplification and phase-shifting network in order to improve the phase orthogonality and amplitude balance of the RF phase shifter.
The local oscillator signal is phase-shifted and amplified to provide a local oscillator drive for the Gilbert mixer of the subsequent stage. The I and Q signals are directly superimposed with the local oscillator signal after being mixed in the mixer, and finally, the modulated signal is output after being amplified by the radio frequency amplifier.
The local oscillator end of the Gilbert integrated modulator is generally differential because the differential input is beneficial to improve the rejection of the local oscillator signal. The reason is that after the local oscillator signal passes through the differential amplifier, the differential input form can minimize the common mode local oscillator signal, thereby improving the carrier rejection of the modulator.
In order to improve the accuracy of the RF phase shift of the modulator, the RF phase shift part of the local oscillator of the general modulator contains a phase shift amplification network composed of a multi-stage RC phase shifter and a differential amplifier.
The first stage of some modulators RF phase shifters is an RC phase shifter, and some first stages are differential amplifiers. In design, the differential input impedance of the local oscillator port theoretically should close to the standard impedance of 50 ohms. The bias of the amplifier at the input end of the local oscillator is provided by the energy of the modulator. When there is no integrated DC blocking capacitor inside the local oscillator end, an additional DC blocking capacitor is required at the input end of the local oscillator.
The inside of I and Q baseband signal input ends is the input stage of the differential amplifier, and the input impedance is generally several thousand ohms. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides safeguard of the signal from outside electromagnetic interference.
Coaxial cable conducts electrical signals using an inner conductor generally a solid copper, stranded copper or copper plated steel wire enclosed by an insulating layer and all enclosed by a shield, typically one to four layers of woven metallic braid and metallic tape. The cable is confined by an outer insulating jacket. Normally, the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor. The benefit of coaxial design is that electric and magnetic fields are restricted to the dielectric with little leakage outside the shield.
Equally, electric and magnetic fields outside the cable are mainly kept from interfering with signals inside the cable. Larger diameter cables and cables with various shields have less leakage. This property makes coaxial cable a good choice for carrying weak signals that cannot endure interference from the environment or for stronger electrical signals that must not be allowed to radiate or combine into nearby structures or circuits. The characteristic impedance of the cable is determined by the dielectric constant of the interior insulator and the radii of the inner and outer conductors.
A restricted cable characteristic impedance is essential because the source and load impedance should be matched to make positive maximum power transfer and minimum standing wave ratio. Other significant properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, and shield quality. A coaxial RF connector radio frequency connector is an electrical connector intended to work at radio frequencies in the multi-megahertz range.
RF connectors are typically used with coaxial cables and are intended to preserve the shielding that the coaxial design offers. Coaxial cable is a type of transmission line, used to carry high frequency electrical signals with low losses. It is used in such applications as telephone trunk lines, broadband internet networking cables, high speed computer data busses, carrying cable television signals, and connecting radio transmitters and receivers to their antennas. It differs from other shielded cables because the dimensions of the cable and connectors are controlled to give an accurate, even conductor spacing, which is needed for it to function efficiently as a transmission line.
Widespread applications of coaxial cable include video CATV distribution, RF, microwave transmission, computer and instrumentation data connections. Radio frequencies are electromagnetic waves operating between 10 khz and 1 Ghz propagated without guide wire or cable in free space.
If you have a personal computer that lets you use your home TV set as a video display device, then the computer has a RF generator. This means that this device is generating a RF carrier to carry the video signal information. The term is still used today, even though it now includes video and control signals as well as audio. What precisely does modulating the signal entail? Modulation is transmitting a signal over an extensive range, which will cause to it to suffer from attenuation. Attenuation is a general term indicating a decrease in power loss of signal from one point to anothe r.
This loss can be a loss of electrical signal. The loss is measured as a ratio of input power to output power. The antenna or cable box RF signal. Is that the RF Modulator? You are focusing too much on the RF modulator. It is not needed in the setup I described. I got it!!! The tv had the white, red and yellow cables and it connected to the ones on the back of the VCR!!
The Sharp tv didnt have any of the cables. I do have to remember that when I want to play a tape, make sure that the Imput says Video on it. Usually I post in the sports section and never but you guys have been extremely helpful and patient.
If I need any help, I know where to turn. Im back. Everything else works fine. I can play back tapes, I have the clock set on the VCR. The only thing is that everytime I try to record something, I get a blank blue screen. I see the time on the bottom that it is recording but when I play it back there is no picture. Last edited: Dec 31, Hi Mets. This VCR does not have a built in tuner, so you must connect to something that can receive the cable signal.
If you have a cable box, you should use that. These are items 2 and 8 o the diagram on page 8. If you don't have a cable box, you are going to be forced to find some sort of a tuner. You could use the older broken VCR for the tuner, or obtain a cable box from the cable company. I went back through the posts and you never say if you have cable service. These used to be free, but that time period has expired.
Most cable companies are in the process of removing channels from the old NTSC system and converting them to the more efficient digital system. In order to get approval to do this, the were forced to supply a free box for several years. I probably confused you again, but it does get complicated. In summary, in order to record from the cable or antenna, you need a tuner box because your new VCR doesn't have one. You can get a digital tuner box from Amazon for over the air broadcast, or, if you are on cable, give them a call and they will install one for you.
Previous Next. Post reply. Insert quotes…. Users who are viewing this thread. For this modulation, we need an RF modulator. When the wave reaches the other end, it needs reconversion.
This reconversion to the weak signal or video form is called demodulation. For this purpose, a demodulator is useful.
First of all, the input inserted in the modulator is the signal that you need to modulate. A comparator compares the two signals and then transfers them to the low noise amplifier. You need a deterrent measure to accompany some power losses during this transmission. So, this compensation of power happens due to the low noise amplifier.
It amplifies the signals. The next component in the RF modulator is the mixer. The mixer functions to integrate the signal coming out of LNA with the radio frequency wave. In this way, a modulated signal forms. Nowadays, all the signal modulation completes digitally.
Before transmission to the TV channels, all the signals modulated with the radio waves get a suitable frequency. This specific frequency is the frequency of a particular cable channel that you tune out while channel surfing with your remote.
The exceptional audio and video quality enjoyable these days is the result of these RF modulators. These RF modulators perplexed with video and audio devices made the life of peoples easier.
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