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Detailed RF transceiver system architecture

Release date:2021-12-28Author source:KinghelmViews:523


Today we will continue to learn about RF system architecture. When designing transceivers for wireless communication systems, we may consider many factors, such as performance, cost, energy consumption, weight and volume. For example, in the current 5g AAU design, more and more operators may pay attention to the energy consumption, weight and volume of the AAU. After all, the electric tiger is a little overwhelmed, and the tower also has corresponding load requirements. Therefore, we see that more and more AAUs with small volume, light weight and low energy consumption have been developed. For example, Ericsson launched a revolutionary product with originality on September 29 - a medium frequency Mini AAU product weighing only 12kg - air 3268. Air 3268 weighs 12kg, has a volume of 23L, has 32 channels, the total carrier bandwidth is 200MHz, the output power is 200W, and the power consumption is reduced by about 10%.


 From Ericsson's official website www.ericsson.com com

This depends on what architecture our RF system chooses. The most commonly used architectures of RF systems are superheterodyne architecture and zero if architecture.


No1 superheterodyne architecture

At present, most wireless communication systems use superheterodyne structure. For example, in 2G, 3G and 4G communication systems, this kind of superheterodyne transceiver is the most common. Compared with other structures, this structure has better performance. However, on 5g, the simpler zero if structure is more commonly used because.

Let's first look at the history of superheterodyne architecture. It was invented by Edwin Howard Armstrong during and just after World War I and patented in 1918. The most awesome thing about this man was that he began to study radio when he was still in high school. A 125 foot high radio mast was erected at his parents' home in Yonkers, New York, to receive weak radio signals at that time. When he was still in college in 1912, he invented the feedback circuit based on Lee de forest three terminal audio tube, provided the first available electric amplifier, submitted the patent of regenerative receiver in 1913, invented superheterodyne receiver in 1918, and invented FM broadcasting.


Speaking of superheterodyne architecture, Maybe many students don't quite understand the word "superheterodyne". I have always wondered why it is called extrapolation and whether there is interpolation. The word "heterodyne" was put forward by Reginald Aubrey Fessenden in 1901. He called the idea of mixing to produce new signals "heterodyne" ", and a receiver architecture with primary mixing structure is given, which is called heterodyne receiver, as shown in the figure below: it requires a mixer to bring the modulated RF signal into the modulated IF signal, which is applied to the I / Q demodulator to bring the modulated low and medium frequency band into the baseband of zero if.


Armstrong improved the heterodyne receiver and invented the superheterodyne receiver. Superheterodyne is a receiver with two or more mixing structures, as shown in the figure below. In a superheterodyne receiver, two mixers are needed to convert the modulated RF signal into a modulated IF signal. The first mixer brings the RF signal into the high if signal, and the second mixer brings the high if signal into the low IF signal. This applies to the I / Q demodulator, which changes the low IF signal into a zero if baseband signal. When it comes to mixing, we are familiar with it: when the signal received by the receiver from the antenna and the signal generated by the local oscillator are input into the mixer to obtain the IF signal, or the IF signal is mixed into RF signal in the transmitter, it is superheterodyne. In the superheterodyne structure, we convert the signal through a mixer. This frequency conversion process may occur more than once. The superheterodyne architecture will have multiple if frequencies and if modules.


After understanding the basic structure of heterodyne and superheterodyne, we introduce the structure diagram of superheterodyne transceiver commonly used in wireless communication system, as shown in the figure below. The superheterodyne receiver link usually includes RF part, if part and baseband BB part.

The RF part of the receiver includes a duplexer as a frequency preselector, a low noise amplifier (LNA), an RF band-pass filter (BPF), an RF amplifier as a mixer preamplifier, and an RF to if down converter (mixer).

The down converter is followed by an IF amplifier (FA), followed by an if BPF for channel selection and suppression of unwanted mixing products.

The I / Q demodulator is the second frequency converter, which down converts the signal frequency from if to BB. The demodulator consists of two mixers, It converts the IF signal into I and Q signals - two 90 "phase shifted BB signals. Low pass filter (LPF) The mixer is followed in the I and Q of each channel to filter out unwanted mixing products and further suppress interference. The filtered I and Q BB signals are amplified by the BB amplifier, and then the ADC converts the amplified BB signal into a digital signal for further processing in the digital baseband. Similar to the superheterodyne receiver, the superheterodyne transmitter is also composed of BB, if and BB.

The gain control of the if part accounts for about 75% or more of the whole gain control range. It is rare to realize gain control in the analog BB part of this radio architecture. The reason is that the BB part in the receiver or transmitter has I and Q channels, and it is difficult to maintain the amplitude imbalance of I and Q channels within the allowable tolerance within the variation range of BB gain.

No.2 direct conversion / zero if architecture

The superheterodyne receiver with mixing module is introduced above. Is it possible not to use mixing module? Therefore, RF scientists began to use direct frequency conversion radio transceiver around 1980. Direct frequency conversion means that the RF signal directly enters I / Q demodulation without going through the IF stage and is converted to the baseband signal, and no IF signal is generated in the middle. Therefore, it is also called zero IF receiver, as shown in the figure below.

As shown in the figure, the lo (local oscillator) frequency is set to the required frequency, so the received signal is directly converted into baseband I (in-phase) and Q (quadrature phase) signals. In this architecture, both DAC and ADC operate at baseband sampling frequency. The transceiver based on this zero if architecture is called} zero if transceiver.

The direct frequency conversion architecture has many superior characteristics. The RF signal received by the receiver does not need to go through the IF stage and directly goes to the I / Q demodulator and enters the baseband part, which reduces the expensive if modules in the superheterodyne architecture, such as mixer and if filter, so the cost and size of this part can be reduced, As described in "zero if architecture, this post explains thoroughly", zero if architecture is easier to integrate into an RFIC.

No3 direct RF sampling

Further, can we conduct direct RF sampling and directly sample digital signals into RF signals for transmission and reception? Of course, it depends on the conversion rate of AD / DA. If it can directly achieve RF consideration, it is not impossible. And the conversion rate of AD / DA is also increasing. The sampling rate of analog-to-digital converter (ADC) and digital to analog converter (DAC) of major semiconductor companies is several orders of magnitude faster than that of products ten years ago. For example, in 2005, the sampling rate of the world's fastest 12 bit resolution ADC was 250 ms / S; By 2018, the sampling rate of 12 bit ADC has reached 6.4 GS / s. Due to the improvement of these performances, the converter can directly digitize the RF frequency signal and provide sufficient dynamic range for modern communication and radar systems.

The above figure shows the receiver architecture of direct RF sampling, which is only composed of low noise amplifier, appropriate filter and ADC. The receiver in Fig. 2 does not need to use a mixer and lo; The ADC directly digitizes the RF signal and sends it to the processor. In this architecture, you can implement many analog components of the receiver through digital signal processing (DSP). For example, you can use direct digital conversion (DDC) to isolate terminal signals without using a mixer. In addition, in most cases, you can replace most analog filters with digital filters in addition to anti aliasing or reconstruction filters.

Since no analog frequency conversion is required, the overall hardware design of the direct RF sampling receiver is much simpler, which can achieve smaller composition structure and lower design cost.

Ending

In addition to the several common RF transceiver architectures mentioned above, there are many others, such as the neutral architecture between superheterodyne and zero if - low if architecture, and the software radio SDR architecture that we have been in contact with for a long time but has not yet been called a reality. Maybe one day, we can see the real software radio in the application of wireless mobile base stations.

Back to the 5g AAU mentioned at the beginning of the article, how to achieve small volume and lightweight, I want to leave the development of RF architecture, which is also a myth.


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