At present, communication systems such as wireless base stations place very high requirements on reception sensitivity and large signal performance. This article focuses on the mixer-related issues and introduces several key performances of the mixer and the basic parameters provided in the data sheet. The article discusses how to choose the best mixer to optimize the performance of the receiving channel.
Introduction Wireless base station communication standards, such as GSM, UMTS, and (current) LTE, define lower limits for different parameters, including receiver sensitivity and large-signal performance. These key indicators pose design challenges for each radio frequency functional module in the wireless base station. In the received signal path, mixer performance mainly affects the receiver's sensitivity and large-signal performance. This article introduces the key performance and parameters of the mixer, which helps to choose the best mixer when designing the receiving channel.
Wireless base station receiver We first analyze the typical receiver block diagram in a wireless base station (Figure 1). Because the received signal undergoes two consecutive down-conversions and is converted to a lower frequency, these receivers are called superheterodyne receivers. As shown in the figure, the signal is received through the antenna and then filtered by the first-stage RF filter, which is usually used to filter out unwanted signals. Subsequently, the filter output is amplified by an LNA (low noise amplifier), which usually has a very low noise figure.
![Figure 1. Typical block diagram of a wireless base station receiver]()
Figure 1. Typical block diagram of a wireless base station receiver
The amplified signal is filtered again by the second-stage RF filter, which filters out unwanted signals that limit the performance of the mixer and also limits the frequency range. After filtering, the signal with limited bandwidth is sent to the first mixer, where it is mixed with the LO (local oscillator) signal and down-converted to the IF frequency. Depending on the receiver structure, the IF signal can be further down-converted to a lower secondary IF frequency, and then sent to baseband for demodulation processing.
Now, we begin to study the mixer in the receiving link. Because the main factors affecting receiver sensitivity and large signal performance are mixer parameters, they should be carefully analyzed.
Mixer parameters The noise figure of the mixer represents the SNR (signal-to-noise ratio) attenuation from input to output, which is usually expressed in logarithm (dB), as shown in Equation 1:
![Formula 1.]() | (Formula 1) |
Another important parameter is the conversion gain (or conversion loss). The conversion gain is an important basis for judging whether the mixer is configured as an active architecture or a passive architecture. Passive mixers do not contain components that amplify signals, and there is insertion loss (called conversion loss); while active mixers contain active devices that can provide conversion gain.
Two configurations can be used to implement an active mixer: an integrated mixer based on a balanced (Gilbert cell) architecture design, or a passive mixer combined with an IF amplifier stage to provide gain rather than loss. Because the integrated mixer has amplification capability, no additional IF amplifier stage is required to compensate for insertion loss.
![Formula 2.]() | (Formula 2) |
The conversion gain (or loss) is expressed in logarithm and the unit is dB. As shown in Equation 2, it is a function of frequency and is defined in the entire operating frequency range of the mixer. In order to ensure the best reception performance, the change of conversion gain / loss should be as small as possible within the specified frequency range.
Since the wireless base station usually works in a temperature fluctuating environment, the specifications of the conversion gain / loss in the entire operating temperature range should be given, and the variation should be as small as possible. Under normal operating conditions, the smaller temperature variation range requires less design margin, and the design margin is very useful for system planning. Therefore, the temperature range is a very important factor in the design.
The characteristics of the mixer under large signals use a mixer parameter called "1dB compression point" (the index is also called the compression point (IP1dB)) and 2nd and 3rd order intermodulation intercept points (IP2 and IP3) Said. According to the linear expression shown in Equation 3, the IP1dB compression point is used to predict the corresponding input power when the mixer gain is reduced by 1dB:
| POUT = G × PIN | (Formula 3) |
When two large signals with almost the same frequency are applied to the input of the mixer, the mixer should also be able to convert weak signals. This performance is usually expressed as the third-order intercept point (IP3), which together with the noise figure represents the dynamic range of the mixer. The larger IP3 indicates that the linearity of the mixer is higher. The mixer data should also provide the input and output intercept points of the mixer. Using Equation 4, the OIP3 (output intercept points) can be calculated from IIP3 (input intercept points), and vice versa:
| OIP3 = IIP3 + G | (Formula 4) |
In the formula, OIP3 is the output intermodulation intercept point of the mixer, IIP3 is the input intermodulation intercept point, and G is the conversion loss or gain. As a result, for passive mixers, the conversion loss of the mixer is reduced by OIP3. In order to achieve the overall noise figure required by the receiver, the insertion loss should be compensated at the RF or IF gain stage (noise figure is another parameter that must be considered when designing the receiver).
The main advantage of passive mixers and active mixers is that they can also be used as upconverters. In other words, its input signal can be converted to a higher frequency. The upconverter is usually used for the transmission link, which transforms the IF signal to the final transmission frequency. Because the passive mixer can be used for both the transmit link and the receive link, only one device needs to be ordered or a device inventory is reserved.
The "direct down-conversion receiver" directly down-converts the input signal to baseband without IF signal. For this type of receiver, the data sheet of the mixer should specify another important parameter, namely the isolation between ports. This parameter is used to measure the isolation between the LO signal and the mixer input signal. If the isolation between the ports is insufficient, the LO will mix with its own signal, resulting in a DC offset at the output of the mixer, thereby reducing receiver performance.
As the mixer transforms the frequency, it will generate new frequency components (called mixer spurs). A comprehensive analysis of spurious components should be conducted, especially (2RF-2LO), (3RF-3LO) and higher-order spectral components, which coincide with the IF frequency and directly affect receiver performance. This phenomenon is usually indicated by the 2x2 and 3x3 indicators in the mixer data sheet.
In addition to these parameters, the degree of integration must also be considered. Integrating the mixer core with the LO amplifier, balun, and LO switch is very beneficial for some applications.
General PCB receiver layout improves design flexibility. At present, using the same circuit board layout for different frequency ranges can effectively reduce the load of development work. Only a few key components need to be modified to design the 900MHz GSM receiver system for the 1800MHz GSM system.
Pin-compatible mixer series products are ideal for applications that use the same general PCB layout to support multi-band wireless architectures. The ultimate goal is to develop a circuit layout for multiple standard wireless base stations that supports GSM, UMTS, WiMAX â„¢ and LTE applications.
For example, in the receive link, a passive mixer similar to the MAX2029 can down-convert the received signal, while the same mixer can up-convert the IF signal in the transmit link and convert it to the final transmit frequency. All the external components are integrated in the circuit shown in Figure 2: LO buffer amplifier, balun and LO switch.
![Figure 2. Block diagram of a passive mixer]()
Figure 2. Block diagram of a passive mixer
As a downconverter, the MAX2029 can provide 36.5dBm IIP3, 27dBm IP1dB, 6.5dB conversion loss, and 6.7dB noise figure. Because the SiGe process of the MAX2029 greatly improves device performance, it is very suitable for base station applications that require ultra-high linearity and low noise figure.
2RF-2LO suppression (72dBc at -10dBm RF input signal) helps reduce the filtering requirements of harmonic components near the center frequency, thereby simplifying filter design and improving cost performance. The MAX2029 extends the low-end frequency range from 815MHz to 1000MHz. As part of a series of pin-compatible mixers (including the MAX2039 and MAX2041), the MAX2029 allows the receiver to use the same PCB layout to support designs with different frequency ranges and different communication standards.
The active mixer can be either a balanced (Gilbert unit) design or a combination of passive mixer and IF amplifier. For example, MAX9986 uses the second configuration. The lower noise figure allows the mixer to use a very low RF gain, which helps to improve the linearity of the receiver. On the other hand, if the gain of the mixer front stage is increased in order to reduce the series noise figure, the mixer must have a sufficiently high linearity to ensure the overall linearity index of the receiver.
Choosing the correct mixer When searching for mixers from the Internet, it is difficult to filter out the full list of technical indicators of different mixers, and an optimization choice needs to be made. Fortunately, we provide a web-based parameter search tool to help you complete this task. Design engineers can use the parameter search tool to quickly find the most suitable IC. All search requirements can be displayed on one webpage, and the screening results and related models are listed. Changing any search condition will immediately refresh the model list. The search function includes: click the filter box, slide filter control, multi-level filter and other prompt tools. Provides a most convenient way to find models.
The search results shown in Figure 3 list the active mixers designed for base stations with a gain of 10dB. The recommended model is MAX9986. Click the model link, you can directly enter the device's quick browsing web page to find related data, application notes and other more information.
![Figure 3. The web tool is able to list products that meet the filtering criteria — before the user makes a decision!]()
Figure 3. The web tool is able to list products that meet the filtering criteria — before the user makes a decision!
Using Maxim's web tool to perform parameter search, you can get the product model that meets a set of screening conditions-before the user makes a decision. The "smart" search algorithm only displays devices that meet specifications. The user cannot choose to exclude all models. The parameter search tool uses the latest version of Web 2.0 technology, without the need to install any plug-ins on the user's system.
Industrial Fast Ethernet Switch is 10/100Base-TX to 100Base-FX Industrial Fast Ethernet Ethernet Switch. It will extend transmission distance of network up to 120Km via fiber port, and the UTP port can operate at 10M or 100M application.
industrial fast Ethernet switch is packaged in a compact IP40 case that allows either DIN rail or panel mounting for efficient usage of cabinet space. It accepts an integrated power supply with a wide range of voltages for worldwide operation. It can also be equipped dual-redundant, reversible polarity 12V DC to 48V DC power supply inputs for high availability applications requiring dual or backup power inputs.
Industrial Fast Ethernet Switch
Industrial Fast Ethernet Switch,Mini Industrial Ethernet Switch,Industrial Fast Network Switch, Industrial Ethernet Switch Din Rail Mount, Industrial Slim Type Fast Switch, Industrial 10/100M Ethernet Switch
Shenzhen N-net High-Tech Co.,Ltd , http://www.nnetswitch.com