Comparison of Passive Down-Conversion Mixers and Active Mixers

The LTC554x family of passive down-converting mixers covers a frequency range of 600MHz to 4GHz, providing high conversion gain and low noise figure (NF) with high linearity. These mixers are targeted at wireless infrastructure receivers, which require high gain mixers to overcome the high insertion loss of today's highly sensitive IF SAW filters. While traditional passive mixers typically have 7dB of conversion loss, the new LTC554x mixers have an integrated IF amplifier, as shown in Figure 1, which produces a total conversion gain of 8dB. This allows for an additional 15dB of loss in the IF filter while still enabling the receiver to meet sensitivity and spurious-free dynamic range requirements.

Figure 1: LTC554x passive mixer in a receiver application

Active Mixers vs. Passive Mixers

Most integrated circuit mixers are based on active or current steering topologies. Linear Technology has a wide range of active mixer products, such as the LT5527 and LT5557, and these active mixers are widely adopted because they are easy to use and have low power consumption. However, the 2dB to 3dB conversion gain of these active mixers is not high enough to meet the requirements of some wireless infrastructure designs. In addition, active mixers generally have a higher noise figure than passive mixers with comparable linearity. The LTC554x mixer family uses a passive mixer core to achieve the lowest noise figure with high linearity. Table 1 compares the performance of the LTC5541 passive mixer with the LT5557 active mixer. As shown in the table, the passive mixer has about 5dB higher gain, about 2dB lower noise figure, and 1.7dB higher IIP3. However, the DC power consumption of the LT5557 is much lower.

Table 1: Comparison of active and passive mixers at 1.95 GHz

Large Signal Noise Figure

Another important mixer performance parameter is the large signal noise figure. As in amplifiers, the NF of a mixer is the ratio of the input S/N to the output S/N. All mixers will have an increased NF when driven with a high level RF signal. In receiver applications, this phenomenon is also known as “noise figure under blocking conditions”, where the “blocking” signal is a high amplitude signal in an adjacent channel. The increase in noise figure occurs because the mixer’s output noise floor is proportional to the product of the RF input amplitude and the LO path noise (ARF·NLO).

在许多场合中接收器需要在存在强大阻塞信号源的情况下检测一个微弱的信号。如果阻塞信号使噪声层充分地上升，那么所需要的弱信号就有可能淹没在噪声中。图2显示了LTC5540的噪声指数相对于RF输入功率的变化。噪声指数在低输入值时接近小信号值，但是当RF信号功率上升时，ARF x NLO的贡献就变得显著了，而且噪声指数增大了。在+5dBm 的高RF输入值和0dBm的标称LO功率时，噪声指数仅比小信号值提高6dB，达到16.2dB。该图中也很明显的是，大信号噪声随着LO功率值的提高而改进，因此如果需要，甚至可以实现更高的性能。

Figure 2: LTC5540 noise figure versus RF blocking signal value

Although the problem of elevated noise figure cannot be completely eliminated, performance can be improved through careful design. All devices in the LTC554x family have excellent large signal noise figures, as shown in Table 2.

Table 2: Large Signal Noise Figure of the LTC554x with a +5dBm Blocker

Comparison of computational performance in a receiver chain

The following receiver chain analysis demonstrates the benefits of these new passive mixers. Figure 3 shows a typical, single-conversion base station receiver chain that compares the overall system performance of the same receiver using the new LTC5541 passive mixer and the LT5557 active mixer (see Table 3). The LTC6400-26 IF amplifier with a gain of 26dB is used in the LT5557-based link, while the LTC6400-20 with a gain of 20dB is used in the LTC5541-based link. This keeps the overall receiver gain nearly the same in both cases. In each case, a highly selective SAW filter is used at the output of the mixer, as required for high-performance base stations. As shown in Figure 3, the receiver chain using the LTC5541 passive mixer has a 0.76dB lower noise figure and a 1.6dB higher IIP3. This results in a higher signal-to-noise ratio (SNR) and a larger spurious-free dynamic range (SFDR) for the LTC5541-based receiver.

Figure 3: Comparison of a typical wireless base station receiver chain consisting of an LT5557-based receiver and an LTC5541-based receiver

Table 3: Cascaded receiver performance summary

Comparison of measurement performance in a transmitter DPD application

In its simplest form, a single-conversion digital receiver consists of a downconversion mixer, a lowpass or bandpass filter, and an analog-to-digital converter (ADC). In high linearity base station transmitters, this type of receiver can be used as a digital predistortion (DPD) receiver. In this application, the most important performance parameters are linearity, gain flatness, wide IF bandwidth, and of course simplicity. Unlike the receiver applications described previously, in DPD applications, the noise figure is not a critical parameter due to the large amplitude signals coupled from the transmitter output. The LTC554x mixers are ideal devices for DPD receiver applications because they have high linearity, high conversion gain, and flat IF output response when the frequency is changed.

Figure 4 shows a prototype DPD receiver using the LTC5541. This receiver is designed and tested for 1.95GHz applications and has a wide IF bandwidth of 185 ± 60MHz. For comparison, another receiver is designed using the LT5557 active mixer. The LT5557-based DPD receiver requires an external IF amplifier before the bandpass filter to compensate for the 5dB lower gain of the active mixer. The main advantage of the LTC5541 is that it does not require this IF amplifier. In addition, as summarized in Table 4, the LTC5541-based DPD receiver has higher SNR, higher IIP3, and lower harmonic distortion.

Figure 4: Prototype DPD receiver block diagram

Table 4: Measurement results of prototype DPD receiver (RF = 1950MHz, IF = 185MHz)

in conclusion

新的LTC554x无源下变频混频器系列提供了当今无线基础设施接收器所需的高性能。这些混频器兼有高变频增益、低噪声指数、阻塞噪声條件下卓越的噪声指数和高线性度，可改进总体系统信噪比和无寄生动态范围。其卓越的性能还为改进DPD接收器性能做出了贡献，同时，LTC554x系列的600MHz至4GHz频率覆盖范围使这些器件能在多种接收器应用中使用。