Active Mixers Deliver High IP3

Oct. 1, 2003
A pair of active double-balanced mixers provides the frequency range and linearity needed for a host of broadcast and wireless communications applications.

Active mixers have long been associated with the ability to work at low local-oscillator (LO) levels, but falling short on linearity performance. Fortunately, a pair of active mixers has been developed based on a high-frequency bipolar process from Linear Technology. These plastic-packaged mixers are suitable for frequency upconversion and downconversion functions, respectively, with linearity comparable to passive diode mixers.

Passive diode mixers are widely used in wireless and cable-television (CATV) infrastructure equipment for their high linearity. Their need for high LO signals requires the addition of LO amplifiers, which also add cost and compromise isolation. The high LO levels can lead to signal leakage, and the need for additional filtering. Passive diode mixers also suffer from high sensitivity to the LO-signal input amplitude, therefore requiring tight control of LO signal flatness. Passive mixers also typically have 6 to 7 dB of conversion loss, calling for additional amplifier gain to compensate for the loss.

As an alternative to passive diode mixers, the LT5511 upconverter mixer and the LT5512 downconverter mixer were developed to provide high linearity at lower LO levels (see table). They provide comparable input third-order-intercept (IP3) performance as a diode mixer, but with 20 dB lower LO drive. The LO leakage is about 20 dB better than for a passive mixer, with about 7 dB higher output IP3 than a diode mixer.

The linearity of these active mixers has been achieved, in part, through careful optimization of the mixer core designs. Their integrated LO amplifiers are optimized for stable, high-speed switching. Stability problems common to other designs often require the addition of ferrite beads and resistors to the application board on which the mixer is mounted. The active mixers can be driven by single-ended LO sources, even beyond 2.5 GHz. They are designed to tolerate wide variations in input LO power with negligible impact on mixer performance.

Both the LT5511 upconverter (Fig. 1, top) and LT5512 downconverter (Fig. 1, bottom) mixers use an optimized double-balanced mixer core with the transistors' bases driven by an integrated LO buffer amplifier. Precision integrated bias circuits ensure high performance over temperature; both ICs provide an enable control input for a power-down function. The mixers have differential RF, LO, and intermediate-frequency (IF) ports that allow flexibility of impedance matching for use in a wide variety of applications. Careful die layout and well-planned package pinouts yield good port-to-port isolation and linearity. The LT5511 is packaged in a 16-lead TSSOP while the LT5512 is supplied in a 16-lead, 4 × 4-mm QFN housing.

The LT5511's IF input is simple to match. Two resistors set the current through the mixer core, while two DC blocks (for symmetry) provide DC isolation between the IF+ and IF- input ports. A capacitor across the IF ports reduces LO leakage. An IF balun performs an impedance transformation and single-ended-to-differential conversion. With a differential signal source, such as a digital-to-analog converter (DAC), it may be possible to eliminate the input balun.

The RF output match is realized with a pair of inductors, or transmission lines, followed by a balun. The match can be optimized with a shunt capacitor at the output. The LO port is matched with a shunt 62-Ω resistor and a DC blocking capacitor for frequencies below 1.5 GHz. At higher LO frequencies, the match requires only a shunt-series inductor-capacitor (LC) combination.

The LT5512's RF input port is easily matched for applications between 10 MHz and 3 GHz. RF matching consists of one capacitor and a balun for single-ended-to-differential conversion. The center tap on the RF balun also provides a DC return path for the RF buffer amplifier. The IF output is also simple to match, requiring two bias chokes and a capacitor to set the output IF. An IF balun may be used for differential-to-single-ended conversion. The LO port is matched with a shunt resistor and DC blocking caps. A balun is not required for the LO port.

The active mixers are well suited for use in a CATV downlink transmitter for analog and digital television. In a simplified block diagram for this application (Fig. 2), variable attenuators required for precise output power control are not shown. In this architecture, a 44-MHz input signal from the DAC is filtered and upconverted to 1230 MHz by the LT5511 using a fixed LO. A bandpass filter attenuates the image frequency and any unwanted spurious. The resulting signal then drives the RF input of the LT5512. A wideband LO feeds the LT5512 to downconvert the 1230-MHz IF to the desired output frequency in the 54-to-870-MHz band. In this application, all spurious products within the CATV band must be 60 dB below the desired signal level.

The good linearity of these active mixers also makes them well suited for low-distortion cellular applications, including multichannel receivers (Fig. 3). In this receiver, the LT5512 downconverts PCS input signals to a 140-MHz IF. The mixer's differential IF output is matched directly to a differential SAW filter, eliminating the need for an IF balun while preserving the benefits of differential signal processing to the ADC.

On the multiple-carrier transmit side, the LT5511 can be used to upconvert multiple carriers from a DAC directly to the transmit frequency (Fig. 4). The LT5512 is then used in a pre-distortion feedback loop to downconvert a sample of the transmit signal for digital processing.

To demonstrate its versatility, the LT5511 was evaluated in a low-frequency downmixer application requiring high linearity (Fig. 5). In this case, the mixer had an input frequency range of 20 to 50 MHz and an output frequency of 10 MHz. On the IF input, the values of the components have been increased to accommodate the lower frequencies and a series-shunt LC combination prior to the transformer was added to optimize the impedance match. The output match is very simple, requiring only the balun, a series inductor, and shunt capacitor. On the LO port, a larger DC blocking capacitor was used for the lower-frequency coverage. At an IF of 25 MHz and an LO level of −10 dBm, conversion gain was 0 dB, input IP3 was +18.8 dBm, and LO leakage to the output was −41 dBm (Fig. 6). The measured input second-order intercept (IP2) performance was +61 dBm.

Another example of the flexibility of these mixers is a triband downconversion application for the LT5512 in which a bandpass RF input matching network supports operation at the 900, 1800, and 1900 MHz GSM bands (Fig. 7). The IF port is matched for 270-MHz operation. The design achieves 0.7 dB conversion gain at 900 MHz with 0.3 dB conversion gain at 1800 MHz and only 0.2 dB conversion loss at 1900 MHz. The input IP3 levels at 900, 1800, and 1900 MHz, respectively, are +18.3, +18.2, and +20.6 dBm.

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