Mixers, Detectors Serve MM-Wave Bands

Sept. 16, 2011
These DC-biased mixers and detectors can be used as the basis for high-speed or UWB millimeter-wave receivers and transmitters through 110 GHz.

Millimeter-wave frequency bands offer invaluable bandwidth for a variety of commercial, industrial, and military applications. Of course, with increasing frequency comes increasing challenges in the design of the high-performance components needed for signal processing at frequencies of 30 GHz and higher. With ultrawideband (UWB) communications systems, for example, high-speed amplitude-modulation (AM) modulators and detectors are needed in support of fast data transmission over millimeter-wave bands. Fortunately, a series of broadband balanced diode mixers can be used to add AM to millimeter-wave carriers; this is achieved by "triggering" via a transistor-transistor-logic (TTL) signal sent to the mixer's intermediate-frequency (IF) port. The above approach has been demonstrated in support of UWB and high-speed data transmission at data rates exceeding 2 Gb/s at millimeter-wave bands as high as 94 GHz.

Millimeter-wave frequencies, which essentially extend from 30 to 300 GHz, offer 10 times the potential usable bandwidth as the centimeter-band frequencies from 3 to 30 GHz. Unfortunately, for free-space propagation, signal loss increases dramatically with frequency, rendering millimeter-wave communications links shorter-range but inherently more secure systems than their lower-frequency counterparts. For any millimeter-wave link, signals must be generated, transmitted, and detected, which requires the use of high-frequency components capable of such functions as frequency generation and translation (such as oscillators and frequency mixers), amplification (such as amplifiers), and reception (detectors and low-noise amplifiers for receivers). In addition, supporting components, such as power dividers, couplers, and filters, are needed for signal processing.

Because of the limitations of coaxial connectors at higher frequencies, waveguide components usually employ waveguide interconnections designed for use at specific frequency ranges. For example, for operation from 50 to 75 GHz (V-band frequencies), rectangular waveguide (WR) with inside dimensions of 0.148 x 0.074 in. is used to connect signals from one component to the next. Additional rectangular waveguide bands include 26.5 to 40.0 GHz (Ka-band, WR-28), 33 to 50 GHz (Q-band, WR-22), 40 to 60 GHz (U-band, WR-19), and 75 to 110 (W-band, WR-10).

Spacek Labs is a supplier of a variety of components for millimeter-wave transmission and reception, including balanced mixers. The firm's mixers can be designed to cover any full waveguide band from K-band to W-band, or optimized for a narrower portion of any of those bands. Whether used as frequency upconverters or downconverters, the mixers are designed for local oscillator (LO) power levels to+10 dBm, although DC-biased versions are also available for use with lower-output LO sources from 0 to +3 dBm.

When one of these DC-biased mixers is used in an AM transmission application, it can usually provide respectable isolation between the LO and RF portsabout 20 dB. It operates as a high-speed amplitude modulator by sending a transistor-transistor-logic (TTL) signal to the mixer's intermediate-frequency (IF) port while adjusting the DC bias for optimum on/off ratio. This approach has yielded video bandwidth in excess of 15 GHz, which is equivalent to a several-Gb/s data rate.

In a high-speed AM setup (Fig. 1), the millimeter-wave carrier to be modulated is fed to the LO port at a level of 0 to +10 dBm. When the mixer is "on," the insertion loss is typically about 6 dB. When it is "off," the insertion loss is as high as 26 to 30 dB. Optimum results require careful adjustment of LO input power, DC bias level, and video input level. To boost the "on" signal to +10 dBm or higher, broadband RF amplification is available, typically at standard waveguide bands.

These same standard or DC-biased millimeter-wave mixers can also be used to construct an UWB or high-speed-data superheterodyne receiver, along with a wideband IF amplifier and broadband waveguide detector. Most commercial detectors are limited in bandwidth due to an RF blocking capacitor at their video output port. But the DX-2 series of waveguide detectors from Spacek Labs achieves full waveguide band coverage with a high-resistance load, offering tangential signal sensitivity (TSS) of better than -30 dBm for capturing low-level signals over the full band.

Without the video blocking capacitor, these wideband zero-bias Schottky-diode detectors achieve video bandwidths in excess of 3 GHz (the detector's video output port should be loaded with an impedance of 50 Ω for optimum performance). The detector sensitivity is about -10 to -13 dBm, but in a millimeter-wave receiver, a low-noise preamplifier will typically be included to improve the low-level receive sensitivity. If a video bandwidth of less than 1 GHz is required, the video port can be loaded with a higher impedance (as much as 1 kΩ) so that the detector sensitivity at the narrower bandwidth might be improved to about -20 dBm. Figure 2 shows the relationship of detector sensitivity for a typical load reduction.

To demonstrate the use of its balanced mixers in the AM transmitter stages and its Schottky diode detectors in the receiver stages, Spacek Labs developed several custom transmit-receiver systems for high-speed data transmissions at millimeter-wave frequencies. For example, a 45-GHz radio was designed as an extension to an optical Ethernet system operating at data rates to 1.25 Gb/s. The millimeter-wave signal at 45 GHz was obtained from a PIN diode detector, amplified, and transmitted over a distance of about 1 km to a receiver. A model DQ-2 detector with a 50-Ω load was used at the receive end, with two low-noise preamplifiers. A variable attenuator was used to adjust the optimum detector input level. For the reverse path, a model MQ-10B DC-biased balanced diode mixer was used. The TTL signal input to the mixer's IF port was adjusted along with the DC bias to generate a good eye diagram at the opposite receiver output. The RF input signal was generated by a 23-GHz phase-locked oscillator (PLO) and a frequency doubler (Fig. 3).

In another application, a 60-GHz transmitter was developed, along with a companion receiver, for data transmissions in excess of 2 Gb/s at that carrier frequency. The transmit signal was generated by means of a 20-GHz PLO and a frequency tripler. The output power from the TTL-driven model M60-10B balanced diode mixer was 0 dBm maximum and -23 dBm minimum. The companion receiver (Fig. 4) employed a model DV-2P full-band detector and a model 5840B video amplifier from Picosecond Pulse Labs. The amplifier, with a fast 30-ps risetime, provides 21-dB small-signal gain over a 13-5-GHz bandwidth. The complete transmit-receive system achieved a transmission rate of 2 Gb/s or higher over its 60-GHz band (Fig. 5).

Similar transmitters and receivers were developed for high-speed-data and UWB communications applications at different millimeter-wave bands. For instance, a model R80-UWB receiver for use at 75 to 84 GHz was developed with 9-GHz bandwidth centered at 80 GHz. It incorporates a high-speed, broadband IF/video amplifier from Picosecond Pulse Labs to assist with the signal-processing chores. Additional system designs were also developed at operating center frequencies of 35 and 94 GHz.

As has been demonstrated, DC-biased, balanced waveguide mixers are invaluable for use as modulators in millimeter-wave AM transmitters, as well as for front-end devices in millimeter-wave receivers that must process large amounts of information at high data rates. The ZBS series of detectors from Spacek Labs offer video bandwidths in excess of 3 GHz; by coupling them with low-noise preamplifiers and/or video post-amplifiers, they are suitable for use in UWB or high-speed-data receivers that must handle large amounts of transmitted information. The firm offers a full line of millimeter-wave components from 18 to 110 GHz.

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