A Digital Quadrature IF Mixer

A Digital Quadrature IF Mixer

Anaren Microwave Inc.
East Syracuse, NY

A digital quadrature IF mixer (DQIFM) combines two proven technologies to produce a single RF-to-digital solution for front-end conversion in microwave signal processing applications. The device incorporates broadband RF stripline technology with high speed digital conversion, providing an in-phase and quadrature digital signal representation of the RF input spectrum. The DQIFM consists of a stripline QIFM integrated with a PCB-implemented video/digital processing section. The QIFM provides two equal-amplitude in-phase/quadrature IF outputs. The DQIFM’s functional block diagram is shown in Figure 1 .

QIFM Operation

The QIFM is implemented in stripline circuitry and consists of two balanced mixers and two hybrids. The LO is introduced to the two in-phase mixers and the RF is introduced through a quadrature hybrid. This process introduces a 90° phase difference between the RF and LO signals applied to the mixers and produces two in-phase/quadrature IF outputs that are of equal magnitude. One IF output leads or lags the other depending on whether the RF is greater than or less than the LO frequency. If the LO signal is a fixed frequency, the desired IF output can be produced by an RF signal above or below the LO frequency and is expressed as RF = LO ± IF.

The operation of a QIFM can be understood by following the signal voltage vectors. With a high level LO signal b (typically greater than 0 dBm), an RF input a produces IF outputs equal to k1(a/Ö 2) and k2(a/Ö 2) where k1 and k2 are proportional to the conversion losses in mixers 1 and 2. The vectors show the relative phase of the IF outputs for the upper (RF – LO) and lower (LO – RF) sideband responses. When the RF is below the LO frequency, the IF response (LO – RF) from mixer 1 lags the output of mixer 2 by 90°. When the RF is above the LO, the IF response (RF – LO) leads the output of mixer 2 by 90°.

The operation of a QIFM is shown in Figure 2 .

Another way of looking at the operation of a QIFM is to realize that the two IF outputs are proportional to a sinq and a cosq , respectively. (q is the phase difference between signals a and b.) Note that q can be a fixed angle if the two frequencies are identical, or can be time varying at the difference frequency (Dw t = IF). Since the IF outputs are proportional to sin(a – b)t and cos(a – b)t, respectively, and since sin(–q ) = –sinq and cos(–q ) = cosq , one output changes phase by 180° relative to the other when a > b or a < b.

The two IF voltages (a sinq and a cosq ) are the in-phase and quadrature components of the correlation of the RF and LO signals. The correlation time is the inverse bandwidth of the IF outputs. The QIFM is identical in design and application to a correlator (phase discriminator) provided one of the QIFM inputs (a or b) is strong enough to act as an LO. (A correlator uses DC-biased diodes, which enable it to operate with both a and b signals at low levels.)

The A/D Circuit

The DQIFM adds the high speed analog-to-digital (A/D) circuitry to provide the digitized quadrature output signals. The in-phase and quadrature videos are differential signals that provide good noise immunity and common-mode rejection for the DQIFM. The video signals from the mixers are buffered and amplified prior to filtering by a balanced five-pole lowpass filter. The filtering is designed for a nominal 2 dB bandwidth of 15 MHz with 13 dB of alias signal rejection at 25 MHz offset from the LO, as shown in Figure 3 . After filtering, the video signals are further amplified prior to the A/D converter (ADC). Within this final stage of amplification, the gains and offsets are adjusted to match the quadrature signals.

The A/D conversion function is performed using a 10-bit monolithic sampling ADC. An advantage of sampling with the in-phase/quadrature approach over a single-ended design is that, for the same instantaneous bandwidth, the ADC can operate at half the clock rate of a single-ended design, which increases the ADC effective number of bits and provides better performance.

An example of the good spectral processing that is achieved using this approach is shown in Figure 4 . In this test, an RF signal at –5 dBm (offset 10 MHz above a 2 GHz LO) was applied at the RF port. The LO is 2 GHz at a 19 dBm power level. The in-phase and quadrature digitized data were stored for 8192 samples at a 40 MHz clock rate. The sampled data then were processed digitally through a fast Fourier transform (FFT) using a Hamming weight. The plot shows good spectral purity with the image at –35 dBc. For this test, the DC offsets were removed digitally prior to FFT processing.

Product Implementation

The DQIFM comprises a multilayer laminated assembly that combines the RF, digital and analog circuitry, as shown in Figure 5 . The RF portion is a multilayered stripline assembly containing surface-mount, blindmate coaxial connectors, an in-phase power divider, 3 dB hybrids, detector diodes and terminations.

The RF and LO inputs are distributed through the stripline layers and converted to IF through recessed detector diodes installed directly on the stripline layer. This configuration allows the RF stripline section to be totally enclosed within a plated assembly and eliminates the need for additional shielding or isolation. The video and A/D section comprises eight PCB layers and is bonded directly to the RF section. The detected in-phase/quadrature differential video signals are fed to the ADC section through plated through holes.

By scaling the RF components, the RF section can be modified to operate anywhere in the 0.5 to 6 GHz frequency range with the complete assembly packaged easily within a 3U (or smaller) VME slot, depending on the operating frequency. The assembly operates from ±5 V DC supplies, dissipating less than 2 W of power. Table 1 lists the DQIFM’s key specifications.

Applications

DQIFMs can be used in several applications, such as making fine frequency measurements where the amount of resolution is limited only by observation time and the stability of the observed and LO signals, which allows the user to measure RF to single-hertz resolution. DQIFMs also can be used as demodulators for phase-shift keying or as network analyzers to evaluate other RF components. In addition, DQIFMs can be used in signal parameter extraction for signal analysis and characterization. Since the QIFM essentially is a phase discriminator, DQIFMs also can be used as phase discriminators when the LO and RF are located at the same frequency provided there is enough power at the LO port to bias the diodes. Enhancements can be achieved in image-rejection performance by conducting digital calibration where the residual phase, gain and offset imbalances are removed digitally to further drive down the image term.

Conclusion

The DQIFM provides a compact and efficient implementation for the front-end converter in microwave signal processing applications. It is a low cost, single-package RF-to-digital solution with small size and low power. In-phase/quadrature processing enhances the ADC’s efficiency while maintaining instantaneous bandwidth. Enhancements can be achieved in image rejection by performing digital calibration where the residual phase and gain imbalances are removed digitally to further drive down the image term. What was once a complex interface of transitions from RF to video to digital is now an easy-to-utilize component that can be incorporated easily by the system or subsystem designer.

Anaren Microwave Inc.,
East Syracuse, NY
(800) 544-2414.