A Frequency Discriminator With a Small Footprint

Product Feature

A Frequency Discriminator With a Small Footprint

Merrimac Industries Inc.
West Caldwell, NJ

Delay-line frequency discriminators have long been fundamental components in radar warning receivers, instantaneous frequency measurement receivers (IFM) and angle of arrival interferometers. They offer a high performance alternative to I and Q networks in these applications, and deliver instantaneous multi-octave bandwidths, short pulse measurement capability, high frequency accuracy and broad dynamic range. However, when realized with discrete components, they can be relatively large and heavy considering the function they perform. The PHC-2D-1.0G dual-speed delay-line discriminator is designed to replace discrete designs while delivering better performance in only half the size and five percent of the weight.

The PHC-2D-1.0G discriminator is designed to operate from 750 MHz to 1.25 GHz, although other frequency ranges up to 40 GHz in multi-octave bandwidths can also be produced. The overall network consists of six basic building blocks: a power distribution network, two delay lines, two phase correlators (each including a power divider and three quadrature hybrids) and eight diode detectors.

Fig. 1 The PHC-2D-1.0G discriminator's block diagram.

The discriminator assembly, shown in Figure 1 , consists of five power dividers, six quadrature hybrids and eight video detector circuits, all contained in eight layers of bonded material that measures 2.0" x 2.0" x 0.25" and weighs 1.5 oz. Input and output ports consist of an RF input port, two pairs of delay-line ports and eight detected video outputs in a surface-mount configuration. Two phase correlators are employed in the PHC-2D-1.0G discriminator to provide connection to the individual delay lines. The first is connected to a long (fine) delay line to provide accurate frequency measurement resolution, and the second is connected to a short (coarse) delay line to resolve frequency ambiguities generated by the fine delay-line correlator.

The discriminator is fabricated in Merrimac's Multi-Mix technology, which produces multi-layer integrated circuits that can include both active and passive components such as MMICs, mixers, filters, quad hybrids, power dividers and couplers. The technique allows single-and multi-function components to be fabricated in much less space than conventional discrete designs, without sacrificing performance or durability. High temperature and pressure-fusion bonding is employed to unify the eight layers of material, and no bonding films are required, which significantly reduces dispersion and insertion loss. The device is realized on a Rogers microwave laminate, which is very temperature stable and has a coefficient of thermal expansion almost identical to that of copper and aluminum.

The discriminator achieves many of its benefits because of the inherent characteristics of the Multi-Mix process. For example, rather than having to lay out circuits in the same plane as would be in a design based on discrete components, the multi-layer technique inherently provides a higher level of integration, which reduces the component's overall size. Discrete designs also require the video circuit to be separate from the RF board, which requires a separate assembly and jumpers to connect the two functional blocks. Also, the discriminator does not require heavy coaxial delay lines, since printed circuits are used instead.

Operational Theory

The delay-line discriminator splits an input sinusoid into a delayed ( ) and undelayed path with relative phase angle between paths of

The frequency of the sinusoid is determined by measuring this phase difference. Each discriminator produces two outputs

= Asin( )

= Acos( )

The phase difference is then determined from performing the operation

Since the delay value is known, the frequency can be determined from the measured phase angle as

The discriminator can be configured in many ways. For example, multi-speed discriminators, multi-correlator arrays, digital frequency discriminators, IFMs and channelized IFMs, can all be produced in Multi-Mix with footprints that are extremely small. Various additional functions can also be integrated within the structure. Typical examples include a limiter or post-limiting bandpass filter, and a log video amplifier to provide log video output, selectable video bandwidth, and RF, video or digital outputs. The discriminator can also include digital post-processing with analog-to-digital converters, timing and control circuits, error correction and calibration. Table 1 lists the discriminator's performance specifications. More information about the PHC-2D-1.0G discriminator can be found at the company's Web site at www.merrimacind.com, or via e-mail at mmfm@merrimacind.com.

Merrimac Industries Inc., West Caldwell, NJ (973) 575-1300, www.merrimacind.com.
Circle No. 303