Frequency Discriminators for Broadband Applications

Frequency discriminators are widely used in electronic warfare systems for the instantaneous frequency measurement of intercepted signals, allowing rapid identification of threat emitters. This paper reviews microwave discriminator architectures based on multiple delay line, phase measurement with parallel digital processing, for operation over decade bandwidths. An alternative reflection mode discriminator architecture is introduced which exhibits the advantages of simplicity, compact realisation and can be designed over broad bandwidths. The principle of operation, circuit design techniques and frequency measurement performance are described and examples are given of mature designs which have been successfully manufactured in production quantities. INTRODUCTION Instantaneous Frequency Measurement receivers (IFMs) are widely used in ESM systems for rapidly identifying the frequency of pulsed signals. In dense signal environments the frequency parameter allows pulse train deinterleaving to be performed and imminent threats to be identified. The essential component used in the IFM is the frequency discriminator which converts the input microwave frequency into a quantity such as voltage which can then be further processed to give the desired frequency measurement output. This paper outlines the architectures used in IFMs based on conventional discriminators designed with broadband coverage and goes on to describe a circuit configuration which can be easily designed for single band operation over decade bandwidths. ARCHITECTURES Frequency discriminators conventionally use the delay line correlator as the basic measurement element [1] as shown in figure 1. Figure 1. Basic delay line correlator The input signal is split and fed into a phase detector via a delayed and non-delayed path respectively. The phase difference between these two paths is related to the input frequency by = radians Where = 2 and = delay length Figure 2. shows a realisation of such a discriminator using hybrid couplers and detectors to give four output voltages related to frequency by = 1 + cos = 1 − cos = 1 + sin = 1 − sin And the phase angle calculated by = − − For long delay lines the cyclic nature of results in ambiguous frequency measurement with unambiguous bandwidth UBW given by Td RF O