GB2320986A - Delay line frequency discriminator - Google Patents

Abstract

RF input 20 in split 14 into two components fed to a recombining coupler 15, one via a delay line 17. Coupler 15 provides two signals each containing both components to two detectors 18,19 whose outputs are differenced (not shown) to provide a highly linear discriminator. The detector outputs are combined to control diode 23 to maintain the input level constant. Further linearising is achieved by compensating for reflections at mismatches in the detectors. The circuit is constructed in microstrip form and provided with absorbent material to prevent spurious radiation.

Description

FREQUENCY DISCRIMINATOR AND RADAR TRANSMITTER INCORPORATING SUCH A DISCRIMINATOR This invention relates to a frequency discriminator and to a radar transmitter in which such a discriminator is used more particularly but not exclusively a radar transmitter for forming part of a target seeker mounted on board a guided weapon, for example a terminally guided, ballistically launched bomb.

It is especially desirable for a such a radar transmitter to be of small size and low cost so the use of large scale integration (LSI) of the electronics is desirable. Microwave source components suitable for LSI tend to have highly non-linear drive voltage to output frequency characteristics. This raises a problem if the radar output signal is to comprise a repetitive frequency sweep (chirp) with the frequency varying linear sawtooth generator driving a microwave source is used, the frequency of the source output will be non-linear with time. The source could be driven by a voltage waveform shaped to compensate for its non-linearity, but, especially since the source linearity may depend on external parameters such as antenna loading, complex and time consuming set up of each transmitter may also be necessary. Another proposal is to top the source output, down-convert and use a low-frequency discriminator to derive a feed-back signal for controlling the source frequency. However, the down-conversion step increases the complexity and volume of the transmitter.

Thus an object of the invention is to provide a radar transmitter using a microwave source which may have a non-linear voltage to frequency characteristic but in which linearity is achieved by way of a feedback arrangement not involving any frequency down-conversion. A further object is to provide a small compact frequency discriminator capable of operation in the microwave band, eg in the frequency range from 75 to 95 GHz.

According to one aspect of the invention a delay line frequency discriminator comprising a delay line and a reference line through which components of an R.F. signal are transmitted, decoupling means for producing said components; means for spliting each component into a signal for each of a pair of detectors, which compare the signals, wherein the signal at each detector is a combination of said components;a levelling means which receives a signal from said detectors so that the input power is kept at a constant level; further comprises a frequency adjusting means; an absorber for absorbing any spurious radiation; and a means for using reflections from said pair of detectors to improve the extent of a linear range in the discriminator.

Reference will now be made by way of example to the accompanying drawings, in which: Figure 1 is a block diagram of an open loop transmitter; Figure 2 is a block diagram of a closed loop transmitter; Figure 3 is a cross-sectional view of a microstrip line; Figure 4 is an adjustable frequency discriminator with a leveller; Figure 5a is a graph of the Diode 1 (D1) response; Figure 5b is a graph of the Diode 2 (D2) response; Figure Sc is a graph of (D1-D2) response; Figure 5d is a graph of the slope of figure 5c; Figure 6a is a graph of power versus normalised frequency response of the discriminator around outputs to diodes D1 and D2; Figure 6b is a graph of the slope versus normalised frequency of figure 5c; Figure 7 is a cross-sectional view of a microstrip covered with a sheet of dielectric material; Figure 8 is a plan view of a resonant absorber in a microstrip circuit; and Figure 9 is an isometric view of the figure 8 resonant absorber.

Ballistic weapons of the future will require "SMART" radar systems, small in size, low in cost and maybe readily manufactured. The advent of surface mount technology and large scale integration of the electronics has promoted the development of small low cost millimetric transceiver systems.

The millimetric transceiver presents a considerable challenge when considering low cost, small size and ease of manufacture. A particular problem is encountered when wideband chirp is required with linearity of the order of 1%. The source has a frequency/voltage characteristic far from linear (eg + 20% from ideal) and is dependant on antenna loading etc. This tends to rule out pre-shaping of the oscillator drive waveform as set up requirements are complex and increase manufacturing time. Down-converting the source using a low frequency discriminator in a closed loop configuration increases the complexity and volume of the transceiver. A direct measurement of the source frequency can be made with a "simple" delay line discriminator which when used as the feedback element in a closed loop will provide high linearity chirp. The simple delay line discriminator offers low cost, small size and can be readily manufactured.

When an open loop transmitter as is shown in figure 1, transmits an input signal 1 from a modulator 2 through an oscillator 3, the output 4 has non linear frequency time characteristics. This is due to the non-linear voltage-frequency characteristic of the oscillator which is a fundamental property of microwave sources in general.

In order to obtain a linear frequency - time characteristic a closed loop control may be used to compensate for the oscillator non-linearity. This is shown in figure 2. The modulator output 5 is passed via a junction 6 and an amplifier 7 to an oscillator 8. A discriminator 9 and another amplifier 10 act in a feed back loop 11 to adjust the resultant input signal in order that the oscillator output 12 has a linear frequency-time characteristic.

The discriminator in the feed back loop may take the form of a microstrip on a thin low dielectric constant substrate. Figure 3 shows a microstrip delay line discriminator in which the microstrip line 13 has a nominal substrate thickness of about 76cm, dielectric constant of about 2.1 and line width of 0.1 3mm which has a characteristic impedance of approximately 7 On . Figure 4 shows an embodiment of the invention. It consists essentially of two branch line couplers 14 and 15, a reference line 16, a delay line 17 and a pair of mixer diodes D1 and D2 numbered 18 and 19 respectively. The delay line 17 and one of the mixer diodes when it is operating as a phase detector have the combined effect of a frequency discriminator.

An R.F. input signal 20 is split approximately equally between the reference line 16 and the delay line 17 by the branch line coupler 14. The fourth port or isolated port 21 is terminated in a matched R.F. load 22. In an ideal discriminator no power would flow into this port as it would be correctly terminated in the characteristic impedance for which the device was designed. However, due to imperfections in circuit manufacture and reflections from the diodes, 18 and 19, the matched R.F. load is required.

The power split is a function of the branch impedances.

These impedances have also been adjusted to maintain a proper match over the operating frequency band. In the discriminator design, both branch line couplers, 14 and 15, have been designed to provide half power to each arm (ie a 3dB coupler). The power entering coupler 15 from the reference line 16 splits approximately equally between the diode ports 18 and 19. Similarly, the R.F. signal from the delay line (17) path is split evenly between the diode ports. The outputs of the diodes 40 and 41 respectively, are a function of the phase difference between the delay line and reference line paths. Figure 5 (a) and (b) shows the ideal diode output responses with a path difference of 3254L (where < L = microstrip wavelength) between the delay line and reference line. Figure 5 (c) shows the difference response and Figure 5 (d) the slope of this characteristic.

The microstrip discriminator was modelled using for example, Super Compact and the power arriving at the diodes 18 and 19 versus normalised frequency is shown in Figure 6 (a). Figure 6 (b) shows the differential of the difference curve of figure 5 (c) which has a percentage linearity of =0.1% over a frequency deviation of +0.002to where fO is the central frequency of the frequency range. The sum of the diodes 18 and 19 is a measure of the input power variation which must be removed to obtain the correct discriminator characteristic. When compared with a voltage reference an error signal is derived which controls the shunt levelling diode 23 and hence levelling the sum channel.

The mixer diodes 18 and 19 and the levelling diode 23 are matched to the microstrip line by a respective open circuit stubs 24,25 and 26. The outputs from diodes 18 and 19-may be filtered via respective low pass filter 42 and 43.

In an ideal system all the power incident upon diodes 18 and 19 would be absorbed and processed. However, due to matching problems etc some of the incident signal is reflected back to coupler 15 and then throughout the circuit and eventually back to diodes 18 and 19. This makes the signal received more complicated.

The reflected components may be added to or subtracted from the main signals received at the diodes 18 and 19 in order to improve the extent of the linear range of the discriminator. The reflected and main signals must be in phase for this addition or subtraction to be possible.

The leveller diode acts as an attenuator in order to keep the output constant. The present arrangement may be replaced by a double diode.

The R.F. load may be replaced by a load diode, eg a PIN diode which may be adjusted in order to control the reflected signals so that they can be in phase with the main signal and therefore used to extend the linear range.

The circuit patterns of the discriminator are defined on the substrate using conventional printed circuit photolithographic techniques and are thus potentially cheap to produce. The mixer diodes used are commercially available GaAs MOTT diodes. A possible substrate for the discriminator is a 0.5or rolled copper RT-Duriod 5890, a pure unreinforced PTFE laminate.

In the delay line 16 of the discriminator one may place a dielectric frequency adjuster which can vary or preset Fo.

Since the microstrip line wavelength (L L ) is given by the expression,

where Ere = Effective dielectric constant A O = Free space wavelength The effective dielectric constant can be increased by covering the microstrip line 13 with a dielectric cover 45.

The analysis of a microstrip line covered with a low loss sheet material as shown in Figure 7 is known from Bahl et al. "Design of Microstrip Antenna Covered with a Dielectric Layer" IEEE Trans. of Antennas and Propagation, Vol AP-30 No. 2 P314-318.

Increasing the effective dielectric constant by covering the microstrip delay line with a dielectric layer increases the electrical length and hence the centre frequency of operation decreases. With a dielectric frequency adjuster 28 as in Figure 4, made from, for example, TPX (Er = 2.1) of thickness 0.53mm covering 30% of the microstrip delay line (17) enables the frequency to be decreased by 34%.

A Microstrip line radiates considerably at any point where there is a change in the electric field pattern which occurs at a bend or discontinuity such as a change in line width. The spurious radiation from the microstrip circuit combined with close proximity enclosed may interfere with discriminator performance ie its linearity.

A resonant absorber 29 made from, for example, carbonyl iron/epoxy resin maybe placed over the microstrip substrate to minimise the effect of the spurious radiation. Figures 8 and 9 show views of the resonant 29 absorber which is generally a semi-circular channel following the circuit pattern as illustrated in the cross-sectional view on x - x of Figure 8.

Claims (7)

1 A delay line frequency discriminator comprising a delay line and a reference line through which components of an R.F.

signal are transmitted, decoupling means for producing said components; means for spliting each component into a signal for each of a pair of detectors, which compare the signals, wherein the signal at each detector is a combination of said components;a levelling means which receives a signal from said detectors so that the input power is kept at a constant level; further comprises a frequency adjusting means; an absorber for absorbing any spurious radiation; and a means for using reflections from said pair of detectors to improve the extent of a linear range in the discriminator.

2 A delay line frequency discriminator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Amendments to the claims have been filed as follows 1. A frequency discriminator and lineariser system, comprising; means for receiving an input R.F. signal; first splitting means for splitting said input R.F.

signal into a reference path and a delay path, said delay path including a delay line which delays the signal applied thereto; second splitting means, receiving signals at proximal points of said reference line and delay line respectively for splitting said signals from said reference line and delay line into two paths; first detecting means, coupled to a first of said paths from said second splitting means, for producing a first output representing a phase difference between outputs of said reference line and delay line; second detecting means, coupled to a second of said paths from said second splitting means having a different sense than said first path, for producing a second output representing a phase difference between said outputs of said reference line and delay line; error means, coupled to outputs of said first and second detecting means, for detecting an extent of non-linearity of the system based on said outputs of said first and second detecting means; and means, receiving an output from said error means, for attenuating said R.F. signal between said receiving means and said first splitting means to compensate for said non-linearity. - -- 2. A system as in claim 1 wherein said first and second detecting means comprise diodes. - --

3. A system as in claim 1 or claim 2 wherein said attenuating means comprises a levelling diode responsive to a signal. - --

4. A system as in any preceding claim wherein said error means comprises an adder for adding outputs of said first and second detecting means, and means for comparing an output of said adder with a reference to produce an error signal, said error signal being coupled to said attenuating means to control its level. - --

5. A system as in any preceding claim wherein said delay line includes a frequency adjusting means which maintains a specific centre frequency thereof. - --

6. A system as in any preceeding claim further comprising a resonant absorber for absorbing any spurious radiation from said system. --

7. A frequency discriminator and lineariser system substantially as hereinbefore described with reference to and as illustrated in Figs. 4-6 and 8 and 9 of accompanying drawings.