3.1

TEM transmission lines

The most natural way to obtain desired value of time delay is the use of a section of TEM transmission line. The most popular way is the use of dielectric-filled coaxial line. However, for respectively higher time delays the coax lines present relatively big dimensions and weight. The signal attenuation also increases, especially for higher frequencies. The physical implementation of coaxial delay lines commonly appears in a form of big coils made from thin coaxial transmission line, usually semi-rigid cable. Commercially available solutions have typical working frequency range and bandwidth 0÷6 GHz, available delay 1÷250 ns, but at the cost of insertion losses from 0.2 to 50 dB [14].

3.2

Planar structures

In order to obtain smaller overall dimensions transmission delay lines may be realized in the form of planar structures made on dielectric substrate with high permittivity. The most common structure is a section of microstrip or coplanar line. On the other hand, there are additional ways to obtain higher values of propagation delay based on using various planar structures, and they are as follows:

The multilayer planar delay lines are commonly realized in LTCC (Low Temperature Co-fired Ceramics) technology. The available parameters are: bandwidth 1GHz at carrier frequency about 10 GHz, insertion loss lower that 1 dB, time delay up to several ns.

3.3

Delay lines with acoustic waves

The use of acoustic surface waves techniques (SAW) assumes conversion of electrical signal to mechanical wave, which propagates in the form of material vibration along its surface. The device that uses this technique consists of input electrical-to-mechanical transducer, a specially prepared layer of piezoelectric material and inversely operating mechanical-to-electrical transducer at the output. The propagation velocity of acoustic wave is thus much lower than for electromagnetic waves because of the fact that the acoustic wave travels at velocities of 3000 to 12000 m/s. The SAW devices allow obtaining delays from few nanoseconds to several hundred nanoseconds with frequency bandwidth up to 800 MHz. Their main disadvantages are the increased insertion losses that can reach values more than 25 dB and the frequency limitation to about 2.5 GHz. It should be pointed out that these devices are not tunable [23].

Another, similar technology is bulk acoustic wave (BAW) that uses mechanical waves excited to propagate through whole volume of prepared material. This allows realizing greater delays compared to SAW devices, ranging from few hundreds of nanoseconds to several thousand microseconds [24].

BAW technology overcomes the frequency limitation of SAW one (2.5 GHz) and allows reaching frequencies of about 20 GHz and a frequency bandwidth up to 2 GHz. Similarly to SAW technology, the insertion losses are high, more than 30 dB, and depend on the realized time delay and the operating frequency. The examples of commercially available BAW device parameters are: f₀=1GHz, B=0.5GHz, τ=4 μs, IL = 47 dB and f₀=10GHz, B=2 GHz, τ=0.5 μs, IL = 50 dB.

4.1

Transmission lines periodically-loaded with varactor diodes

In some applications, a transmission line may be substituted with the set of cascaded lumped inductances and capacitances in the form of multistage lowpass filter. This lumped elements corresponds to distributed unit inductance and capacitance of the transmission line. A transmission line in the meaning of delay line may be realized in this technology. Typically used number of stages ranges from a dozen to several tens and depends on assumed group delay values.

Propagation of microwave signal (with respect to parameters not field pattern) along such a structure is similar to propagation in transmission line as far as the signal wavelength is long compared to elements dimensions.

This, so called, artificial transmission line, may be additionally loaded by periodically placed varactor diodes, connected in parallel to lumped capacitances [25] – Figure 1.

Figure 1.

Schematic of artificial transmission line with periodically connected varactors

The design and structure of the whole line is optimized in order to obtain the possibility of changing the propagation constant (thus group delay) in line by applying bias voltage to varactor diodes thereby causing the change of equivalent artificial line shunt capacitance.

Another solution is the use of planar transmission line e.g. coplanar line, which is loaded at regular spacing by a series of varactor diodes, that serve as voltage dependent shunt capacitors (Figure 2).

Figure 2.

Schematic of a nonlinear transmission line (NLTL) with periodically connected varactors

This type of lines is also called nonlinear transmission line (NLTL).

Artificial transmission lines, are lowpass filters i.e. they exhibit an upper limit of frequency band. In order to obtain bigger values of time delay higher values of unit capacitance and inductance are needed and it lowers maximum frequency of propagating signal.

Additional disadvantages of such a line are:

This distortions are higher for higher delay values due to varactor bias near to zero volts.

4.2

Fiber optic delay lines

Microwave delay lines may be realized with the use of optical techniques. Most commonly used solution is the conversion of electric signal to intensity modulated light signal and then transmission through an optical fiber. At the output an optical receiver converts modulated optical power back to electric signal. The time delay is introduced during the propagation along the optical fiber. The value of time delay is proportional to the fiber length. Propagation velocity of the optical signal inside the fiber is lower than speed of light in vacuum and single-mode optical fiber overall diameter is about several tenth of millimeter. Therefore it is possible to obtain large time delay using enough long optical fiber rolled with relatively small overall external dimensions [26].

The input circuit of optical delay line comprises a light source i.e. laser with direct-modulation or CW laser with external modulator e.g. Mach-Zehnder device. At the output an optical receiver in the form of PIN photodiode is commonly used with optional electric signal amplifier.

To sum up, the microwave optical delay line is a kind of analogue microwave optical link with appropriately assorted length of the optical fiber. It exhibits advantages and disadvantages inherent to optical techniques.

Advantages:

Disadvantages:

The optical delay lines has possibility of tuning the delay value [27]. This feature may be obtained with the use of laser source with tunable wavelength and optical fiber with high dependence of group velocity versus light wavelength.

The example of commercially available optical delay line parameters is: B=0.05 ÷13 GHz, τ=500 ns, IL = 39dB@1GHz.

4.3

Reflective varactor-controlled load

In some applications a precision phase shifter with group delay control is needed. The control range is usually small and equal to about a half of the wave period. The structure consists of one 3dB quadrature coupler (0 and 90 deg of output phase shift) and two varactor diodes with driving circuits [28]. An outline of such a circuit is shown in Figure 3.

Figure 3.

Schematic of reflective phase shifter with adjustable load.

The hybrid coupler may be designed with the use of distributed elements i.e. transmission lines or lumped elements as LC structure with specially optimized values.

Disadvantages:

Advantages:

5.1

Cascaded line with switched delay sections

Cascaded line with switched delay sections consists of N stages where N is the number of bits. Each stage refers to one bit and comprises two SPDT switches and two sections of delay line, the first is a reference line τ₀ (“zero delay”), the second is a line with assorted basic time delay τ_n [29-30]. The subsequent stages contain basic time delays that fulfil the rule:

where n is the number of the bit, and τ₁ is the time delay difference between states (or in other words delay at lowest state). For example, for N = 3 bits the basic time delays equal τ₁, 2τ₁, and 4τ₁ (Figure 4).

Figure 4.

General scheme of digitally-controlled delay line with switched delay sections, example for number N of bits equal to 3.

The delay lines: the references and the basics may be connected between each other by means of synchronically operated SPDT switches. The overall number of available delay values (and states) is 2N, and the obtained time delay is between Nτ₀ (the first state) and (2N-1) τ₁ (the last state). General scheme of digitally-controlled delay line with switched delay sections, for N=3, is shown in Fig. 3. Assuming that the value of basic time delay τn contains the amount of reference delay τ₀, the first state introduces the time delay equal to Nτ₀ and the last state (2N-1) τ₁ + Nτ₀. Hence, the time delay difference with respect to the reference path is from 0 to (2N-1) τ₁.

There is a number of advantages and disadvantages related to digitally-controlled delay lines. For cascaded line with switched delay sections they are as follows.

Disadvantages:

Advantages:

5.2

Trombone line

The another solution of digitally-controlled delay line is a line with digitally-added basic delay sections called the trombone line [31,32]. In this solution there is no reference sections, the line consists of two pathways of cascaded basic delay line sections, each of them having the same value of delay equal to half of the lowest state delay 0.5τ₁. Each pathway is a microwave two-port network, it comprises an input/output port and the second port is connected to matched load. There is a tap in point between every cascaded two basic lines and corresponding taps placed on two pathways are connected by means of SPST switch, usually realized as a pair of transistors in cascode connection. The number of basic delay sections per pathway equals required number of delay states. The SPST switches are normally-open, the onset of a chosen device causes to switch-on a delay patch of corresponding length i.e. value of time delay. Once a switch is activated the whole structure of delay line starts conducting a microwave signal between the input and output ports.

General scheme of trombone delay line with added delay sections is shown in Figure 5.

Figure 5.

General scheme of digitally-controlled delay line with digitally-added basic delay sections (trombone line).

As far as the trombone line is concerned, there is no additive accumulation of insertion losses of the switches. In this line there is only one switch in conducting state during the state of delay. However, there are additional power losses due to signal absorption in matched loads ending the signal pathways. Physical structure of the line results that for N available delay states there are N basic delay stages required.

Both types of digitally-controlled delay lines mentioned above require properly designed basic delay units i.e. having insertion and return losses as low as possible. The basic delay lines may be realized in any kind of technology as far as the requirements indicated above are fulfilled.

5.3

Reflection-type line with switched delay sections

The delay line structure may be constructed with the use of reflective sections of transmission line [33]. The reflective scheme means that one end of a section of transmission line is short-circuited and the second end serves as input and output of delayed signal. In order to separate the input and output signals a microwave circulator is used. An outline of such a circuit is shown in Figure 6.

Figure 6.

General scheme of one stage reflective delay line with microwave circulator.

The short-circuited section of a transmission line serves as a unit delay element. In order to obtain switching of this value a SPST switch is used, which in open state allows signal to enter the delay section and in close state provides the short-circuit at the input of delay section. The microwave circulator may be replaced by a microwave coupler (3 dB 90 deg), as it is shown in Figure 7.

Figure 7.

General scheme of one stage reflective delay line with microwave 3dB quadrature coupler.

The following scheme may be multiplied to obtain multi-state solution. This situation is shown in Figure 8.

Disadvantages:

Advantages:

Figure 8.

General scheme of one stage reflective delay line with microwave 3dB quadrature coupler.

5.4

Analogue tapped cascaded delay line

In order to have a number of analogue correlations simultaneously another special kind of delay line may be used. This is the analogue tapped delay line.

The analogue tapped delay line consists of a number unit delay sections and microwave couplers connected in a cascading way (Figure 9 [34]). The couplers have the same coupling factor. One delay line stage consists of one unit delay line with one coupler. The output ports for subsequently delayed signals are based on coupled signal ports. The time delay for every stage is equal to the number of the stage multiplied by the unit time delay τ₁. General scheme of analogue tapped delay line is shown in Figure 9.

Figure 9.

General scheme of analogue tapped cascaded delay line.

Every signal tap may be regarded as one time gate and may be connected to one signal correlator.

The analogue tapped delay line allows obtaining all time-gate signals almost simultaneously, as the signal propagates down the cascaded structure. However, in this case a signal switch (SPDT or SPST) implemented to set the one desired value of time delay is not needed.

The analogue tapped cascaded delay line offers several advantages:

To compare, there are disadvantages:

Another way to design the analogue tapped delay line is the optimization of microwave coupler’s coupling factor in order to obtain the same signal power at each tap.