GB2225682A - Phase modulators - Google Patents

Abstract

A phase modulator comprises a rigid helical coil 11a, 11b mounted on one side of a rigid substrate, a plurality of capacitor diodes 12a, 12b mounted on the other side and connected through the substrate to the coil at regular intervals and an arrangement not shown for applying a bias voltage to vary the capacitances of the diodes 12a, 12b. The bias voltage may include an a.c. voltage superposed on a d.c. voltage. Amplifiers A1-A3 compensate for attenuation losses and also function as limiting stages in respect of incidental amplitude modulation. <IMAGE>

Description

PHASE MODULATOR.

This invention relates to a phase modulator or adjusting circuit.

A known phase adjusting arrangement comprises a commercially available R.F. oscillator coupled to a so-called "trombone" delay line to facilitate static phase or adjustment of the signal. The equipment is, however, awkward to use and relatively expensive, and will only allow static phase adjustment.

According to the present invention there is provided a phase modulation arrangement including a rigid substrate, a rigid helical coil mounted on one side of the substrate, a plurality of capacitor diodes mounted on the other side of the substrate placed at regular intervals along the length of the coil and connected thereto through the substrate, and means for applying a bias voltage to said diodes to vary the capacitances thereof.

In a preferred embodiment the bias voltage applying means includes means for superimposing an a.c.

signal on a d.c. bias voltage.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a basic delay line constructed of helical coils and capacitor diodes, and Figure 2 is a phase modulator arrangement.

The circuit of Figure 1 is formed as two delay line length lOa, lOb in series. Each line length is constructed from a rigid helical inductor coil lla, llb with a set of capacitor diodes 12a, 12b connected at regular intervals to the turns of the coil. An input signal from a source R8 is fed, via a first amplifier Al, to one end of the first coil lla. The other end of coil lla is connected via a second amplifier A2 to the second coil llb. The output from the second coil is via a third amplifier A3 to a load RL. With the diodes biased for minimum capacity, i.e. very reverse biased, the signal is delayed by the electrical length made up of the distributed inductance of the helical coil and the minimum capacity presented by the diodes.The length of the delay line is increased by raising the capacity Cl, C2 distributed along the line by reducing the bias on the diodes, making them less reverse biased. As the bias is reduced and diode capacities C1, C2 are increased a shunt loss is introduced resulting in some signal attenuation. Also the cut-off frequency of the line sections is reduced, which tends to produce attenuation. The signal level is therefore restored by the amplifier stages A1-A3, the gain of which complements approximately the average loss of a line section throughout the range of the capacitance change.

Figure 2 shows a practical circuit for a phase modulator. The constructional details are not shown but are as follows. A single sided copper clad circuit board (not shown) has four rows of metal interconnect pins drawn through the board through clearance apertures in the copper cladding. The pins are cut short close to the unclad side of the board and rigid helical coils Ll-L4are affixed, the pins being spaced at alternate turns of the coils and soldered thereto. The coils are constructed of 20 turns of gold plated 19 s.w.g. copper wire and the coils are approximately 7.5mm diameter. On the other side of the circuit board four sets of ten capacitor diodes D1-D40 are connected to the interconnect pins. The diodes are also all connected in parallel via a potentiometer 20 to a d.c. bias circuit 21.Each of the coils L1-L4 is connected to an individual transistor amplifier stage VT1-VT4. The four sections of the delay line thus formed are connected in series. In addition, provision is made for superimposing on the d.c. bias circuit 21 an a.c. modulating signal via input 22. The d.c. bias in the present example is nominally -5V and the a.c. modulation is of the order of 30KHz. The signal input to the circuit is at terminal 23 and the output is at terminal 24.

The limitations of each section of the circuit are decided by the capacity range of the diodes and the loss they introduce in the section. Typically the diode have a capacitance range of 2-5pF. For this limiting reason it is necessary to cascade a number of sections in order to obtain as large a phase shift as possible without excessive amplitude modulation. However, amplitude modulation does appear and therefore the amplifier stages can be constructed as amplitude modulation limiting stages to offset this. Using the d.c. phase shift control approximately 450 of phase shift modulation can be achieve over each line section, making 1800 0-to-peak total on the complete circuit.

Claims (4)

CLAIMS.

1. A phase modulation arrangement including a rigid substrate, a rigid helical coil mounted on one side of the substrate, a plurality of capacitor diodes mounted on the other side of the substrate placed at regular intervals along the length of the coil and connected thereto through the substrate, and means for applying a bias voltage to said diodes to vary the capacitances thereof.

2. A phase modulation arrangement according to claim 1 including means for superimposing an a.c. signal on a d.c. bias voltage.

3. A phase modulation arrangement according to claim 1 or 2 wherein the rigid coil is divided into two or more sections connected in series and each section has a separate set of capacitor diodes connected thereto, each coil section incorporating an amplifier stage, the coil sections with their associated amplifier stages being connected in series, the bias voltage applying means being comnected to the sets of diodes in parallel.

4. A phase modulation arrangement substantially as described with reference to the accompanying drawings.