awa_3-7490r.doc

1.

INSTRUCTION BOOK N0. 3-7490R

A.W.A. ULTRA-HIGH FREQUENCY SIGNAL GENERATORS

TYPES 8R7490 & 9R7490

1. DESCRIPTION

1.1 Construction and Circuit Arrangements:

                            This Instruction Book describes the A.W.A  U.H.F. Signal Generators bearing the following name plates.  The corresponding A.W.A. Type numbers are indicated since they do not appear on the nameplate.

Nameplates and Type Numbers

The A.W.A. U.H.F. Signal Generator, types 8R7490 and 9R7490 employs in its r-f sections the special technique necessitated  by the frequency band in which they operate.   The r-f oscillator  is of the Hartley type, using an acorn triode, and a short circuited transmission line forms the tank circuit.   The line is loaded by a variable capacity to provide the frequency adjustment.  The r-f output is obtained from a mutual inductance coupling to the line arranged  in the form of a piston attenuator, capable of an absolute calibration in decibels.   An 80 Ω concentric output cable is provided.   The whole of the r-f section is enclosed in a brass box, heavily silver-plated, and all leads entering the box are well filtered. The  variable condenser rotor and stator are milled from solid brass, and are insulated by polystyrene and micalex respectively.

                            The r-f voltage developed is monitored by a diode voltmeter, and may be held                            constant by varying the plate voltage of the oscil­lator valve (“SET CARRIER” control).   The                         voltmeter employs two acorn diodes, one being used in conjunction with a potentiometer                        (“SET ZERO” control) to supply a “backing-off” potential.

                            Modulation is provided by a Colpitt's type oscillator, and  is applied to the r-f               oscillator in such a way that the modulation depth does not vary as the plate voltage is               adjusted.   External modulation may also be employed.

                            The power, supply section uses a full-wave valve rectifier and a double-section                          choke-input filter.   A mains switch, pilot light, r-f mains filter and fuses are provided.

2.

INSTRUCTION BOOK N0. 3-7490R

A.W.A. ULTRA-HIGH FREQUENCY SIGNAL GENERATORS

TYPES 8R7490 & 9R7490

SECTION 1 (Contd.)

                            The instrument is contained in a cast aluminium case, all com­ponents being mounted               on two vertical panels.   The larger of these panels carries the r-f oscillator box, and a chassis               mounting the modulator, power supply filter and voltmeter “backing-off” diode and spare                                    valves. On the front of this panel are mounted the frequency, output, set carrier and set                           zero controls, carrier and modulation switches, terminals for external modulation, and                                the output monitoring meter, The smaller panel carries the r-f mains filter, mains                               switch, fuses, power transformer, rectifier and               pilot light,   A partition in the case effectively               screens these components from the signal generator proper.   The case is provided with               carrying handles and with cleats on which the output and power cables may be wound when               not in use.

1.2              Summary of Characteristics

(a)                            Frequency Range

                                          140-300 Mc.

                            (b)              Output Voltage

                                                        Maximum value:  About 0.4 volt.

                            Monitoring:  Input to attenuator held constant by manual control and valve voltmeter.

(c)              Attenuator Range

                                          0-100              db. below maximum output

              (d)              Output Connection

                            80 Ω unbalanced unterminated transmission line

              (e)              Modulation

                                          Internal: 400 c/s ±10 c/s. distortion less than 2%, depth approximately 2%

External:              Required input - 40V R.M.S. for 30% modulation (approximate)

                                          Input Impedance - approximately 20,000 to 17,000 Ω (resistive)

              (f)              Power Supply

                                          220-260V, 50-60 c/s, approximately 30 watts.

3.

INSTRUCTION BOOK N0. 3-7490R

A.W.A. ULTRA-HIGH FREQUENCY SIGNAL GENERATORS

TYPES 8R7490 & 9R7490

SECTION 1 (Contd.)

(g)                            Dimensions

              27” x 10 ½ ” x 11” over all projections

(h)                            Weight

65 lbs.

4.

INSTRUCTION BOOK N0. 3-7490R

A.W.A. ULTRA-HIGH FREQUENCY SIGNAL GENERATORS

TYPES 8R7490 & 9R7490

2. INSTALLATION

2.1              Adjustment for Supply Voltage

                            The Mains Unit fuses should be placed in the correct clips to suit the mains voltage.  Three transformer taps are provided, namely 220, 240 and 260V.  One fuse should be in a clip marked with the voltage to be employed, and the other fuse in the clip marked common.  The fuses are accessible when the Mains Unit is removed from the case.

2.2               Insertion of Valves:

                            The valves should be inserted in their respective sockets, which are plainly marked, and top connections made where required.  The two valves in the oscillator box are accessible through cover plates without removing the back of the box.

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