RA.98 INDEPENDENT  SIDEBAND  ADAPTOR

HIGHLIGHTS

Suitable for reception of I.S.B., S.S.B. and D.S.B. signals

Mechanically operated automatic frequency control

Monitoring facilities for both sidebands separately

Memory system maintains alignment during fading

The Independent Sideband Adaptor type RA.98D is designed to permit reception of independent sideband and single sideband signals with the RACAL type RA.17, RA.117 or RA.217 high-stability Communications Receivers (or other communications receivers having a 100 kHz i.f. output). It is equally suited for the SSB reception of DSB transmissions, with the advantages of minimising the effects of selective fading and reducing adjacent channel interference, by offering the choice of either sideband.

The unit incorporates two 118 kc/s oscillators, one of variable frequency and one crystal controlled. The variable frequency oscillator is fitted with a motor-driven variable capacitor, which adjusts the tuning by means of an automatic frequency control (a.f.c.) system. This locks the adaptor circuits to the incoming frequency to within a very small margin of error.
The a.f.c. system compensates for drift in the adaptor, receiver or distant transmitter.
The tuning frequency is corrected to plus or minus 3 c/s over a carrier drift range of plus or minus 1 kc/s.

A carrier derived a.g.c. output (d.c.) is available from the adaptor for the purpose of controlling the receiver gain, both to counteract fading and to keep the input level to the adaptor within the optimum working range.

The adaptor is self-contained, having its own power unit and audio circuits. Two outlets are provided, one for each sideband, together with a further outlet which can be switched to either sideband for monitoring purposes. Independent gain controls are provided for each channel.

For use with emissions having a full or pilot carrier, an electro-mechanical a.f.c. system is incorporated which enables the adaptor to follow frequency drift and remain aligned to within a few Hz over excursions of up tp 1 kHz. A memory element keeps the adaptor tuned, even in the temporary absence of signal. Special care has been taken to avoid malfunctioning by unwanted signals in the vicinity of the pilot carrier.

The RA.98 is designed for standard rack mounting, with a panel height of 133 mm. (5.25 in.) and is supplied complete with dust cover. All electrical connections with the exception of headphones are made at the rear of the unit.

The RA.98D is for use either with full or pilot carrier signals, using the a.f.c. circuits, or for the reception of emissions with suppressed carrier in very high stability systems. An inbuilt crystal-controlled oscillator provides the 118 kHz injection signal. (Although primarily designed for 100 kHz input, the unit can be operated from inputs of 450 .. 465 kHz by the addition of a small converter, type RA.73C).

The incoming signal, centred on 100 kHz is mixed with the output from a 118 kHz crystal-controlled oscillator. Each sideband, after passing the appropriate filter, is mixed with the output from a high-accuracy 18 kHz oscillator in a product detector. The final product takes the form of an a.f. signal from 300 Hz to 6 kHz in bandwidth.

To operate the a.f.c. system, the carrier is passed through narrow bandwidth filters to a discriminator circuit developing a control voltage to adjust the frequency of the 118 kHz oscillator. A low-inertia d.c. motor, suitably geared and provided with a magnetic clutch, is energised by this controlling voltage and adjusts the tuning capacitor of the 118 kHz oscillator. The system is designed to avoid de-tuning during fading or temporary disappearance of the incoming pilot carrier, as the adaptor remains correctly aligned on the frequency at which the carrier was last received. The a.f.c. system has a capture range of +/- 50 Hz and is capable of following a captured signal over a range of +/- 1 kHz, with a residual error of less than 3 Hz. The stability of the 118 kHz crystal-controlled oscillator is such that the frequency of the re-inserted carrier will remain within 2 Hz of an incoming signal of constant frequency.

To avoid the a.f.c. system locking on interfering signals adjacent to the carrier frequency, the carrier channel passband is restricted within narrow limits. This protects the system from unwanted signals at more than 50 Hz from the operating frequency.

During suppressed carrier working the a.f.c. system can not be used, as no carrier is present to which it can be locked. In this method of working the variable oscillator and the a.f.c. system is switched off and the crystal oscillator is brought into circuit, providing a stable frequency with which the receiver input ca be mixed.

Tubes

• Independent Sideband Adaptor RA.98D (brochure)       (Publication No. 278-3)

• RA.98A
  The RA.98A is fitted with British type fixed coaxial plugs (PL1 and PL2) on the rear panel.
• RA.98B
  The RA.98B follows the North American practice with fixed coaxial receptacles (SKT1 and SKT2).
  The monitor jack sockets also differ in type.
• RA.98D
  The RA.98D is fitted with an internal 118 kc/s crystal oven and crystal oscillator assembly
  and has an additional position (" XTAL OSC. ON ") on the A.F.C. switch.
• RA.98E
  The RA.98E has an additional transistorized coupling circuit for operation with transistorized receivers (RA.217, RA.1217).

The RA.98E is fitted with a small transistorized coupling circuit in the a.g.c. output line in order to render the a.g.c. source compatible with the gain-controlled circuits in the RA.217. This coupling circuit requires a -16 V d.c. power supply which is provided from an external source (RA.217, MA.397, etc).

Not all RA.98 versions have this internal switch. The switch corrects the sideband inversion which would otherwise render the L.S.B. and U.S.B. panel markings incorrect when a RA.117 receiver is connected instead of a RA.17 receiver.   I have two RA.98A's and one has the switch (see picture), the other only has the text markings on the chassis printed, but there is no hole and (obvious) no switch.