Racal Instruments RA-117 User Manual page 11

R.F. TUNE
R.F./I.F. GAIN
A.V.C.
A.F. LEVEL
LIMITER
"S" METER
SPEAKER
V.F.O.
Operation
If maximum sensitivity is not required, the antenna need not be
tuned unless strong unwanted signals are present. It should be
noted that the presence of very strong singnals anywhere within
the spectrum may cause crossmodulation unless the aerial is tuned.
Under these conditions, CARE MUST BE TAKEN TO AVOID
TUNING THE INPUT TO THE INTERFERING SIGNALS in-
stead of the signal required. Familiarity with the tuning controls
will facilitate this.
The R.F./I.F. GAIN control is operative both in the MAN. and the
A.V.C. position of the SYSTEM switch. In the MAN.position of
the SYSTEM switch the setting of the control should be always at
a minimum consistent with satisfactory A.F. level. The following
should be noted when the SYSTEM switch is in the A.V.C. posi-
tion. Reducing the I.F. gain results in a reduction of a A.V.C. loop
gain together with the a degraded A.V.C. characteristic. There-
fore when in the A.V.C. position, it is desirable that the R.F./I.F.
GAIN control be set to maximum. A possible execption of this oc-
curs when receiving interrupted signals in which the carrier is peri-
odically switched off; in this case , receiver noise could be trouble-
some during the quiet intervals.
The choice of time-constant depends upon conditions. The LONG
time-constant (1 second) should be employed with the choice sig-
nals, the SHORT time-constant may be used with high speed teleg-
raphy or voice. For hand (low) speed telegraphy, the MAN. position
of the SYSTEM switch should be used (refer to R.F./I.F. GAIN)
The preset control sets the A.F. level in a separate A.F. stage for
feeding a 600-ohms, 10mW line. It is unaffected by the position of
the main A.F. GAIN control. IT IS MOST IMPORTANT that the
A.F. LEVEL is not turned towards its maximum position unless
the 10mW 600-ohms winding is suitable terminated.
When swithced into use, the LIMITER reduces the effects of noise
peaks exceeding the level of a 30% modulated signal. It does not
introduce noticeable distortion below a 30% modulation level.
With the METER switch in the R.F. LEVEL position the meter
indicates the signal diode current. In the A.F. LEVEL position, the
10mW, 600-ohms output only is monitored. A calibration mark is
provided at 10mW.
The loudspeaker may be switched ON or OFF as required. The two
telephone jack sockets remain in circuit in either position of the
SPEAKER switch. The insertion of a telephone jack disconnects
the loudspeaker.
This switch should be set to the EXT. position when a external 3.6
– 4.6 Mc/s source is applied.
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