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10. Spurious Responses
ORIGINS OF SPURIOUS RESPONSES
1. In a high sensitive receiver, precautions against internally generated spurious responses
are essential. To this end, various sections of the receiver have been carefully screened
and the power supplies filtered.
2. Any reduction in the screening efficiency or the failure of any filtering component may
results in spurious signals being generated. It is therefore essential to ensure that the
bonding surfaces are clean and that all securing screws are tight. Spurious responses
in the receiver may occur from the following main causes:-
(1) 37.5 Mc/s break-through from the second mixer V9 to the third mixer V25.
(2) Break-through of 1 Mc/s harmonics.
(3) Break-through of B.F.O. harmonics.
(4) Responses at 3
.
800 and 4
.
000 Mc/s due to second v.f.o. break- through.
(5) Responses of 1
.
7 and 3
.
4 Mc/s due to 1
.
7 crystal oscillator break-through.
(6) Response of 3
.
2 Mc/s due to 6 Mc/s break-through.
CHECKS FOR SPURIOUS RESPONSES
3. Spurious responses are measured relative to receiver noise in the following manner:-
When response is located, the receiver is de-tuned from it just sufficiently to
render the beat inaudible. The A.F. gain is then adjusted to provide a convenient
noise reference output (1mW) and the receiver retuned to the spurious signal for
maximum output. The dB rise in audio output is a measure of the spurious signal
level relative to receiver noise.
Standard conditions of test:
No connection to aerial socket
System switch to MAN.
R.F./I.F. Gain at MAX.
B.F.O. on
3 kc/s bandwidth
R.F. ATTENUATOR at MIN.
Set V.F.O. switch to INT.
37
.
5
MC/S BREAK-THROUGH TO THIRD MIXER
4. Switch R.F. RANGE Mc/s to WIDEBAND 75-ohms. This response will be indicated
as a beat note which varies rapidly in frequency with respect to the KILOCYCLES
scale, i.e. a change of 1 kc/s on the scale results in a much larger change in the note. It
will also move along the KILOCYCLES scale if the MEGACYCLES dial is adjusted
slightly. This response may be eliminated by adjusting the 37
.
5 Mc/s strap (L300 at
second mixer anode).
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