Circuit Description - Heathkit Q MULTIPLIER Assembly And Operation Manual

INTRODUC TION
The Heathkit model QF-1 Q Multiplier when used with a communicati 'ns receiver, provides ad-
ditional selectivity or signal rejection as desired. Because the peak .»r null effect of the Q Mul-
tiplier may be tunedacross the receiver's IF bandpass, its operatiin is more flexible than that
of a crystal filter. The Q Multiplier may be used with a receiver which already has a crystal
filter to obtain two simultaneous functions. For example, the receiver's crystal filter could be
set to peak the desired signal and the Q Multiplier used to null an adiacent signal.
The QF-1 requires very little power in either filament AC or B+, consequently, it can derive
its power from any receiver without overloading the receiver power supply. When properly con-
nected and adjusted, it will not decrease the receiver sensitivity a'd may even increase it. In
peak position, an audible gain will be noted on CW signals; on phone signals a carrier gain will
be noted on the S meter. However, the audio will be down slightly due to narrowing of the trans-
mitted sidebands.
The reduced sideband reception will tend to attenuate the higher audio frequencies. This is more
than compensated for by the increased readibility against the surrounding QRM.
CIRCUIT DESCRIPTION
The Q Multiplier functions as a very high Q circuit coupled in parallel across the IF transfor-
mer for peak or by inversion through another triode tube section, in series for null. Any high Q
circuit tends to have a very sharp point of resonanceas either a series or parallel tuned circuit.
Asthe Q of the circuit is increased, the side slope of the resonant peak or dip becomes steeper.
A parallel circuit has a very high impedance at resonance and a series circuit a very low im-
pedance, consequently when circuits of this type are connected in sh:nt across the receiver IF
circuit, the following action takes place. Asa parallel circuit, any .ignal passing through the
IF at the resonant frequency of the Q Multiplier sees a very high shunt impedance and is not at-
tenuated, whereas signals slightly off the resonant frequency fall along the slope or "skirt" of
the Q Multiplier bandpass and seeing a relatively low impedance, are shunted to ground. See
Figure 1. By means of the variable capacitor in the QF-1, this peak can be tuned to any place
in the IF bandpass to peak one signal and attenuate all others.
Figure 1
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