Coll 35/31 'Bahrain: application for W/T amateur transmitting licence for employee of Bahrain Petroleum Company' [‎70r] (139/250)

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anode volts of V 2 . We can thus find R, which
will be equal to the difference in anode voltage
of V, and divided by the anode current of
C 3 is put in the circuit to bypass the resistance R
for the modulation frequencies and is usually of the
order of 1 to 2 p.F. In order to produce very low
frequency modulation satisfactorily, the reactance
of L 3 at the lowest modulation frequency must be
greater than the impedances of Vx and V , in parallel.
A GUIDE TO AMATEUR RADIO
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of V 2 is not at zero D.C. potential, and the windings
of the transformer or batteries employed to light
this filament must be suitably insulated. It is also
necessary to have good insulation between the
primary and secondary of the low frequency trans
former which feeds the grids of the modulator.
In Fig. 11B the positions of modulator and modu
lated amplifier are reversed. The modulator filament
is in this case at earth potential and the modulated
amplifier filament and grid circuits are at a D.C.
potential above earth. This means that we must
employ some form of inductive coupling, for example,
link coupling for the H.F. valve.
In this type of modulator there is no L.F. loss
due to the modulation choke, but we must beware
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Fig. 9.
Simple Choke Modulation Circuit.
In Fig. 10B the required anode voltages are
applied directly to the respective valves by means
of low frequency chokes L 3 and L 4 . In order to
couple the two anodes together, condenser C 4
is connected. In Fig. IOC the maximum per
missible swing of V 2 is increased by means of a
suitably designed transformer T. The anode
swing is of course increased by the turns ratio,
but notice that the impedance of the load is now
equal to the anode voltage of Vx divided by its
anode current divided by the square of the turns
ratio. An important feature of this circuit is
that the direction of the windings on T can be so
arranged that some of the D.C. magnetisation of
the core is balanced out.
Push-pull Class C power amplifiers may be
modulated in an exactly similar manner by any
of these three circuits.
Series Modulation
Instead of feeding the modulator and modulated
amplifier in parallel through a choke, we can feed
them in series as shown in Fig. 11. This obviates
the use of a cumbersome and expensive low
frequency inductance. The driven H.F. amplifier
is still the load for the modulator and the conditions
stipulated for choke control still hold. The voltage
drop across the modulator will require to be
considerably greater than across the modulated
amplifier in order to secure full modulation as in
the choke control case. The valves are chosen to
efiect this with the required H.F. drive on the
amplifier and bias on the modulator.
Two arrangements are possible. In Fig. 11A
the modulated amplifier is connected with its
cathode to the negative of the H.T. supply and the
modulator V 3 is connected in series with the positive
lead. This means of course that the filament