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15.2 The task of the radio frequency amplifier
The r.-f amplifier employed in a broadcast frequency receiver may differ de-
cidedly from that used in a receiver serving other purposes or tuned to other frequen-
cies. A broadcast receiver, for example, must be capable of amplifying at any fre-
quency between 500 and 1500 kc. It must be easily changed from one frequency to
another, and its amplification at all frequencies within this band should be uniform. It
is selects and amplifies too, its task is much more difficult to perform, as we shall see.
The energy thrust upon the other from a given broadcasting station is a com-
plex bit of wave motion. If the microphone is idle, what comes, from the antenna may
be considered as a very narrow band, at say 600 kc, called the carrier wave. If a single
tone, say 1000 cycles, is put into the microphone, the antenna current has frequencies
of not only 600 kc in it but 599 and 601 as well, and when music is broadcast the fre-
quencies in the antenna may be varying between zero and 5000 cycles above and be-
low the carrier from instant to instant. These frequencies on either side of the carrier
are called the side bands. The characteristics of the transmitter must be such that each
of these audio frequencies is given equal power compared to the others. The reso-
nance curve of the antenna system of the transmitter, then, must not be sharp but must
be rather flat or dull. It must have a rather flat top from 5 kc below to 5 kc above its
carrier frequency. If audio frequencies up to 5000 cycles are transmitted, each station
requires a channel 10 fee wide for its transmission, and if there are 1000 kc available
there are 1000 channels or places for 100 simultaneous transmissions.
At the listening station, the receiver must be able to pick out any of these sta-
tions, and to receive it without being bothered by others on other channels. This
means that a receiver with a good degree of selectivity is one which will receive and
amplify signals on the band from 595 to 605 kc but not recognize a signal in the adja-
cent channels, that is on the channel extending from 605 to 615 kc. In other words, to
cope with conditions in the broadcasting band a receiver should have "ten kilocycle
selectivity".
15.3 Variable-MU or remote-cutoff pentodes
In certain pentodes, such as the 6K7 and the 6SK7, the grid helix has a variable
pitch, so that some of the turns are closer together than others. The closely spaced
turns have a greater control over electron flow from the cathode than those with
greater separation, and as a result the mutuachar-acteristics of the tube are considera-
bly modified. As the control grid is made more negative, cutoff of the electron stream
is approached much more gradually than if the grid spacing were uniformly close.
Tubes of this type are known as variable-mu, or remote-cutoff, or super-control pen-
todes. A typical example is the 6K7.
In radio receivers, which are required to respond to signals covering a wide
range of intensity, the grid bias is made to depend upon the amplifier output, and as a
result the sensitivity is automatically reduced when a strong signal is being received.
This type of control, known as automatic volume control or a.v.c. is incorporated in
practically all modern receivers.
44
15.2 The task of the radio frequency amplifier
The r.-f amplifier employed in a broadcast frequency receiver may differ de-
cidedly from that used in a receiver serving other purposes or tuned to other frequen-
cies. A broadcast receiver, for example, must be capable of amplifying at any fre-
quency between 500 and 1500 kc. It must be easily changed from one frequency to
another, and its amplification at all frequencies within this band should be uniform. It
is selects and amplifies too, its task is much more difficult to perform, as we shall see.
The energy thrust upon the other from a given broadcasting station is a com-
plex bit of wave motion. If the microphone is idle, what comes, from the antenna may
be considered as a very narrow band, at say 600 kc, called the carrier wave. If a single
tone, say 1000 cycles, is put into the microphone, the antenna current has frequencies
of not only 600 kc in it but 599 and 601 as well, and when music is broadcast the fre-
quencies in the antenna may be varying between zero and 5000 cycles above and be-
low the carrier from instant to instant. These frequencies on either side of the carrier
are called the side bands. The characteristics of the transmitter must be such that each
of these audio frequencies is given equal power compared to the others. The reso-
nance curve of the antenna system of the transmitter, then, must not be sharp but must
be rather flat or dull. It must have a rather flat top from 5 kc below to 5 kc above its
carrier frequency. If audio frequencies up to 5000 cycles are transmitted, each station
requires a channel 10 fee wide for its transmission, and if there are 1000 kc available
there are 1000 channels or places for 100 simultaneous transmissions.
At the listening station, the receiver must be able to pick out any of these sta-
tions, and to receive it without being bothered by others on other channels. This
means that a receiver with a good degree of selectivity is one which will receive and
amplify signals on the band from 595 to 605 kc but not recognize a signal in the adja-
cent channels, that is on the channel extending from 605 to 615 kc. In other words, to
cope with conditions in the broadcasting band a receiver should have "ten kilocycle
selectivity".
15.3 Variable-MU or remote-cutoff pentodes
In certain pentodes, such as the 6K7 and the 6SK7, the grid helix has a variable
pitch, so that some of the turns are closer together than others. The closely spaced
turns have a greater control over electron flow from the cathode than those with
greater separation, and as a result the mutuachar-acteristics of the tube are considera-
bly modified. As the control grid is made more negative, cutoff of the electron stream
is approached much more gradually than if the grid spacing were uniformly close.
Tubes of this type are known as variable-mu, or remote-cutoff, or super-control pen-
todes. A typical example is the 6K7.
In radio receivers, which are required to respond to signals covering a wide
range of intensity, the grid bias is made to depend upon the amplifier output, and as a
result the sensitivity is automatically reduced when a strong signal is being received.
This type of control, known as automatic volume control or a.v.c. is incorporated in
practically all modern receivers.
44
