Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

Problems of Receiver Design 743
shows schematically the operation
super-heterodyne system.
of the
79.
HOW THE FREQUENCY-CHANGER WORKS
FIGURE 66 shows a typical frequency
changer. Let e s sin st be the voltage of
signal frequency picked up by the tuned loop.
Let e h sin ht be the voltage of the heterodyne
oscillator's frequency picked up by the small
coil coupled loosely to the heterodyne oscillator.
The grid potential of the tube is the
sum of these two and the C battery voltage,
so the plate current will be
= K
i,, [B + n (C + e s sin st + e h sin ht)|-
+ (small terms that we need not consider here.)
2
= K [(B + M C) + /* (es sin st + e h sin ht)]
= K (B + jLtC) 2 which is direct current
-f-2k/x (B + /xC) (es sinst -f- e h sin ht) which
are amplified currents of the signal and
heterodyne frequencies.
-
+ k/z- (es sin' 2 st -f e h 2 sin -ht) which reduces
to direct currents and frequencies twice the
signal frequency and twice the heterodyne
frequency.
+2kju 2 e s sin st Ch sin ht which is the only term
we have any use for, because it splits up into
two parts, one of them Kju 2 e s e h cos (s + h ) t
which is the sum of the signal and heterodyne
frequencies, and of no interest to us, but the
other is k n-esQh cos (s h) t which is the new
frequency that we are going to use. The
tuned circuit that connects to the fixed frequency
receiver picks up only this frequency.
It is obvious that this new frequency can be
adjusted to any desired value by simply adjusting
the frequency of the heterodyne oscillator.
For instance, suppose the fixed frequency
receiver is built to work at 100,000
cycles and the radio waves are coming in at a
frequency of 1,000,000. If we adjust the
heterodyne to oscillate at 900,000 cycles the
new frequency will be the difference of the two,
or 100,000, which is just right to be picked up
and received by the fixed frequency set. On
the other hand if the heterodyne oscillator is
adjusted to 1,100,000 the difference will again
be 100,000 so that there are evidently always
two possible settings for the heterodyne condenser
either of which produces the proper
frequency for the fixed or intermediate frequency
receiver. Sometimes there is less interference
experienced when using one of these
settings than the other but usually
it makes no
difference. From the coefficient of the new
frequency term it is evident that its strength
depends upon the amount of voltage picked up
from the heterodyne oscillator as well as the
signal itself. Hence this should be made
large by making the coupling to the heterodyne
oscillator sufficiently close. The C
battery voltage should be greater than the
heterodyne voltage in order to keep the grid
at all times negative. The best C and B
battery voltages and best coupling can be
most simply found by experiment.
80. PROBLEMS OF "SUPER" DESIGN
IN THE actual construction of a superheterodyne,
we are caught between two
fires. On the one hand, if we build the fixed,
or intermediate-frequency receiver to operate
at a fairly high frequency, say one or two
hundred kilocycles, we will have difficulty in
making it sensitive and selective enough.
On the other hand, if we use a very low intermediate
frequency, say 30 kilocycles, we are
TO HETERODYNE
OSCILLATOR
-L.
TO FIXED FREQUENCf
RECEIVING bT-
THESE TWO TUNED CIRCUITS TAKE THE
PLACE OF THE BAND FILTER WELL ENOUGH
FOR MOST PURPOSES
FIG. 66
likely to run into two troubles. The first is
that the quality tends to be bad on account of
the selectivity being too great, and the other is
that unpleasant complications occur in operating
the set, due to the signal and heterodyne
frequencies being so nearly equal. As the
heterodyne condenser is varied there may be a
click when the heterodyne frequency passes the
value for which the signal circuit is tuned.
Also, the same setting of the heterodyne condenser
will often bring in two different stations
at once whose frequencies are really
different
by twice the intermediate frequency used, and
when the latter is
very low, these two frequencies
are too close together for the signal
circuit or loop to select one to the exclusion of
the other.
In view of these considerations three courses
seem to be open: (i) to effect the best compromise
between the advantages and disadvantages
of high and low intermediatefrequency
amplification, the choice depending
upon what is desired of the set and the location
where it is to work, (2) to use the best intermediate
frequency for amplification and quality
and use a frequency changing device employing
special circuits so arranged that only