Heathkit SB-401 - BALANCED MODULATOR

Page 130 ese ees) HHATHEIT
BALANCED MODULATOR (Figure 6-6)
When the audio signal from V1B and the RF
signal from carrier generator cathode follower
V2C is applied to the 4-diode balanced modu-
lator, two different frequencies are produced,
This ring-type balanced modulator uses diodes
D101 through D104,
One of the two signals produced by the balanced
modulator is the sum of the audio and carrier
frequencies; the other signal is the difference
between the audio and carrier frequencies, These
Signals are the upper and lower sidebands,
The carrier signal is applied across the modu-
lator diode ring in a balanced circuit, consisting
of one winding of the balanced modulator trans-
former, resistors R108 and R111, and Carrier
Null control R109, The Carrier Null control and
variable capacitor C109 are used to balance out
the carrier signal in the modulator, leaving
only the upper and lower sideband signals at
the modulator output,
The output from the balanced modulator is the
result of combining the audio and carrier signals,
Neither the audio or carrier signal appears in
the output, but the effect of the audio signal un-
balancing the nulled circuit at an audio rate
produces the sum-and-difference frequencies
called sidebands, With no audio, there is no
output from the balanced modulator,
With the Mode switch in the CW position, a
small amount of DC voltage is applied through
R401 to upset the balance of the modulator
when the Transmitter is keyed, This unbalance
produces an output on the CW carrier frequency
of 3395.4 kHz from the balanced modulator,
which is controlled by CW Level control R16,
ISOLATION AMPLIFIER (Figure 6-7)
The sideband signals from the balanced modu-
lator are coupled through balanced modulator
FROM AUDIO
CATHODE
FOLLOWER
| C/Or
TI
3.395MHZ
BALANCED
MODULATOR
TRANSFORMER
FROM CARRIER —
GENERATOR =
CATHODE FOLLOWER
OLSB
CW LEVEL
(GANGED WITH THE
MIC LEVEL) a —
Figure 6-6
transformer T1 to the grid of isolation ampli-
fier tube V3, The secondary of the transformer
is tuned to the CW carrier frequency, V3 serves
to isolate the balanced modulator from the
crystal filter and provides proper impedance
matching to the crystal filter, A secondary pur-
pose of V3 is to provide the automatic level
control (ALC) function in the Transmitter, The
ALC function will be discussed later,
The output of V3 is coupled through capacitor
C118 to the crystal filter, During receive
periods, this stage is biased sufficiently to cut
the tube off and prevent any signal leak-through,