Circuit Description - Heathkit SG-8 Assembly And Operation Manual

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INTRODUCTION
The >Heathkit model SG-8 Signal Generator has been designed for simplicity of construction and
stability of operation. A little care taken during the process of construction will reward the kit
builder with a signal generator which will give excellent performance and will be both pleasant
and easy to use. This generator has been engineered so that it will be valuable to the radio re ­
pairman, ham and experimenter over a wide range of uses, in addition to being a rugged and
highly dependable piece of test equipment. It is made with high quality parts, conservatively
rated, and will give long and efficient service. The model SG-8 Signal Generator utilizes factory
adjusted coils, thereby eliminating the necessity of having costly equipment available to cali ­
brate the finished kit.
At the same time the audio voltage is also applied to the external audio connector marked OUT ,
and may be used as an audio signal source for testing amplifiers, etc. With the modulation
switch in the EXT. position, any external audio source of any frequency may be used to modu ­
late the RF output of the signal generator. The 604 audio oscillator tube then becomes an am ­
plifier stage for the external signal.
The use of the cathode follower as a buffer stage has the following advantage: The characteris ­
tics of a "cathode follower" are very low output impedance with an extremely high input imped ­
ance. The extremely high input impedance produces little or no loading on the oscillator circuit .
The very low output impedance provides a very stable output from the signal generator. The
effect is such that a varying load on the output of the signal generator will produce little or no
frequency instability of the oscillator. The signal generator incorporates a step attenuator for
coarse control of radio frequency output ahd a continuously variable attenuation circuit for fine
control of the radio frequency output.
The audio oscillator is also a Colpitts circuit. By the use of the large inductance of an iron-
core choke and relatively high capacity in the capacitive divider, it is made to oscillate at ap ­
proximately 400 cycles. This type of oscillator was again chosen for its stability and purity of
waveform. With the modulation switch in the INT. position, 400 cycles audio voltage is ap ­
plied through a resistance network to the grid of the cathode follower stage. This audio voltage
is impressed as an amplitude modulated signal upon the RF output from the cathode follower .
In a Colpitts oscillator circuit, feedback necessary to maintain oscillations is obtained from a
capacitive reactance divider across the inductance of the frequency determining circuit. In the
model SG-8, a capacitive divider is obtained by the use of a split-stator condenser. The ad ­
vantage of this will be made apparent from the following explanation. For example, take a ca ­
pacitive divider composed of fixed capacities. As the frequency across the condensers is in ­
creased, the reactance of the condensers is decreased. Therefore, using a fixed capacitive
divider, as the frequency is increased reactance would become lower until a point was reached
where oscillations could no longer be maintained. By the use of a split-stator condenser for
both tuning and as the divider network as the frequency is increased, the capacity of the con ­
denser dividing network is decreased. The reactance of a condenser for any given frequency is
inversely proportional to the capacity. This tends to maintain the reactance of the network fair ­
ly constant, thereby permitting oscillation over a wide range of frequency.
CIRCUIT DESCRIPTION
The RF portion of the SG-8 Signal Generator consists of a 12AU7 twin triode tube. One triode
section of this tube is used as a Colpitts oscillator. Theother triode section is used in a cathode
follower type circuit and acts as a buffer between the oscillator and the output of the signal gen ­
erator. Four inductances wound on forms comprise the coils for bands A, B, C and D. They are
switched into the circuit by means of the band switch. TheE band coil is unique in its construc ­
tion insofar that the heavy buswire of which it is composed actually forms the connections be ­
tween the band switch and the tuning condenser for all of the lower frequency coils. When the
band switch is placed in E band position, a short is placed across the leads of the E band coils ,
thereby making it a closed circuit inductance which is capable of tuning from 2 to 100 mega ­
cycles.