Figure 1 Block Diagram
By Peter C. McNulty, WA1SOV
Here is a wideband circuit design of a Double Sideband Exciter. The
circuit can be used to generate a low level signal with suppressed carrier
or with any percentage of carrier injection that is desired. The circuit
is simple and will easily provide an output 1 v p-p into a 75 ohm load
depending on selected gain. The exciter can be used as a Direct Conversion
exciter by providing an oscillator at the operating frequency. In an application
where another stage of mixing will be used, an I.F. oscillator can be provided
to the exciters input. A composite AM signal will be output from the exciter
and it can drive succeeding stages of a transceiver or other transmitter.
The following schematic illustrates the design. There are no frequency
limiting components involved so the output will provide as good a quality
as is placed at the audio input. The block Diagram of Figure 1 illustrates
the basic operation of the exciter.
Figure 1 Block Diagram
The output of the Mixer stage is the product of the audio input and
the oscillator input. This simple method produces a double sideband output
with suppressed carrier signal. It is important to match the amplitudes
of the input signals to around 1 v p-p so the output will not exceed the
dynamic range of succeeding stages. The Summer stage will allow carrier
to be injected to the signal for double sideband full or some percentage
of suppressed carrier depending on the adjustment of the injection potentiometer.
The output of the Summer stage is passed to a gain of 2 output buffer.
This stage will drive 75 ohms at a low level of around 10 mwatts rms. Subsequent
gain stages can be added to increase power output as long as they are operated
linearly. One caution should be observed. There are no filters to limit
harmonic content of this exciter. As long as sine wave inputs are used
for the oscillator, the output will be very clean and free of harmonic
content. However the rig will only be as clean as the oscillator input
provided. If the inputs are overdriven (above 1 v p-p) the output also
may tend to saturate and produce excessive harmonic content so be careful
with adjustments.
Figure 2 Schematic Diagram
Figure 2 shows the schematic diagram of the low level DSB system. Two
Operation Amplifiers have been chosen for the design. The Analog Devices
AD828 is a dual Wide Band Video amplifier with adequate bandwidth for the
HF spectrum. The LM833 is also dual amplifier and possess extremely good
noise characteristics. The LM833 was chosen to buffer the audio section.
The AD835 is a wide band four-quadrant multiplier, which is used as the
Product Mixer in this application.
U1A buffers the oscillator input for the Mixer. U1B provides an additional stage of buffering for the Carrier injection function in the Summing stage of the AD835. R5 allows adjustment of the Oscillator level. R6 provides adjustment for Carrier level injection. U4A is set for a gain of two and will drive 75 ohms across the load. If additional gain is needed, further amplification can be added to this simple exciter.
When building this circuit a PC Breadboard can be used as long as lead
lengths are not excessive. Since this is an RF circuit, layout of the design
is critical and coupling of input to output leads should be avoided. So
pay close attention to the routing of wires. Do not route input wiring
over output wiring. Be sure to properly decouple all power supply lines
as shown. Additional decoupling may be necessary in high RF field environments.
The design should be enclosed in a metal enclosure to provide shielding.
External power supplies should be routed into the box via appropriate feedthrough
capacitors.