32 February 2017
BUILD IT YOURSELF
About a year after I had
designed the RF generator
(N&V June 2014), I started
to toy with the idea of
adding a sweep function to
it. A couple of months later,
I completed the design and
prototype of this adapter
that has relatively simple
circuitry with a non-critical
layout; it’s quite economical
to build; and draws its
power through a short cable
that plugs into a connector
at the rear of the RF
generator. This is a five-conductor cable that
consists of three power
leads and two interface
leads. This combination is
easy to use and has
adequate performance.
Although it is focused for
use on my original generator
design, the basic
architecture can be
modified for use with
commercial RF generators
that are based on varactor
diode tuning.
Add Sweep Function to Your
RF SIGNAL
GENERATOR
By Robert Reed
There were many of these units manufactured during the ‘70s and ‘80s, and one may be sitting on your test bench now. One shortcoming is that my RF generator design has no overlapping RF bands such as my
sweep generator has (N&V December 2013). This may not pose too much of
an inconvenience and will be touched on later in this article. Again, the main
body of this article will be directed towards my design but with occasional
side notes pertaining to commercial equipment.
Theory of Operation
The basic architecture of this adapter generates a ramp for driving the
generator’s varactor diode and also a synchronized ramp wave driving the
scope’s horizontal axis. Features are Auto (ramp) or Manual tuning, Set
Sweep Start and Stop Frequency, Variable Sweep Rate, and various alert
lamps, plus an overload alarm (over-driving varactor).
Referring to Figure 1, the sweep ramp is formed by Q1, Q2, IC1a, and
associated components. Q2 is configured as a constant current generator;
this current level is controlled by a fixed R6, R7 bias voltage on its base and
the resistance of R8, R9 (in SLO position) and the 5K Sweep Rate
potentiometer (pot) connected to its emitter. Varying the emitter resistance
produces a change in the collector current which charges C2 in a very linear
fashion and at a rate set by that emitter resistance value.
