consumer product service and non-critical design work.
Although they lack the features and quality of higher-end
generators, they have several glaring advantages: ease of
use, the speed to run the gamut of their entire output range,
plus (the biggest merit) very low cost. No array of
pushbuttons or programming here. Just flip a switch, turn a
knob, and rapidly get to where you want to go. Their
intended market is directed at ham operators, hobbyists, or
people that like to tinker with electronics which would be
people just like us. Having tried several commercial general-purpose generators over the years, I felt that better
performance could be achieved which set the stage for
designing one from scratch. These generators were priced in
the $180 to $250 range.
The one I am presenting in this article will have superior
performance in all specs and (assuming the builder has a
moderate junk box of components) can be built for about
$50. This does not include a commercial housing, which at
a minimum would cost $75 and up. As I will explain later in
this article, there is a procedure for forming your own.
Theory of Operation
The heart of this unit is the RF deck. If you read my
article in the December 2013 issue (180 MHz Sweep
Generator), you will see a very similar design here. I covered
the theory of this section quite thoroughly in that article, so
I won’t dwell on it too much now. I have used this style in
seven or eight different designs over the years, which ran
the gamut of simple one frequency oscillators to complex
phase lock loop synthesizers, and it has always been a solid
To begin with and referring to Figure 1, the RF deck is
based on a marvelous chip developed by Motorola in the
early ‘70s. (So popular, in fact, that it is still being produced
today — almost 40 years later!) The IC is a MC1648 DIP.
However, current versions are in SMD form and go by the
name of MC100EL 1648, but are still available in the DIP
version. This chip is a member of the ECL (emitter coupled
logic) family and is basically a high speed LC oscillator
circuit. It can cover a large range of frequencies from MF,
HF, and well into VHF portions of the RF spectrum. It is
easy to use and has built-in AGC (automatic gain control)
that can be tailored to your particular needs.
In this design, various resistors (R13-R16) are switched
in as the frequency bands are switched. The tank circuit
impedance will vary over a tremendous range when a fixed
capacitance tuning value is used for all bands, but by
trimming the AGC bias for each range it will do an excellent
job of maintaining a leveled amplitude output from the tank
circuit. It also has an output buffer amplifier that I use in a
non-traditional way for an external counter.
June 2014 27
; FIGURE 1.