S3 — a four-position range switch to allow coarse
adjustment of the sweep or frequency dispersion. P7 is the
fine adjustment of the range selected, and feeds IC2D.
As mentioned earlier, CW tune and Vernier are for
manual tuning or sweep start frequency. These two control
voltages are summed in IC2C and fed to IC2D where they
are summed with the ramp voltage. That output (Vt) is then
sent to the VCO on the RF deck for biasing the varactors.
The output of IC2D has an intended range of - 5.0 volts;
more specifically, -0.3 to - 5.0 volts to manually tune one
complete octave for the band selected. Its output may also
contain a ramp voltage of 0 to - 4. 7 P-P.
When a - 4. 7 volt ramp is added to the minimum CW
voltage of 0.3 volts, we have a range of -0.3 to - 5.0 volts —
exactly the same as the manual tuning voltage range and
the optimum tuning voltage range required by the VCO.
So, we can sum any combination of CW and ramp voltage
that will stay within that -0.3 to - 5.0 voltage range. Since
(obviously) we could easily sum both of them to a level that
would not only drive the VCO out of its linear range but
also drive IC2D into saturation (producing a flat spot on the
sweep ramp and giving erroneous readings on a CRT
trace), this is a no-no! So, how would we ever be aware this
That is the job of comparator IC2A to warn us of this
situation. Its trip point is set for about - 5. 2 volts. When this
level is reached, the trip level drives Q3 into saturation
which, in turn, lights D3 (labeled ‘Reduce Freq’ on the front
panel). You’ll see a slow flashing at first, then a constant on.
Since the very peaks of the ramp in the tripping mode are
of such short duration, they would not hold the comparator
on long enough to see any visible light from D3. Capacitor
C3 was added to give adequate hold time for these short
The calibration pot P8 sets the level of the trip point.
One feature I added that is optional is the ‘Stop Sweep’
Alert LED D4 at the upper right of Figure 3. The original
Stop Sweep switch was changed from an SPST to a DPDT
(S1A and S1B) with an LED added to the second pole.
Now when the sweep is stopped, the LED lights up and
alerts me to this.
There were times (that for the life of me) I couldn’t
figure out why the generator was not working properly,
The generator’s final specifications
are shown in Table 1.
I built the internals in three separate modules so that
each could be separately tested before installing in the
chassis: the RF deck, sweep board, and power supply. The
RF deck was built on a 2” x 2. 5” single-sided circuit board
in typical RF prototype fashion. Figures 4 and 5 show the
bottom and top views, respectively.
First, I draw out all the components on a sheet of
quadrille graph paper with each intersection representing
0.1” spacing. All components are included — both top and
bottom mounted. These are drawn as you view the top of
the board, but also using ‘X-ray’ vision to see the bottom
(copper side) of the board.
After completing the layout on graph paper, the hole
locations are transferred to a scrap of perf board of the
same dimensions as the actual circuit board and held in
place with double-sided tape. This also makes a nice drilling
guide for a clean appearance. Ideally, use a #56 drill
If you have never worked with RF frequencies before,
two rules are paramount. First, a ground plane type of
construction as used here is mandatory; the other is to
keep component leads as short as possible. There is an old
saying among RF design engineers: “If you can see the
leads, they are too long.”
A few other things to mention are the MMIC amplifier
is an SMD device and is located in the lower right of Figure
4. It required two “islands” to be ground out for its input
and output terminals; these were the only islands needed
on this board. The other is that two insulated standoffs
were mounted close to the MC1648 P10 and
32 December 2013
Frequency Span 2 MHz to 180 MHz in 10 bands with a 25/30% overlap.
Frequency Stability Better than 200 ppm after warm-up.
RF Output + 10 dBm ( 2,000 mV P-P) of clean sine wave into 50 ohm termination.
Output Flatness ± 0.1 dB 2 MHz to 80 MHz ; ± 0.7 dB 70 MHz to 180 MHz; 50 ohm term.
Attenuation 39 dB (approx. 100:1) in 3 dB steps.
Sweep Linearity 100% sweep <1-2 error 10 sweep < 0.2 error