variable ones are even harder to come by.
I decided to build a compromise attenuator (as shown
in the lower middle of Figure 2) that would give 39 dB of
attenuation (~100:1) and a resolution of about 3 dB
increments, and yet still keep attenuator costs to an
acceptable level. The four step attenuator filled this
requirement and should be adequate for most testing.
The sweep board shown in Figure 3 is where all the
required VCO tuning voltages (Vt) are derived.
For use as a stand-alone CW (continuous wave)
generator, the sweep portion is turned off by virtue of
setting the Dispersion switch S2 to its CW position. Now,
VCO tuning is done manually by the Main & Vernier tuning
controls (P5 and P4). These are fed from the divider
network of R25, R26, with a generous amount of filtering
(we want this line to remain very quiet with respect to
Actual sweep voltage is derived from a very linear ramp
voltage produced across C2 and fed to the input buffer
IC1B. The ramp is generated by a constant current source
(Q2) feeding C2. The rate of that ramp is controlled by R8
and P1 — about 10 Hz to 100 Hz.
Opening S4 (SloSweep) introduces R9 into the circuit,
producing a very slow rate of 20 seconds (more on this
later). Without further control, C2 would just keep charging
until it reached - 13 VDC and sit there forever, so it needs
to be reset periodically; that is the job of IC1C and Q1.
This is a comparator circuit set to trigger at approximately
- 5. 5 VDC. The actual level is not that critical but
repeatability of its trip point is hence the 1% metal film
resistors used here — not for accuracy, but for stability.
R4 adds a small amount of hysteresis for noise
rejection and consequent jitter. C1 is very important to
hold the output reset voltage long enough (about 20 µs) to
allow Q1 to completely dump the charge on C2, and then
the whole charging process repeats itself. IC1B’s output
feeds off in two different directions: an inverting amp
(IC1D) with a gain of one to drive an oscilloscope’s
horizontal or ‘X’ axis; and the other to a calibration pot (P2)
for setting the exact amplitude of the ramp voltage into
IC1A. IC2B outputs a DC voltage of the exact level that
matches the peak amplitude of the ramp voltage coming
out of IC1A. This level is calibrated by P3.
The Set Sweep switch (S2) is used for setting the
; FIGURE 3. Sweep generator.
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