scope’s 10:1 probe. If all levels are correct, you should see
2,000 mV P-P at the RF output jack with no attenuators
switched in and terminated in 50 ohms. The MMIC’s gain is
about 21 dB.
On to the sweep board. Attach a DMM to the output
of IC2B (S2 throw) and adjust P3 (DC calibration pot) for
exactly - 2.35 VDC. Display that level on one channel of a
properly calibrated oscilloscope. On the other channel,
display the ramp voltage from the output of IC1A (the
other throw of S2). Adjust P2 (ramp calibration pot) so that
the P-P ramp level exactly matches the - 2.35 VDC level.
Now, the Set Sweep switch compares a DC voltage to
the ramp’s peak amplitude regardless of the adjustment of
Dispersion switch (S3) or Dispersion level (P7). This allows
for a stable frequency reading when setting the dispersion
or sweep stop point frequency. When the Set Sweep switch
is flipped back to its sweep position, the ramp’s peak will
always equal that DC set point and consequently sweep
the frequency to that exact setting.
Remove the scope probes and attach one probe to the
output of IC2D. Adjust CW main to obtain two or three
volts DC here, then turn up the ramp amplitude until the
combination of the two equals - 5.0 volts at the ramp’s
negative peak. Adjust calibration pot P8 until LED D3 just
starts to flash. This is the alert signal of exceeding the
maximum range of linear tuning voltage. This completes
At this point, a brief explanation of the front panel
controls is in order. With the Dispersion switch (S3) set at
its CW position, the Main Tune control will manually tune
one octave of frequency for any band selected. This allows
the unit to be used as a common CW signal generator. The
Vernier tuner (P4) which normally sits at its midpoint
position allows a ±2% shift for fine adjustment.
In Sweep mode, the Start Sweep frequency is adjusted
as just explained. Switch the Dispersion Range (S3) to an
applicable position and S2 to the Set Sweep position and
adjust the Dispersion level (P7) to the desired sweep upper
limit frequency. Any value of zero to one octave of sweep
can be achieved, depending on the start and stop
frequency adjusted by these controls.
Place S2 back in Sweep for sweep operation. The
Sweep Rate control (P1) will allow sweep rates of 10 to
100 Hz, and is adjusted for the best CRT presentation.
Normally, the faster rates are desired, but for tight filters it
will have to be slowed to reduce distortion. The SloSweep
mode switch S4 (mine is attached to P1) is used in
conjunction with the Stop Sweep switch. This produces a
slow moving dot across the CRT screen that can be
stopped at any point by operation of S1. This effectively
makes IC1B a sample and hold circuit, and will maintain
that dot position for several minutes before any noticeable
movement of that dot occurs.
In actual testing, impedance matching will insure
accurate results, but if the input and output to the device
under test (DUT) have adequate isolation then a decent test
can be accomplished. Two example tests may make the
usage a little clearer:
Example 1 — To test the resonant frequency of an
8. 5 MHz parallel resonant circuit, set the generator to full
sweep at 5-10 MHz (band 3). Feed a signal into the DUT
of the desired level through adequate isolation. Connect
the scope’s vertical input probe to the output of the DUT.
Connect the scope’s horizontal input to the Scope
Horizontal jack from the generator and adjust for exactly
10 graticules of trace. We are now sweeping at 500 kHz
per graticule division ( 5 MHz/10 divisions).
You will see a definite peak in voltage as the trace
travels across the screen. Let’s say it peaked at seven
graticule divisions; the resonant frequency is 8. 5 MHz
(sweep start frequency at 5 MHz plus seven divisions at
500 kHz per division). The sweep rate can be slowed to
20 seconds with S4 (at this point, the horizontal trace will
be a slow moving dot rather than a solid line), and the
moving dot will stop right at its peak with the Stop Sweep
switch (S1A) and the exact frequency read from the
Counter Output jack.
Similarly, the dot can be stopped at the - 3 dB point
(bandwidth) or any unusual aberrations for marking
frequency. The dot may be stopped anywhere along the
horizontal axis to mark frequency.
Example 2 — You may want to check out the
frequency response of the wideband amplifier you just
prototyped. Start with the lowest band at 100% sweep and
just rotate the band switch as you would a potentiometer.
Stop at the - 3 dB point, also noting any unusual events
along the way. You can go to SloSweep to mark the
frequency at any points of interest.
Due to the generator’s exceptional flatness of
amplitude, switching through the bands always maintains
the same input level for accurate test results. At first, these
controls and their interactions may seem a overwhelming
but I guarantee you that after a few hours of playing with
them on the test bench, they will become “old hat.”
The above test examples were very basic just to
familiarize users with the operation of this generator, but far
more involved tests really will make a sweep generator
appreciated as to its ability to gather info in a short amount
of time. That part I will leave to the builder to gain more
knowledge from the Internet and other sources.
I wish I had more space to go into more detail on all
the different aspects of this project, but since I don’t I have
included several websites in the Parts List that contain a
wealth of information.
So, there you have it. There are a lot of variables here.
You can build this generator exactly as described or add
bands (two vacant switch positions are still available), delete
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