for now as they will provide the Vt on the sense lead to
perform the next step. To calibrate the over-range LED,
you will need the RF generator connected and both its
tuning controls fully counter-clockwise. You should see
about - 6. 5 VDC at the sense lead input to the adapter.
Adjust the 5K cal pot feeding IC1d in a direction to turn
off the LED. Then, slowly rotate in the opposite direction
until it just starts to turn on. There will be a small amount
of hysteresis here which is intended.
This ends the calibration. These pots will never again
need adjusting other than troubleshooting or periodic
calibration checks (every five years?). At this point, I will
just do a “drive-by” on each control from the top to the
bottom of the front panel:
ON/OFF: S5 disconnects the Vt out signal output.
SWEEP RATE: Varies from 10 to 100 Hz; 15 seconds per
sweep when S2 is in SLO position.
SELECT: Connects ramp to Vt out in Auto position; pure
DC to Vt out in Manual position.
MANUAL TUNE: Applies DC only to Vt out and moves a
spot across the entire scope’s X axis.
SWEEP WIDTH: Coarse adjustment of the total frequency
span desired; of RF band in use.
VERNIER: Provides a fine adjustment of the Sweep Width
RUN/SET: S3a is used in conjunction with the RF
generator and is for setting the start and stop frequency of
The setup for testing requires both instruments to be
connected together and sufficiently warmed up. If your
generator does not have an internal frequency counter,
one will need to be connected externally. Connect the
adapter Scope Output to the horizontal input of your
scope and adjust the scope for exactly 10 full graticules in
length. With the RF generator’s tuning controls at the full
CCW position, switch to the RF band of interest. On the
adapter unit, set S1a in AUTO position and S3a in SET
position. Set the Vernier at the minimum (CCW) position.
Select the S4 SWEEP WIDTH control for the approximate
span of the RF band selected. For example, if you were
using the 5-12 MHz range and were only interested in a 2
MHz portion of that band, you would place this on 30%
giving you a 2.1 MHz maximum span ( 12-5 x 0.3 = 2.1).
These settings apply to any portion of that band
whether it is upper, lower, or middle.
Adjust the RF generator’s tuning controls for the
desired start frequency and then adjust the adapter’s
VERNIER control for the desired stop frequency. Reset S3a
to the RUN position. Connect the RF generator to the
input of the DUT (device under test) and the output of the
DUT to the scope’s vertical input and begin testing.
Obviously, since the scope trace data isn’t labeled up like
a full blown spectrum analyzer, you will have to be doing
some simple math in your head during any testing. If you
work in units of ‘10s,’ it’s really a no-brainer.
Let’s apply this to the above setup. We want to
examine a narrow band pass filter with a moderate “Q” in
the 11 MHz range. We have adjusted the SWEEP WIDTH
CONTROL for 30% of any given portion of the 5-12 MHz
band. We would like a center trace frequency of 11 MHz
and probably a sweep width of 100 kHz per graticule
division, giving us a total span of 1 MHz which will require
a 10. 5 MHz to 11. 5 MHz actual span. The start frequency
of 10. 5 MHz will be adjusted with the RF generator
controls, and then the stop frequency of 11. 5 MHz will be
adjusted with the adapter’s VERNIER control. In an ideal
world, the center graticule frequency would be exactly
11.0 MHz with this setup, but in reality — due to inherent
non-linearity in the varactor — it may be off the mark. This
may be of no concern in wide bandwidth filters, but is
more important in narrower filters. This is where switching
S1a to Manual sweep control comes into play.
Now, we can move a dot across the screen and land
at any point on the trace. Since it tracks the ramp
precisely, we can read the frequency that the Auto mode
reads whenever its ramp would cross that given dot
location. In my experience, if the dot planted on the exact
midpoint of the trace and the desired center frequency
was in error, the scope trace could be shifted slightly to
center everything up. Rarely would it require more than a
fraction of a division, and a slight shift would not affect
the test nor lose but a small fraction of the total trace due
to a very small part of it being shifted off screen. However,
the wider the band pass of a given DUT, the less
important exacting frequencies become.
Back to our test. Our display shows a flat topped pass
signal with a 3 dB reduction on graticule lines 4 and 6.
Now we know that our filter has a band pass of 200 kHz
(two full graticules at 100 kHz per division) with a “Q” of
55 as per the common formula of center frequency
divided by the band pass of the 3 dB corner frequencies.
What the sweep trace will really show is the symmetry
and steepness of the skirts (how fast the filter builds or
drops off the pass frequencies), plus any other spikes or
aberration. If all this setup sounds confusing, believe me it
is not near as bad as shown. After an hour or so of playing
around with the finished unit (even without a DUT), the
operation will become almost intuitive. To clear the air a
little more, I have taken scope pictures of a few tests I
have run along with the actual setup.
Figure 5 is a display showing the final tuning of my
high fidelity AM radio’s 455 kHz IF strip. These filters are
stagger-tuned to broaden out the pass band and enhance
the fidelity of the detector stage. The setup was: 455 kHz
center frequency; 415 kHz start frequency; 495 kHz stop
frequency; with a total span of 80 kHz at 8 kHz per scope
graticule, and showing a 10 kHz band pass at the - 3 dB
Figure 6 is a sweep of a band rejection filter that I
wanted to verify its location and performance: 19 MHz
center frequency; 12. 5 MHz start frequency; 26. 5 MHz
stop frequency: Total span 14 MHz at 1.4 MHz per major
36 February 2017