Some Ramblings...........This project was built mostly from junkbox parts,
very few were actually purchased specifically for the Sweeper. As such,
component substitution is wide open. The enclosures for both the Sweeper and
the VCO were recycled from other uses. I merely use air conditioning duct
sealer tape to cover any holes that I can't incorporate into the design. Notice
the liberal use of the tape in the VCO box, covering a rather large hole where
a battery holder used to be. Likewise, there's lots of room in the VCO
enclosure and none in the sweep generator. For the front and rear panel
lettering, I use printable sheets of adhesive-backed 8 1/2 x 11 paper,
available at any office supply store. The panels are laid out using Windows
'Paint' program, with many test-prints on plain paper until the dimensions are
correct. Then print the layout on the adhesive paper, cut to size and carefully
apply to the enclosure, before any parts are mounted. If done on a color
printer, a nice front panel can be made. For further protection, a clear
adhesive sheet can be applied over the panel.
The use of an optical isolator is
not necessary for the marker output. I used it because it was very simple and I
don't like to mix negative supply voltages with 5 volt logic. Transistors don't
usually open, they short!
If the 'Spot' pushbutton is held on,
the VCO frequency can be varied between the setup points with the 'Marker' pot.
This can be handy if you don't really care about setting an exact start and end
frequency, but just want to see the response at a certain frequency.
If you are sweeping an unknown
circuit or item, such as an unmarked crystal filter or resonator, start with
the widest sweep possible. Once the peak is located on the display, the 'Min' and 'Max' pots may be used, without
the 'Min/Max' switch, to center and widen the peak for detailed measurements.
To adjust the sweep width displayed
on the oscilloscope, select ‘External Horizontal Input’ and use the ‘Horizontal
volts / division’ and fine adjustment knob to display the desired sweep. The
terminology used for these controls on various scopes may be different, but
should be very close. In any case, the amplitude
of the sweep signal determines the sweep width. The controls will probable
allow for a wide range of sweep adjustment, from perhaps a ½ inch sweep, to a
scan that extends well beyond both edges of the oscilloscope screen. Here lies
another way to expand the scope display. By expanding the sweep beyond the
physical limits of the screen and centering the display of interest, this
widens the displayed peak, although the marker will also extend beyond the
scope screen. The start and end VCO frequencies remain the same, but a smaller
portion of the sweep is displayed, effectively decreasing the ‘frequency per
division’.
As the sweep speed is increased, by
increasing U2’s clock frequency, the displayed information becomes less
accurate. As on a spectrum analyzer, increasing the sweep speed tends to act as
a low pass filter on the display. This is because the scope trace is scanning
so fast that the screen persistence won’t allow display of sharp peaks and
valleys. Likewise, a very slow sweep
rate will display every little nuance in the signal, but measurements are more
difficult because of the opposite persistence effect ; it disappeared! I included the four position DIP switch to select
the best sweep speed for whatever type of oscilloscope is used, and once set,
it does not need to be adjusted further.
A signal that I was going to bring
out to the rear panel of the sweeper is the U2 carry output, RCO on pin 13.
This is a low signal, one clock pulse wide, that occurs at the beginning of
each sweep. It could be used as a trigger to sync other equipment to the
sweeper. It is not used on the prototype.
The prototype was built on several
small printed circuit boards. More often than not, I find that it is just as
easy and quick to make a pcb, as it is to wirewrap or perfboard a circuit using
IC’s. The boards are hand-drawn, and drilled with a handheld Dremel tool. They
are not fancy, but functional. Warmed-up Radio Shack ‘PCB Etchant Solution’
will etch a board in about ½ hour. First I lay the board out on .100 inch graph paper that I make on my
computer using Microsoft Excel, or it can be purchased at an office supply
store. Then tape the drawn pattern to the blank PCB. I drill the board next and
smooth and clean it with a pot-scrubber, like ‘ScotchBrite’. I then use a
‘Sharpie’ or freezer-marking pen, available in most any grocery store, to draw
the traces on the blank board. Then I etch the board, clean it once more with
the scrubber, and it’s done. Dispose of the used etchant down the toilet, and
don’t splash it anywhere. It will permanently stain anything it touches.
Finally, I just found that
Mini-Circuits (www.mini-circuits.com),
sells a neat ‘designers kit’ of seven DIP-sized VCO modules that cover the
frequency range of 25 MHz to 1.025 GHz. They run on 12 volts and use a tuning
voltage of 1 to 16 volts. Digi-Key sells a small, inexpensive DC to DC
converter SIP, made by Power One, which runs off 5 volts, to take care of the
increased tuning voltage. The VCO kit is part number K-POS2 and costs $79.95.
If I decide to build something up, I will put all the details on this webpage.
Email me with any questions,
comments or ideas at wayne@350rx7.com.
Wayne
Maxwell KD4YGU