rotary tool, knife, or whatever.) The remaining thru-holes
were drilled and the remaining components were mounted.
R4, C4, 5, 6, and the regulator are all thru-hole components
and mounted from the other side of the board (phenolic
side). The R4 lead length is not critical, just make sure it fits.
The MMIC amp has one tapered leg with a
corresponding color dot on its case — this is the RF input
terminal. This was then soldered into place along with the
two chip capacitors C2 and C3. Depending on the
enclosure used, the cable installation will vary. In my case,
the cables pass through grommeted holes in each end, and
were pushed through the enclosure before soldering them
to the circuit board. In prepping the cable ends, keep these
leads short. Expose only about 1/8” of insulation on the
center conductors. Twist the braids to form a wire for the
ground lead, then push these leads into
their corresponding holes and solder
them to the board as close as possible.
Ideally, there will only be about 1/8” of
exposed wire/insulation at these
connections. After all the soldering was
completed, the board was secured to
the enclosure on two pre-installed 1/2”
standoffs. One final comment about
overall construction — plan ahead and
after every machining operation, make
sure the respective parts fit as intended
before proceeding with soldering. Add
the final power leads, connectors, etc.,
and the probe is ready to use. A word
of caution here: Double-check all
connections and polarities before initial
power-up as the MMIC amp operates at
4 VDC and will not tolerate reverse
polarity when fed from an 18 volt
supply.
verify that. Upon completion, it would be reassuring if you
could do the same even if you had to temporarily beg,
borrow, or steal the necessary equipment to do so. One
thing I want to point out here is that the probe has a 50
ohm output impedance and will perform best with scopes
that have a 50 ohm vertical input port (most scopes having
>150 MHz BW have this port). In lieu of this port, there are
50 ohm BNC/BNC terminations available that can be
attached to the standard 1 meg vertical input jack. Bear in
mind that using that input, there will be an increasing VSWR
with increasing frequency due to the scope’s 20 pF of input
capacitance at that jack. So, displays will be somewhat
skewed above 100 MHz. Upon completion of this probe, I
ran side by side comparison tests against a variety of my
common 10X passive probes. The tests were performed on
Testing the Probe
For a simple circuit, this probe
required many dozens of tests
throughout its design cycle to verify
performance vs. circuit changes, so it
came as no surprise to me that the
finished unit operated as expected. In
that cycle, various ground lead lengths
of 1” through 6” were tried with only
small changes in performance. Even
though ground leads degrade
performance somewhat, I wanted one
for convenience of use. After some
consideration, 4” was the best
compromise and the circuit was
optimized for that. This works because
the low Z style probe is much more
immune to ground lead problems than
other style probes. With everything
completed, the probe should work as
advertised, but I still ran a battery of
tests with capable test equipment to
TCPmaker for control over the Web
Easy as 1-2-3:
1. Define Your Data, with variable names that
YOU create.
2. Lay Out Your Content (with gorgeous
web-ready screen controls that you can grab
on to) using TCPmaker’s drag & drop Visual
Page Designer.
3. Generate Your Code, for all Microchip C
compilers, to “wire it all together.”
NO PC PROGRAMMING AT ALL – just point your
web browser at your device!
Simple Upgrade Management System
Low cost integrated system manages upgrades
across different processors & connectivity types:
> Lets your end users upgrade your PIC
firmware safely, simply, and securely.
From the makers of HIDmaker FS:
November 2010 51
