The prescaler described here connects
to the front end of a 100-MHz
frequency counter to extend its coverage
range to 10 GHz. The circuit is
a far less expensive way to measure frequencies
above 100 MHz than buying
a 1- or 10-GHz frequency counter. The
divider was built inexpensively. Creating
this low-cost prescaler required some
tradeoffs, and some rules were not followed
(discussed later in the article).
When you use this or any similar
prescaler, the two least significant
digits will be lost. If the frequency
being measured is 9,123,456,789 Hz,
the frequency will be displayed as
9,123,456,7XX Hz. Having the last
two digits not shown shouldn’t be a
problem, considering the frequencies being measured.
A second source of error is the frequency counter itself. The
counter should be tested for accuracy. You can do this by connecting
the output of a 100-MHz signal generator to the counter
and counting for one or 10 seconds. Also, the counter time-base
oscillator should be calibrated using WWV or a similar source.
IC1 is a 10-GHz, divide-by-2, static divider; IC2 is a 7-GHz
divide-by-5; and IC3 is a 1-GHz divide-by-10 (see the figure).
All components are mounted in a cast aluminum box measuring
about 7.5 by 4.7 by 2 in. (190 by 120 by 50 mm) deep.
Electronic components are mounted on 0.062-in. (1.57-mm)
double copper-sided printed-circuit board (PCB) that’s 7 by
2 in. (175 by 50 mm). The dimensions of the box and board
aren’t critical.
J1 was soldered to the top and bottom of the PCB. This
ensures mechanical stability for the input capacitor and IC1.
For further support, the hole in the box is only slightly larger
then J1. A recess was milled around the hole to be able to screw
on an SMA connector. Protection diodes at J1 would degrade
sensitivity, so they were omitted. IC1’s bottom was not grounded
as it should be.
A Hittite evaluation board was purchased for IC2 because
of the close spacing of the leads: 0.025 in. (0.65 mm). Evaluation
boards are available for IC1 and IC2 at $164 (U.S.) each
from Hittite. A set of ICs is $26
(U.S.). A PCB would simplify
construction and reduce costs.
For a prescaler that goes only
to 1.3 GHz, consider building
one with IC3 only. Although IC3 is rated to work at
1 GHz, it works okay
at 1.3 GHz.
Power-supply current
is about 170 mA.
Before connecting the
prescaler electronics,
verify that the input to
the 7805 voltage regulator
is 8 to 15 V and
the output is +5 V. All
capacitors below 0.1
μF are chip caps.
The prescaler has
some limitations. Its
lower frequency limit
is about 200 MHz.
Also, IC3 oscillates
with no input. And,
dynamic range is
somewhat limited (see
the table). The sensitivity
limitations are probably due in part to construction methods.
The prescaler has been working for over a year, performing a variety of tasks.
