diodes with their lower voltage drop allows for the DIP
meter supply voltage to be between 6V and 26V. I
selected the 5V regulator because of its very low dropout
(LDO) voltage and quiescent current. You can easily use a
standard 7805 in the same package if you will not be
using battery power.
Keep in mind that the circuit will use whichever
power source has the higher voltage. So, if you have a
battery installed, be sure that any external power supply
you use is higher than the battery voltage. Otherwise, you
will still be using the battery.
My first prototype of the DIP meter used RA7 as the
signal to drive the decimal point of the display. This
seemed to be the obvious choice since the other bits of
register A are used for the segments. The measurements
for the DIP meter function were quite stable and accurate
enough for three digits.
However, when I decided to
implement a frequency
counter, I found that the
accuracy and stability of the
internal oscillator were not
good enough for six digits.
This PIC allows an external
CPU clock only on RA7, so
I had to find another pin for
the decimal point.
Due to some other
changes I made, RC2 turned
out to be the easiest one to
use. The oscillator I selected
has an accuracy of 100
ppm, so it is plenty accurate
for the DIP meter and okay
for most frequency counter
applications. There are many
oscillators available in this
26 July 2016
■ SCHEMATIC 2.
What is a DIP meter and what can you do with it?
Basically, a DIP meter is nothing more than a variable
frequency oscillator (VFO) with a means of reading its
frequency. It can also be considered as a very low power
transmitter which puts out a constant unmodulated RF
signal (CW). Its main use is to determine the frequency of
a tuned circuit.
A Grid DIP meter (Eico) was the second piece of test
equipment (after my Heathkit VTVM) I purchased when I
got into ham radio. I used it to measure the resonant
frequency of the tuned circuits in the transmitters I built
to ensure they were set to the correct frequencies. I also
used it to measure and then adjust the resonant
frequencies of my various antennas throughout the years.
A DIP meter can be used to measure the resonant
frequency of a tuned circuit (parallel or serial), a trap, or
an antenna. If the antenna is multi-band, you can find
each of its resonant frequencies.
If you want to know the values of the L and C in a
tuned circuit, you can use formula 2 (in the text) if you
have a known capacitor you can put across the circuit.
Measure the frequency using the DIP meter both with and
without the known capacitor. Plug them into the formula
and you will get the L value. You can then calculate the C
knowing the L, and the frequency without the known
extra capacitor. One of the tables in FLC Calculations.xls
(available at the article link) makes use of this formula to
determine the inductor values required for the different
bands of the DIP meter.
You can determine the length of either a shorted or
open section of coax by measuring its resonant
frequency. I used this formula to verify the length of a
piece of coax:
λ inches = 11811 VF/FMHz, VF = Velocity Factor
Attach a short piece of wire to one end of the coax
between the center and shield. The wire should be just a
couple of inches and formed into a single loop. Now, find
the lowest resonant frequency using the DIP meter. If the
coax is open at the far end, the coax is 1/4 wavelength. If
the coax is shorted at the far end, the coax is 1/2
In my test, the coax was 119” and the measured
frequency was 15. 7 MHz. Using the formula above:
Length = 11811*.65/( 4* 15. 7) = 122”
which is quite close to the actual length. I then shorted
the far end and measured the resonant frequency as 32. 7
MHz, which is very close to double the 15. 7 MHz
measured before. In this case, Length = 11811*.65/( 2* 32. 7)
= 117 — also very close to the actual length.
The variation from the actual length can be due to
several factors, such as the VF may not be exactly . 65 (my
coax is over 20 years old), the adapters I used to get the
loop on the end, or the loop itself. I did notice that a
longer loop caused the frequency to be lower by 2 MHz
with the far end open. This causes quite a difference in
the calculated length.
There are many articles on the web you can find by
simply entering “what can you do with a DIP meter” into
your favorite search engine.