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Coupled Frequency Meter
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Near Field Communications
A 0 to 2 MHz ferquency meter with a
Minimum Mass Wireless Coupler, based on
the ATMega8.Range to the Minimum Mass Base Unit is 10 to 15 cm. Since
the frequency meter is battery operated, it can be floated from ground,
making life in and around the test bench a lot simpler. No need to
connect it to the computer or LCD display - just turn in on and place
it next to the base unit.
The basic Minimum Mass Wireless Coupler technology is described and
links to other projects on this site that use the Minimum Mass Wireless
Coupler are located on the web page, Minimum
Download the AVRStudio assembly source for the program: vlffmtr2313.asm
Download the AVRStudio assembly source for the include file: vlfcw2313.inc
The all parts except the loop
antenna fit inside a battery case.
The loop was squashed a little
bit to make it fit the shape of
The circuit is as simple as it
can get and still work.
Wiring is not critical.
A simple one chip frequency meter was the first instrument I updated
with the Minimum Mass Wireless Coupler. The frequency meter is
self-contained, including batteries. The only external electrical
connection is pair of wires to connect to the signal being measured. To
use it, I merely attach the input leads to the signal I want to
measure, place the meter next to the base unit and switch it on. The
activity LEDs blink on both enclosures as the frequency meter sends its
command menu and -voila! -the menu prints on my computer screen and the
system is ready for taking measurements.
After the frequency meter sends its command menu, it "disconnects" from
the RF channel. In other words, it does not respond to commands or send
data until it receives an ASCII control-A character, which is its
reconnect command. A control-B character from the keyboard
disconnects the meter from the channel again. Other devices use
different control codes, for example, the scanning voltmeter uses
control-C do connect and control-D to disconnect. This discipline
allows multiple Minimum Mass Wireless Coupler equipped devices to share
the same Base Unit without interfering with each other.
The circuit shown on this page uses the simplest possible
implementation of the Minimum Mass Wireless Coupler. As long as the
input signal swings through the chip's logic switching thresholds, it
will be counted. As the battery voltage drops lower and lower, the
1N916 protection diodes will keep the input pin on the controller from
exceeding the maximum current shown on the data sheet. A
further protection is the upside-down transistor connected as a low
current zener diode, just in case the batteries a large signal is
applied to the input leads when the power is off.
A battery saver shuts down the controller and power to the bias
resistors if the circuit does not receive a command for a long period
of time. This one feature has saved me from having to replace the
batteries a number of times already.
With respect to firmware, this is basically the RS-232 Freq. Meter/Pulse Generator Based on
from elsewhere on this site. I just removed
the pulse generator part of it and replaced the calls to the UART with
calls to the Minimum Mass Wireless Coupler driver, which are provided
by the .inc file.
The red "operating" and the green "ready" LEDs from the original design
were not used becasue this is a battery powered device, and with that
scheme, one LED is to be on all the time the unit is powerd up. As it
is, there is one yellow LED that indicates activity on the Minimum Mass
Wireless Coupler and that is enough.
Contents ©2005 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/
First posted in March, 2005
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