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drive a single LED. It would run for x
hours (say five), then turn off; 24 hours
after it was first activated, it would
automatically turn back on for the
predefined time.
I've found several ideas, but most
of them surrounded the 555 chip
which has a very limited time frame.
The PIC16F628 has an internal
R/C oscillator. with a 32 kHz option.
The current at three volts and 32 kHz
is 15 micro-amps, making it perfect for
a battery-powered device. The only
external components are the LED and
a 390 ohm resistor. You write delay
loop routines to create a 19 hour loop
and a five hour loop for a 24 hour
repeating LED.
Dennis Hewett
Frontenac, KS
[#11102 - November 2010]
Fluke Scope and RF
I have a Fluke 99B Series II
scopemeter (100 MHz).
I’ve started working with amateur
radio and would like advice on using
this scope to measure RF. I think I need
an RF probe. I currently have the set of
10:1 probes that come with the scope.
Can the scope — using the proper
probes — be used at the actual antenna
feed output from the transceiver?
You certainly can use your Fluke
scopemeter to work with amateur
radios. However, you must be
extremely careful if you decide to
monitor transceiver output. Most HF
transceivers have an output of 100
watts (or more) which can seriously
damage your instrument.
Assuming that a transceiver has a
100 watt output — typical for many
such units — the output voltage at full
power will be 71 Vrms or 99V peak; at
200 watts, the output will be 100
Vrms or 141 Vpeak. Vrms = (P x R)1/2
and Vpeak = (P x R)1/2 x 1.4.
Therefore, it is essential to protect the
scopemeter and use an attenuator
between the transceiver and
instrument. A safe approach is to use a
100:1 attenuator while the transceiver
is also connected to the proper 50
ohm load as shown in Figure 1a. Do
NOT key the transmitter ON without a
matching load. For best results,
resistors R1 and R2 should be at least
0.5 watts high frequency types.
Consider using surface-mount
resistors, 2010 size. These resistors
have better high frequency
Figure 1
February 2011
75
