Specifications

CIRCUIT IDEAS
RUPANJANA
Address OP CODE LABEL Mnemonic Comments
21 CD C3 09 21 JMP AA Repeat for next period
DELAY SUBROUTINE
25 00 1B NEXT DCX D
25 01 7A MOV A, D
25 02 B3 ORA E
25 03 C2 00 25 JNZ NEXT
25 06 C9 RET
Address OP CODE LABEL Mnemonic Comments
DATA
20 50 00 MSD of period no.
20 51 00 LSD of period no.
20 52 00 MSD of minutes
20 53 00 LSD of minutes
20 54 00 MSD of seconds
20 55 00 LSD of seconds
R
adio frequency probe is used to
directly measure the level of
RF
RMS
voltage present across two
points. It is one of the most useful test
instruments for home brewers as well as
for communication equipment service/de-
sign labs.
RF voltage level being measured pro-
vides useful information only when the
probe has been designed for use with a
specific multimeter. The design of
RF
probe is a function of the meter we in-
tend to use it with. If a meter with a
different input resistance is used with the
probe, the reading will be incorrect. The
value of
R
X
(refer figure) is so chosen that
when this resistor is connected in paral-
lel with input resistance of the multim-
eter, the peak value is about 1.414 times
the
RMS voltage. Resistor R
X
has to drop
this excess voltage so that meter indica-
tion is accurate. If we know the input
resistance of the meter, we can calculate
the value of
R
X
with the help of the fol-
lowing relationship:
Let meter
DC input resistance
X 1.414 = R
Y
Then R
X
= R
y
– meter DC input resis-
tance
For example, if meter input resis-
tance is 20 meg-ohm,
R
y
= 28.28 meg-
ohm and
R
X
= 8.28 meg-ohm.
We can convert the
RF voltage level
DESIGNING AN RF PROBE
N.S. HARISANKAR, VU3NSH
( E ) so mea-
sured
across a
given load
resistance
( R ) to RF
watts ( W )
using the
following
relation-
ship:
Power
P
= E
2
/ R
watts ( W )
For example, if RF probe voltage read-
ing across a load resistance of 50 ohms is
found to be, say, 15.85 volts, the power in
the load = 15.85 x 15.85 / 50 = 5W approx.
In other words, for 5-watt power in a
50-ohm load, the voltage across the load
is 15.85 volts.
The rectified
DC voltage at the cath-
ode of diode
D 1 is at about the peak level
of the
RF voltage at the tip of the probe.
Use shielded cable in between the probe
output and meter. It will act as feed-
through capacitance and thus avoid RF in-
terference. The maximum
RF input volt-
age level depends on the peak inverse volt-
age
( PIV ) of diode D 1. The shielded lead
length is too large to give accurate re-
sults at
UHF . Please refer Tables I and II
for ready conversion of
RF voltage level ( RMS ) to
equivalent power across a 50-ohm load and deduc-
tion of
R
X
value for a given meter’s DC input resis-
tance respectively.
Table II
Meter DC Impedence Rx
20 Meg-ohm 8.25 Meg-ohm
10 Meg-ohm 4.14 Meg-ohm
1 Meg-ohm 41.4 kilo-ohm
20 kilo-ohm 8.28 kilo-ohm
TABLE I
Voltage to Watts Conversion
for 50 ohms Termination
RMS (V) RF Power (W)
2.24 0.1
3.88 0.3
5.0 0.5
7.08 1
12.25 3
15.90 5
20.0 8
22.4 10
38.75 25
41.85 35
50.0 50
23