AMATEUR RADIO
By GARRY CHATT, VK2YBX
A low cost RF sniffer probe
and preamplifier
This low cost RF Sniffer will avoid loading
problems when measuring critical low-level
oscillator circuits in amateur gear. It has a gain of
30dB from 1-500MHz and plugs directly into your
CRO or DFM.
One of the most commonly encountered problems when working
on RF equipment is how to measure
the frequency oflow-level oscillator
or mixer circuits. Most frequency
counters have a typical input sensitivity of 5-10 millivolts and require direct connection to the circuit being measured.
But direct connection to critical
oscillator circuits can create problems, due to loading effects of the
probe. Even a xto CRO probe with
an input capacitance of several pF
can "pull" the operating frequency
of an oscillator so much as to ma:ke
100
any reading meaningless.
Another problem with direct connection is that you must locate the
correct point on the circuit to connect the probe. This can be time
consuming if a circuit is available
and just about impossible if the circuit is not available.
A far easier method of confirming the operating frequency of an
oscillator mixer or amplifier is to
use a "sniffer probe". This typically consists of a miniature antenna
followed by a broadband amplifier
of modest gain. The antenna is
simply placed close to the circuit to
100
FX1 115
FERRITE BEAD
r---e-- HW.......,..--...--.--w-H- -1---.---;.---.::::::::~--+12v
4700
.01
.01
1200
4.10
(8.201/8.20)
RF SNIFFER
SC04-1-0688
1200
+·
E
Fig.1: the circuit consists of three virtually identical stages based on RF
transistors Qt, Q2 and Q3. Each transistor is connected as a common
emitter amplifier to give around 30dB of gain from 1-500MHz.
72
SILICON CHIP
The circuit is housed in a plastic jiffy
case and plugs directly into your CRO
or DFM.
pick up the signal. The amplified
signal can then be fed to a frequency counter or oscilloscope.
Circuit details
Fig.1 shows the circuit of a simple amplifier that will do the job. It
consists of three virtually identical
AC-coupled stages, with RF transistors Qt, Q2 and Q3 .
Qt, Q2 and Q3 are all 2SC3358
(or equivalent) RF transistors which
have a gain bandwidth product (fT)
of 7GHz (4.5GHz for 2SC2369} and a
typical hFE of 120. Each transistor
is connected as a common emitter
amplifier and each emitter load is
4.10.
The total gain over the three
stages is 30dB (with a 12V supply)
from 1MHz to 500MHz, and the output stage (Q3} is capable of driving
a 50-ohm load.
The circuit can be powered from
Fig.2: the circuit is built RF-fashion on the copper side of a small piece of unetched PCB material. Follow this coded
photograph carefully during construction and be sure to keep lead lengths as short as possible.
r
f
30mm - - - - - ~ - - - - - - - 9 D m m
STftlP BACK OUTER
SHEATH AND BRAID
co"
HEATSHRINK
TUBING
--------.-!
\
RG58
CABLE
PARTS LIST
~
BNC
PLUG
Fig.3: the sniffer probe is made from RG58 cable, heatshrink tubing and a
BNC plug. The dimensions shown are provided as a guide and are not critical.
Above: close up view of the sniffer probe. The heatshrink tubing isolates the
end of the braid from components above ground potential.
a 9V battery, although this will give
lower gain than the 12V supply
depicted on the circuit.
The unit is best constructed on a
piece of unetched circuit board
material measuring 77 x 15mm.
This provides a good ground plane
on which to mount the components.
1 blank (unetched) piece of
PCB material, 77 x 15mm
1 ferrite bead (eg, FX1115,
DSE Cat. L-1 430)
1 small rubber grommet
1 plastic jiffy case, 83 x 54 x
28mm
2 BNC panel-mount sockets
1 BNC plug (for sniffer probe)
1 120mm-length RG58 coaxial
cable
2 banana plugs (1 red, 1 black)
3 2SC3358, 2SC2369 or
BFR90/91 RF tansistors
(available from Dick Smith
Electronics\
7 .01 µF ceramic capacitors
Resistors (0.25W, 5%)
2 X 1 kO, 3 X 4 700, 1 X 3300, 3 X
1200, 2 X 100, 6 X 8.20
Miscellaneous
Heatshrink tubing, 1-metre twincore cable (for supply leads),
1-metre RG58 coaxial cable terminated with BNC plugs (for connection between RF Sniffer and
DFM)
JUNE 1988
73
Above: the completed RF Sniffer in action. The unit avoids loading problems when you are checking out low-level
oscillator circuits in all sorts of RF gear. Power for the circuit is derived from an external + 12V bench supply.
The input and output coupling capacitors are soldered directly to BNC sockets.
The earth lugs on the sockets a,;e soldered to the groundplane on the PCB.
Because the unit will be
operating up to UHF, it is important
that lead lengths be kept to a
minimum. By mounting components
on the copper side of the PCB blank
as shown in the coded photograph
(Fig.2), lead lengths can be kept to
practically zero.
A useful aid to construction is a
pair of tweezers, which can be used
to hold components in place as they
are soldered. Do not be afraid to apply sufficient heat to component
leads to ensure a good soldered
joint. Most semiconductors are
rated at 300°C within 2mm of the
74
SILICON CHIP
semiconductor junction for 15
seconds or so, which is ample time
to ensure a good connection.
A simple probe can be made by
terminating a 120mm-length of
RG58 cable in a BNC connector
(Fig.3). At the other (sniffer) end of
the cable, strip back 30mm of the
outer sheath and braid from the inner conductor. A short length of
heatshrink tubing can then be
shrunk over the end of the sheath
and braid. This will prevent the
earthed braid from coming into contact with components that are
above ground potential.
The unit should ideally be built
into a metal case but you could also
use a plastic zippy case coated internally with conductive nickel
spray, to provide shielding.
However, we did not find shielding
necessary and simply mounted the
unit in a standard plastic jiffy case
(see parts list).
You will have to drill holes in
either end of the case to accept the
input and output BNC sockets. Once
these have been mounted, the PCB
assembly can be dropped into position and secured by soldering the
earth lugs on the BNC sockets to the
groundplane. The free ends of the
input and output coupling
capacitors are then soldered to the
centre terminals of the sockets.
The prototype was powered from
an external 9-1 ZV source via a
twin-core cable fitted with banana
plugs (for connection to a bench
power supply). The other end of the
cable passes through a grommetted
hole in the side of the case. Terminate the leads as shown in Fig.2
and don't forget the ferrite bead
over the positive lead.
Alternatively, you can fit a
3.5mm DC socket so that the unit
can be powered from a DC
plugpack supply. Another option is
to power the unit from a 9V battery
housed inside the case. If this option is chosen, an on/off switch
should be fitted as the quiescent
current is around 40mA.
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