Fullscreen HTML5Med Res (??MB)HTML4

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. ~