By Bryan Ackerly, VK3YNG.
If you are considering building an 80m foxhunt receiver and don’t own a signal generator, this project may prove useful. It provides a stable signal source at a number of different output levels with more than 100dB range. It can also double as an 80m fox-or mini-transmitter.
This circuit was designed as a simple alignment source for the VK3YNG 80m foxhunt receiver. Anyone constructing one of these receivers is encouraged to build one of these circuits if they don’t own a reasonably good signal source.
This circuit is capable of providing reference signal levels between –120dBm and +6dBm in 40dB steps. This level range is typical for ARDF and other foxhunting on 80 metres and is useful to ensure that a receiver is capable of handling the signal range required to hunt transmitters right to their source on this band.
The schematic is shown in figure 1 . U1A forms the main oscillator. R2 provides bias to operate U1A in its linear mode. C2 and C4 provide parallel loading to ensure Y1 oscillates at its marked frequency. This oscillator runs continuously.
U1D and U1C along with R4, R5 and C5 form a low frequency square wave oscillator at approximately 4Hz. This circuit was taken from an old National Semiconductor databook. On each alternate cycle, C5 is discharged through R5 until U1D toggles. R4 apart from providing a feedback path to U1D inputs also limits discharge of C5 through the input protection diodes on U1D as the voltage at C5 effectively swings greater than the supply rails.
The low frequency oscillator signal is then supplied to U1B which effectively gates the 3.5MHz signal by the lower frequency oscillator. This generates a “pulsing” signal that is easily identifiable.
For situations where a continuous signal is required, fitting R1 at the input of U1D forces the low frequency oscillator to stop. As both inputs of U1D are high its output will be low therefore forcing the output of U1C high which gates the 3.58MHz signal from U1B continuously on.
C3 provides DC blocking for the output. The output of this circuit should provide up to 6dBm into 50 ohms for a 3.3V supply.
The attenuation network is a little unusual. Although it is not immediately obvious it is actually a voltage divider arrangement. It takes into account the fact that in most cases only one output will be used at a time, and because the attenuation is so great there is little influence from each stage. Using the –40dBm output as an example, a voltage divider is formed by R6 and R10. Actually the effective value of R10 is closer to 20 ohms when all of the series and parallel resistances are taken into account. This network results in a power attenuation of about 46dB. The series combination of R9 and R10 provide an effective source impedance which is close to 50 ohms to allow good matching into coax cable.
The same arrangement is used for the –80 and –120dBm outputs. The actual output levels should match fairly well for all outputs except the –120dBm one. This is because leakage limits its level somewhat and in practice the level is between 5 and 10dB higher than this. For the purpose of the circuit this should not be a problem. If you require a more accurate level, putting a shield around R8, R13 and R14 may help. Note that leakage increases significantly with frequency and this circuit arrangement will require a lot more care if used at higher frequencies unless surface mount components and double sided board are used. For a single sided board though the results at 3.5MHz are very good and quite repeatable.
Construction is straightforward. There are no special components. Mount the resistors and capacitors first as shown in figure 2 . Make sure that all components are mounted flat against the board. Any elevated components especially around the attenuator will compromise the circuits operation. Trim the leads close to the board once soldering is completed.
Next mount and solder the integrated circuit taking note of orientation. Connect the +3V and associated ground connection to a battery holder.
If you are not using a properly made PCB , good results should still be possible using “Dead bug” construction techniques, but keep all signal carrying components flat against the board to minimise unwanted coupling.
A short run of RG174 or similar 50 ohm coaxial cable should be used to connect the required output signal. It may be tempting to consider a rotary switch to select the output level. However, the additional leakage through the switch may compromise the accuracy of the attenuation. If the transmitter is to be mounted in a proper box it may be better to provide four BNC or RCA output connectors instead. At this frequency, the impedance of the connectors is of little consequence. What is important here is that they are well shielded. Make sure that sockets are spaced a reasonable distance apart to minimise leakage.
If the board is to be properly boxed up, a single pole “on-off” switch and power LED with an appropriate dropping resistor can be added if desired. Another switch to change between “continuous” or “pulsed” operation could also be fitted in place of R1 if desired.
There is no alignment required, just a simple check to make sure that the circuit is functioning. The first test should be done with R1 installed. Connect a short length (about 500mm) of wire to the +6dBm output. Use a nearby communications receiver of some sort and tune it to 3.58MHz. A continuous signal should be heard.
Next remove R1 and make sure that the signal then appears “pulsed” at about 4 times per second.
A 500mm wire antenna should enable the transmitter to be heard up to 50m away. A full length 80m dipole should be able the transmitter to be heard several kilometres away. Be careful using an antenna that is this efficient as there is no output filtering and the transmitter out put is very rich in harmonics. If a long antenna is to be used, an output filter should be considered. A suitable filter is shown in figure 4 . The expected response of this filter is shown in figure 5 . (At least 36dB of attenuation is required at 7MHz to make sure harmonics are below –30dBm.) This filter is only needed on the +6dBm output and can be built dead bug style.
R1 - Zero ohm Resistor or link. (*see text)
R2 – 10M 1/4W or 1/8W resistor
R4, R5 – 1M 1/4W or 1/8W resistor
R6 – 6K8 1/4W or 1/8W resistor
R7, R8 – 2K7 1/4W or 1/8W resistor
R9, R10, R11, R12, R13, R14 – 27R 1/4W or 1/8W resistor
C1, C3, C5 – 100nF X7R 5mm Monolithic Capacitor
C2, C4 – 33p NPO Ceramic capacitor
U1 – 74HC00A DIP Quad 2-input NANAD gate
Optional parts:
Metal box, Battery Holder, LED and 270R resistor, SPST power switch, SPST mode switch, 4x BNC or RCA sockets. For Low Pass filter: 2x3.3uH inductor, 2x 1nF Ceramic or Monolithic capacitor, 1x 1.8nF Ceramic or Monolithic capacitor.
*Note: R1 is fitted when using this board as an alignment signal source.
R1 is removed when using this board as a pulsed "fox-or-ing" transmitter
Power supply is 3 Volts nominal (Use 2x AA Alkaline batteries)
The Victorian ARDF group web page: http://www.ardf.org.au
The Author’s web page: http://www.foxhunt.com.au
The Author’s email address: info@foxhunt.com.au