TESTING & ALIGNMENT:
1.
Alignment is straight forward, although some form of
signal source or VHF signal generator is an enormous help.
Set all 4 ferrite cores in the Toko coils to approximately
3mm below the tops of the cans.
The ferrite cores in the
Toko coils are very fragile so only the correct plastic
hex trimmer tool should be used. Metal screwdrivers
etc will easily break the cores.
2. Using a current regulated powersupply
apply +12vdc
to the board & check that the current is 100mA or less.
Connect a suitable speaker to the board & turn the
Volume1 trimpot up until noise is heard from the receiver.
Make sure that the Squelch trimpot is turned off,
( Fully Clockwise ).
3.
Put the AFC switch in the AFC position. Adjust the
100kohm 25 turn trimpot to the middle of its range. Set the
fine tune trimpot to mid position. Short the antenna input
to ground & measure the DC voltage at the AFC switch
with a multimeter. The switch is now changed to the
Normal position & the Offset1 trimpot is now adjusted to
read the same DC voltage.
4.
With the antenna input still shorted, adjust the Offset2
trimpot for 0 Volts on Pin 16 of the MC13135P.
5. The next lot of adjustments 5 to 7 are with the AFC
switch in the normal position.
We start with alignment of
the first voltage controlled oscillator, VCO. For a receive
frequency of 137.500MHz, the oscillator is tuned 10.7MHz
higher to a frequency of 148.200MHz. The frequency can
be measured by connecting a frequency counter to the
VCO output pin on the PC board, or by monitoring the
signal on a suitable FM receiver tuned to 148.200MHz.
Adjust the oscillator coil L5 until the local oscillator carrier
is on frequency. The tuning of L5 will be quite large with
only a slight adjustment but just get it close to
148.200MHz. As the VCO is freerunning any movement
of the board, or temperature changes will cause the VCO to
change frequency. It is preferable that the board be
mounted on 10mm or higher PCB spacers on each corner
of the board in a suitable box. For initial alignment, the
board can be supported with the PCB spacers on the
workbench .
6.
Next connect a signal generator tuned to 137.500MHz
to the antenna input. Adjust the 100kohm 25turn trimmer
if required until the signal is heard from the speaker.
Adjust Inductors L1 to 4 for maximum output on the signal
meter. Modulate the signal generator with a 2.4kHz
sinewave with 30kHz deviation & adjust the 455kHz
demodulator coil for minimum distortion on the audio
signal from the speaker. An Oscilloscope can be connected
to the audio output & the demodulator coil adjusted for
minimum distortion of the sine wave.
7.
If you are using the optional channel switch with
additional 100kohm 25turn trimmers, then set each one to
the required frequencies of 137.300MHz, 137.500MHz,
137.620MHz etc using the signal generator to tune them in.
All frequency tuning should be done with the AFC in the
normal position. When all frequencies have been tuned
then the AFC should be turned on.
8.
Most alignment above can be made without test
equipment by receiving a Weather Satellite signal.
RECEIVER BOARD CONNECTIONS:
1.
The PC audio output is suitable for connection to a Sound
Blaster or similar card. Use suitable shielded audio cable
with a 3.5mm stereo plug on one end. Connected the center
of the cable to the tab on the plug that corresponds to the far
end of the 3.5mm plug. This is the left channel. Plug the
3.5mm plug into the microphone input of the sound card.
With the software supplied with the sound card, set the
mixer record volume level to +21, & the microphone input
to about half level. Check that there is noise from the PC
speakers.
The PC audio level trimpot on the Receiver
should be turned fully clockwise.
For the recommended
Software, refer to the separate Notes on Weather Satellite
Reception.
2.
The Antenna connection can simply be soldered directly
to the board. An Optional F type R/Angle socket can be
fitted to the board to allow the use of RG6 cable with F type
connectors for the antenna cable.
3.
Any 8 to 32ohm Speaker can be used on the Receiver.
The Speakers Volume1 is independent of the Line Output
Volume2.
4.
For an S Meter any low cost surplus meter can be used.
The S meter is not really required, but does give an
indication on how strong the satellite signals are.
5.
The AFC / Normal connection can be connected to an
external toggle switch. The switch should be set to the
normal position when tuning in a signal, before the AFC is
turned on.
6.
A PLL Synthesizer Option is currently not available.
Spare holes are located in the PC board near the VCO coil
L5 for the PLL loop filter components.
OPTIONS & NOTES:
1.
The 470ohm
Option 1
resistor sets the capture & hold in
range of the AFC circuit. The capture area with the 470ohm
is /- 40kHz. The capture area can be lowered to
/-15kHz by replacing the 470ohm with 0ohms.
2.
The 1uH SMD Choke,
( L1.0u ) Option 2
can be fitted to
feed +12 volts up the antenna cable to power an optional
preamplifier.
The choke should not be fitted if a
preamplifier is not used.
An external preamplifier should
not be required if low loss RG6 or similar cable is used to
the antenna. An external preamplifier may cause overload of
the receiver from strong out of band signals.
3.
An optional PLL Synthesizer could be connected to the
receiver for automatic scanning of the Weather Satellite
Copyright
©
Mini-Kits
www.minikits.com.au
Revised
17/11/2014
137MHz Weather Satellite Receiver
EME117 ver3
DISCONTINUED
PRODUCT
