RAT5 Manual version 1.2.0
RAT5 Information Manual version 1.2.0 © GW4GTE 2010
Page 20 of 29
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Antenna PL1/1,2
A case mounted antenna connector may be planned but for testing a couple of short
pieces of unscreened wire twisted together will suffice to attach an antenna connector
of your choice to the antenna input PL1.
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LO output PL2/1,2
A buffered LO out has been provided for LO alignment and for future use. The level is
200mV P/P.
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Power Source PL3/1,2
Any Voltage from 9V to 15V is suitable. Absolute maximum is 18V.
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SQL / Mute PL3/5, 6
This is a dual function connection. As an output, a voltage of (supply volts -1) is
present on squelch mute. An input of around 5V to 12V can be used to override the
normal squelch function and force a mute.
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Loudspeaker (LS) PL3/9, 10
Use an 8 ohm or higher impedance loudspeaker or headphones.
5.3 Connecting an S-Meter.
PL4 (located near the tuning trimmer VC1) provides an output suitable for driving an external
S-meter. A meter with a movement of 500uA FSD would be ideal. As the same line is used to
drive the squelch circuit, the extra loading may have a slight effect on the squelch trigger
position of RV2 but the effect is small and of no consequence.
A 10k series variable resistor can be used to set the meter level as required.
5.3.1 Calibration
Calibration of the S-meter can be attempted. If a calibrated signal generator is not
available, one suggestion is to set the meter FSD by comparing with the S-meter of
say a transceiver. Based on a level of S9 +30dB for FSD the following levels were
noted on a test receiver using a 500uA FSD meter:
Signal level
Reading Signal level
Reading
S9 + 30dB (-43dBm, 1580uV)
500
S7 (-85dBm, 12.5uV)
240
S9 + 20dB (-53dBm, 500uV)
460
S6 (-91dBm, 6.3uV)
90
S9 + 10dB (-63dBm, 158uV)
430
S5 (-97dBm, 3.2uV)
10
S9 (-73dBm, 50uV)
400
S4 (-103dBm, 1.6uV)
Zero
S8 (-79dBm, 25uV)
360
S3 (-109dBm, 0.8uV)
zero
The rapid fall-off in level below S7 is understandable as the TDA1072 is intended as
a car radio receiver handling millivolt level signals. A circuit could be devised to
compensate for the non-linearity at lower signal levels.
5.4 Operation with a Transmitter
The receiver settle-time after switch on is virtually zero, so integrating RAT5 into a transceive
setup is no more difficult than arranging a relay to switch the supply along with an antenna
changeover relay.
Alternatively, the Squelch/Mute line at PL3/6 can be used. This line has a dual function. Used
as an input, a voltage between 5V and 12V will mute the audio regardless of the setting of the
squelch control. This allows the receiver LO to stay running.
