Specifications

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3. Verify the DC center of the demodulated data is stable. The random data
should be randomly timed 1s and 0s, and have little to no AC component
on them. See pictures below for details.
4. If necessary, change the ATR9 setting so that the long data bit have the
same amplitude and DC value as the shorter ones.
10.6. RX DC Offset Verification
To verify that the DC center of the received signal is near the nominal value,
perform the following steps:
1. Generate a test signal, 1kHz tone, 1.5kHz deviation, on the receive
frequency. Connect the signal generator to the antenna input. Set the RF
level to -90dBm.
2. Measure the average voltage at TPRX1 on the modem circuit board.
3. The DC voltage should be between 1.0 and 2.0 volts, and the p-p voltage of
the sine wave should be greater than 50mV and the DC bias on the signal
should be less than 2 volts DC.
4. There is no calibration required in this step, but if the specification is not meet,
the “quad” coil in the radio may need adjustment to pull the DC level into
spec.
10.7. RX Carrier Detect
The DCD signal from the modem may be generated by detection of RF, using an
internal signal called RSSI (Receive Signal Strength Indication). The RSSI signal
is an analog signal representing the strength of the RF carrier. It is compared
with a pre-set value, and if it is above this value, Carrier Detect is asserted. The
pre-set value may be change with the ATRA command.
When the modem is used in the Streaming Mode (non-packetized operation, you
may wish to configure the modem to require RF carrier detect to be active for the
modem to receive data. Use the ATRF command to enable/disable this feature.
To set the RF carrier detect level, perform the following steps:
1. Generate a test signal, 1kHz tone, 1.5kHz deviation, on the receive
frequency. Connect the signal generator to the antenna input. Turn the RF
level off.
2. Watch the RSSI DAC value by issuing the AT$5 command. The current
reading will be sent out the serial port.
3. Adjust the RF level to a point where you would like the RSSI to trigger the CD
signal. This is factory-set to -110dBm.
4. Send the ATRA nnn command. Set nnn to be equal to the RSSI setting you
would like the CD signal to trip at. The BUSY LED will also indicate an RF