SEA 245 - Theory of Operation; General Overview of SEA 245; SEA 245 Receiver Details

5-1
5 THEORY OF OPERATION
5.1 GENERAL
The SEA 245 is a double conversion MF/HF SSB transceiver of up conversion
design. The first intermediate frequency (IF) is 45 MHz and uses a relatively
narrowband (8 kHz) crystal topping filter in conjunction with front end low and high
pass filters to provide excellent image, spurious and harmonic rejection. This type
of broadband design results in a minimum of tuned circuits. The second
intermediate frequency of approximately 40 kHz permits the use of DSP
oversampling techniques to provide secondary selectivity.
Receiver baseband recovery uses IF based DSP circuitry. The filtered,
downconverted 40 kHz IF signal is fed into the ADC and DSP circuitry provides
programmable receiver filtering and demodulation.
Transmitter baseband generation is likewise DSP based and uses the DSP/CODEC
circuitry and an I/Q modulator, together with an appropriate DSP algorithm to
generate the desired baseband signal at 45 MHz.
The frequency control circuitry in the SEA 245 uses a combination of two PLL-
based frequency synthesizers and the system DSP engine to provide the various
frequency conversions.
The first conversion oscillator is the 90-150 MHz VCO, which uses a PLL-based
loop with a reference of 8 kHz. This oscillator is then divided by two to 45-75
MHz. The resulting coarse-tuned local oscillator has a resolution of 4 kHz, very fast
settling time and a low noise floor.
The second conversion VCO operates at 45.040 MHz. In the transmit mode, the
VCO operates at 45.016 MHz for the transmitter I/Q modulator circuitry. The loop
reference frequency is 8 kHz.
All frequency determining circuitry is locked to the master clock oscillator, a 12.288
MHz OCXO.
The SEA 245 operating system resides in the Front Panel/Controller Assembly. The
operator communicates with the operating system firmware through the 18 key
keypad. The Front Panel/Controller Assembly is actually a SEAbuss(c) Controller
designed to communicate with the Mainboard Controller through the standard
SEABUSS interface. The SEA 245 SEABUSS is designed to support a single SEA
2450 Remote Controller in addition to the Front Panel.
5.2 THE RECEIVER