Elecraft PX3 Owner's Manual - Manual (Page 33)

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Theory of Operation
Figure 21. PX3 Simplified Block Diagram.
The input to the PX3 is a pair of signals from the KX3 transceiver that represent the in-phase (I) and quadrature
(Q) components of the received RF signal. These signals are also buffered and sent back out so you can connect
them to a personal computer or other device for additional processing. Both the I/Q input and output use
differential amplifiers to isolate the grounds so that isolation transformers are not needed. The low-noise
amplifier and high-dynamic-range analog to digital converter (ADC) ensure that weak signals can be received
without overloading from strong signals. The ADC output is passed to a Microchip dsPIC digital signal
processor/controller IC, which processes the digital I/Q signal from the ADC for presentation on the 480x272-
pixel color TFT LCD display
The "circuitry" shown inside the processor box in the block diagram above is actually implemented as software
routines. The FFT is the fast Fourier transform, which is a software version of a hardware spectrum analyzer. It
reads the incoming signal and calculates the frequency spectrum. Further software routines calculate the power
of the spectrum, take the logarithm, and then scale and offset the result so that it reads correctly in dBm on the
display.
The dsPIC also acts as a controller for the rest of the circuitry. For example whenever the user changes the span,
both the ADC decimation rate and the clock frequency are re-calculated. In that way, the optimum sample rate is
used for any span, which optimizes the display update speed and ensures that each horizontal pixel on the
display represents a distinct frequency, with minimum bleed-over between pixels.
One firmware task is to maintain communications with the KX3 transceiver over one of the RS232 ports. A
special PX3-specific command set has been implemented to maximize communications efficiency. In addition, a
special PX3-KX3 communications protocol was set up to ensure that the PX3 gets the information it needs when
it needs it, so that the PX3 acts as a fully-integrated extension of the KX3. Communications between the
computer and the KX3 is passed transparently through the PX3 so that the computer "thinks" it is talking
directly to the KX3.
In addition to two UART (universal asynchronous receiver-transmitter) ports for the two RS232 connectors, SPI
(serial peripheral interface) and I
2
C (inter-integrated circuit) interfaces are provided for future option modules.
Those interfaces, plus power supplies and other signals, are passed via a 40-pin connector through the power
supply board.
For best efficiency, the +3.3 V and +5 V internal power supplies are supplied by a pair of switching DC-DC
converters from the +12 VDC input. The input power is approximately independent of the voltage, which means
the lower the input voltage the higher the current.