SCS PTC-IIex, Manual

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14 Circuit Description
It is also possible for the PTC-IIex to control the output power of the transceiver, so that
the power to maintain the link may automatically be adjusted to an optimum value. No
more power than needed is being used.
For the signal input, the DSP uses a Sigma/Delta analog to digital converter with a 16 bit
dynamic range (14 bit effective), which enables the normally necessary anti-alias filter to
be dispensed with. With the exception of the decoupling OP-AMP, at the input and output
of the DSP, no further components in the signal path are required. The DSP contains a
built-in static RAM, which, in the PTC-IIex, is further expanded with two additional, very
fast, static RAMs. This enables difficult algorithms, for example 4096 point FFT, to be
used. The size of this RAM is 256k words (24 bit) and is not variable.
The exact receive frequency of PACTOR-II/III is achieved very quickly, and reliably,
adjusted by software, using a newly developed tracking method, a frequency deviation
between connected stations up to 100 Hz is allowed. The complex modulation scheme,
often used, but very difficult to handle because an adjustment to only a few Hertz was
necessary, is gone.
14.3 The Power supply
The PTC-IIex contains two power supply input options, which can be used optional. It
may be supplied either via a special DC input connector, or directly from the HF
transceiver, via the connecting cable and socket. The two options are decoupled via
diodes, and feed a switching regulator. This is very efficient, and generates the 5 V supply
for the digital section. The supply voltage can vary between 10 to 20 V DC. The current
requirement, due to the use of the switching regulator, is dependant upon the supply
voltage, the Packet-Radio modems used and the processor speed – usually it is 200 mA at
13,8 V. The higher the supply voltage, the lower the current consumption. This reverse
proportionality is due to the fact that the power consumed is a product of voltage and
current, and must be virtually the same, before and after the regulator. The power supply
input of the PTC-IIex contains special filtering, so that the switching harmonics from the
regulator cannot reach the outside world. The operating voltage is internally fused with a
self-resetting fuse.
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