Elecraft K2 Owner's Manual - Manual (Page 117)

116
E
LECRAFT ®
Sheet 3: Filters and I/O Controller
The band-pass and low-pass filters are switched with latching relays to
minimize loss and current drain. Only five band-pass filters and seven DPDT
relays are required to cover nine bands (160-10 m). This is accomplished by
switching fixed capacitors in or out using two additional relays. For example,
on 160 meters, relay K3 places C13 and C14 across the 80 m band-pass filter.
But relay K3 also used to switch the 20 meter band-pass filter to 30 meters by
shorting C21 and C23 to ground. K6 places C32/C34 across the 15 m
inductors to select 17 meters, or C44/C46 across the 10 m inductors to select
12 meters. The band-pass response is a compromise on 80 and 160 meters but
on all other bands is similar to what would be obtained with separate filters.
The low-pass filters also serve double-duty in most cases; five filters cover 8
bands (80-10 m). The 30/20 m filter uses three pi-sections to provide good
roll-off of the 20 MHz second harmonic when operating on 30 meters. Most
of the filters are elliptic, aiding attenuation of specific harmonics. But elliptic
filters are not needed on 40 and 80 meters since these each cover only one
band. The 2
nd harmonic attenuation provided by the push-pull power
amplifier is quite good even pre-filter (sheet 4).
DPDT relays are used for the low-pass and band-pass filters rather than the
traditional SPDT approach which requires twice as many relays. This is
possible by virtue of careful guard-banding techniques on both top and
bottom of the PC board in the filter areas. Isolation between input and output
of each filter is excellent across the entire frequency range.
The T-R switch (D1-D5) provides very high isolation using low-cost silicon
diodes with a PIN characteristic (1N4007). Q2 is a very high-voltage
MOSFET that provides a ground path on receive for D3 and D4, but on
transmit this transistor can easily handle the high voltages present on the
power amplifier collectors.
U1, a 28-pin PIC microcontroller (16F872 or 16F872A), drives all of the
latching relays and a few other I/O lines. U1 is referred to it as the I/O
controller (IOC) because it handles nearly all I/O functions for the main
microcontroller. It also has the job of determining whether the 160
m/RXANT option board is installed by sensing the presence or absence of the
two relays on the module. Finally, the IOC contains all of the per-band and
per-memory initialization data in ROM, which is sent to the main
microcontroller as needed to initialize EEPROM data tables. A number of
different regional band plans and other customized parameters can be
accommodated in U1’s data tables.
The latching relays are wired with a single common drive line so that when
one relay needs to be turned on or off, the others are pulled in the opposite
direction. This arrangement requires no drivers of any kind. U1’s I/O lines are
protected from relay transients by its own internal shottky clamping diodes to
6 V and ground. Measured transients are well within the current rating of the
clamping diodes. Transients are reduced in amplitude by the series resistance
of the other non-switched relays and U1’s own MOSFET driver impedance.
The relays are rated at 5 V nominal (250-ohm coils). The actual impressed
voltage is in the 5 V to 6 V range, depending on ambient temperature,
reflecting the best and worst-case sink/source current limits of the 16F872.
The IOC communicates with the main microcontroller over the 1-wire
AuxBus. U1’s 4 MHz clock is turned off and the device is in sleep mode at all
times, except when it is processing an AuxBus message, so there is no digital
noise on receive. The main microcontroller runs from a 5 V supply, while the
IOC runs from 6 V. The AuxBus is designed to accommodate devices
running at both voltage levels.
Sheet 4: Transmitter Amplifier
Q5 and Q6 are class-A pre-driver and driver stages, respectively. Q5’s bias is
provided directly by the 8 V transmit line (8T), while Q6’s bias is switched
on by the 8T line but is gated by Q10. This is necessary because the DAC
output that supplies the bias voltage for the driver is used as the crystal filter
bandwidth control voltage on receive. The bias to Q6 can be varied under
firmware control to optimize efficiency for CW vs. SSB and at different
output levels. This is useful in maintaining high overall efficiency during
battery operation.
Q7 and Q8 form a conservatively-rated push-pull power amplifier that can
easily supply 10 watts or higher output on all bands. Q11 and Q13 are used as
a bias voltage regulator. The bias regulator is effectively out of the circuit on
CW because of the large size of resistor R62, resulting in approximately a
class-B bias level. On SSB, resistor R63 is grounded by the I/O controller,
causing much more current flow through Q13 and stabilizing the bias for
class AB operation.