22
Circuit Details
Note:
The schematic for the 4-band Filter board appears on the following two pages. The schematics for
the K1 RF and Front Panel boards can be found in Appendix B of the K1 Owner's Manual.
Sheet 1
One of crystals X1-X4 is selected for premixing by relays K1 and K2 (there are four possible settings of the
two relays). Miniature latching relays are used rather than diode switching in order to minimize the amount
of PC board area required, as well as to keep receive-mode current drain at zero.
The balanced output from the premixer (K1 RF board, U7) is routed to P1 pins 1 and 2. This 3k-ohm
impedance signal is used to drive the premix band-pass filter (T1, T2, and associated capacitors) via link
coupling. The link-coupled input and output, combined with high-Q toroid cores, ensure high output from
the premixer over its entire frequency range (12 to 26 MHz, depending on the band). Relays K3 and K4
select four combinations of capacitance to provide resonance on each band. For example, with both relays
reset (as shown in the schematic), only trimmers C1, C2 are in-circuit (15/17 meters). Setting K4 puts C3
and C4 in parallel with C1 and C2 (20 meters).
T3, T4, and associated capacitors make up the RF band-pass filter, which is used in both transmit and
receive modes via the T-R switching diodes on the RF board. Since this filter must cover a 3:1 frequency
range (7-21 MHz) with only a single inductance value, both input and output use tapped connections to
their associated transformers (i.e.,
autotransformer
coupling). The tap points are adjusted to provide
optimal power transfer at both ends of the filter. K5 and K6 select the appropriate capacitors.
Sheet 2
Two low-pass filters are used, one to cover 7-10 MHz, and the other 14-21 MHz. 10-element Elliptic filters
are used to ensure excellent 2
nd
harmonic suppression on all bands. Separate relays are used at input (K7)
and output (K8) to maintain good end-to-end filter isolation.
Microcontroller U1 is connected to the K1's auxBus network line, which the main microcontroller on the
Front Panel board uses to send band-change and other commands. When a band-change command is
received, U1 pulses the appropriate relay coils bidirectionally to switch them on or off. (The required states
for each relay on each band are shown in Note 1 on sheet 1 of the schematic.) In addition to controlling the
Filter board relays, U1 controls the state of the Attenuator relay on the RF board.
