Elecraft T1 ATU Owner's Manual - Manual (Page 11)

11
Using Your HF Transceiver to Listen to the INFO Report
The INFO report is sent via both the yellow LED and a weak RF signal. To find the best signal, disconnect
the antenna from the T1 to reduce background noise, switch in a wide receive filter, then tune your
transceiver’s VFO across any band while the T1 sends data (see page 7). Some frequency ranges to try
include 1.8-1.9, 3.8-3.9, 7.0-7.2, and 14.0-14.2 MHz. Note: The reason the signal frequency varies from
one T1 to the next (and drifts a bit) is that its MCU runs from a ceramic resonator rather than a crystal.
Circuit Details
(Please refer to the schematic, next page.) The ATU uses seven inductors and seven capacitors in an L-
network. The capacitance can be placed at the transmitter or antenna end of the network via relay K8. Each
inductor and capacitor has its own DPDT relay, with the individual sections of each relay placed in parallel
for reliability. The relays are selected under control of the ATU's microcontroller, U1. Latching relays are
used so that they will not consume any power except when the operator is actually tuning.
The T1 uses a comprehensive, three-stage tuning algorithm to ensure selection of the best of its 32,768
relay combinations. The coarse phase takes the longest because the impedance is an unknown (using band
data via remote control can reduce this time by eliminating some L and C values based on the operating
band). High SWR “subtrees” of L/C possibilities are also “pruned” during this phase. Fine and very fine
matching stages try successively narrower L/C groups. Once a match has been found, it is saved in
EEPROM for later consideration when retuning. A number of such matches are saved, so the tuner will
rarely need to tune completely from scratch. If band data is available, it is used to load the last-used
network values on each band. Further details on using band-data appear on page 8.
T1, T2, D1, D2 (etc.) form a directional coupler for SWR and power measurements. This type of bridge is
inherently balanced over a wide frequency range and requires no adjustment. The bridge outputs are
connected to A-to-D inputs on U1. U1 measures these voltages and converts them to SWR or power
readings, using averaging and linearization techniques to improve accuracy. Before matching starts, the T1
determines the peak power of the transmitted signal, then uses a large fraction of this value as a qualifier
for SWR readings. In this way, SSB voice as well as any other modulation method can be used for antenna
tuning, with little loss of accuracy compared to the use of a constant carrier.
The Control board provides the user interface (LEDs D6-D8 and switches S1-S2) as well as the interface
between the remote jack (J3) and the microcontroller (U1 on the main board). Since the Control board is a
plug-in module, future variations can be accommodated. D3 is a reverse-polarity protection diode for Q1
and U2. Q1 is turned on whenever the user presses the PWR/TUNE switch, providing about 9 volts to U2,
the 5-volt regulator. This wakes up the microcontroller, which loads the present settings from EEPROM
and verifies proper relay configuration. R10 and R11 form a voltage divider for battery voltage checks.
DATA is a bidirectional logic line used by the T1 to request band data from remote-control adapters.
The INFO report RF output is generated by placing a digital pulse train on the VREFL line during “ON”
code elements. This modulates the 4-MHz oscillator and its subharmonics, providing audible CW signals
throughout the HF and VHF range. The signal is coupled to the transceiver via leakage paths in the SWR
bridge and L-C network.