Some considerations
about Xiegu G90 internal ATU tuning problems.
1. The ATU is a "relay" based.
2. The ATU is driven by the Main Unit MCU
STM32F429.
3. The mismatch (SWR ratio) is measured with a
Bruener phase compensated bridge, like the one in this picture (too lazy to
draw it myself):
Here is a picture of the SWR bridge from the Xiegu G90:
The outputs of the bridge are fed into a
dual OpAmp then sent to the MCU ADC where the values are used to calculate the
SWR that is used to find the match solution and it is also sent via UART to the
Front Panel MCU to be displayed on the LCD.4. In some FW versions the ATU seems to do a
better job in matching various non-resonant antennas. FW revisions that use high RF power gave more accurate
matching solutions than those that uses low RF power.
5. How the tuning sequence is done (in a
simplified and presumable description):
Frequency is stored in EEPROM;
-LC relay configuration is stored in
EEPROM;
-SAMPLE(Direct and Reflected are
sampled);
-oldSWR is calculated; oldSWR is
stored in EEPROM;
-change is applied to LC relay configuration;
-SAMPLE;
-newSWR is calculated;newSWR is
stored;
-newSWR is compared with oldSWR;
decision is made upon this (continue or stop finding a match solution).
In high quality "tuners", the RF is
applied only during the SAMPLE and SAMPLE is made when the relays are settled,
in a stable state.
In the G90 the RF is
present always during the TUNE process.
Depending on the voltages applied function of
time, the relays can be in 3 states: ON. OFF and UNSETTLE, the last being an
uncertain position between the first two ones and this parameter is defined as “operating
time” which, depending on manufacturer can or cannot include the bouncing time.The
bouncing time is a time interval in which the contacts are closing and opening
erratically for mechanical reasons.
If the SAMPLE is done during the
bouncing or inside the operating time interval, the result can be massive
altered. (bouncing during the RF presence can make the LC’s to “ring” on
various harmonic frequencies also).
This is particulary
valid on those relay ATU’s that are working very fast and the ATU in the Xiegu
G90 IS A VERY FAST one!
Because all of this, a
wrong LC configuration can be selected for that particular frequency thus
allowing the radio to transmitt full power on a mismatched antenna system.
This can be a source for
errors in ATU sequence and those can be identified very quick by activating the
tune sequence several times on the same frequency and the same antenna system.
Beware! The short press will recall a matching solution already stored in memory for that frequency therefore a long press is desirable, in order to erase that memory and start the matching process from the scrap. (Thanks Dave).
If the results are different, most probable is due to settle time. This time
interval is in the 5-10msec range.
The problem can be
mitigated by various means, all in the FW code; the most efficient one is to
switch on RF after a 10msec timeguard from the LC solution is applied to the
relays.
6. Errors in SWR value
can came from the intrinsec construction of the SWR bridge. The major source of
error are the diodes in the bridge. Injecting low RF (around 1W) into a Bruener
bridge based on Si diodes (even Schottky) can be into the non-linear range of the
transfer function which also is altered by variation in the ambient
temperature. (https://www.analog.com/en/technical-articles/integrated-diode-based-rf-detectors.html#)
Another factor that alter the SWR ratio could be the loses on the Reflected path. On loosy antenna systems, the Reflected power is attenuated by some factors like the lenght in the coaxial cable. This leads to a lower voltage in the non-linear transfer function of the diode therefore reducing more and more the reflected detected voltage and finally affecting the SWR ratio to a point that the MCU "feel" a proper match is achieved even if a non-match solution is set in the LC relays.
Therefore, the SWR calculated
value could reflect or not the true SWR. The result is a wrong tuning solution.
This problem can be
mitigated by applying one of these solutions:
a. Replacing the Direct and Reflected detection
diodes with lower threshold ones. The actual diodes used in the Xiegu G90 has
to be measured to find out if they are normal Si or Schottky.
b. Biasing the actual diodes to the linear transfer
range.
c. Replacing the diodes with logarithmic amplifier
like AD8307
Actually, the last method can be the most successfull one it is also the
most complicated and require major modifications that might be not
reproductible by most hams so the first and the second one could be further
explored.
