I am getting ready to order parts (again) and build the tuner. To prepare for it, I have updated the blog (again). But I am still in the middle of building the new EME 5m dish project at the DVRA shack. This might take time.
I have switched from the KiCad 6.0 circuit CAD tools to KiCad 8.0. A definite improvement. The tuner schematic is a total of 3 pages. They are the RF path, the directional coupler and the main page which is mostly the microprocessor circuit. I will start with the RF path page.
The inductor and capacitor networks, from the previous discussions, should be obvious. Relays K3, K6, K7, K9, K11, K13, K15 and K17 control taking the inductors in and out of the circuit by shorting them when necessary. Relays K4, K5, K6, K10, K12, K14, and K16 short the capacitors to the ground by putting them in the circuit. K2 and K18 put the capacitor network in parallel with the load or with the source depending on the location of the load in region 1 or region 2 of the Smith Chart. Relays K1 and K19 (2 Form C) either bypass the LC network or put in in the RF path.
Relay Driver
The relay driver is a Toshiba TBD62783A which is a "source type DMOS transistor array". Normally, one side of the relay coil is connected to the power supply and the other side is grounded using some type of active device. A flyback diode is incorporated to protect the device. This device uses a P-Channel MOSFET to connect one side of the relay to +13.8 volts (I am using the general shack power supply through Anderson Power Poles to power the tuner) while the other side is permanently grounded. It also incorporates the fly back diode inside the device. Its input is compatible with 3 volt logic. As you can see from the schematic, it drastically simplifies the board layout compared to the alternatives.
If you note, the routing of signals to the relay drivers and from the relay drivers to the relays is a bit random. It is specifically chosen to simplify the board layout. For the same reason, there are two bypass signals from the microcontroller and the signal to put the capacitors across the load or the source, even though they are inverse of each other, are separately sourced.
Relays:
I do not plan to open or close the relays with the power applied. So, the two parameters that matter for my design are the open contact breakdown voltage and close contact current carrying capacity.
First, I will review some items about how the LC network:
- The inductors and inductor relays form a series circuit. The "inductor current" is either carried by the inductor when the relay is open or by the relay when the relay is closed.
- If the inductor relay is closed, the voltage across it is zero and if it is open the voltage across it will be the same as inductor voltage
- The capacitors and capacitor relays form a series circuit. The "capacitor voltage" is either the voltage across the capacitor when the relay is closed or across the relay when the relay is open.
- If the capacitor relay is closed, the current through it is the capacitor current and if the relay is open, it is zero.
- When the relay is open, the dielectric strength between the contacts (1 Form A and 1 Form C) should be good enough not to break down at voice peaks at 800 watts peak power.
- When the relay is closed, the rated carry current should be sufficient to support the current at 200 watts average power.
The two blocks in the upper left and right are the abstraction of the two pages that I have already discussed. The block in the middle is the Raspberry Pi Nano.