pa0nhc SWRsweeper v2
Place on the PCB top and solder at the PCB bottom :
- Capacitors (low inductance multi-layer types) with the shortest leads possible.
- The LED
The chassis type BNC bus must be
screwed in a side of the (Hammond 1590Y) box. The position is critical.
The BNC central pin should be lined-up and soldered into the top or bottom of the PCB, at the island connected to R1/R2/C2.
The ground lug can be soldered to the ground island near R1/2.
From now on: USE ANTI STATIC MEASURES (wrist wrap + grounding also of the soldering iron) to prevent damage to ICs.
You need only to solder the pins which are connected to other points on the PCB
- Press and solder the pins of the Arduino board and DDS board
in the holes of the main board.
- On the Arduino board : close Jumper J1 with a drop of solder.
- On the DDS board : solder a resistor 6k8 in parallel to R4 (see photo).
I MPORTANT: This will adjust the RF output of the DDS to a correct level.
Solder both ICs onto the bottom surface :
Do not overheat ICs to prevent damages to them. After each
soldered pin, wait a while to let it cool down.
At one corner of the IC is a mark for pin1.
- First position the IC correctly.
- Solder pin 2 (the "common" pin of the AD8361ARTZ)
- CHECK for exact positioning of all pins over the PCB solder points.
- Carefully solder each pin and let cool down.
Important: above 2.5Vdc input, the ADC inputs A0 and A1 on the Arduino board will be overloaded. Max input voltage is 2.4Vdc.
- Connect a 50 Ohms dummy load.
- Let the DDS sweep once between 0.1MHz and 1MHz. The 1MHz signal then stays present.
- Using a 10:1 probe and oscilloscope, on C2/R1/R2 should be measured abt. 620mVpp pure sine wave (NO distortion).
- Remove the probe.
- On Arduino A0 and A1 should be measured abt. 1.73Vdc and 0.86Vdc.
- The graph on the laptop should indicate VSWR 1:1.0
As both ICs can show little differences in electrical properties, corrections in the INO Arduino software can be needed for optimal VSWR plots.
VSWR plot corrections.
- Reset the sweeper by removing power.
- Reconnect power. No RF signal is generated now.
- Measure the "offset" voltages on Arduino pins A0 and A1, and write them down.
- Open in the
Ardiono program the INO software file.
- Go to lines 140 / 141 and change the values of "OFFSETA0" and "OFFSETA1".
Example: measured offset voltage A0 = 0.073 Vdc, "OFFSETA0" must be 0.073 .
- In the Arduino
program, adjust the sweep between 1MHz and 3MHz.
- Check the VSWR graphs of induction-free 1% test resistors of 10 Ohms, 39 Ohms, 50 Ohms, 82 Ohms, 150 Ohms.
- Open in the Ardiono IDE program the INO software file.
- Goto line 149 and change the value of "VSWRCORR" a little.
The value of "1" is the neutral processing value. The optimal value for your sweeper can be a lower or higher value than 1. You can estimate it by calculating the ratio between the shown VSWR and the ideal VSWR value of test resistors.
- Set sweep range
to 0.05MHz to 50 MHz.
- Check the flatness of VSWR graphs of test resistors 10 Ohms, 50 Ohms, and 150 Ohms. They should be very flat at least up to 10MHz.
The shallow bump in the graph of the prototype below is caused by very small differences (a few percents) in conversion gain characteristics between both ICs, and the fact that the RF voltages in the circuit drop gradually above 10MHz.
In my case, for VSWR values up to 1.5 worthwhile VSWR measurements at DUTs can be made up to 50MHz.
Don't worry, be happy.