While I was looking for the methods how to control the multiple antenna beam, I came up with a new idea of using the direct digital synthesizer (DDS). DDS is a frequency synthesizer used for creating arbitrary waveforms and the frequency, amplitude and phase can be controlled by digital commands. The famous vendor for DDS chips is Analog Devices and this company’s AD series are widely used since 2000’s. One of the famous RF products using this DDS chip is a low cost vector network analyzer (VNWA). With the help of this low cost chip, one German genius whose call name is DG8SAQ could design and fabricate the VNWA below 1,000 USD.
http://sdr-kits.net/VNWA3_Description.html
After I came up with an idea of using DDS, I started to read some books and search at youtube. (Youtube is really amazing source of engineering information.) Fortunately, I found soon that Analog Devices has already prepared the DDS chip which provide 4 channel outputs and sync function between each other DDS chips. The name of the DDS chip is AD9959 and I easily found one Chinese vendor is selling the low cost 4 ch signal generator which is controlled by AD9959.
http://www.ebay.com/itm/121850671649?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
It took 10 days for delivery from China to Japan. 10 days are very common when the product was sent by China Airmail. The signal generator consists of two PCB layers. The upper PCB layer is the control unit with STM32F103 micro controller. The lower PCB layer is the RF circuit with AD9959 DDS chip. There are 4 SMA connectors as RF output ports on the right side of the lower PCB layer. It works with USB power and there is control jumper pins for transmitting the serial data from PC to the upper control unit.
When you look this board from the top view, you can see five buttons on the lower side. These buttons are using changing cursor location, increasing and decreasing the parameters. It was not so difficult to operate this signal generator with these button because LCD screen shows me what the parameter was changed directly. The frequency range is from DC to 200 MHz. The pictures below show that operating frequency is 10 MHz. The phase difference between channel 0 and channel 1 was set to be 90 degrees. The Amplitude is set to be maximum and its corresponding Vpp seems to be 0.5 V. The resolution of amplitude 10 bits/0.5 V ( 2^10=1024( 0~1023) ). We can see the two channel signals are shown on oscilloscope and its phase difference is 90 degrees. Very Amazing!
I am so satisfied with this signal generator. I could not expect how easy I can control the phase of multiple signal source easily. The price is also so reasonable This product is also equipped with the schematic file. It is an open-source hardware. Finally, I would like to say thank for AD9959.
P.S. Its maximum output Vpp is 0.5 V (=~496.0 mV)
V_eff = 1/2 * 1/sqrt(2) * Vpp = 1/2 * 1/sqrt(2) * 0.5 = 0.177 V
The output port is connected with 50 ohm system, therefore the relating output power will be
P_output = (V_eff)^2 / Z = (0.177)^2 / 50 = 0.000627 W = 0.627 [mW] =~ -2 [dBm]
