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Thursday 28 December 2017

Homebrew SDR HF Transceiver - Quadrature Divider

We are now at the point where we need to take the signal from the Si5351, divide it by 4 and produce two outputs, one of which is 90 degrees out of phase from the other.

A good tutorial on this process is here although we will divide by 4 rather than 2. The datasheet for the 74HC74 is here. The 74AC74 and 74HC74 are more or less interchangeable, the difference being that one can handle higher frequencies than the other.

There are many circuits on the Internet for quadrature dividers but I used one that is used in the SoftRock Ensemble receiver, here. The only difference being that the outputs to QSD CLK0 & CLK1 are not used, however, note that pin 9 does need to be connected to pin 2 (it took me a while to figure that was the reason I couldn't get it to work). Pin 14 is fed with 5V from the 7805 regulator and pin 3 is fed from CLK0 output on the Si5351. The outputs from pin 5 and 8 are individually fed to the LO ports on ADE-1 #1 and ADE-1 #2.

Below is my version, it looks a bit messy but it works.

The screenshot from my oscilloscope shows the two out of phase outputs from the quadrature divider in yellow and blue. The purple line shows the that these two outputs have been divided by 4 and are at a frequency of 7.100 MHz.

We are now ready to see what this thing can receive by taking the I & Q outputs from the audio LPF's to a stereo jack plug (I to left channel, Q to right channel or vice versa plus ground) and plugging it into our computer to listen to the signals using SDR# but finally a picture of the completed RF splitter, mixer, AF low pass filter, regulator, quadrature divider board.

I was going to mount the Si5351 breakout board onto the two posts top right but I think the board uses imperial screw size holes as my M3 screws will not quite fit.

Next post: Connecting to SDR#

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