I'm playing with direct sequence spread spectrum. I'm applying a PRBS and a
70 MHz carrier to an SBL-1 mixer, and then de-spreading it by applying the same PRBS to a second SBL-1 mixer. It's interesting to see the resultant signals on the spectrum analyzer. I've almost completed an early / late synchroniser using the Tau-Dither method.
I needed to down-convert the despread signal to an intermediate frequency of
6 MHz. Other designs use two mixers: one to de-psread at 70 MHz; and a second to down-convert the resultant narrow-band signal. Since these are both switching mixers, I wondered if it might be possible to de-spread and down-convert in one go, using the same SBL-1, by XORing the PRBS with a square-wave 64 MHz local oscillator, and driving the mixer LO port from the XOR gate. This seems to work in my breadboard lash up; but it's early days. What's the catch?
One thing I have noticed: XOR gates have different propogation delays depending on the state of the other input, which leads to a phase shift on the 64 MHz LO. I'm using a 74F86 XOR gate.