A
Andrew Holme
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.
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.
