Effect of using inverse image on demodulating symmetrical signals
Effect of using inverse image on demodulating symmetrical signals
(OP)
I am working with a supplier to have them build a repeater for a comm system. The repeater has to repeat AM signals (not SSB), a BPSK signal, and a BPSK signal that has been spread by a spreading code.
The supplier asked me if they were to use the image out of a mixer that was the inverse (frequency components reversed) if this would be OK.
Obviously, this would be bad for an FM signal, but I don't think it will cause problems demodulating AM or the BPSK signal...not totally sure though. I'm especially not sure if it will cause demod problems with the spread signal.
Thanks for your help
The supplier asked me if they were to use the image out of a mixer that was the inverse (frequency components reversed) if this would be OK.
Obviously, this would be bad for an FM signal, but I don't think it will cause problems demodulating AM or the BPSK signal...not totally sure though. I'm especially not sure if it will cause demod problems with the spread signal.
Thanks for your help





RE: Effect of using inverse image on demodulating symmetrical signals
You're instinct about the spread spectrum is correct. Many such systems are based on frequency hopping in the RF spectrum and would thus be hopping in an inverted manner if inverted.
AM should survive, unless it were vestigial sideband AM used to send analog video.
BPSK often describes how the data is imposed on a subcarrier. That subcarrier is then imposed on an RF carrier. So you have to be careful to understand the complete modulation scheme.
So to summarize, it's not worth the trivial cost savings. And such an approach would probably make the SS not work without a fix.
RE: Effect of using inverse image on demodulating symmetrical signals
RE: Effect of using inverse image on demodulating symmetrical signals
The BPSK signal is generated digitally onto a 70 MHz carrier +/- 0.5 MHz depending on the desired freq fractions of MHz. Then it is upconverted probably without inversion to the final transmit freq.
Tracy Blackford
Comm and GPS Systems Engineer
RE: Effect of using inverse image on demodulating symmetrical signals
But if each of these signals has its own RF carrier, then the order of those carriers would be reversed. This would perhaps require retuning.