Effect of using inverse image on demodulating symmetrical signals

[blkfrd]

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

 

[VEBill]

All this to save another local oscillator? Not worth it.

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.

 

[VEBill]

I spelled "Your" incorrectly.

 

[blkfrd]

Thanks. It is a cost driver and because of the frequency scheme, it becomes a bit more complicated than just another LO. The SS signal is not a frequency hopping type. Its a direct sequence where a gold code is used to spread the signal.

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

 

[VEBill]

Then it sounds like all your signals are perfectly symmetrical about their respective carrier frequencies. If true, then nobody would even know that you've inverted them.

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.