pcbpcb-designRF
I am combining the layout for two PCBs, one with an RF output SMA jack, and the other with an input jack, into a single PCB.
We were originally connecting the separate boards with a ~3" segment of SMA coax cable.
In the combined layout, can I simply join the output and input together with a short where the SMA jacks were previously, or are there other considerations to make at this junction?
Ground loop currents (from other AC equipment) are not going to be significant if your two modules share the same power supply and that power supply is an isolated type like from a wall-wart.
Also, if you are transmitting a balanced signal and the receiving end has balanced termination resistors, the current flowing in the screen is not significant : -
The signal down one coax will produce a current into one of the termination resistors and this current will not flow back through the screen but continue to flow back thru the "centre" wire of the other coax (via its termination resistor - this assumes that the coax wires are largely the same length (i.e. the two signals received are largely antiphase) and that the two signal levels are largely identical.
If in doubt use screened/shielded balanced twister pair like this: -
40 MHz corresponds to a wavelength of 7.5 meters. So long as you limit your driving rise and fall time to avoid exciting high harmonics, you should be able to transmit over 10 inches (~25 cm) without thinking too much about transmission lines and controlled impedance.
Are IDC ribbon cables sufficient at 40MHz?
For this distance, I'd say yes. Provide as many ground lines as you can afford to to avoid cross-talk between lanes and reduce radiation and interference.
Will my FPGA (and especially the ADC) even be able to drive the 10" cable?
You can work out the capacitance of the line and check it against the FPGA datasheet to see.
Or you can just buffer with a 74LVC244 or other logic buffer part and you'll be good. I would provide a location for a series resistor at each output to allow you to increase the rise and fall times if needed.
However in the timing section they specifiy 100Kohm load outputs.
In this case (for the ADC outputs), buffering is probably a good idea.
Best Answer
It would probably be ok. The loss in the connectors and the cable is probably negligible. I would make sure that the impedance of the PCB trace is the same as the connectors and cable to avoid mismatch between in- and output of the circuits.