2
\$\begingroup\$

I plan to build two RF modules.

  • One is an analog down converter with a balanced output IF of ~50 MHz
  • The second in a fast ADC @ 100 MSPS with balanced input.

I want to have them separated for modularity and later reuse.

Everything will use 50 Ohm coax and SMA connectivity.

My question here is how to connect them together:

  • I can use two coax. One for the positive and one for the inverted signal? But the signal are balanced... What will be the impact of that solution? And then I would have two path for the GND because the coax shielding is connected at both ends... GND loops?
  • I can first internally convert to an unbalanced signal inside my first module and use a single coax between the modules.

The two modules will by powered independently. If I use a single coax, the GND currents (PSU etc..) will combine with the return current of my signal in the shielding of the coax. This would alter the signal. Right? But with two coax, the GND current will apply in both cable and alter my two signals the same way. Because they are balanced, this would have no impact. I am still right?

Or do I have to strongly connect together my GND of the two modules elsewhere in order to reduce the GND current in the coax? But because I will have to connect the shield of the coax to GND at both end in order to allow the return currents to flow, what about the GND loops?

What is the common approach, for RF lab equipment for example, when a RF source has to be connected to a spectrum analyser? This is the same kind of issue, the two instruments are connected to the earth, but at the same time, they are connected together by the shielding of the coax. Gnd loops? But this works, as fas as I know.

The issues here is that I don't have any clue of the strength of each described effect.

Edit:

The modules will be powered by a single wall wart that will deliver 12V. But each module will have it's own voltages requirements and will use DC/DC converters or LDO.

\$\endgroup\$
2
  • \$\begingroup\$ I think in this case it's fairly relevant how the devices are powered/shielded. Are they connected to the mains or battery-powered? \$\endgroup\$ – clabacchio Feb 20 '14 at 8:05
  • \$\begingroup\$ What sort of ground loop currents are you worried about - earth currents from other equipment at AC frequencies finding their way along the screen/shield of the coax? \$\endgroup\$ – Andy aka Feb 20 '14 at 9:53
2
\$\begingroup\$

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 : -

enter image description here

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: -

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ +1 for Twinax. You can simply terminate one signal to the other - via 2*50R = 100R so the centre point is roughly 0V (+ some DC) \$\endgroup\$ – user_1818839 Feb 20 '14 at 11:05
  • \$\begingroup\$ @Brian I've had problems with that type of termination on that particular cable due to slight timing differences down each core. It produced a common_mode signal and that effectively is unterminated without two individual resistors. Pain in the butt in reality but I'll never use a single resistor on twinax any more except for lengths less than 5m. \$\endgroup\$ – Andy aka Feb 20 '14 at 11:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.