John wrote:
For the most part, you don't want transformer isolation unless you plan on using balanced lines. There are worse things than ground loops out there, and lifting a coax shield away from ground is a great way to find all of them.
You certainly need the shield grounded at RF, but you don't necessarily need it grounded all the way down to DC. A fairly common solution that works well for many applications is to use isolated connectors (not connected directly to the chassis) with a 10nF capacitor from each connector shield to the chassis immediately adjacent to the connector. The capacitor grounds the shield at RF but allows very little current to flow at the mains frequency. NOTE: You want the bypass capacitor right at the connector, to make sure the RFI current loop is tiny and cannot radiate.
That said, I often build distribution amplifiers with chassis-grounded connectors and have never had ground loop problems. But then, I pretty much always design transformer-isolated DC supplies with low-field transformers and linear regulators into everything I build, and do not use wall-warts, desk-warts, or switching supplies.
If one were building the dist amp I posted the other day and wanted to use chassis-grounded connectors, the per-stage transformers could be simplified from 1:1:1 to 1:1 -- omitting the third winding -- with the transistor collectors capacitor-coupled to the output connectors. In this case, I would put ~100k resistors across each output to hold the connector ends of the capacitors at 0v even with an open load.
For isolation amplifiers used with mixer-type frequency comparators, which can be very sensitive to ground loops because of the low mixer output frequency, I do use RF transformers and bypass the shields to chassis.
You definitely don't want 10.7 MHz IF transformers, unless you are just trying to build a thermometer.
Hear, hear. Best regards. Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
