Re: Idle Question About the 485

Tom Gardner

No disrespect taken :)

Looking at the circuit diagrams, the 485 has two separate attenuators, one 1Mohm and one 50ohm. A relay connects the BNC input to the relevant attenuator. Overload causes the relay to switch to the 1Mohm attenuator.

OTOH, the 24x5 has a single 1Mohm attenuator, and a relay simply puts a 50ohm resistor in parallel with that.

Using a NanoVNA to (imperfectly) measure the VSWR of my 485 shows it is between 1.00 and 1.03, rising to 1.04 at 350MHz.

OTOH, my 2465 varies between 1.01 and 1.15, and is 1.12 at 350MHz.

On 21/10/21 15:06, Jeff Dutky wrote:

no disrespect intended, but I'm looking directly at my 2465, and it has five input coupling positions for channels 1 and 2, labeled in order: 1MΩ AC, 1MΩ Gnd, 1MΩ DC, 1MΩ Gnd, and 50Ω DC (with a red border around the 50Ω DC label). It also has, according to the manual, overload protection in 50Ω DC mode, and will revert to 1MΩ Gnd position in such a case. The precise mechanism of the overload protection seems to be more direct in the case of the 2465, which appears to have a thermal sensor that triggers the switch back to 1MΩ input, but the effect is the same.

-- Jeff Dutky

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