AIM4170 Input Protection?

[g3nrw]

Bob,

Can you say a few words about the provisions for input protection on the AIM4170?

Before plugging in the antenna cable, I always bridge the inner and outer of the 259 plug with right thumb and forefinger, and at the same time clasp the 259 socket on the 4170 with left thumb and forefinger. This way I figure that any static charge drains away before plugging in (Note -- this procedure is not recommended during a thunderstorm :-)

The reason I'm asking is that I intend to recommend my radio club buys a 4170, to loan out to club members. Mindful that no particular individual will own it, it's possible that the instrument may be subjected to less than ideal handling, and I need to be sure that the input circuitry is robust enough to survive.

Thanks.

--
73
Ian, G3NRW

[Bob]

Shorting the transmission line is a good idea before connecting it to any electronic equipment. I highly recommend that. If there is any doubt about the amount of charge involved, use a screw driver first to discharge the line.

The AIM is protected in several ways:
1. There is a relay in series with the RF connector and the measurement circuitry. The relay is closed only when the red led is on. If you listen carefully, you can hear the relay click when it opens (red led goes off).

2. There is a gas discharge tube connected directly from the RF connector to ground to handle high voltages that can build up on the transmission line.

3. There is a 100K resistor from the RF connector to ground to keep the line discharged when the relay is open.

4. There are back to back silicon diodes in the measurement circuit to clamp voltages greater than about 0.5V.

The input to the AIM is DC coupled, so be sure there is no DC control voltage present on the line.

Always be careful when connecting the transmission line. A large static charge can build up even in a wind storm with no rain present in the area. Do not leave the AIM connected to an antenna when it is not being used.

Check the DC ground between the antenna and the DC power and PC with a voltmeter. Sometimes these grounds can be different by several volts.

-- Bob

[g3nrw]

Thanks Bob. That was exactly what I wanted to hear.

--
73
Ian, G3NRW

[therian]

How about VNA-2180, what kind of protections both ports have, and what is absolutely max voltage/current before irreversible physical damage?

[Bob]

The VNA2180 has similar protection. There is a relay in series with both ports and a gas discharge tube to ground. The VNA can stand 3.1V rms (200mw) on Port A and B without damage.

The maximum stray input on Port A while measuring impedance (like an antenna) is about 2V pk (80mw). The max input on Port B is 1Vpk (10mw).

73/ Bob