Jim Lux 2020/11/12 11:50
On 11/12/20 11:30 AM, Stephen Laurence wrote:
> Er......
>
> Perhaps I (as a retired doctor) should shut up.
>
> However there can never be absolute protection in any sphere of life.
> When I was working, I could never give an /absolute /guarantee my
> patient would wake up from their anaesthetic.
>
> Back-to-back protection diodes might be excellent protection, but, the
> degredation of performance of a £50-£100 device may not be justified
> compared to the less effective protection of a 300ohm resistor. Most
> users of this forum probably would not blink if they took the family for
> a “cheap meal out” for the cost of buying one of these vna devices. I
> pour the cost of 1-2 vnas into my car every time I fill up (ok, it is a
> gas guzzler).
The problem is that a 300 ohm resistor won't protect it. It does nothing
to reduce the peak voltage seen by the RF switch when the ESD first hits.
The diodes are cheap - maybe the next rev of the board will include
them. This is an ideal application - the parasitic C from the diode will
be calibrated out, and the nonlinear junction effects are filtered out
because the VNA is a narrow band tuned receiver.
>
> Although I have never owned, or even seen one of the fancy professional
> vnas used in the field for aerial erection and maintenance, it would not
> surprise me if there are not expendable write-offs such as connectors,
> leads etc, every time they are used, costing more than a nanovna.
Generally not - They're designed to be robust to typical things like DC
voltages, ESD, etc. Hook up line voltage, and it's toast, but that's
not super common.
And, in commercial practice, it's standard to ground the coax center pin
to the shield and to something else before connecting it.
One thing you will see is "connector savers" - a plug/jack combo that
you put on the thing, so that if you bang it and damage the threads, you
don't have to replace the connector (which is expensive).
The damage I've seen has been things like:
1) hooking up an instrument to an amplifier that normally has a DC
blocking capacitor on the output. 15 or 24V will cook the input of the
instrument, sometimes.
2) inadvertently putting high power in - wrong switch position, wrong
cable, hit the wrong button - the diode won't save you when it's 100 Watts
3) Something in the breadboard on the bench shorts, putting DC on the
instrument input (oops, that ezhook came loose when I brushed the wire,
and it shorted 15VDC to the RF output.
4) Mechanical damage - dropping it, dropping something on it, setting it
down on the ground and getting a rock stuck in the connector, not seeing
it, then mating it, driving the rock further in.
5) Mechanical damage from mating with the wrong connector (K connectors,
I'm looking at you!) which has the same thread and size, but has a
different center pin configuration)
6) Ungrounded bench, person is live with line voltage (due to leakage
capacitance from line to chassis), but doesn't realize it and touches
circuit, putting a (high impedance) 120V 60Hz into the circuit. Not
enough to feel the shock (around 1-5 mA), but certainly enough to cook
the instrument input.
In 40 years, I think I've broken or seen someone else break things just
about every way possible.
>
> Returning to aspects of my original profession, I wonder how many of us
> do a risk analysis of their aerial testing and setting up, regarding
> their own wellbeing (tripping, falling off ladders etc) and cost of
> hospitalisation, loss of income etc, compared to the cost of their Vna.
>
> Moan over.
>
> PS. I have not *yet *blown one of my four nanovna devices yet, despite
> gutting them to experiment regarding improved performance.
>
> Steve L. G7PSZ
>
