Electronics > RF, Microwave, Ham Radio

RF overload indicator circuit

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bson:
Thanks for all the help and ideas - it works very well up to my ability to test it - which is only 200MHz currently.  I should really get myself some proper RF gear (SA/VNA - been eyeing the 8753D, a power meter, and an RF generator).  I did screw up the PCB by tapping the output after the source load instead of prior to it  :palm:, and have an updated board in the works (also tightening up spacing on the diodes and adding a bit of top side ground pour), but I just bodged it with an x-acto knife and a very short piece of 30awg single strand conductor.  The DC offset is there when it trips but I decided to just use an external DC Block instead for better bandwidth control.  Works like the proverbial charm!  :-+

tggzzz:
Consider using a 65LVDS2-series LVDS receiver, as recommended by Winfield Hill and John Larkin. Advantage: you can vary the threshold voltage, thereby varying the sensitivity. FFI: https://groups.google.com/forum/#!topic/sci.electronics.design/ZgNl_0PNnjg

Alternatively, the 400MHz (i.e. ~500MHz) Tek 485 scopes detect too much power and flip a mechanical relay before the 50ohm input gets too hot. You could easily reuse the circuit shown in the manual.

bson:

--- Quote from: tggzzz on June 06, 2016, 08:08:49 am ---Consider using a 65LVDS2-series LVDS receiver, as recommended by Winfield Hill and John Larkin. Advantage: you can vary the threshold voltage, thereby varying the sensitivity. FFI: https://groups.google.com/forum/#!topic/sci.electronics.design/ZgNl_0PNnjg

--- End quote ---
It looks like the maximum differential voltage on the 65LVDS2 is 1V, which is a bit low.  Also, it's not clear that it tolerates DC input on either input.  Its output drive is 12mA, which I suppose is workable.

I'd still probably need the emitter follower and '14 hysteresis, so it would really only substitute for a diode.  But on the plus is it has only 3pF input capacitance so its load is even less than a diode.  And, of course, no DC offset!

ealex:
Hello

This is the overload module used by a R&S SMS2 signal generator:



I'ts able to detect DC offset applied to the input and RF overvoltage.

tggzzz:

--- Quote from: bson on June 14, 2016, 01:43:51 am ---
--- Quote from: tggzzz on June 06, 2016, 08:08:49 am ---Consider using a 65LVDS2-series LVDS receiver, as recommended by Winfield Hill and John Larkin. Advantage: you can vary the threshold voltage, thereby varying the sensitivity. FFI: https://groups.google.com/forum/#!topic/sci.electronics.design/ZgNl_0PNnjg

--- End quote ---
It looks like the maximum differential voltage on the 65LVDS2 is 1V, which is a bit low. 

--- End quote ---

You could use a divider to make it less sensitive. Sensitivity is usually an advantage!


--- Quote ---Also, it's not clear that it tolerates DC input on either input.

--- End quote ---

LVDS has a DC component, so yes, it can tolerate DC - and requires it for proper operation! You would bias the inputs to halfway between the min/max levels, and AC couple the RF input.

The data sheet gives the max/min input voltages.

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