Electronics > RF, Microwave, Ham Radio

RF overload indicator circuit

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bson:
I'm looking for ideas for a simple overload indicator.  Here's my naive stab at it.  The RF signal comes from a TI THS3202D CFB wideband amplifier (MSOP-8 with thermal pad, 28dB gain from two stages insides an SMD PCB shield).  Its behavior as the output goes beyond ~18dBm (@50R) is it shoots straight to ~3Vrms and oscillates at ~500MHz.  Now, I don't really have a problem with this per se, it doesn't really cause any harm, I'd just like an indicator showing this is occurring.  The RF signal is DC up to 300-500MHz depending on what gain and compensation I populate the board for.



The series D1, D2, D3 will have a Vf of ~2.1V, or ~16-17dBm.  So they'll clamp just below 18dBm, causing the THS3202D output to sink into C1, charging it and triggering U1, lighting the indicator LED.  When the condition clears C1 bleeds out through R4.  The hysteresis of the HC14 guarantees the LED has a minimum pulse duration and won't blink at a rate too short to see.

Some potential issues:
* If the amp output sits right at the threshold it can, because of the hysteresis of U1 "drive" the detector circuit without any actual indication, clipping and distorting.  This is a very narrow boundary though since the output of the amp will very quickly charge up C1.  But it's theoretically possible.
* The 4148's have about a 4pF junction capacitance.  This will add a load on the output, though I suspect possibly insignificant.  I'm not entirely sure how the junctions will combine; three 4pF caps would result in a 1.33pF net capacitance, but these aren't capacitors...

If this works, then it would be a nice little compact addition that takes very little board space.  The LED would be a 3mm PTH device actually stuck into a front panel hole and hooked up with a simple 100mil 2 pin header.

Any diode more suitable than 1N4148 - with lower capacitance?  Clearly the continuous current from 2V down through 33k is trivial.  The peak current to charge C1 should be of no major concern.  I'm kind of new to poking around with RF in general and I'm slightly (but not overly) concerned with stability with the series 1N4148 loading the output.  Unfortunately it's kinda hard to test without actually making the board...  If it simply won't work I'd rather find out before I have a handful of boards!

The +5V is already on hand since I operate the THS3202D at +/-5V.
R2 will be determined empirically depending on what LED I use; 1k is just a placeholder.
U1 is a single-gate 74HC14 in SOT-353 (SC-70-3).

Any better, simple approaches I'm missing?

bson:
Here's the current incarnation of the amplifier board.  It's about 22x32mm.  The idea is to mount two of these against a front panel with bulkhead SMA connectors, side by side in a shielded enclosure with a power supply.  They'll be fixed +14dB (or +10dB, +20dB or whatever else they're populated for) and can be used either individually or cascaded.  Each one would have its own individual overload indicator.  Since they'll be mounted directly using bulkhead connectors they won't need the corner holes... I'm inclined to remove those.  (I just habitually add corner holes to pretty much everything.)

 

(Apologies for the crappy phone photos...)

bson:
One idea I had to handle the capacitive load is to split the 50 ohm source resistor into 10+40 and put the overload detector between them.  But I think gain would have to be rather low (and hence bandwidth high) for 4pF to significantly eat into the phase margin.

Another potential problem is that the detector only operates on the positive half of the output, so when it trips it may cause a sharp negative spike, but it should I think ( <= famous last words!) it's bound to be short enough to not really be much of a DC offset.  The amp can source 115mA so will when it trips the diodes it's going to load up C1 real quick.

I will probably have to run the HC14 off 3.3V.  Doh.
I need to rethink the triggering altogether, but that's a small problem; I can always replace the third diode with a transistor.

Actually, maybe just splitting the output and feeding one half of it into a half-wave rectified integrator with a bleed resistor, and using the integrator to trigger is a better approach.

bson:
New plan.  I guess I'm talking to myself here.   :D

Whales:
For detecting negative spikes: have you considered replacing your two diodes with a bridge rectifier arrangement?

Would the capacitance of the diodes matter?  They won't pass your carrier but they will pass your original signal.

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