How to protect this UHF amplifier input

[ConKbot]

Is there enough room to fit something like a https://www.digikey.com/product-detail/en/littelfuse-inc/SESD0402X1BN-0010-098/SESD0402X1BN-0010-098CT-ND/5233522 SESD0402X1BN in the radio? The loading is quite low, and the 10v clamping voltage should be enough to save the gate. The 0402 size package with pads on the bottom only is a bit of a pain to solder, but if you can get it to straddle from a  TX line to ground somewhere in the circuit, it could be fit in.

[peter-h]

There probably would be but it is too late since both radios are reassembled.

I will build a BNC inline adaptor with that device. It looks amazing. 0.1pF... I need to find out how to do an inline adaptor which maintains 50 ohms all the way.

[peter-h]

OK... here goes a dumb question

If I just solder two 50 ohm BNCs back to back, there will be something like 25mm of PTFE-covered centre conductor which will have no shield. There will also be about 5mm of centre conductor which will be completely bare - where I will solder the voltage limiter.

How bad will this be at ~300MHz?

I could create a shield for the PTFE section, either with a machined brass tube or by painting it with silver paint, if necessary.

I will probably use a little metal box for the whole thing, just to make it neat.

[T3sl4co1l]

BNC...what?  Cables cut open?  Connectors?

Impedance is about distance between the conductor and the shield.  Maintain that, and you're set.

Discontinuities on the order of 1/20 wavelength long (i.e., ~5cm) are not much impact on the signal.

You still want shielding, otherwise you might be letting interference right back in.

Tim

[peter-h]

There is already a BNC-BNC joint in the cable, so I would just be inserting one connector's worth of loss.

[richnormand]

OK... here goes a dumb question

If I just solder two 50 ohm BNCs back to back, there will be something like 25mm of PTFE-covered centre conductor which will have no shield. There will also be about 5mm of centre conductor which will be completely bare - where I will solder the voltage limiter.

How bad will this be at ~300MHz?

I could create a shield for the PTFE section, either with a machined brass tube or by painting it with silver paint, if necessary.

I will probably use a little metal box for the whole thing, just to make it neat.

Typically I use a small blue Pomona box with the two BNCs of choice already installed at each end. Inside the box you can use a short shielded cable to reach the component. Typically I'll use a small length of rigid coax.  For smaller stuff a 1" to 2" length of 3/8" copper pipe to which I insert the threaded connector part into and then solder (some light machining required). Done properly both are waterproof too. If critical then check for VSWR or by TDR to make sure all is OK.

[peter-h]

Excellent - thanks. I started looking for little BNC boxes on Ebay but you reminded me of the "ITT Pomona" boxes which used to be everywhere in the 1970s... Digikey sell a 2931. I have some semi-rigid RG coax too.

[peter-h]

I have finally built it. Any comments would be appreciated. I wonder if the way the semi rigid coax shield is grounded is ok. Unfortunately there is no easy way to connect to the BNC connectors directly. I could have cut the BNC pins down in length to reduce the unshielded portions. But this is for about 300-350MHz.

[richnormand]

That looks like a nice job.

If I remember correctly you mentioned a 330MHz working frequency so, once with the cover installed (and grounded to the case, assuming the cover is metal. Shave some of the paint at each corner.), the losses should be tolerable.
If easy to do,  you might double up on entry and exit grounds but I doubt if it will make an appreciable difference.
You might want to do a few receiver tests with the box in and out to see if you can measure any loss in sensitivity.

[Marco]

Actually, the datasheet says it's got zener diodes internally.  Curiously, they don't give an ESD rating, so it's not clear if they're good for anything at all...

Irrelevant now, but Infineon BF998 is  AEC Q101 qualified, which includes ESD rating AFAIR.

[peter-h]

I don't think this is ESD. The inductor at the radio input should kill that - as posted above. I think it is RF, at just the "right" frequency.

[T3sl4co1l]

Well, ESD is a wideband pulse, a direct strike will still excite a tuned circuit. An RF pulse is an RF pulse. 

Tim

[peter-h]

A late update: the insertion loss of this little device has been tested and it is unmeasurable, at some fraction of a db.

Thank you all for your input

[richnormand]

and the receiver front end is still OK or only time will tell?

Thanks for the feedback

Cheers.

[peter-h]

Only time will tell... however I would be amazed if it gets blown up again.

I also tested the box with a 0-30V power supply, via a ~10k resistor to limit the current, to make sure it actually worked as a clamp. That 0402 chip was so small that checking the soldering needed a x40 microscope.

[peter-h]

A further update: It worked perfectly in flight on a real ILS - as expected after being tested with an IFR4000 tester and found to have an insertion loss below what is measurable.