EBAY Cheap SDR Receiver Input Protection

[@JohnTarbox]

Seems like you would have been better off building your own from scratch.

[@acestudioscouk-Ace-G0ACE]

Glad to see this and thanks for your work on it. Thanks too to those who have provided comments below. It's all helping me learn... and hopefully protect my SDR!

[@ohyeawhosurpoppy]

Great to here and see your channel active

[@WECB640]

Steve, add two more (back to back) diodes between C1 and L1 and connect them to GND. If the first half of the circuit is unable to sink the entire signal, the second half (nearest the receiver) will shunt whatever is left over. The design engineer may have also "assumed" that some users would connect this device in reverse (not paying attention to the labels). The circuit works in either direction as shown in the schematic.

[@n4lq]

I'll agree with your second statement but not the first. L2 can't shunt anything unless the RF appears on the RX side. By then it's too late.

[@tpcdude]

.. relay has flight time .. enough to smoke the SDR

[@JxH]

The neon bulbs are supposed to be in *parallel* with the signal path, so that any voltage higher than about 70 volts will trigger the discharge and help to protect things. This technique goes back endless decades. The two back-to-back diodes are presumably the same sort of thing, except triggering at much lower voltages (~0.7v), which why they're on the receiver side of the circuit (further from the antenna). So, given this *parallel* concept, I have no idea why you'd want grain of wheat incandescent bulbs across the circuit. From here, I can't see why your example didn't work.

[@JxH]

P.S. Keep in mind that incandescent bulbs have a cold resistance about 10x *lower* than when they're hot (illuminated). So none of this is making much sense.

[@n4lq]

The SDR would be long gone before the voltage reached 70v. Neon bulbs wire fine for tube rigs. Diodes like this can cause IMD with strong signals so it's a trade off.

[@JxH]

In some designs, an incandescent bulb (e.g. grain of wheat) would be used in *series* to limit the current through the back-to-back diodes, in the event of a nearby powerful transmitter. In the design shown here, it seems that they're using a series capacitor to perform that function (assuming it was even designed correctly). As mentioned already, if you see a neon bulb, then it's in *parallel*, across the signal path (as can be apparently be seen in the video here: PCB layout). Re: "70v". The neon bulbs might also help protect the downstream back-to-back diodes (again, assuming the circuit is actually designed correctly). It makes no sense to unsolder a neon bulb or two and replace them (in the same PCB holes) with incandescent bulbs. Parallel vs series design concepts; so different connections are required. If all of these are triggered, then your SDR will be overloaded, so nitpicking about IMD from the diodes might be inappropriate. It's likely that the SDR doesn't have the same sort of high performance dynamic range of a top-ranked HF rig. It may also have its own IMD-susceptible protection circuits anyway. If these ("70v" and "IMD") are actual issues, then you might wish to consider why you're playing with such protection devices to begin with. These considerations, to a large extend, come with the territory. Perhaps someday soon, they'll have all-in-one SDR radios and Active whips that can be easily installed at more-remote locations on your own property. Perhaps solar powered and Wi-Fi-linked, and installed far enough away that overload from your kW transmitter is no longer an issue. All-in-one so that the inexpensive SDR is built-into the base of the whip antenna ('Active' High-Z broadband HF whip, an existing technology). If such all-in-one devices were cheap enough, and with the right SW, then receiver arrays become cheap and trivial. Hammer them in (big circle), align their solar panels towards the Sun, and go inside to start playing.