Need help in understanding how this RF detector works

[bard1988]

Hello,

I'm having a little trouble with understanding how this detector works, i will be really grateful if someone could explain me how it works.

https://www.testequipmentdepot.com/usedequipment/pdf/8471d.pdf


Thank you in advance,
Dubovski Bar

 

[biff44]

it works by using a single schottky diode to rectify the RF signal and turn it into DC volts.
from the rf connector, there is a tiny (low parasitic) 50 ohm chip resistor placed in shunt. i.e. between the RF connector center pin and the case ground. Then right next to it there is a series schottky diode, followed by a capacitor (probably in the ~470 pF range) in shunt. then the capacitor voltage is output to the bnc connector.

 

[jiripolivka]

The schematic fails to show how the RF power is coupled to the sensor.
In fact this is a simple diode detector with 50-Ohms input impedance, and a capacitive coupling to one/two detector diodes.

 

[bard1988]

Still can't quite understand, maybe you can upload a picture that describes it better? You mentioned that schematic fails to show how the RF power is coupled to the sensor , can you please describe it?

I'm having trouble with the fact that the 2 RFin cables are connected to the same place (one through 50 ohm)...

 

[FvM]

I'm having trouble with the fact that the 2 RFin cables are connected to the same place (one through 50 ohm)...

The schematic is an equivalent circuit, not showing the real connection of the rectifier diode inside the detector module. The real circuit is like described in post #2 and #3.

 

[bard1988]

The schematic is an equivalent circuit, not showing the real connection of the rectifier diode inside the detector module. The real circuit is like described in post #2 and #3.

I hope that im not rude, but is there any chance someone could post a scheme which showing the real connection, sorry for all the trouble :/

 

[volker@muehlhaus]

but is there any chance someone could post a scheme which showing the real connection

It is not in the datasheet, so how can we know?

 

[FvM]

I think, bard1988 is asking for a typical zero bias diode detector circuit with 50 ohm input termination. Unfortunately I don't have it at hand, bif44 and jiripolivka already described the principle setup. You'll also find examples in detector diode datasheets.

In contrast to the posted equivalent circuit, most Avagotech datasheets and detector matching application notes show a DC short at the RF side. They don't have an input load resistor.

 

[vfone]

It's true, most of the detectors shows on the application notes a choke as a DC short at the input, but 8471D use a 50 ohms resistor at the broadband input, which also is needed for DC return (otherwise the detector cannot generate a DC voltage).