Need explanation of a circuit function/purpose

[robismyname]

schematic rf

I attached a section of the reference design schematic for a PLL circuit (ADL4252).
I woul dlike for someone to help me understand what the following discretes are doing, please:

1) C16
2)R13
3)R14
4)R12
5)R15
6)C17
7)C15

Thanks..........

 

[E-design]

Re: circuit function/purpose

18 Ohm (≈Zo/3) resistors form a 50 Ohm 3-way splitter. 51 Ohm resistor used for termination (should be 50 ideally). Caps used for RF coupling (value not critical)

 

[robismyname]

Re: circuit function/purpose

Caps used for RF coupling (value not critical)

What would happen if I placed a BP filter right after C15. I attached BP filter below. Will this be a critical issue?

 

[E-design]

Re: circuit function/purpose

If your filter is matched to 50 Ohms you should be ok. Use L & C's for a filter

 

[robismyname]

Re: circuit function/purpose

Caps used for RF coupling (value not critical)

So these caps have a coupling application vs. bypass?

 

[E-design]

Re: circuit function/purpose

The caps are selected to have a low impedance (few ohms) at the required frequency.

 

[robismyname]

Re: circuit function/purpose

The caps are selected to have a low impedance (few ohms) at the required frequency.

In the diagram I sent with the ADF 4252 schematic RF Out on VCO2 has a frequency range of 2260MHz - 2343MHz. Using the capacitance reactance formula I get a capacitive reactance of .70 ohms for 2260MHz and .68 ohms for 2343MHz.

So since Z = R+jXc. and Xc = .68 ohms what would R be?

 

[swingbyte]

Re: circuit function/purpose

The R should be zero - with a perfect capacitor in a perfect world. In practice R will depend on the type of capacitor used - but will be very small if cap is good

 

[hemant03]

Re: circuit function/purpose

The "R" in the equation is the ESR or the Equivalent Series Resistance of the cap. It is generally mentioned in the specsheet of the cap.

Thanks,
- Hemant

 

[robismyname]

Re: circuit function/purpose

Caps used for RF coupling (value not critical)

What would be the ramafications if the coupling capacitors were not present in the circuit? I understand that the coupling capacitor blocks DC.

But

1) why do you want to block the DC? What does this help with?
2) what would happen if DC and high frequencies were allowed to pass?

 

[swingbyte]

Re: circuit function/purpose

Dc blocking capacitors are used to isolate the bias conditions of one stage from the next. I haven't checked the datasheet, however, usually one doesn't know what the internal circuit of the ic is and such caps are indicated in the application notes.
If they were not included, the signal may be severely degraded as one stage of amplification is incorrectly biased and doesn't work or may become unstable under certain conditions.
There are other reasons for capacitors in circuits - the more circuits you study the more reasons you will find.

 

[robismyname]

Re: circuit function/purpose

Use L & C's for a filter

. Please provide brief expanation for why RC filters ill advised here?