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DC Blocks

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Mark Hannah:
Hello All,

While researching DC Blocks, several questions came to mind or rather conclusions I'm not completely comfortable making.

- Inner/Outer DC Blocks - Pasternack & Fairview Microwave (which are outside my price point because of wider bandwidths) list Inner, Inner/Outer and Outer blocks. Mini-Circuits' block diagrams appear to show a Inner design if I'm reading the data sheets correctly (yes?). Is there any application in our "industry" that would require the other two options and why?

- Return Loss (following specs from Mini-Circuit's DC Blocks) - I have discovered Return Loss is counter intuitive (at for me) where the higher value is better. One source I found mentions 20 dB of return loss provides 99% power into the antenna. In my situation, I'll need a N to BNC adapter at some point depending on the system I'm using so all things equal an adapter should not make a difference except for a tiny amount of insertion loss. The price of the two versions are within $1 of each other so also out of the equation. And my SA has a N connector. So... Is there any preference or advantage to purchasing a N to N type with a return loss of 40 dB at 505 MHz vs a BNC to BNC type with a return loss of 30 dB at 500 MHz?

Thank you in advance,
Mark

PS. Links to the Mini-Circuit data sheets if interested

https://www.minicircuits.com/pdfs/BLK-6+.pdf
https://www.minicircuits.com/pdfs/BLK-222+.pdf

Henry Cohen:

--- Quote from: Mark Hannah on June 24, 2020, 08:28:59 pm ---While researching DC Blocks, several questions came to mind or rather conclusions I'm not completely comfortable making.

- Inner/Outer DC Blocks - Pasternack & Fairview Microwave (which are outside my price point because of wider bandwidths) list Inner, Inner/Outer and Outer blocks. Mini-Circuits' block diagrams appear to show a Inner design if I'm reading the data sheets correctly (yes?).
--- End quote ---
Correct.



--- Quote ---Is there any application in our "industry" that would require the other two options and why?
--- End quote ---
Not really in our little world of RF, unless one suspected a catastrophic fault of reverse polarity on the bias voltage from some interconnected device. I sometimes use inner/outers when connecting our LMR repeater systems into an existing antenna system.


--- Quote ---- Return Loss (following specs from Mini-Circuit's DC Blocks) - I have discovered Return Loss is counter intuitive (at for me) where the higher value is better.
--- End quote ---
Correct, sort of. Actually, it's the lower value, mathematically, that's better, but as in many industries, in an effort to abbreviate speech and typing, we leave off the negative sign ("-") since we assume most people understand what return loss actually is. Very simply, the least amount of energy returned - reflected back (this is the 'loss') - the better, since that means the energy kept on going down the transmission line or out the antenna.



--- Quote --- One source I found mentions 20 dB of return loss provides 99% power into the antenna.
--- End quote ---
20dB represents 99% of the energy in question, in any path. 10dB is 10%.



--- Quote ---In my situation, I'll need a N to BNC adapter at some point depending on the system I'm using so all things equal an adapter should not make a difference except for a tiny amount of insertion loss. The price of the two versions are within $1 of each other so also out of the equation. And my SA has a N connector. So... Is there any preference or advantage to purchasing a N to N type with a return loss of 40 dB at 505 MHz vs a BNC to BNC type with a return loss of 30 dB at 500 MHz?
--- End quote ---
40dB is 10x better than 30dB, but 30dB is providing 99.9% energy transfer and at that point, the extra 10dB RL performance is meaningless at the minute power levels we work with: Choose the one that best fits your workflow and inter-connectivity.

Mark Hannah:

--- Quote from: Henry Cohen on June 24, 2020, 09:28:17 pm ---Correct, sort of. Actually, it's the lower value, mathematically, that's better, but as in many industries, in an effort to abbreviate speech and typing, we leave off the negative sign ("-") since we assume most people understand what return loss actually is. Very simply, the least amount of energy returned - reflected back (this is the 'loss') - the better, since that means the energy kept on going down the transmission line or out the antenna.

--- End quote ---

Not a single article I found, explaining return loss, mentioned leaving off the negative sign for simplification reasons and instead solely used words to explain the concept. How does removing one character help abbreviate speech and typing? Especially when it comes to teaching someone what return loss is. I imagine the resources I came across are avoiding a deep dive into math.

Regardless, knowing this detail now solidifies the concept in my mind.

Thank you.

Russell Ault:

--- Quote from: Henry Cohen on June 24, 2020, 09:28:17 pm ---20dB represents 99% of the energy in question, in any path. 10dB is 10%.

--- End quote ---

Just double-checking my math: a device with a return loss of 20dB will allow 99% of the original power through because -20dB represents 1% of the original signal, so a device with a 10dB return loss should still pass 90% (because -10dB represents 10% of the original signal), right?


--- Quote from: Mark Hannah on June 25, 2020, 09:01:08 am ---Regardless, knowing this detail now solidifies the concept in my mind.

--- End quote ---

+1 (If I squint really hard and think about it the implied negative starts to make a bit of sense, but knowing this makes it so much clearer.)

Thanks!

-Russ

Henry Cohen:

--- Quote from: Russell Ault on June 26, 2020, 01:58:16 pm ---Just double-checking my math: a device with a return loss of 20dB will allow 99% of the original power through because -20dB represents 1% of the original signal, so a device with a 10dB return loss should still pass 90% (because -10dB represents 10% of the original signal), right?
--- End quote ---

Correct. Here's a table showing the relationship between VSWR, reflected and transmitted power.

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