RC filter problem

[000SHREDDER000]

Hi guys, I am back. I want to filter above 900 Mhz and below 400 mhz UHF band with a band pass RC filter for a very noisy place, but for some reason it doesn't work, my values was two 100ohm resistor, one 1.8pico cap and one 3.8pico cap. I choose resistor value to 100 ohm because coaxial antenna cable resistance can effect my filter values it's about 3 meter. I don't know anyway?!!! Any suggestions please?

 

[danadakk]

Of course stray C and L at these frequencies has to be carefully managed. A sneeze (so's to speak)
can introduce pF and nH worth of strays due to board layout, variation with Temp, component
tolerances.......

How are you confirming performance, VNA, Scope and swept oscillator,......?

Here is your RC filter, not knowing Zin or Zload ( so I used Zin = 0, Zout = infinity) -

Here is LC filter (more typically the way you would handle an RF filter, assuming 50 ohm in/out Z) -

RF Tools | LC Filter Design Tool

Regards, Dana.

--- Updated Aug 18, 2020 ---

Here is a second order LC design -

Regards, Dana.

--- Updated Aug 18, 2020 ---

Here is your filter with 50 ohm in/load Z's -

Regards, Dana.

 

[000SHREDDER000]

Of course stray C and L at these frequencies has to be carefully managed. A sneeze (so's to speak)
can introduce pF and nH worth of strays due to board layout, variation with Temp, component
tolerances.......

How are you confirming performance, VNA, Scope and swept oscillator,......?

Here is your RC filter, not knowing Zin or Zload ( so I used Zin = 0, Zout = infinity) -

View attachment 163536


Here is LC filter (more typically the way you would handle an RF filter, assuming 50 ohm in/out Z) -

View attachment 163537

RF Tools | LC Filter Design Tool

Regards, Dana.

--- Updated Aug 18, 2020 ---

Here is a second order LC design -

View attachment 163538


Regards, Dana.

--- Updated Aug 18, 2020 ---

Here is your filter with 50 ohm in/load Z's -

View attachment 163540


Regards, Dana.

I am not at my home here I have no tools for analysis. Components values are standard instead of exact. Also as I said my input connected to a 3 meter coaxial cable with an anttenna and output connected to a digital receiver device so I don't know impedance or capacitance of out or input!!!

 

[danadakk]

So how can you tell the filter is not working, or working for that matter ?

As far as tools these days a hot item is Nano VNA, its cost < ~ $60.

Or use a RTL SDR dongle, ~ $20, and that at least gets you a basic spectrum analyzer.
You could build a cheap broadband noise generator that would at least get you basic
info.



Regards, Dana.

 

[000SHREDDER000]

So how can you tell the filter is not working, or working for that matter ?

As far as tools these days a hot item is Nano VNA, its cost < ~ $60.

Or use a RTL SDR dongle, ~ $20, and that at least gets you a basic spectrum analyzer.
You could build a cheap broadband noise generator that would at least get you basic
info.



Regards, Dana.

My problem is not money. Anyway thank you for good information.

 

[BigBoss]

I am not at my home here I have no tools for analysis. Components values are standard instead of exact. Also as I said my input connected to a 3 meter coaxial cable with an anttenna and output connected to a digital receiver device so I don't know impedance or capacitance of out or input!!!

Component values have been given by mathematical computations, it's not an issue. You should play around those values with practical standard value. Of course you will need a circuit analysis program but there are many free ones on the internet.
Cable length is not a problem since the Characteristic Impedance does not change during length. If the Antenna has 50 Ohms, the Impedance will be 50 Ohms at the end of the cable.

 

[000SHREDDER000]

Component values have been given by mathematical computations, it's not an issue. You should play around those values with practical standard value. Of course you will need a circuit analysis program but there are many free ones on the internet.
Cable length is not a problem since the Characteristic Impedance does not change during length. If the Antenna has 50 Ohms, the Impedance will be 50 Ohms at the end of the cable.

How I can measure output impedance from a digital receiver device then?!!! My filter output for a digital receiver.

 

[danadakk]

You can use classic substitution and a small amount of math, although at RF
i think a Nano VNA the better approach.

“Z-Substitution” Method for Measuring UNUN Loss Into Real-World Load Impedances – KD6RF Blog

vtenn.com

Substitution approach is

You know the load, its Zsource you are trying to establish.

You can write loop equations for Zsub = 0, measure Vload.
You then plug a "nominal" value in for Zsub and you have a
new set of equations and measurement.

Solve them for Zsource and do the simple algebra to determine
Zsource. Note this does not yield the reactive breakdown of Zsource,
just is magnitude.

Regards, Dana.

 

[BigBoss]

How I can measure output impedance from a digital receiver device then?!!! My filter output for a digital receiver.

You will measure the Input Impedance of the Receiver.But they have generally 50 Ohms Input Impedance.

 

[BradtheRad]

Here's a plain series LC bandpass filter, although it's not necessarily better than the methods presented above.

The inductor can be a few loops of wire around a lollipop stick. It's easy to change Henry value by pushing loops closer together or stretching them apart.

The capacitor can be a 'gimmick' kind made by twisting together two wire ends about an inch or two worth.

L:C ratio makes a difference to the shape of the rolloff curve. Hence you need to customize L & C values to the load.

Three layouts with different loads and different L:C values. Center frequency 600 MHz.

 

[000SHREDDER000]

Here's a plain series LC bandpass filter, although it's not necessarily better than the methods presented above.

The inductor can be a few loops of wire around a lollipop stick. It's easy to change Henry value by pushing loops closer together or stretching them apart.

The capacitor can be a 'gimmick' kind made by twisting together two wire ends about an inch or two worth.

L:C ratio makes a difference to the shape of the rolloff curve. Hence you need to customize L & C values to the load.

Three layouts with different loads and different L:C values. Center frequency 600 MHz.

View attachment 163553

If I add one resistor from output to ground for increasing load resistance can it incrace signal strength?

 

[danadakk]

Sweeping load from 1 ohm -

 

[BradtheRad]

If I add one resistor from output to ground for increasing load resistance can it incrace signal strength?

Post #12 is a helpful graph answering your question.

 

[000SHREDDER000]

Sweeping load from 1 ohm -


View attachment 163562

Maybe it can affect filter's performance, so I decided to not incrace output impedance.