Held in design of 15MHz BPF, LPF and HPF

[neazoi]

Hello, I design a cheap bare bones spectrum analyzer for HF **broken link removed** and I require 3 filters to be made.
One LPF with a quite sharp cut off near 15MHz (RF front end).
One HPF with a quite sharp cut off near 15MHz(RF front end).
A quite sharp BPF for 15MHz (post mixer).

The quality of these filters will affect the SA performance. When I say "quite sharp", this is not a specific specification, but the sharpest the better. Obviously a compromize between number of poles and performance has to be done. The only requirement is that I would like to use molded choke inductors (for easiness) and non-variable capacitors for the filters.

The impedance of the LPH and HPF inputs is 50R (input of the SA). I have no idea about the output impedance required.

Can anyone make some simulations for me about these filters?
It would be very helpful as I do not know how to design them.

 

[chuckey]

It is conventional to quote the source and load impedance and the in band loss.
Frank

 

[BradtheRad]

An LC tank makes a sharp bandpass filter.

The ratio of L:C affects the bandpass Q.



Notice the different LC ratios. Center frequency 14.5 MHz. Input resistor is 50 ohms.

 

[volker@muehlhaus]

As always, I recommend Nuhertz Filter Free for this task. This is the no-cost version of the Nuhertz filter synthesis software.

**broken link removed**

You can choose between different designs and try different filter orders, so it really makes sense that you use this yourself and decide what design & specs work best for your needs.

 

[neazoi]

As always, I recommend Nuhertz Filter Free for this task. This is the no-cost version of the Nuhertz filter synthesis software.

**broken link removed**

You can choose between different designs and try different filter orders, so it really makes sense that you use this yourself and decide what design & specs work best for your needs.

Thank you for the suggested software, I am trying it now. How can I see the response curve?

I used to use rfsim99 which is straight forward as well.

I am not a filter expert so I do not know if my results are correct. Anyway, the biggest problem is that I do not know the load impedance, as this is not 50R.
Is there any guidelines to design is and being able to accept wide load or source impedances?
Simple 50R shung resistors int he input and output will do I think, but they will attenuate the RX signal much.

 

[volker@muehlhaus]

Thank you for the suggested software, I am trying it now. How can I see the response curve?

With the user interface set to "Advanced" mode, the button is on the top right side.
Frequency settings at the bottom.

Anyway, the biggest problem is that I do not know the load impedance, as this is not 50R. Is there any guidelines to design is and being able to accept wide load or source impedances?

Undefined impedance gives undefined response for high/low pass. Some series or parallel resistors to create an approximate source/load range would be possible. Without that, I don't see how you can design for a specific frequency. All this filter stuff is based on L and C impedance relative to some reference impedance (source/load).