Capacitive vs. Inductive Filter Design

[WA2SQQ]

I’ve been experimenting, making some low and high pass filters for an inexpensive SDR receiver, using Elsie software. Can anyone explain the difference, advantages or disadvantages, of using a capacitive vs. inductive input filter design? In case any of you are not familiar with Elsie, it’s an incredible piece of software with many YouTube tutorials available.

http://www.tonnesoftware.com/elsie.html

[DMOD]

I’ve been experimenting, making some low and high pass filters for an inexpensive SDR receiver, using Elsie software. Can anyone explain the difference, advantages or disadvantages, of using a capacitive vs. inductive input filter design? In case any of you are not familiar with Elsie, it’s an incredible piece of software with many YouTube tutorials available.

http://www.tonnesoftware.com/elsie.html

For what purpose, Power Supplies or RF filters?


Phil - AC0OB

[VE3ELQ]

I’ve been experimenting, making some low and high pass filters for an inexpensive SDR receiver, using Elsie software. Can anyone explain the difference, advantages or disadvantages, of using a capacitive vs. inductive input filter design?

http://www.tonnesoftware.com/elsie.html

I fully agree Elsie is a great design tool. I design all of my filters with it, so many features, plot, tune part, optimizer and more.

C or I input depends on the application.  For example a pulse width modulator filter starts with a square wave which demands an inductive input, a capacitor would appear as a dead short for the leading edge, not good.

For transmit low pass filters in theory it could be either but inductive input seems to work better so thats what I use.
For your application ahead of a receiver it probably wont matter.  

To make multi layer inductors try this app: http://electronbunker.ca/eb/InductanceCalcML.html
For single layer this one: http://electronbunker.ca/eb/InductanceCalc.html
Both have proven to be quite accurate. Best to make them a little higher in value then measure them and reduce the turns to get what you want.

73s  Nigel

[WA2SQQ]

My applications are for HP and LP filters, strictly for receiving applications (ahead of lower cost SDR receivers)

[WBear2GCR]

aka RF "front end" filtering?

[WA2SQQ]

YES

[W4AMV]

The choice of first element and the resulting final network is based on realizing all of the final element values. Ideally, all the element values provided from the software will have a self resonant frequency well removed from either the low pass or high pass corner of the final filter to be used. So try either and then look at the final values ELSIE provides. Recognize that ELSIE provides values of the IDEAL elements. In reality you will not see this. It is worthwhile to take the values ELSIE provides and then dial in the SRF effects. You can do this in LT SPICE and get a realistic frequency response.

Alan

[WA2SQQ]

ELSIE has the option of substituting the standardized values. I actually made the filter to notch out the FM broadcast band. Elsie calculated about -52db of attenuation. Measured it was -47db and it proved to be very effective.

[W4AMV]

Yes, standard values are fine. However, the SRF is usually not provided by the manufacturer. Although some do, it usually is provided for small L and C component values. In many cases particularly at lower frequencies below a few hundred MHz, the effect may be neglected as your example proved.

One other comment on the choice of L or C for the first component. If the terminations are REAL, it may not make much difference other than what I mentioned in the prior post. If the terminations have reactance, L or C, then proper choice of the first component in the filter may permit you to eliminate or at least reduce or absorb that element into the filter design. Can simplify your design,

Alan