I want to build a low pass filter for 1296 and have scaled a design out
of the handbook. I would like to use chip inductors and chip capacitors
for the filter. I was looking at thin film chip inductors (Digi-Key)
and they list the self resonant frequency. For an inductor passing
1296, what should the minimum self resonant frequency be? Is 10 GHz
enough? The filter is a 5 element Chebyshev Low Pass Filter, inductive
input (3 coils in series with a cap at each inner junction to ground),
Fco=1290 MHz, F-20dB=2600 MHz. Any other component specs I need to be
aware of?

Scott
N0EDV

Scott wrote:



I want to build a low pass filter for 1296 and have scaled a design out
of the handbook. I would like to use chip inductors and chip capacitors
for the filter. I was looking at thin film chip inductors (Digi-Key)
and they list the self resonant frequency. For an inductor passing
1296, what should the minimum self resonant frequency be? Is 10 GHz
enough? The filter is a 5 element Chebyshev Low Pass Filter, inductive
input (3 coils in series with a cap at each inner junction to ground),
Fco=1290 MHz, F-20dB=2600 MHz. Any other component specs I need to be
aware of?

Scott
N0EDV

10Ghz would be *almost* an order of magnitude away from the frequency of
interest. That should be more than sufficient. The higher the self-resonant
frequency, the less parallel capacitance is involved which makes "trimming"
the filter less involved. At this frequency your capacitors are going to be
pretty small. Parasitic capacitances, such as in the inductor, can make
things look pretty screwy in a big hurry.

tim ab0wr

Scott wrote:
I want to build a low pass filter for 1296 and have scaled a design out
of the handbook. I would like to use chip inductors and chip capacitors
for the filter. I was looking at thin film chip inductors (Digi-Key)
and they list the self resonant frequency. For an inductor passing
1296, what should the minimum self resonant frequency be? Is 10 GHz
enough? The filter is a 5 element Chebyshev Low Pass Filter, inductive
input (3 coils in series with a cap at each inner junction to ground),
Fco=1290 MHz, F-20dB=2600 MHz. Any other component specs I need to be
aware of?

Scott
N0EDV

Inductance of the capacitors, capacitance of the inductors, and loss of
both will all affect the quality of the filter. Parasitic inductances
and capacitances in the connecting lines and mounting pads will play a
role also. Your problem probably won't be so much getting decent
response to 2600 MHz (although it won't be easy), but maintaining good
attenuation for some range above that. Take a careful look at the
highest frequency you have to reject and the required attenuation at
that frequency, then decide whether your components will behave as
expected up there.

Roy Lewallen, W7EL

Thanks for the replies...I will probably give it a go and see what
happens. I will order values of inductors and capacitors on both sides
of my calculated values and play with the circuit until (if) I can get
it to work OK. Maybe an appropriate helical filter might be the way to
go...

Scott
N0EDV

Scott wrote:
I want to build a low pass filter for 1296 and have scaled a design out
of the handbook. I would like to use chip inductors and chip capacitors
for the filter. I was looking at thin film chip inductors (Digi-Key)
and they list the self resonant frequency. For an inductor passing
1296, what should the minimum self resonant frequency be? Is 10 GHz
enough? The filter is a 5 element Chebyshev Low Pass Filter, inductive
input (3 coils in series with a cap at each inner junction to ground),
Fco=1290 MHz, F-20dB=2600 MHz. Any other component specs I need to be
aware of?

Scott
N0EDV

Scott wrote:

I want to build a low pass filter for 1296 and have scaled a design out
of the handbook. I would like to use chip inductors and chip capacitors
for the filter. I was looking at thin film chip inductors (Digi-Key)
and they list the self resonant frequency. For an inductor passing
1296, what should the minimum self resonant frequency be? Is 10 GHz
enough? The filter is a 5 element Chebyshev Low Pass Filter, inductive
input (3 coils in series with a cap at each inner junction to ground),
Fco=1290 MHz, F-20dB=2600 MHz. Any other component specs I need to be
aware of?

Scott
N0EDV

Why not a microstrip filter? Is it that you must have response from DC
all the way to 1296? Would a "no-tune" bandpass filter be more appropriate?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Scott wrote:
I want to build a low pass filter for 1296 and have scaled a design out
of the handbook. I would like to use chip inductors and chip capacitors
for the filter. I was looking at thin film chip inductors (Digi-Key)
and they list the self resonant frequency. For an inductor passing
1296, what should the minimum self resonant frequency be? Is 10 GHz
enough? The filter is a 5 element Chebyshev Low Pass Filter, inductive
input (3 coils in series with a cap at each inner junction to ground),
Fco=1290 MHz, F-20dB=2600 MHz. Any other component specs I need to be
aware of?

Scott
N0EDV

Adding to what others have already posted, you can simulate the filter,
with parasitic resistances and reactances and the inevitable
transmission line effects, in the freeware RFSim99. You have to be
careful to think about the physical layout, and see everything longer
than a couple millimeters as a transmission line. The data sheets on
the discrete parts should give you a decent idea about the parasitic
R/L/C of each package. RFSim99 includes the ability to model
self-resonance and Q in inductors and capacitors without adding more
parts to the schematic; just check the "use physical model" box for
those parts you want to specify for Q and SRF.

And I'd echo what Tim wrote, too: you might want to consider a
distributed filter, especially if you can make a PC board with
reasonable accuracy. Also, if the goal is to kill harmonics of a
single narrow band, you may do better with notches at the harmonic
frequencies.

Cheers,
Tom

Well, it might, but I'm not 100% sure how to determine the lengths and
widths of the microstrips. (On my present project, I'm trying an open
1/4 wavelength 50 ohm stripline that I can trim down in length in an
attempt to make that line a "short" at the 2nd harmonic.)

The filter only needs to pass from about 1296 to 1300 MHz. I like the
idea of a helical filter (except for the cost!) with steep skirts and
since my I.F. will be 432 MHz, it will be far enough away so the image
will be quite a ways removed and should be way down in level after the
filter.

Scott
N0EDV

Tim Wescott wrote:




Why not a microstrip filter? Is it that you must have response from DC
all the way to 1296? Would a "no-tune" bandpass filter be more
appropriate?

That's exactly what I'm trying on my present project at 902. I have a
50 microstrip line on the circuit board where the RF connector hooks up.
I will etch it to be a 1/4 wavelength at 1802 MHz (2nd harmonic) and
see how that works. If it works OK, I may try the same at 1296...I
might also go with a helical filter...they work nice but can be a bit
pricey!

Scott
N0EDV

K7ITM wrote:


Also, if the goal is to kill harmonics of a
single narrow band, you may do better with notches at the harmonic
frequencies.

Cheers,
Tom

Scott wrote:
That's exactly what I'm trying on my present project at 902. I have a
50 microstrip line on the circuit board where the RF connector hooks up.
I will etch it to be a 1/4 wavelength at 1802 MHz (2nd harmonic) and
see how that works. If it works OK, I may try the same at 1296...I
might also go with a helical filter...they work nice but can be a bit
pricey!

Isn't it possible to make a helical resonator with a helix of wire
inside a box made from soldered-together pieces of PC board?(*) A filter
could be made from a series of these. The Q might not be optimum, but
should be good enough for a lot of purposes.

(*) I actually made a two-resonator filter for 450 MHz using this method
long ago, but it didn't work well. I came across it recently and checked
it with a network analyzer -- I had no decent test equipment back when I
made it -- and found that the sections were severely undercoupled. But I
haven't tried modifying it to see if it could have been made to work as
intended.

Roy Lewallen, W7EL

Yes, that's all a helical filter appears to be...a coil inside a metal
box. I have no idea how many turns, turn spacing and anything else that
might be important to the design of one. Fortunately, Downeast
Microwave sells a bunch of different helicals for a LOT less than
Digikey, but I need the 1305 MHz set and I don't see them at DEMI, so
I'll have to fork out $26 each from Digikey (and probably will end up
using two) ;(

Scott
N0EDV




Roy Lewallen wrote:
Scott wrote:

That's exactly what I'm trying on my present project at 902. I have a
50 microstrip line on the circuit board where the RF connector hooks
up. I will etch it to be a 1/4 wavelength at 1802 MHz (2nd harmonic)
and see how that works. If it works OK, I may try the same at
1296...I might also go with a helical filter...they work nice but can
be a bit pricey!


Isn't it possible to make a helical resonator with a helix of wire
inside a box made from soldered-together pieces of PC board?(*) A filter
could be made from a series of these. The Q might not be optimum, but
should be good enough for a lot of purposes.

(*) I actually made a two-resonator filter for 450 MHz using this method
long ago, but it didn't work well. I came across it recently and checked
it with a network analyzer -- I had no decent test equipment back when I
made it -- and found that the sections were severely undercoupled. But I
haven't tried modifying it to see if it could have been made to work as
intended.

Roy Lewallen, W7EL