best op-amp for 10.7MHz BP filter

[[email protected]]

Hi Group,

I would like to design a 10.7 MHz (common IF RX) BP filter using a quad
or dual op amp package.

voltage will be gnd and 5 or 12 volts.

gain will be x5 or x10.

the signal, out of the IF will be small.

BW will be +/- 500KHz.

What is the best op-amp for this application? And, possibly, a freeware
software package that would help create the design.

Thanks,

PDRUNEN

 

[Michael Black]

Hi Group,

I would like to design a 10.7 MHz (common IF RX) BP filter using a quad
or dual op amp package.

voltage will be gnd and 5 or 12 volts.

gain will be x5 or x10.

the signal, out of the IF will be small.

BW will be +/- 500KHz.

What is the best op-amp for this application? And, possibly, a freeware
software package that would help create the design.

Thanks,

PDRUNEN

You need to switch your thinking. Just because you see active
filters at audio does not mean you should use them at RF. In
all but a handful of cases, you'd be using LC circuits at
10MHz, and given that you have a standard IF, the norm would
be a ceramic filter. The latter may not be the solution, given
your bandwidth, but since we don't know your actual application
one can only guess.

Note that since 10.7MHz is a standard IF frequency, you can get
IF transformers for the frequency, so the solution may be to
build up something from them.

As for active elements, they'd be there to compensate for
the loss in the LC circuits, and to provide the gain you need
overall, as opposed to synthesizing a filter. Common would
be simple transistor gain stages, or an IC intended for IF
amplifier use.

Michael

 

[Meindert Sprang]

You need to switch your thinking. Just because you see active
filters at audio does not mean you should use them at RF. In
all but a handful of cases, you'd be using LC circuits at
10MHz, and given that you have a standard IF, the norm would
be a ceramic filter.

Why ? I agree a ceramic filter would be good if you need a few to a few 10's
of kHz bandwidth. But why should you use LC circuits? Because everybody used
to do it that way or is an LC filter still better than a modern 1GHz GBW
opamp with a few cap's and R's? Just curious..

Meindert

 

[Frank Raffaeli]

Why ? I agree a ceramic filter would be good if you need a few to a few 10's
of kHz bandwidth. But why should you use LC circuits? Because everybody used
to do it that way or is an LC filter still better than a modern 1GHz GBW
opamp with a few cap's and R's? Just curious..

Meindert

Assuming that:
1) you are demodulating a signal between 100 kHz and 800 kHz bandwidth.
2) fidelity of the demodulated baseband signal is important.

Then the group delay (phase linearity) of the filter is of primary
importance.

A ceramic filter can give nearly flat group delay and good bandpass
charactaristics. The cost varies from a few pennies to 0.40 cents in
production quantities. Here is a good catalog with application info:

http://www.murata.com/catalog/p11e.pdf

A loosly coupled IF transformer can give nearly flat group delay and
good bandpass charactaristics. Depending upon the coupling between the
primary and secondary, and the loaded Q, you can alter the bandwidth
and group delay charactaristics. Fancier passive L.C. I.F. transformers
/ filters have multiple mutual couplings. Try searching on "helical
filter", for instance.

It is challenging to do this as well with an active analog filter, even
at this low of a frequency.

Yes, a passive filter or IF transformer will give the same or better
performance at 1/10 the price.

If you want near perfect group delay and passband charactaristics, use
a wideband flat group delay ceramic filter, digitizer and DSP, assuming
cost is no object.

I have seen active filters used for digital video IF's

Frank Raffaeli
http://www.aomwireless.com/

 

[Meindert Sprang]

Assuming that:
1) you are demodulating a signal between 100 kHz and 800 kHz bandwidth.
2) fidelity of the demodulated baseband signal is important.

Then the group delay (phase linearity) of the filter is of primary
importance.

A ceramic filter can give nearly flat group delay and good bandpass
charactaristics. The cost varies from a few pennies to 0.40 cents in
production quantities. Here is a good catalog with application info:

http://www.murata.com/catalog/p11e.pdf

A loosly coupled IF transformer can give nearly flat group delay and
good bandpass charactaristics. Depending upon the coupling between the
primary and secondary, and the loaded Q, you can alter the bandwidth
and group delay charactaristics. Fancier passive L.C. I.F. transformers
/ filters have multiple mutual couplings. Try searching on "helical
filter", for instance.

Thanks for the explanation Frank. Deep down, I knew all this, but not
working with analog stuf much, makes it really stay "deep down" and then you
need someone to stir those dormant grey cells..... ;-)

Meindert

 

[Rich Grise]

....
You need to switch your thinking. Just because you see active
filters at audio does not mean you should use them at RF. In
all but a handful of cases, you'd be using LC circuits at
10MHz, and given that you have a standard IF, the norm would
be a ceramic filter. The latter may not be the solution, given
your bandwidth, but since we don't know your actual application
one can only guess.

Note that since 10.7MHz is a standard IF frequency, you can get
IF transformers for the frequency, so the solution may be to
build up something from them.

I remember many many moons ago, tuning up an IF strip using a
sweep generator and scope. I got a huge kick out of watching
the humps on the response curve move while I tweaked the
cores.

Cheers!
Rich

 

[Joerg]

Hello Meindert,

Why ? I agree a ceramic filter would be good if you need a few to a few 10's
of kHz bandwidth. But why should you use LC circuits? Because everybody used
to do it that way or is an LC filter still better than a modern 1GHz GBW
opamp with a few cap's and R's? Just curious..

Because LC is a whole lot cheaper and you can achieve a much better
filter performance and group delay flatness. With your wide BW spec it
would boil down to two filters in series. One high pass and another one
low pass.

Today it would be possible to do this with opamps but that doesn't mean
it would make economical sense. Actually, it doesn't ;-)

Besides cost and size there is another drawback of opamp filters at high
frequencies. They consume a whole lot of power.

Regards, Joerg

 

[Meindert Sprang]

Hello Meindert,


Because LC is a whole lot cheaper and you can achieve a much better
filter performance and group delay flatness. With your wide BW spec it
would boil down to two filters in series. One high pass and another one
low pass.

Today it would be possible to do this with opamps but that doesn't mean
it would make economical sense. Actually, it doesn't ;-)

Besides cost and size there is another drawback of opamp filters at high
frequencies. They consume a whole lot of power.

Good points, Joerg. I recently designed an RF generator board with 2 DDS
chips and at that time I was very tempted to use a 1GHz GBW opamp as a low
pass filter after the DDS at 400MHz. Eventually I decided to go for the
"good ol' " LC filters....

Meindert

 

[Joerg]

Hello Meindert,

Good points, Joerg. I recently designed an RF generator board with 2 DDS
chips and at that time I was very tempted to use a 1GHz GBW opamp as a low
pass filter after the DDS at 400MHz. Eventually I decided to go for the
"good ol' " LC filters....

That probably saved you several Grolsch pintjes worth of money. Also,
you don't have to call the local power station before tunring it on ;-)

Regards, Joerg