Programmable low pass filter

[Kooky613]

ADRF 6518 is a low cost chip that offers a dual programmable baseband low pass filter with a great range of tunable gain

Unfortunately this chip is obsolete and the other components of the same company are not doing the same job (frequency tunability etc is not the same or they are expensive )

Is there a way to achieve something similar?? do you know any component or do you have any idea to do something equivalent?

TY

 

[danadakk]

What is the freq range you need to do in your application ?

Regards, Dana.

 

[Kooky613]

i want to programm 5-6 different frequency ranges between 10 and 80 MHz ideally

 

[BradtheRad]

Programmable... Do you wish to apply a voltage to a terminal in order to change the rolloff curve? There is the varactor (variable capacitor), aka varicap diode. Often used in modern radio tuners. By changing voltage you change Farad value, thus changing filter behavior.

You can build one filter which you program. Depending on what steepness you want in the rolloff curve, it can contain one or more stages. Then to change frequency range, it should be possible to change voltage on all varactors in unison.

Or you can build 5 filters, adjust each individual frequency range, then switch them in or out of circuit.

 

[Kooky613]

Do you have a varactor at these frequencies ?

The solution with 5 filters is known but it requires a lot of space

Ty for the reply anyway

 

[BradtheRad]

Take a look at methods used in everyday FM radios.

* Components used are capable of operating at 88-108 MHz (faster than your spec 10-80 MHz).

* Electronically controlled tuning. No rotating tuning capacitor nor tuning dial.

* Frequency selection is programmable. Your interface can be several settable buttons (one for each station), or a single 'scan' button.

 

[biff44]

i suppose you could make your own variable filter.
like have a series inductor, a 5 throw switch in shunt, and another series inductor.
you would hand different capacitor values off of the 5 other terminals, and switch the biggest cap in when you want the lowest frequency passband.

it would be a little crude, in that the return loss of the filter may not be very good for the largest or the smallest capacitor selection.

and when i say 5 throw switch, at these very low frequencies, an "analog switch" semiconductor will be fine...you do not need a "microwave" switch

because the switch has some series resistance, the amount of out-of-band rejection you get may be less than you need....also that topology is limited to an N=3 LPF topology....

someone might make a MEMS switch capacitor already....

you also could do a mostly analog approach, and use a varactor diode in shunt, and two fixed series inductors. Drive the varactor diode with a DAC chip. But there will be temperature variations in the filter's cuttoff frequency....

 

[BigBoss]

Varicap supported Low Pass Filters are not common in RFIC circuits. Instead, switching Capacitors technique is used. You can also use this technique with a simple LNA ( as OpAmp) and switch appropriate capacitors values in according with Low Pass Cut-Off frequency.

 

[Kooky613]

If anyone has components list with a schematic for these freqs could be helpfull.

 

[danadakk]

Some possibilities :

Selection Table for Tunable Filters | Parametric Search | Analog Devices

Analog Devices' Selection Table for Tunable Filters lets you add, remove, and configure parameters to display; compare parts and choose the best part for your design.

www.analog.com

Internet Archive: Digital Library of Free & Borrowable Texts, Movies, Music & Wayback Machine

archive.org

Possibly use this approach, apply a filter transformation to LPF -

https://www.rfcafe.com/references/articles/Tunable-BPF-Design-Brad-Baker.pdf

Regards, Dana.

 

[Kooky613]

Analog list has no Low pass at the range i need that is not obsolete

Low pass for direct conversion receiver to select 7/14/28/56/112 MHz channel BW that means 3.5/7/14/28/56 MHz low pass cut off freq

+ the 6518 had integrated vga
But the problem let's say is not the vga but the selective filters
I need a discrete low cost minimum space implementation may the filters be less for only 28/56 MHz cut off freq

Anyway thank for the reply

 

[FvM]

ADRF6518 is 6th order Butterworth. What are your requirements?

 

[Kooky613]

i will use 2 discrete low pass filters 28MHz and 56 MHz but i need help with multiplexers or switches for the selection
a schematic with a low cost solution could help

 

[danadakk]

What filter specs do you have, loss, rejection, Butterworth, equaripple, linear phase.......

Amateur radio Transceivers use lumped, relay switched BPF solutions. Typically 2 pole.
You could modify as LPF.

Are you in a 50 ohm environment ?

Regards, Dana.

--- Updated Apr 1, 2022 ---

Using active approach, convert this to LPF, attached.