455kHz bandpass filter for manufacture.

[vk4ffab]

Yeah the caveat here is he said its for a commercial product. If your design starts with a $250 part, it better have super high margin, if your design starts with end of life and surplus, you better change your design. There are dozens of ways to get single signal reception that do not require the use of stupid expensive or hard to get parts. Just change the IF frequency to one where you can build a suitable narrow filter or do away with analog entirely and go DSP.

IF filters are awfully useful to improve in-band rejection of signals which would otherwise overload the following stage.  No amount of DSP can undue overload of the final mixer or digitizer.

As I recall, Rockwell bought Collins' mechanical filter division for exactly this reason and continued to sell mechanical filters to others but of course later shut it down.

If the IF frequency can be lowered, then bandpass switched capacitor filters become viable.

If you go DSP you do not need an IF at all and thus the IF filter. Just direct convert to audio I and Q and then do the processing in software with Hilbert transform and FIR filters. For the cost of a $2 Arm micro a couple of mixers and quadrature LO from an SI5351A. The whole receiver build would be sub $50. There are plenty of examples of this on the net from Charlie Morris, Pete Juliano etc and this is the way the QSX from QRPlabs, which is being designed the exact same way.

[arildj78]

Inrad still make 455khz narrow filters, but you will be paying in kidneys for them. You cant up convert to say 9mhz and build a narrow xtal filter. All current 455khz filters i know of are for am and are 6khz wide, not 200hz.

The narrower the better, but I'd take 6kHz any day.

[arildj78]

Inrad still make 455khz narrow filters, but you will be paying in kidneys for them. You cant up convert to say 9mhz and build a narrow xtal filter. All current 455khz filters i know of are for am and are 6khz wide, not 200hz.

I just looked at the Inrad filters and you're right, had to pay one kidney, my right arm and my first born child for that thing! Also, they were out-of-stock   

[arildj78]

Is anything stopping you from doing this in DSP? I'm in a similar boat for a 20MHz video FM receiver with 1MHz deviation and after some fiddling around just decided to do it in DSP.

This is a sidegig, and my knowledge is not up to the task yet. In a future implementation I hope to do this with a micro, but for now, I figured that the easiest path would be through the analog domain.

Little did I know... If there are any greybeards out there in search of a Norwegian apprentice, feel free to give me a shout

[fcb]

If you just want to get something up & running quickly, then you can build an 8th order bandpass (20KHz BW) using easily available parts:
https://www.analog.com/designtools/en/filterwizard/

[arildj78]

If you just want to get something up & running quickly, then you can build an 8th order bandpass (20KHz BW) using easily available parts:
https://www.analog.com/designtools/en/filterwizard/

This is the approach I've started on. Do you have any suggestion on how to minimize the effect of component tolerance? In my first try, both the center freq and the gain has a very large tolerance envelope.

[KaneTW]

Use 0.1% tolerance components and it should be well within margin. Note that narrow (high-Q) active bandpass filters very quickly get unrealistically high GBW requirements. GBW requirements for an active filter are proportional to Q * gain * frequency, multiplied by some safety factor to account for nonideal opamps (https://www.ti.com/lit/an/sbaa236/sbaa236.pdf, https://www.changpuak.ch/electronics/downloads/sloa088.pdf).

Traditionally the safety factor is 100 for a max. 1% gain error, but I find that way too high for most applications.

Component nonideality is another good reason to use digital filters. A FIR filter isn't that difficult to design and simulate (I use ScopeFIR for figuring out the frequency response and tap count, then Mathematica for the actual calculations and simulations).

[vk4ffab]

Is anything stopping you from doing this in DSP? I'm in a similar boat for a 20MHz video FM receiver with 1MHz deviation and after some fiddling around just decided to do it in DSP.

This is a sidegig, and my knowledge is not up to the task yet. In a future implementation I hope to do this with a micro, but for now, I figured that the easiest path would be through the analog domain.

Little did I know... If there are any greybeards out there in search of a Norwegian apprentice, feel free to give me a shout

It might be possible to use a high speed quad opamp and do an 8th order high pass followed by an 8th order low pass and get a fairly narrow bandpass that way shoot for say 1K, this will give much better skirts and slope  factor to the filter compared to using the bandpass calculator values. You could try simulating this in LTspice to see if you can get  good enough shape.

But there are probably easier ways to go about this, but its hard to give specifics without knowing much about what you are trying to achieve. As for DSP, its not all that difficult, if you want to go down that rabbit hole, here is a series of 10 vids by Charlie Morris ZL2CTM in New Zealand who has done most of the work for us. Basically you can do it all with a TeensyDuino and a handful of parts.



Rob.

[David Hess]

It might be possible to use a high speed quad opamp and do an 8th order high pass followed by an 8th order low pass and get a fairly narrow bandpass that way shoot for say 1K, this will give much better skirts and slope  factor to the filter compared to using the bandpass calculator values. You could try simulating this in LTspice to see if you can get  good enough shape.

At least one Linear Technology application note briefly discusses using current feedback amplifiers as part of an IF filter circuit but it was not an active filter.  Before fast operational amplifiers became available, sometimes discrete transistors were used for high frequency active filters.  I think the complexity and tolerance requirements would get out of control very quickly when trying to achieve the performance of a resonator but maybe it would be good enough.

[fmkit]

I  just tested simple 2 series 455kHz ceramic resonators filter ( as normally used crystals),   these marked 455E.
Bandpass CF: 435.45kHz,   Bandwidth 2kHz
Sweep 100kHz, center 455 

ps: added sweep of  LT455HW  ceramic filter,  measured BW is 10.4kHz,  2-series res.filter  5 times narrower and much cheaper,  I'll use one on DIY spectrum analyzer to see FM broadcast 19kHz pilot