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Old 21st Oct 2008, 9:20 pm   #1
Mike Brett
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Default bandpass filters

Hello all
I have just finished building a 80 meter superhet receiver using a mixture of discret components and IC chips and now I am in the process of building band converters for it.I need converters for the 40, 20 and 10 meter bands. I have got the local oscillators and mixers sorted I just need the bandpass filters.I have looked at various formular for working out the values and to be honest it goes right over my head. Is there anyone out there that has worked out the values for these common bandpass filters, or could point me to a scource of circuit diagrams. I was thinking of filters that used toko coils or toroids, whichever would be best. I have spent a long time geting my superhet working to perfection so would want my converters to work just as well.
So over to you. Mike
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Old 21st Oct 2008, 9:40 pm   #2
Mark_4CX35000
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Default Re: bandpass filters

You need to explain further the design of your receiver, as this will
decide the complexity of your filter design.

What IC's are you using in the RF stage.
What is the input impedance of these devices (Datasheet will be required).
What frequency range do you wish to use.
What IF frequency and filters are you using.
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Old 21st Oct 2008, 10:31 pm   #3
Skywave
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Arrow Re: bandpass filters

Yes, the design of passive-component filters can be rather daunting: a lot of text-books go very heavily into the math. However, for a lot of off-the-shelf requirements, you don't really need to know all the "how's" and "why's" (fascinating as it is ).

Various editions of the RSGB "Radio Communication Handbook" and the "Radio Amateurs' Handbook" (ARRL) give the basic design equations. Alternatively, there are a few good on-line software design tools on the Web. (Plug in the start data figures and press "Enter". "AADE Filter Design" springs to mind).

Having said all that, you do need to know in advance certain parameters before proceeding: cut-off frequencies, pass-band & stop-band attenuation, source & terminating impedances - & various other specifications - dependant on the type of filter you elect to use (e.g. Chebychev requires an in-band ripple figure; this is non-existant for a Butterworth-type response).

HTH

Al. / Skywave.
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Old 22nd Oct 2008, 2:58 pm   #4
Mike Brett
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Default Re: bandpass filters

Hello all
Thanks for replys, I will do my best to answer.
If you mean by RF stage RF in, I am using a preselector before the mixer and this has no IC,s in it. I have tried to keep all ports to 50 OHm,s.
Not sure what you mean by what frequency range do I wish to use.
I am using a 11.000 MHz crystal ladder filter, IF frequency is 7.000 to 7.500 MHz to cover 3.500 to 4.000 MHz.
Hope this helps. Regards MIke
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Old 22nd Oct 2008, 4:12 pm   #5
Mark_4CX35000
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Default Re: bandpass filters

Preselectors can vary, some are a simple L match which can have
the added bonus of behaving as a low pass filter, while others have
more complex tuning circuits and can behave as a bandpass filter.
It comes down to the design. Therefore you might find a purpose
built bandpass filter doesn't perform any better than your preselector.

I have been through some of your previous messages and you seem
to be using a Philips SA602 mixer IC. Unless that is for a different receiver.

I will have a look around through my notes as I designed a receiver
for 6m band a few year back using the SA612, more modern equivalent
of the SA602.
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Old 23rd Oct 2008, 5:38 pm   #6
joe
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Default Re: bandpass filters

Hi,

Personally I know next to nothing about filters but I came across this website which may be of some use.

It is http://alignment.hep.brandeis.edu/El...PassiveFilters

Good luck

Joe
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Old 23rd Oct 2008, 9:21 pm   #7
Mike Brett
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Default Re: bandpass filters

Hello Mark
The preselector I used can be found on Frank W Harris webbsite. Just put this name in google and it should come up at the top. This preselector works remarkable well for me. The mixer I used is indeed a NE 602,I tried a diode ring mixer first and it was hopeless but the 602 works fine.
Joe
Interesting webbsite but again it takes some understanding.
Regards Mike
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Old 30th Oct 2008, 12:35 am   #8
squier
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Default Re: bandpass filters

go to www.aade.com and download their filter toolbox. Its ****** brilliant and will do just about any filter (xtal or LC) that you care to mention.
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Old 30th Oct 2008, 1:41 am   #9
Skywave
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Arrow Re: bandpass filters

Re - above Post, Squier.

Quite - as per Post 3 above.

Al.

Last edited by Skywave; 30th Oct 2008 at 1:43 am. Reason: Remove quote of immediate previous Post: Forum requirements!
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Old 7th Nov 2008, 9:09 pm   #10
Mark_4CX35000
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Default Re: bandpass filters

Sorry for the late reply, but I have had my hands full of problems for the
past couple of weeks.

Anyway I have attached some JPEG files which show a circuit diagram and
the component values for a simple filter for the SA612. Same filter will work
with the NE602, which is a older version of the SA612 mixer IC.

The input impedance for these filters is 50 ohms, therefore should work with
your standard equipment. The resistor R1 shown in the diagram is a static
leak, I normally added this as it does effect performance, but you may wish
not to bother.

See how you go with these filters as they should be more than good enough
for what you need, but if you find that the filtering could be better then you
can design a filter using the suggested software.
I have not used this software as I prefer to design my own and carryout a
simulation. This allows me to tweak the values to suit my requirements.

Remember the NE602 or the SA612 IC's have a DC voltage present on pins 1
and 2, therefore you will need to use decoupling capacitors on pins 1 and 2.
These are shown on my circuit diagram as C3 and C4. Both are 100nF.

Also the input impedance of the mixer is 1.5k in parallel with 3pF capacitor,
therefore you will need to specify this impedance within the filter software.

You can use the same values as I have used for the decoupling capacitors
if you decide to use a more complex filter.

Mark G7RJV
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