|
Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
|
Thread Tools |
21st Oct 2008, 9:20 pm | #1 |
Retired Dormant Member
Join Date: Dec 2005
Location: Wisbech, Cambridgeshire, UK.
Posts: 452
|
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 |
21st Oct 2008, 9:40 pm | #2 |
Retired Dormant Member
Join Date: Sep 2008
Location: West Yorkshire, UK.
Posts: 45
|
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. |
21st Oct 2008, 10:31 pm | #3 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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. |
22nd Oct 2008, 2:58 pm | #4 |
Retired Dormant Member
Join Date: Dec 2005
Location: Wisbech, Cambridgeshire, UK.
Posts: 452
|
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 |
22nd Oct 2008, 4:12 pm | #5 |
Retired Dormant Member
Join Date: Sep 2008
Location: West Yorkshire, UK.
Posts: 45
|
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. |
23rd Oct 2008, 5:38 pm | #6 |
Rest in Peace
Join Date: Apr 2003
Location: Edgware, North London.
Posts: 307
|
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 |
23rd Oct 2008, 9:21 pm | #7 |
Retired Dormant Member
Join Date: Dec 2005
Location: Wisbech, Cambridgeshire, UK.
Posts: 452
|
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 |
30th Oct 2008, 12:35 am | #8 |
Retired Dormant Member
Join Date: Jul 2006
Location: gloucester
Posts: 7
|
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.
|
30th Oct 2008, 1:41 am | #9 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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! |
7th Nov 2008, 9:09 pm | #10 |
Retired Dormant Member
Join Date: Sep 2008
Location: West Yorkshire, UK.
Posts: 45
|
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 |