Band Pass Filter CF 3395.4MHz Design Help

Thread Starter

W7NUK

Joined Mar 12, 2017
17
I'm looking to learn CW. I have a old Heathkit SB102 which did not have the optional CW filter. And filters if you can find one go for $50 or more! Much more then I'd want to spend considering what I already put into it restoring it up. I'm at least 90% done so far! Just a few items left and its complete! Mostly cosmetic as it operates just fine wihout changing them.

So I was thinking it may be possible to just build a new filter using modern parts. The orginal filter has a center frequency of 3395.4MHz, bandwidth of 400Hz(600@6Db down, 2000@60Db down) and it has input/output impedances of 2kohm. So could a new filter be built to be close to the orginal specs? If yes how would one go about this? What parts are needed? When I looked online I wasn't finding what I needed to build one at a RF frequency. Only audio frequencys and nothing regarding impedances and such. Possibly a active filter? from what I found but I'm not sure since as that has gain and the orginal filter didn't since it's a passive network of crystals and caps. So maybe a resistive network at the output to attenuate the signal to then take the signal out of it?

If someone could help me with this by giving some insight on how to design this that would be awesome! Would be cool to build my own filter which I wouldn't have to worry about drifting over time. Some orginal filters have drifted! Then I could always build a SSB filter in the future should mine drift. If you want to design a full thing I'd love that but at least a little help figuring out how to design one for use at RF with a narrow bandwidth. Including the parts and math needed wi a little decription of how to proceed.

Thanks
 

Papabravo

Joined Feb 24, 2006
12,514
The center frequency of your filter is not inside the nearest amateur band which runs from 3.5 to 4.0 MHz. So I'm curious about how it is applied. I'm not sure the parts you would use today are any different than they were several decades ago.
 

Thread Starter

W7NUK

Joined Mar 12, 2017
17
The center frequency of your filter is not inside the nearest amateur band which runs from 3.5 to 4.0 MHz. So I'm curious about how it is applied. I'm not sure the parts you would use today are any different than they were several decades ago.
The filters reside close to the IF frequency of the radio. The CW crystal is 3395.4MHz, LSB=3393.6MHz and USB=3396.4MHz. The IF of approx 3.4MHz is added to the LMO frequency of 5 to 5.5MHz which is then subtracted from the band crystals frequency which is 12.395MHz(3.5-4MHz) to 38.395MHz(29.5-30MHz) to get to the output frequency for each band. There are more crystals for the bands between but not needed for a brief description.

Hope I didn't botch that too bad. That just so happens to be the IF frequency Heathkit choose at the time. I think Collins had 9MHz or so. So I hope I have that correct. But you might then be able to understand how the radio gets to the output. The CW filter is narrow at 400Hz to filter out nearby signals. The SSB filter is 2.1KHz at a CF of 3394.6MHz or close. Can't recall without pulling the radio out of the case.
 

Thread Starter

W7NUK

Joined Mar 12, 2017
17
So I found a couple band pass filters online. So one has high Z for out of band and has 3 inductors and 3 capacitors. But 2 capacitors and 1 inductor are so low value I think it would just be the stray capacitance/inductance of a wire. The rest I can find on Mouser which is a 390mH(2 in series for 790mH approx?) and a 390nF capacitor. So should I attempt to build something? Or should I just buy a factory crystal CW filter for $50-60? Because the price of a filter and key and learning material/time is over $100 easy. And I've already spent more then enough bringing to life.

So I'll sleep on it and see how much more I can find for this. I found through QST a capacitance keyer Tkey-1 which goes for $70. Nice little keyer. So that's a little more pricey for that and a filter then I want. But that is on the cheap end of paddles! Those can go way over reasonable fast! So let me know how I should go about this.

Thanks
 

BR-549

Joined Sep 22, 2013
4,938
That's 3395.4 KHz, not MHz. I would suggest restoring to original specs with the supplied options.

Then get a breadboard and start experimenting with active filter op-amps. You can put a CW filter in line with your headphones.
 

MrAl

Joined Jun 17, 2014
6,642
Hi,

I thought my eyes were going bad when i saw something around 4GHz.
I had to look and re-look at that several times :)
4 MHz is doable for hobby radio guys.
 

Thread Starter

W7NUK

Joined Mar 12, 2017
17
Yea sorry about the mistyping. Everything in the manual reads as 3395.4 and I'm thinking MHz as I read it written in KHz. Two lines of thought lead me wrong. So I may have to try getting a few parts and see what I can build. I will have to undig my signal generator. Got half my stuff packed up! Wrong time to work on projects as the house is on the market.

Would be cool to build my own but seeing the parts needed makes me a little worried. Some capacitors are 0.0028pf some inductors are 0.00ish uh. Seems like stray cap/inductance of wire but if I do it as wire I think it will short everything out. Could be wrong.

Just don't know really how to start on this. Never did anything like this before! I would try a filter for headphones, if I had some. But I also don't know what frequency CW comes through the speakers as when zero beat properly. Well I guess it's about time to learn anyways. Time to experiment!
 

BR-549

Joined Sep 22, 2013
4,938
Please don't try to modify your radio. Google......cw active audio filters.

They all take normal common cheap parts.
 

MrChips

Joined Oct 2, 2009
19,415
I am not sure I understand what you are trying to do.

In order to "hear" your CW signal through your headphones you need a beat-frequency oscillator (BFO).
This is basically an adjustable oscillator close to the intermediate frequency (IF).
If the BFO frequency is exactly tuned to the IF you hear nothing. You have to shift the BFO frequency so that you will hear the difference between the BFO frequency and the IF.

Hence if you want to hear the CW as a 400Hz tone, you set the BFO to either 400Hz below or above the IF.

You can also use the BFO to recover single side-band (SSB) voice transmission.
 

BR-549

Joined Sep 22, 2013
4,938
The radio does that in cw mode. he gets the cw tone.......but with a wide noisy bandwidth.

AN active filter can center.....500....300....100 Hz around that tone. If he likes 400 hz tone...set main tuning for 400 note. I prefer 700-800 myself. So I built my filter for that.
 

Thread Starter

W7NUK

Joined Mar 12, 2017
17
T
Please don't try to modify your radio. Google......cw active audio filters.

They all take normal common cheap parts.
Thanks for the info on that. I'm looking at it online. And don't worry. Whenever I modify my equipment I like to make them as easly reversible as possible. I highly believe in preserving vintage equipment. But why not enjoy it too? As long as it can be reversed easly I don't have a problem. So no drilling holes unless it's a simple easly replaced part.

What I thought for the filter would just be a plug in type unit in place of where the original CW filter would go with minimal mods to the radio itself. But since your idea is much easier and better. So if I could figure out how to implement it into the rig and use the original filter switch to bring it in and out that will be cool. Implement the filter inline with both the speaker and headphones would be perfect. Then either one would could use it. Might be tricky but somehow it could be done.

More research is needed but an idea is nearly there! By the end of the weekend I should know how and what I need to do with it. Then just have to learn CW! Well need to build it first
 

MrChips

Joined Oct 2, 2009
19,415
This is what worked for me in learning code.
Of course, sending is easy. Receiving is the difficult part.

Get yourself a practice key and oscillator.
Record yourself on a tape recorder (if they still exist). Start off with random T, E, then add A, I, M, N.
When you have mastered receiving those, move on to random D, G, K, O, R, S, U, W mixed in with the previous set.

Finally, add the remaining longer letters and numerals.

Of course, start with a comfortable speed and work your way to higher speeds.

(I wouldn't be surprised if there is already a free app on the internet. I haven't checked.)
 

BR-549

Joined Sep 22, 2013
4,938
I'm sure there are multiple websites to practice copying code now days. After you can recognize the code at 5 words a min......listen on air, on the amateur novice ham bands. We all really learn there......and it is a blast.
 
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