Re: Crystal CW filter with Version 4 uBITX boards? #ubitx

Jerry Gaffke
 

Allison,

You mentioned a number of ways to have switchable/variable crystal filters
that have been proven to work in other rigs.
I'm not aware of any commercial rigs just slapping two crystal filters in parallel.
There's likely a reason.

Though perhaps the reason is that they didn't have
such an easily programmable oscillator at the time.

If I ever get time, and get everything else done I'm expected to do,
I'd like to build two pluggable filters for a uBitx, say 12 and 1.8432 mhz.
Simple, and should just work.
Scan each with some sweeperino like thing, verify they are working properly.
I'm fine with pluggable filters on a $120 rig, I don't often have to switch between
CW and SSB mode, and generally have the cover off anyway.
Would rather have pluggable than more of those d**n relays.

Could be worth trying just running them both in parallel, sweep the combination
and see how much worse it is.
Images should not be a problem, since we also have the 45mhz filter in there.
Since everything is unshielded, will likely be more birdies.
There may well be weird reactances present off frequency, though I'd expect the filter
that is on frequency with it's 200 ohm load to be of much lower impedance.
Having never swept a crystal filter, I could easily be wrong.
But could work well enough to be a keeper.

There are lots of corners cut on the uBitx that you never see in commercial gear.
That's why it's $120, not $600. 

Jerry


On Sat, Jul 7, 2018 at 10:23 am, ajparent1/KB1GMX wrote:
When and if you get it to work please post your results and what it took to make it work.

Yes the ports are 50 ohms, that's never been secret but if you have two 50 ohm devices
(source and load) and then parallel a third 50 ohms device what is that, not 50 ohms.
Only the IF and the Balanced mod are 50 ohms, the filter is 50 ohms in the band pass and
whatever outside it.  

I never said the idea to switching IF frequency will not work. ONly paralleling filter will not work well
due to one more or less shorting out the other.  You have already written that part up.  I did expllain
those of us that can design would do it and it would work well.  Also every stinking cheap am/fm
radio has a dual frequency if both 455 and 10.7 and thats been done for over 50 years.
Been in engineering or over 50 years, my specialty is making the won't work things,  work.
But that unusually means some fancy design or understand how it works to start with.

Two things.   I told you how to do it and it will work if you do that.   I said your hack
of paralleling them relies on things that will be guaranteed to work poorly unless lucky.
Also Farhan said 1.842mhz filter and you didn't try that.  

Now your own words:
>>>The idea of switching the IF frequency to switch filters appears to be sound - just need to find the magic crystal/capacitor/transformer combination and for that I need someone with better knowledge of crystal filters and better test equipment that I have.<<<
I have both and your proceed to disagree.  
Until other post their work it is vaporware.  I have your posting it didn't work.

>>>>So far I've not been able to get a signal through filters at 4.6,  6.2 or 8 MHz.<<<<

You didn't try 1.8432, and you did get some signal but the attenuation is high.  
Silly question did you try the filter without the 12mhz filter shorting it?  That
would prove your filter works.  You could also do that using a bench jig.
You only need signal generator and a scope maybe a few resistors to simulate
a 50 ohm load.

Me I'd mod the 12mz and make it a jones filter (varicap tuned variable bandwidth)
as used in some older Tentec and the Elecraft K2.   I like that as it means the board
only needs parts added (varicap diodes and resistors to a control point) and no
hacking it up.  It is an old idea and discussed before.


Allison

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