CW Audio Filter Kit

Started by RadioRay, August 16, 2014, 12:06:19 AM

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RadioRay

I've been FINALLY begun having fun building low end, receivers and projects, after having a million-and-one other things on my plate.  Filtering on some of these receivers is either none-at-all, like the RockMite or very marginal.  So, I decided to build the NEScaf - switched capacitor audio filter kit from the New England QRP Club.  It's center frequency is normally peaked at 600hz, when the detent pot is centered, allowing a wide swing of center frequency, in case the other station does not know how to zero beat your signal, or you want to use the filter to slew through a pile of stations.  I also has a pot for bandwidth, which goes from unfiltered, down to a very narrow setting - which I cannot remember.  Please see the link below.

http://newenglandqrp.org/nescaf

I fit it all into an Altoids tin (Wintergreen - blue) along with a 9 volt battery and tried it out on just about everything in the house.  In sort: it works!  This really works well.  You just run a patch cord (3.5 mm stereo on each end) from the audio out of your receiver to the filter and I like to use earbuds/phones when copying CW. The controls are bandwidth & center frequency (there is an internal trim pot to set audio gain/volume) and you use the receiver volume control/AF gain for your volume control as per normal. I found that using this set-up GREATLY improves a general coverage 'plastic' shortwave radio when listening to CW. You suddenly have a narrow CW filter, at least an audio filter. Even on my 2 transistor , regenerative 40m band receiver, this filter turned it into a MUCH more usable receiver. 

I would highly recommend this kit. Even for those rigs with some filtering, adding an audio filter after good IF filtering is usually quite helpful.


de RadioRay ..._  ._
"When we cannot do the good we would, we must be ready to do the good we can."  ~ Matthew Henry

KK0G

I've heard nothing but excellent reviews of the NEScaf filter. Damnit Ray, now I'm gonna have to order one, thanks a lot. ;D
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

cockpitbob

Thanks for the report.  That's a project I've had my eye on for a while.  I want to mate it with my MTR-2 (that I still haven't finished building).  With the LM386 audio amp it should be able to drive a small speaker too.  Some days I don't want the ear buds.

gil

Looks like a great product... Awesome for a Rock-Mite. I don't think you'll need it on your MTR Bob. The built-in 400Hz filter is pretty good... The Rock-Mite, well, let's just say I usually enjoy music while listening to CW!

Gil.

underhill

I'm working a weekly CW sked now with a guy up in Redding, CA (I'm just north of  Long Beach, ca) on 40M, and right at the appointed hour, the band suddenly gets quite congested.  For me and my weak cw skills, very difficult to maintain copy!  My 857 doesn't have the optional 300hz filter, so very difficult, at my end.  Well, my cw buddy uses one of these little filter units, is quite satisfied with it, and suggested it so I picked one up.

It's a slightly different approach for a similar result, from 4 state qrp club http://www.4sqrp.com/HiPerMite.php

200Hz bandpass centered on 700Hz.  Planning to stuff it into an altoids box as well.

Would be built by now, but I discovered that the 857D has a special 240Hz cw filter in it's DSP selection, which works amazing on the QRM!.  Sill have a use for it, so still going to build it, and now I know that the 200Hz bandwidth is just fine.

Just thought I'd throw out there another option available.

Allan

vwflyer

I just built a HiPerMite. Had I seen the one from New England QRP Group first I probably would have gotten it for its adjustable center freq and bandwidth for a few dollars more. My HiPerMiteWorks pretty well even though it's center freq isn't exactly what my Kenwood's side tone is. I even put it in a blue wintergreen altoids tin. I built it as per the instructions for zero audio gain but I swear that it is adding gain to my audio signal. It was a real easy build.

RadioRay

#6
Audio gain is a funny thing.  If you're listening through a 2800Hz wide SSB filter with a tiny (50 Hz) CW signal in there, a lot of other-than-signal NOISE is amplified, not only the signal.  When the filter cuts out most of the noise and amplifies 'mostly' the signal, your signal to noise ratio (which is really what matters most) jumps sky high!  You may have the same sound energy in the headphones, but you WILL have a much louder signal with the filter engaged. The NEScaf has an internal trim pot for just this reason, so that AF gain can be adjusted for when the filter is in use.

I choose the NEScaf for the same reason that you did: adjustable center frequency and bandwidth. Not all rigs use the center same center frequency as the Hi-per-Mite, and many hams do not zerobeat to you, causing them to be outside of your filter center, unless you have an RIT and know how to use it. 

-...-

Hey Allan!

Glad that you're doing the CW sked.  THAT is the #1 way to transform 'I kinda knew the code when I passed my license' into "Sure, I speak Morse code."   ;-)  I had an 857D and the built in audio filter makes quite a difference.  That also has a variable passband filter, ja?



73 de RadioRay ..._  ._
"When we cannot do the good we would, we must be ready to do the good we can."  ~ Matthew Henry

underhill

Quote from: RadioRay on August 17, 2014, 02:03:08 PM
.

-...-

Hey Allan!

Glad that you're doing the CW sked.  THAT is the #1 way to transform 'I kinda knew the code when I passed my license' into "Sure, I speak Morse code."   ;-)  I had an 857D and the built in audio filter makes quite a difference.  That also has a variable passband filter, ja?



73 de RadioRay ..._  ._

Hi Ray

What you say about filters and signal to noise is quite true, if I had been aware of the NEScaf at the time, would have been considered certainly.

Out of the box, the 857D, and for that matter my 817ND have a nice IF shift function, and passband, and while sidetone is set on both at 700Hz, that is variable as well.  Side note, these filters are in menus, which are more challenging 'on the fly', but using the radios on something other than FM or Digital modes is bringing the familiarity up fast.

Weekly sked not daily, but still it's consistant, and 20W into NorCal frm my area working.  I'm not worrying about real qrp at this time, focus is getting better with CW, the rest will come.

Allan