[CW] NEScaf switched capacitive audio filter

[Richard Knoppow]

    Even simple single crystal filters ring if too narrow. Some 
earlier receivers had crystal filters with bandwidths as narrow 
as 50Hz. They ring on both signal and noise. Somewhere around 200 
to 400 Hz seems to be the practical limit. The old style crystal 
filters, especially the patented Hammarlund filter, have a good 
notch which can take out some QRM. An audio filter can have a 
notch just as effective except it works on audio, not RF so can't 
eliminate some cross mod effects of a strong hetrodyne signal. 
Generally, the narrower the filter the more side effects it has. 
There is no argument that a good audio filter can help a lot. 
This goes back to the 1950s and the Select O Ject filter made 
commercially by National. Same sort of thing as this more modern 
version, audio peak or notch filter. Could give low cost 
receivers like the S-40 or NC-57 a fighting chance.

On 6/18/2018 1:15 PM, pa0wv wrote:
> Petr Ourednik schreef op 2018-06-18 07:28:
>> Hi all,
> 
> As generally known your brain filters, so cw in wideband noise 
> (300-2800 Hz) and CW band limited to 250 Hz are decodable with 
> the same error rate.
> 
> However when you narrow the filter to 50 Hz you can copy better 
> because that filter is narrower then your brain filters.
> 
> Everything without QRM only white band limited noise noise in the 
> 3 cases the same strength. and the Morse code the same PEP in the 
> 3 cases.
> 
> Which cases? Listen by dpwnloading the 3 sound files on my 
> website https://pa0wv.home.xs4all.nl/cw_archive/
> 
> Small filters , mechanical or electrical, has often severe 
> ringing. You have to test with Morse code, and switched capacitor 
> filters build around an IC of Maxim are 8 pole chebyschev. 
> certainly not phase linear.
> 
> The 3 example files are produced bij 1025 element FIR filters.
> 
> PA0WV
> 73=30
>>
>> as Mike wrote I can confirm it is great feature for your rig. I 
>> tested
>> few of them and
>> after proper building it run for the first shot and works like 
>> champ.
>> You will be amazed with the improvement of your radio using the 
>> NESCAF box.
>>
>> 73 - Petr, OK1RP
>>
>> On Sat, Jun 16, 2018, at 1:54 AM, w9oj at indy.rr.com wrote:
>>> The NEScaf is a switched capacitive audio filter. It is 
>>> designed to be
>>> simple to build and use.
>>>
>>> It is more flexible than previous filters for QRP rigs. It has 
>>> two
>>> panel-mounted controls — a single center-detent potentiometer 
>>> that sets
>>> the center frequency of the filter’s bandpass (default value 
>>> settable
>>> between ~415Hz and ~1kHz), and a dual-ganged potentiometer 
>>> that provides
>>> a continuously variable bandwidth control (from ~60Hz to 
>>> ~1500Hz). This
>>> ability to smoothly vary the bandwidth is very useful.
>>>
>>> The NEScaf is designed to be driven with a phones or speaker 
>>> level
>>> signal. No hacking into your rig — just connect it to the 
>>> headphone or
>>> speaker output of the radio. This makes it usable with several 
>>> radios.
>>> The board is small enough, at about 2 inches square, to build 
>>> inside
>>> many radios however, if you so choose.
>>>
>>>
>>> http://www.newenglandqrp.org/wordpress/nescaf-a-switched-capacitor-audio-filter/ 
>>>
>>>
>>>
>>>
>>> 73 de Mike W9OJ
>>>
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>>
>>
>> -- 
>> 73 - Petr, OK1RP
>> -- 
>> B: http://goo.gl/Fd2JhJ
>> G+: http://goo.gl/w3u2s9
>> G+: http://goo.gl/gP99xq
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL