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NEScaf: A Switched Capacitive Audio Filter

Contributed by Dave Siegrist NT1U

The NEScaf is a switched capacitive audio filter. It is designed to be simple to build and use.

NEScaf PC boardIt 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.

This kit is suitable for all experience levels. It uses all thru-hole components, and the construction and adjustment is very straight-forward. It is reasonably priced and will be a nice addition to the QRPer's arsenal of tools.

The NEScaf is only $31 postpaid in the U.S. For your convenience, we offer a connector set for just $4. Visit the NEQRP Club Store to purchase the NEScaf.

Theory

The integrated circuit at the heart of the NESCaf is made up of two CMOS active filters. These filters are extremely configurable (low pass, band pass, notch etc). We have chosen to set up both filters as Butterworth band pass filters and to cascade the filters. Butterworth filters have the characteristic of constant amplitude in the band pass region, while the cutoff knee is not be as sharp as if the filter were configured as a Chebychev. We considered this an acceptable tradeoff, wanting constant volume out regardless of the bandwidth or center frequency setting of the filter. There are two on-board trimmer pots. One is used to "calibrate" the center frequency pot. This allows you to adjust the frequency at which the center detent occurs. If you are using a rig with a transmit offset and sidetone of say, 700 Hz, you can use this trimmer to make that the center-detent frequency. The other on-board trimmer adjusts the audio level into the filters output amp. Using this pot, you can set the overall gain of the filter. This can be used to set the filter for unity gain, if desired. This way, the filter could be switched in and out, and still maintain a comparable volume level in the headphones. The NEScaf has no panel mounted volume control of its own. It is designed so that you use the volume or RF gain control (if present) on the rig it is being used with.

Fine Print

Please Note: A case and audio/power connectors are NOT supplied. Since each builder will have his or her own personal favorite styles for these, we are not including these parts. All board-mounted parts, plus the two panel-mounted potentiometers, a power/bypass switch, and a power-indicator LED are included in the kit. You supply your own case. You can supply your own connectors for audio in and out and power, or you can order a connector kit from us with your NEScaf filter.

The NEScaf was originally kitted with the MF10. That was replaced by the LMF100, a drop-in replacement for the MF10. These were superseded by the LTC1060 in spring 2010.

Comments

Hi there! I was hoping to find out if the NEScaf kit is going to be available any time soon. Everything that I've read about it has been very good and I am eager to be able to get one of my own.

Hi, Scott. Thanks for inquiring about the NEScaf filter kit. I am hopeful that kits will be available again within the next several weeks. I believe all the parts are in, and a kitting session needs to be scheduled. Once available, an announcement will be made here on the club website, and on the various QRP email lists.

73,

Bruce N1RX

I built my NEscaf into a nice black metal box and it works fine but at various times I get BCB interference at certain frequency positions and it is worse when I touch the box.  I assume I can bypass certain parts of the filter with caps but don't know where to start.  Any suggestions?  Thank you.  K8WPE

PS the filter works great to reduce or get rid of local electronic noise in my house and the neighbors plasma TV set.  Please respond to DjWilcox01@aol.com

My older version of the NEScaf has become painfully noisy.  What's the method of choice for getting the pots quiet again?

Thanks,

Chuck, W5USJ

Any thought been given to having an SMT version of the NEScaf filter?

Thanks, Chuck, W5USJ

I have built an SMT version of the NEScaf myself. At this time, the club has no plans to produce it. -Bruce, N1RX

Chuck,

Have you tried cleaning the pots with a good quality contact cleaner?

Bruce N1RX

the kit arives now. so Heinz go on :-)

 

73 DF7FR

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