[amsat-bb] 67 Hz filter

[Stephen E. Belter]

I agree with Mike, this is an audio filter so the RC from PCB traces and other wiring should be ignored.  Choose the capacitor(s) to match common values, then select the resistors with 10% or 20% tolerance.  You can find common values by checking Mouser or DigiKey.

But I do need to ask a question before you build your filter:  What satellite are you listening to that transmits 67 Hz?

None of the SSB or FM amateur satellites *transmit* 67 Hz.  The Fox satellites (AO-85, AO-91, AO-92) transmit Data Under Voice (DUV) which can be annoying depending on the frequency response of your receiver.  DUV isn't 67 Hz, but a range of frequencies less than 300 Hertz.  Most commercial FM receivers filter out most, if not all of the DUV frequencies which are used for CTCSS.  Most SDR receivers allow you to adjust the filtering of the audio so that you can remove the DUV.

If you're building an analog receiver, you'll want to be filtering frequencies below 300 Hz, not 67 Hz.

73, Steve N9IP
--
Steve Belter, seb at wintek.com



On 7/19/18, 8:13 AM, "AMSAT-BB on behalf of Mike Diehl" <amsat-bb-bounces at amsat.org on behalf of diehl.mike.a at gmail.com> wrote:

    It’s a simple high pass audio filter, don’t overthink it. Surely the tolerance values of the components are going to far outweigh any contributions to the circuit from PCB tracings. 
    
    73,
    Mike Diehl
    W8LID
    
    > On Jul 19, 2018, at 07:19, Samudra Haque [TTLLC] <sehaque at tekterrain.com> wrote:
    > 
    > I'm currently wondering if there is a guideline in general to select filter components so that they are realistic / common part values and still get the performance desired? The calculations might result in exact values for C, R, and might not include accommodation for the C* and R* of the PCB traces, or of the associated wiring - what are practical steps to allow for real world filter construction.
    > 
    > Also - suddenly I am wondering if I shouldn't be looking for a vendor who supplies/manufacturers pre-designed filter blocks for inclusion in my projects. Mouser?
    > 
    > Samudra N3RDX.
    > 
    > -----Original Message-----
    > From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Zach Metzinger
    > Sent: Wednesday, July 18, 2018 2:55 PM
    > To: amsat-bb at amsat.org
    > Subject: Re: [amsat-bb] 67 Hz filter
    > 
    >> On 07/18/18 13:43, Ashhar Farhan wrote:
    >> I am brewing a satellite receiver. The 67 Hz CTCSS tone is quite annoying.
    >> Is there anyway to easily filter it out?
    > 
    > How about a simple high-pass filter?
    > 
    > https://www.electronics-tutorials.ws/filter/filter_3.html
    > 
    > --- Zach
    > N0ZGO
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