At 06:31 AM 04/03/07, you wrote: >Hi, > >Could anyone point me in the direction of a simple circuit which will >facilitate the muting or suppression of a 1750 hz tone on the received >audio. > >Thank you for reading....regards, Doug - GM7SVK
How accurate are the encoders in the users radios? Some active filters are very narrow, and an inaccurate encoder will fall outside the notch of the filter. How long is the tone burst? I'm asking because the old tone burst repeaters used 30 years ago here in the colonies operated in one of two modes: "Tone Burst" mode required a tone burst at the leading edge of each transmission... the burst decoder keyed the repeater transmitter, which stayed up until the user unkeyed. "Whistle up" mode (the tern came because users could "whistle up" the repeater) was a modification on the above where the repeater is disabled until the tone burst comes in, then it's in carrier squelch mode until it's been idle for a specific time period (like three minutes). Generally "whistle up" repeaters used wider decoders to allow the users that were too cheap to build an encoder easier access. If you are using "whistle up" mode, one solution might be to not key up the repeater transmitter (or pass audio to it) until the tone burst ends. Then there is no tone to notch... Or have the decoder mute the audio. Then all you will hear is a "chirp". If you do need a notch, here's one idea: One of the large repeater networks here in the states uses a 1064Hz tone for the identification of the point-to-point links as that frequency is right in the middle of the low and high group of DTMF tones, and hence has a minimal impact on any downstream DTMF decoders (i.e. if someone is punching buttons right when the controller decides to ID the DTMF decoder won't see the 1064Hz ID). The most common controller in that system has 7 radio ports and each link port uses a "Single Frequency Tuneable Bandpass/Notch Filter/Generator" S3526B chip made by AMI. The notch frequency is set by a crystal oscillator at 1376 times the audio frequency that you want notched (there's a reason for the strange multiplier - there was a big market for 2600hz notch filters and tone generators in the telephony marketplace, and 3.58Mhz is a very common crystal frequency in the USA. The chip was designed so that 3.58MHz notches (or generates) 2600hz. The filter board used in the controller used one DIP package oscillator to drive all 6 filter chips. In your case a 2.408Mhz source would result in a notch at 1750Hz. Here's the data sheet: <http://www.tranzistoare.ro/datasheets/700/387803_DS.pdf'> This chip has the characteristic found in most very narrow notch filter designs - if you make the notch too narrow it "rings".... In this case it just enough that when it's removed/bypassed you can hear the difference. Extreme purists will want to use something else. Mike WA6ILQ
