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MAX7401 AF filter harmonics to RF?

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neazoi

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I use the max7401 as an AF LPF in a small transceiver.
I use the internal oscillator option with the single capacitor connected to GND (about 300KHz internal osc).
I notice that some noises signals-like appear at the band of interest in the receiver.
Can it be that the internal oscillator of the max7401 causes harmonics that are received in the RF input of the receiver?
 

The oscillator for a switched-capacitor filter is squarewave with many high frequency harmonics. Long wires or close together parts spray those harmonics onto the radio circuitry.
If the radio has an AM demodulator then interference is guaranteed. Separate the filter oscillator from the radio circuit, add shielding on both of them and make a solid ground.
 

The oscillator for a switched-capacitor filter is squarewave with many high frequency harmonics. Long wires or close together parts spray those harmonics onto the radio circuitry.
If the radio has an AM demodulator then interference is guaranteed. Separate the filter oscillator from the radio circuit, add shielding on both of them and make a solid ground.

Even worse! the radio has a product detector on HF!
I guess isolating it will be a nightmare.
So shall I better abandon the idea and use a normal filter instead?
 

Why are you removing normal audio frequencies with an 8th order filter? Voices will only be mumbling vowels with no important consonants. Music will be extremely muffled.
 

Why are you removing normal audio frequencies with an 8th order filter? Voices will only be mumbling vowels with no important consonants. Music will be extremely muffled.
It is an HF transceiver, Audio bandwidth should be limited to 3KHz at max. The main reason is to cut off the high frequency noise whereas provide no significant attenuation to <3KHz. The chip does that but the HF noise it generates makes it unusable I think.
 

Many people normally hear to 20kHz, you are cutting everything above 3kHz. You are removing more than half of the octaves of normal audio frequencies used for speech. Important consonant sounds such as "s, ch, f, sh, th" and many more will be missing. A conversation will be full of "what did you say?". A muffled AM radio and an old telephone did not cut high audio frequencies as steep as 8th order.
 

Many people normally hear to 20kHz, you are cutting everything above 3kHz. You are removing more than half of the octaves of normal audio frequencies used for speech. Important consonant sounds such as "s, ch, f, sh, th" and many more will be missing. A conversation will be full of "what did you say?". A muffled AM radio and an old telephone did not cut high audio frequencies as steep as 8th order.

HF HAM radio regulations state that the max bandwidth for SSB shall be no more than 3KHz or so. This means that the audio that modulates the modulator shall be limited to 3KHz. The received signals contain no more than 3KHz, so there is no need for the receiver to receive all this extra higher noise (actually it is undesirable).
These are narrow band signals not intended for music, but just for communications purposes.
The better you cut off these higher frequencies the less noise in the receiver you have and the less interference from other stations.
In superhets, this is done in the IF but in direct conversion this is done in the AF range.

Oh... and I barely hear to 16KHz, tested. Do you really hear up to 20KHz?
 

I am an old geezer with normal hearing for my age (73), so I got hearing aids that let me hear like when I was young, when I clearly heard ultrasonic motion detectors that were on all the time "because nobody could hear them". I heard the 19kHz pilot on stereo FM and I heard the 15kHz from TV horizontal sweep oscillators.

Today it was very windy and the leaves on the trees were dry and making a "sssss" sound. I had my hearing aids set on the "music" mode where they play normal sounds. I switched to "automatic" mode then the background noise slowly dropped away but high frequencies from speech could still be heard. They use smart new noise reduction not the stupid and old cutting of all high frequencies.

Ot an oo ear ot eeol ay (what can you hear what people say) when consonants are cut off? Do you say Charlie instead of the letter C and say Foxtrot instead of the letter F like old communications and aviation people?
 

I am an old geezer with normal hearing for my age (73), so I got hearing aids that let me hear like when I was young, when I clearly heard ultrasonic motion detectors that were on all the time "because nobody could hear them". I heard the 19kHz pilot on stereo FM and I heard the 15kHz from TV horizontal sweep oscillators.

Today it was very windy and the leaves on the trees were dry and making a "sssss" sound. I had my hearing aids set on the "music" mode where they play normal sounds. I switched to "automatic" mode then the background noise slowly dropped away but high frequencies from speech could still be heard. They use smart new noise reduction not the stupid and old cutting of all high frequencies.

Ot an oo ear ot eeol ay (what can you hear what people say) when consonants are cut off? Do you say Charlie instead of the letter C and say Foxtrot instead of the letter F like old communications and aviation people?

You were lucky to be able to hear up to 19KHz! I remember I could hear the CRT TV sweep oscillators as well.
 

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