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MC2833 distorts sound

Hi Does anyone know why the FM transmitter IC distorts the sound signal at the output? I am using the MC2833 FM transmitter with a crystal that has a rated primary frequency of 7.3728MHz at 11th harmonic (88.4736 MHz) trying to capture it with an FM radio. The carrier frequency is very strong; however, the sound is distorted. Please post any helpful hints.

Thanks

Reply to
mb12777
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How about the fact that you are 26 kHz. away from the center of the band? That is severely mistuned. I'd expect a great deal of distortion.

Jim

Reply to
RST Engineering (jw)

Is it overmodulated?

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Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

Thanks for the reply

I think the MC2833 has a 400KHz bandwidth...

Reply to
mb12777

Thanks for the reply

Overmodulation is not the case...I'm still trying different things

Reply to
mb12777

First, how do you know it's not the receiver that's causing the distortion?

Second, since you want the signal at 88MHz, you are likely using an FM broadcast band receiver. Those are intended to receive relatively wide deviation signals.

The MC2833 is designed for narrow deviation. Admittedly the multiplication to 88MHz will multiply the deviation, but it still may be a stretch. So the "distortion" may actually be that the deviation is too small for the receiver. Or, in trying to get enough deviation, the deviation of the transmitter is not linear.

Michael

Reply to
Michael Black

Huh?

I don't follow what Jim is saying, but the "bandwidth" of the IC is somewhere past 50MHz.

If by "bandwidth" you mean deviation, 400KHz at 11MHz is outrageously large for a crystal controlled transmitter. The datasheet says 10KHz at 16MHz is the maximum deviation.

Michael

Reply to
Michael Black

Overloaded input?

Reply to
Rev. 11D Meow!

You're not on a FM broadcast frequency. Digitally tuned receivers will not lock onto a random frequency.

Reply to
a7yvm109gf5d1

Excuse me, why would you "think" that? Saying that a device has thus and such a bandwidth is a completely bogus statement; it means nothing besides letting us know that you don't have a clue. Besides, these are the "sci" newsgroups where we try to have a modicum of professionalism. We don't "think", we have datasheets that we know how to read.

  1. You are multiplying a 7.37 MHz. crystal up and using the 11th harmonic, hoping to come up with an 88.5 MHz. signal for an FM receiver to hear. As I said, you are 26 kHz. away from the receiver's center frequency. At this mistune of the receiver, I would expect some of the distortion that you are hearing.

  1. The modulation deviation of this device at 16 MHz is 3, 5, and 10 kHz. (min, typical, max). Since the "variable reactance" device is a varicap in series with an external inductor, I'd expect the maximum deviation of this device to be roughly half of these values at 8 MHz., perhaps a little less at 7.37. Let's say, 1.5, 2.5, and 5 kHz. respectively -- just for discussion.

  2. So typically in a x11 multiplier, you would expect around 28 kHz. maximum deviation for a x11 multiplier using the device you have chosen. But the FM broadcast standard is 75 kHz.. You therefore have a very underdeviated signal if you stay within the limits of the device ou have chosen or a very overmodulated and distorted signal if you try and drive the modulator into saturation.

  1. Short answers ... off channel and overmodulated -- and it ain't gonna get any better with this device.

Jim

Reply to
RST Engineering (jw)

Well said Jim, the guy's out to lunch.

Reply to
a7yvm109gf5d1

Thanks Jim

Very impressive lecture...I'll have a second look at the datasheet.

Reply to
mb12777

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