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# MC2833 distorts sound

M

#### [email protected]

Jan 1, 1970
0
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.

Thanks

R

#### RST Engineering $$jw$$

Jan 1, 1970
0
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

M

#### Michael A. Terrell

Jan 1, 1970
0
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.

Thanks

Is it overmodulated?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

M

#### [email protected]

Jan 1, 1970
0
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
Thanks for the reply

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

M

#### [email protected]

Jan 1, 1970
0
Is it overmodulated?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Thanks for the reply

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

M

#### Michael Black

Jan 1, 1970
0
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.

Thanks
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

M

#### Michael Black

Jan 1, 1970
0
Thanks for the reply

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

R

Jan 1, 1970
0

A

#### [email protected]

Jan 1, 1970
0
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.

Thanks

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

R

#### RST Engineering $$jw$$

Jan 1, 1970
0
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.

2. 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.

3. 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.

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

Jim

A

#### [email protected]

Jan 1, 1970
0
Well said Jim, the guy's out to lunch.

M

#### [email protected]

Jan 1, 1970
0
RST said:
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.

2. 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.

3. 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.

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

Jim
Thanks Jim

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

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