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'[EE]: Difference between LM565 and 4046?'
2001\04\18@124307 by Tim Hamel

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Howdy,

While browsing around for an SCA decoder, I came upon one that uses an LM565
PLL. So....I happily went over to Radio Shack hoping they might have it; fat
chance! I found another PLL, 74HCT4046. I'm wondering if I couldn't
"retro-fit" this 4046 to the SCA circuit which requires an LM565. Herein lies
the question, are there any major differences between these two chips? I have
Forrest Mims' "Circuit Scrapbook" which has a section on the 4046, so I
imagine I could try my hardest to redesign the circuit to accomodate a 4046.

Thanks in advance,

Tim Hamel

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2001\04\18@125542 by David VanHorn

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At 12:41 PM 4/18/01 -0400, Tim Hamel wrote:
>Howdy,
>
>While browsing around for an SCA decoder, I came upon one that uses an LM565
>PLL. So....I happily went over to Radio Shack hoping they might have it; fat
>chance! I found another PLL, 74HCT4046. I'm wondering if I couldn't
>"retro-fit" this 4046 to the SCA circuit which requires an LM565. Herein lies
>the question, are there any major differences between these two chips? I have
>Forrest Mims' "Circuit Scrapbook" which has a section on the 4046, so I
>imagine I could try my hardest to redesign the circuit to accomodate a 4046.

Close enough..
They are both PLL chips that are workable in that range.
Your first goal is to get the pll to lock on to a tone at the subcarrier
freq, then to keep it locked when the tone is modulated. This has to do
with the loop filter.


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2001\04\18@141254 by Tim Hamel

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David VanHorn wrote:

> Close enough..
> They are both PLL chips that are workable in that range.
> Your first goal is to get the pll to lock on to a tone at the subcarrier
> freq, then to keep it locked when the tone is modulated. This has to do
> with the loop filter.
>

Eek! I hope my understanding isn't flawed. I thought decoding SCA was a
matter of high-pass filtering the signal at the discriminator in an FM radio,
running it into the PLL (which is tuned for 67kHz or 92kHz). At the PLL, the
error voltage is sent back to the phase-comparator; this is where I'd tap the
audio. To avoid confusion, here is the circuit of interest:
http://www.fineware-swl.com/images/SCA.gif

Is my understanding correct, or do I need to do more studying?


Thanks,

Tim Hamel

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2001\04\18@142609 by David VanHorn

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At 02:10 PM 4/18/01 -0400, Tim Hamel wrote:
>David VanHorn wrote:
>
> > Close enough..
> > They are both PLL chips that are workable in that range.
> > Your first goal is to get the pll to lock on to a tone at the subcarrier
> > freq, then to keep it locked when the tone is modulated. This has to do
> > with the loop filter.
> >
>
>Eek! I hope my understanding isn't flawed. I thought decoding SCA was a
>matter of high-pass filtering the signal at the discriminator in an FM radio,
>running it into the PLL (which is tuned for 67kHz or 92kHz). At the PLL, the
>error voltage is sent back to the phase-comparator; this is where I'd tap the
>audio. To avoid confusion, here is the circuit of interest:
>http://www.fineware-swl.com/images/SCA.gif
>
>Is my understanding correct, or do I need to do more studying?

That's correct.
The PLL may or may not need that prefiltering. That's a function of it's
lock-in range.

Limit cases: A completely stable oscillator, would generate a large error
signal, but getting it in tune has all the conventional problems.

A very easy to slew oscillator would track the subcarrier modulation, but
generate a very low error signal.

The National Semi website has a bunch of tools for PLL design, most if not
all are free.

You'll want to take your signal from the discriminator output, as this
signal is almost always filtered out of the audio path, since it's not
audio.  Nothing like trying to receive a signal that isn't there! :)
The four stages of filtering can probably be reduced to one, or none, with
adequate PLL design.
Just don't give the VCO enough range to follow the audio, or even other
subcarriers.
Each SC is only modulated by a small amount, so that's all the range you need.

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2001\04\20@024730 by Peter L. Peres

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I've never played with SCA but I've used the 4046 several times with
success for modem applications from 300 Bauds to shop music (carrier
current FM on mains). A CMOS or HCMOS data sheet book (perhaps from
Phlips) will have the relevant equations for the loop setup. If you don't
find them I can mail you the ones for CMOS 4046. I suspect that some
constants are different for HCMOS.

hope this helps,

Peter

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2001\04\20@031310 by Spehro Pefhany

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At 11:56 PM 4/19/01 +0300, you wrote:
>I've never played with SCA but I've used the 4046 several times with
>success for modem applications from 300 Bauds to shop music (carrier
>current FM on mains). A CMOS or HCMOS data sheet book (perhaps from
>Phlips) will have the relevant equations for the loop setup. If you don't
>find them I can mail you the ones for CMOS 4046. I suspect that some
>constants are different for HCMOS.

IIRC, these vary significantly by manufacturer (resistor values etc.),
so be sure to check the relevant data sheets.

Best regards,
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2001\04\20@114546 by Tim Hamel

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In a message dated 4/19/01 11:48:52 PM Pacific Daylight Time,
.....plpKILLspamspam.....ACTCOM.CO.IL writes:


> I've never played with SCA but I've used the 4046 several times with
> success for modem applications from 300 Bauds to shop music (carrier
> current FM on mains). A CMOS or HCMOS data sheet book (perhaps from
> Phlips) will have the relevant equations for the loop setup. If you don't
> find them I can mail you the ones for CMOS 4046. I suspect that some
> constants are different for HCMOS.
>

In  the datasheet I have, the VCO freq. formula is fmax=1/R1(C1 + 32pF),
fmin=1/R1(C1 + 32pF).

Only thing I can't figure out is....should C1/32pF be converted to Farads?

For the loop filter, they give two options. The formula for one is fairly
easy, but the other is a bit over my head. I was recently told that the 4046
wouldn't work for SCA because it's digital while the LM565 is analog?

Thanks,

Tim Hamel


> hope this helps,
>
> Peter
>

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2001\04\21@045359 by Peter L. Peres

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The 4046 has two phase comparators and one of them will work ok (phase
comp. #2). Actually #1 may give lower distortion but it may jump frequency
easily. C1 should be in uF with R in KOhms and F in kHz ? I hate mixed
unit formulas. You want to calculate the loop for dF +/- 10kHz (I think ?
check numbers for SCA). Then use a frequency defining cap calculated using
a resistor on pin12 (neglect the loop and pll output).

You may want to use a 220pF frequency set cap (pins 6,7), a 1M pot from
pin12 to gnd to set center frequency (tune) and 62K + 1n8 loop filter
components between pin13 pin9 and gnd. Audio out is pin10 using pulldown
47K or 62K, about 100mV pk-pk with some carrier residue. If the distorsion
becomes unacceptable (over 5%) then add a 470K resistor between pin13 and
GND.

Peter

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