# Question on a modem receiver using MC13135

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#### nickagian

##### Member
Hallo to everybody! I would like some help with the following schematic of a 56k modem receiver, which uses the MC13135 chip. Unfortunately I do not know a lot about these kind of circuits and I was given this one, to analyze its operation. I have studied the Motorola datasheet of the chip -and that of the MC145162, which is also used- but I have still 3 basic questions (perhaps a little bit stupid too )

1)what is the purpose of the filter (?) consisting of L11, R37 and C32?
2)why is the frequency of the IF filter, connected to the 2nd MIXER output, at 448kHz? Is there any special reason for this specific frequency?
3)what is the PLL MC145162 exactly used for? Couldnt we just use a simple circuit whith a crystal for the LOs?

Anybody who knows something of the above, please give me a hint! Thanks...

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#### JimB

##### Super Moderator
1 These components are for the "quadrature decoder" to demodulate the FM signal.

2 448kHz? beats me. I would have expected 455khz (10.7Mhz first IF - 10.245Mhz for the 2nd LO).

3 The PLL allows the receiver to be switched to different frequencies, without the expense of a crystal for each frequency.

JimB

#### nickagian

##### Member
I have one more-last question!! What is the transistor MPF102 used for? Is it just an amplifier? Is that true? (Whenever these Ls are used, I am really confused!! :? )

#### Nigel Goodwin

##### Super Moderator
nickagian said:
I have one more-last question!! What is the transistor MPF102 used for? Is it just an amplifier? Is that true? (Whenever these Ls are used, I am really confused!! :? )

Yes, it's an amplifier, but more importantly though it's a buffer.

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