MC13030 데이터 시트보기 (PDF) - Motorola => Freescale

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MC13030

Dual Conversion AM Receiver

Motorola => Freescale 

Pin No.

26

26

27

27

MC13030

PIN FUNCTION DESCRIPTION (continued)

Internal Equivalent Circuit

Description

VCC

75

Vref

VCO Reference

The first oscillator coil is connected from Pin 26 to 27. Pin 26 must be

bypassed to ground with a capacitor which has a low impedance at the

oscillator frequency. This capacitor also will reduce the phase noise of

the VCO.

VCO

to Mixer1 The VCO is a negative resistance type and has an internal level control

circuit so a tapped coil or one with a secondary is not needed. The level

7.5 k

is fixed at 0.8 Vpp so the oscillator signal does not modulate the tuning

diode, thus keeping the distortion low. The oscillator stray capacitance is

≈ 12 pF and the tuned circuit impedance should be greater than 3.0 k to

guarantee oscillation. Oscillator range is up to 45 MHz so it can be used

for SW receivers.

28

100

28

VCO Out

The output level is 240 mVrms (108 dBµ), high enough to drive any

CMOS synthesizer.

AM CAR RADIO DESIGN NOTES

The MC13030 AM Radio IC is intended for dual

conversion AM radios. In most cases, the 1st IF frequency

(FIF1) is upconverted above the highest input frequency. The

first oscillator (VCO) is tuned by a synthesizer and operates

at Fin + FIF1. For the 530 to 1700 kHz AM band with a

10.7 MHz first IF, the VCO goes from 11.23 to 12.40 MHz.

Therefore, Fmax/Fmin for VCO is only 1.104, so one low–cost

tuning diode can be used. Since the required tuning voltage

range can be made less than 5.0 V, it may also be possible to

drive the tuning diode directly or from the phase detector of

the synthesizer IC, such as the Motorola MC145170,

operating from 5.0 V, without using a buffer amplifier or

transistor.

If the VCO is above the incoming frequency, the image

frequency of the first mixer is at fOSC + FIF1. For the AM

broadcast receiver, it is around 22 MHz, so a simple LPF

can be used between the RF stage and Mixer1 input.

However, if a LPF is used, an additional coil is still needed

to supply the collector voltage of the RF amplifier. For this

reason, a BPF filter was used in the application circuit

instead, since it uses the same number of coils and gives

better performance. It is simply a lowpass to bandpass

conversion. The lowpass filter is designed to have a cutoff

frequency equal to the desired bandwidth. In this case, it

would be 1700 – 530 kHz = 1170 kHz. Then, it is

p transformed to be resonant at 949 kHz, the geometric

mean of the end frequencies: 1700 x 530 = 949 kHz.

A balanced–to–unbalanced transformer is required at the

output of both mixers. The first one is designed so that Mixer1

has enough gain to overcome the loss of the 10.7 MHz filter

and so that the output of the mixer will not overload before the

input. The primary impedance of the transformer is relatively

low, and it may be difficult to control with commonly available

7.0 mm transformers because the number of primary turns is

quite small. It would also require a large tuning capacitance.

A better solution is to tune the secondary with a small

capacitance and then use a capacitive divider to match the

tuned circuit to the filter. This allows one transformer to be

used for either a ceramic or crystal filter. The capacitors can

be adjusted to match the filter. The recommended coil is

made this way.

If the formula: Pin = IP3 – DR/2 is used, the maximum input

level to the mixer can be calculated for a desired dynamic

range.

IP3 = 3rd order intercept level in dB (dBm or dBµ)

DR = dynamic range in dB between the desired signals

and 3rd order intermodulation products

Pin = input level in dBm or dBµ

The RF AGC level can then be adjusted so that Pin does

not exceed this level.

Whether or not a narrow bandwidth crystal or wide

bandwidth ceramic filter is used between the first and second

mixers depends on the receiver requirements. It is possible to

achieve about 50 dB adjacent channel and IM rejection with

a ceramic filter because of the wide dynamic range of the

mixers. If more than this is required, a crystal filter should be

used. If a crystal filter is used, a lower cost CFU type of

455 kHz second IF filter can be used. If a ceramic filter is

used, a CFW type filter should be used because there is no

RF section selectivity in this type of radio.

Since the wideband AGC system is quite sensitive, it can

be set to eliminate all spurious responses present at the

receiver output. However, the RF AGC will sometimes

eliminate or reduce the level of desired signals if there is a

strong signal somewhere in the bandpass of the RF circuit.

The second mixer is designed like the first and requires a

balanced output. Since its load impedance is higher, the

transformer can be designed to be tuned on the primary or

MOTOROLA ANALOG IC DEVICE DATA

9

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