MAX7030 查看數據表（PDF） - Maxim Integrated

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MAX7030

Low-Cost, 315MHz, 345MHz, and 433.92MHz ASK Transceiver with Fractional-N PLL

Maxim Integrated 

Low-Cost, 315MHz, 345MHz, and 433.92MHz

ASK Transceiver with Fractional-N PLL

Intermediate Frequency (IF)

The IF section presents a differential 330Ω load to pro-

vide matching for the off-chip ceramic filter. The internal

six AC-coupled limiting amplifiers produce an overall

gain of approximately 65dB, with a bandpass filter type

response centered near the 10.7MHz IF frequency with

a 3dB bandwidth of approximately 10MHz. For ASK

data, the RSSI circuit demodulates the IF to baseband

by producing a DC output proportional to the log of the

IF signal level with a slope of approximately 15mV/dB.

Data Filter

The data filter for the demodulated data is implemented

as a 2nd-order, lowpass, Sallen-Key filter. The pole

locations are set by the combination of two on-chip

resistors and two external capacitors. Adjusting the

value of the external capacitors changes the corner fre-

quency to optimize for different data rates. Set the cor-

ner frequency in kHz to approximately 3 times the

fastest expected Manchester data rate in kbps from the

transmitter (1.5 times the fastest expected NRZ data

rate). Keeping the corner frequency near the data rate

rejects any noise at higher frequencies, resulting in an

increase in receiver sensitivity.

The configuration shown in Figure 1 can create a

Butterworth or Bessel response. The Butterworth filter

offers a very-flat-amplitude response in the passband

and a rolloff rate of 40dB/decade for the two-pole filter.

The Bessel filter has a linear phase response, which

works well for filtering digital data. To calculate the

value of the capacitors, use the following equations,

along with the coefficients in Table 2:

CF1

=

b

a(100kΩ)(π)(fc

)

CF2

=

a

4(100kΩ)(π)(fc )

where fC is the desired 3dB corner frequency.

For example, choose a Butterworth filter response with

a corner frequency of 5kHz:

CF1

=

1.000

(1.414)(100kΩ)(3.14)(5kHz)

≈

450pF

CF2

=

1.414

(4)(100kΩ)(3.14)(5kHz)

≈ 225pF

Choosing standard capacitor values changes CF1 to

470pF and CF2 to 220pF. In the Typical Application Circuit,

CF1 and CF2 are named C16 and C17, respectively.

Data Slicer

The data slicer takes the analog output of the data filter

and converts it to a digital signal. This is achieved by

using a comparator and comparing the analog input to

a threshold voltage. The threshold voltage is set by the

voltage on the DS- pin, which is connected to the nega-

tive input of the data slicer comparator.

Numerous configurations can be used to generate the

data-slicer threshold. For example, the circuit in Figure

2 shows a simple method using only one resistor and

one capacitor. This configuration averages the analog

output of the filter and sets the threshold to approxi-

mately 50% of that amplitude. With this configuration,

the threshold automatically adjusts as the analog signal

varies, minimizing the possibility for errors in the digital

data. The values of R and C affect how fast the thresh-

old tracks the analog amplitude. Be sure to keep the

corner frequency of the RC circuit much lower (about

10 times) than the lowest expected data rate.

With this configuration, a long string of NRZ zeros or

ones can cause the threshold to drift. This configuration

works best if a coding scheme, such as Manchester

coding, which has an equal number of zeros and ones,

is used.

Figure 3 shows a configuration that uses the positive and

negative peak detectors to generate the threshold. This

configuration sets the threshold to the midpoint between

a high output and a low output of the data filter.

MAX7030

RSSI

100kΩ

100kΩ

DS+

OP+

DF

CF2

CF1

Figure 1. Sallen-Key Lowpass Data Filter

Table 2. Coefficients to Calculate CF1

and CF2

FILTER TYPE

Butterworth

(Q = 0.707)

Bessel

(Q = 0.577)

a

1.414

1.3617

b

1.000

0.618

14 ______________________________________________________________________________________

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