Micrel

MICRF010

June 2005

7

M9999-063005

(408) 955-1690

is desired, use crystals with lower ESR, which normally are

bigger in size, like the HC49 package. “Application Hints

35” provides additional information and recommended

sources for crystals. If using an externally applied signal, it

should be AC-coupled and limited to the operating range of

As with any super-heterodyne receiver, the difference

center frequency. Equation 1 may be used to compute the

315

⎛

⎝

⎜

⎞

⎠

⎟

(1)

mixing” and “low-side mixing.” High-side mixing results in

an image frequency above the frequency of interest and

low-side mixing results in a frequency below. There is

generally no preference of one over the other.

Equation 2 to compute the reference oscillator frequency

64.5

(2)

to REFOSC on the MICRF010. Four-decimal-place

accuracy on the frequency is generally adequate. The

frequencies.

Reference Oscillator

315.0 MHz

9.7941 MHz

390.0 MHz

12.1260 MHz

418.0 MHz

12.9966 MHz

433.92 MHz

13.4916 MHz

Table 1. Recommended Reference Oscillator Values For

Typical Transmit Frequencies (high-side mixing)

Step 2: Selecting C

Extraction of the DC value of the demodulated signal for

purposes of logic-level data slicing is accomplished using

switched capacitor “resistor” RSC, as shown in the block

diagram.

Slicing level time constant values vary somewhat with

decoder type, data pattern, and data rate, but typically

values range from 5ms to 50ms. This issue is covered in

more detail in “Application Note 22.” Optimization of the

The first step in the process is selection of a data-slicing-

level time constant. This selection is strongly dependent

upon system issues including system decode response

time and data code structure (that is, existence of data

preamble, etc.) This issue is also covered in more detail in

“Application Note 22.”

characteristics table as 150kΩ at 315MHz, this value

T

SC

f

9.7941

150

R

Ω

=

(4)

can be calculated using equation (5) with the

τ

(5)

Recommended

τ is 5x the bit-rate.

Step 3: Selecting C

The signal path has AGC (automatic gain control) to

increase input dynamic range. The attack time constant of

the AGC is set externally by the value of the CAGC

capacitor connected to the CAGC pin of the device. To

maximize system range, it is important to keep the AGC

the control voltage attains its quiescent value. For this

reason, capacitor values of at least 0.47µF are

recommended.

The AGC control voltage is carefully managed on-chip to

allow duty-cycle operation of the MICRF010. When the

device is placed into shutdown mode (SHUT pin is pulled

high), the AGC capacitor floats to retain the voltage. When

operation is resumed, only the voltage drop, due to

capacitor leakage, must be replenished. A relatively low-

leakage capacitor such as a ceramic type is recommended