Rainbow Electronics MAX7030 - Manual (Page 13)

MAX7030
Low-Cost, 315MHz, 345MHz, and 433.92MHz
ASK Transceiver with Fractional-N PLL
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AGC Dwell-Time Settings
The AGC dwell timer holds the AGC in low-gain state
for a set amount of time after the power level drops
below the AGC switching threshold. After that set
amount of time, if the power level is still below the AGC
threshold, the LNA goes into high-gain state. This is
important for ASK since the modulated data may have
a high level above the threshold and low level below
the threshold, which without the dwell timer would
cause the AGC to switch on every bit.
The MAX7030 uses the three AGC control pins (AGC0,
AGC1, AGC2) to set seven user-controlled, dwell-timer
settings. The AGC dwell time is dependent on the crys-
tal frequency and the bit settings of the AGC control
pins. To calculate the dwell time, use the following
equation:
where K is an odd integer in decimal from 11 to 23, deter-
mined by the control pin settings shown in
Table 1.
To calculate the value of K, use the following equation
and use the next integer higher than the calculated
result:
K ≥ 3.3 x log
10 (Dwell Time x f XTAL )
For Manchester Code (50% duty cycle), set the dwell
time to at least twice the bit period. For nonreturn-to-
zero (NRZ) data, set the dwell to greater than the peri-
od of the longest string of zeros or ones. For example,
using Manchester Code at 315MHz (f
XTAL
=
12.679MHz) with a data rate of 2kbps (bit period =
250µs), the dwell time needs to be greater than 500µs:
K ≥ 3.3 x log
10
(500µs x 12.679) ≈ 12.546
Choose the AGC pin settings for K to be the next odd-
integer value higher than 12.546, which is 13. This says
that AGC1 is set high and AGC0 and AGC2 are set low.
Mixer
A unique feature of the MAX7030 is the integrated
image rejection of the mixer. This eliminates the need
for a costly front-end SAW filter for many applications.
The advantage of not using a SAW filter is increased
sensitivity, simplified antenna matching, less board
space, and lower cost.
The mixer cell is a pair of double-balanced mixers that
perform an IQ downconversion of the RF input to the
10.7MHz intermediate frequency (IF) with low-side
injection (i.e., f
LO
= f
RF
- f
IF
). The image-rejection circuit
then combines these signals to achieve a typical 46dB
of image rejection over the full temperature range. Low-
side injection is required as high-side injection is not
possible due to the on-chip image rejection. The IF out-
put is driven by a source follower, biased to create a
driving impedance of 330
Ω to interface with an off-chip
330 Ω ceramic IF filter. The voltage-conversion gain dri-
ving a 330 Ω load is approximately 20dB. Note that the
MIXIN+ and MIXIN- inputs are functionally identical.
Integer-N Phase-Locked Loop (PLL)
The MAX7030 utilizes a fixed-integer-N PLL to generate
the receive LO. All PLL components, including the loop
filter, voltage-controlled oscillator, charge pump, asyn-
chronous 24x divider, and phase-frequency detector
are integrated internally. The loop bandwidth is approx-
imately 500kHz. The relationship between RF, IF, and
reference frequencies is given by:
f REF = (f RF - f IF ) / 24
Dwell Time
f
K
XTAL
=
2
AGC2 AGC1 AGC0 DESCRIPTION
0 0 0 AGC disabled, high gain selected
0 0 1 K = 11
0 1 0 K = 13
0 1 1 K = 15
1 0 0 K = 17
1 0 1 K = 19
1 1 0 K = 21
1 1 1 K = 23
Table 1. AGC Dwell Time Settings for
MAX7030