Pin Descriptions
Pin
Name
Description
This pin is connected directly to the transceiver AGCCAP pin, which controls the AGC reset operation. To enable
AGC operation (required for ASK transmission) an external capacitor is placed between this pin and ground. The
capacitor sets the minimum time the AGC will hold-in once it is engaged. The hold-in time is set to avoid AGC
chattering. For a given hold-in time tAGH, the capacitor value CAGC is:
CAGC = 19.1* tAGH, where tAGH is in µs and CAGC is in pF
1
AGC/VCC
For 115.2 kbps operation, a 2200 pF 10ꢀ ceramic capacitor should be used at this pin. The value of CAGC given
above provides a hold-in time between tAGH and 2.65* tAGH, depending on operating voltage, temperature, etc. The
hold-in time is chosen to allow the AGC to ride through the longest run of zero bits that can occur in a received
data stream. The AGC hold-in time can be greater than the peak detector decay time, as discussed below. How-
ever, the AGC hold-in time should not be set too long, or the receiver will be slow in returning to full sensitivity
once the AGC is engaged by noise or interference. AGC operation also depends on a functioning peak detector,
as discussed below. The AGC capacitor is discharged in the transceiver power-down (sleep) mode and in the
transmit modes.
This pin is connected directly to the transceiver PKDET pin. This pin controls the peak detector operation. An ex-
ternal capacitor between this pin and ground sets the peak detector attack and decay times, which have a fixed
1:1000 ratio. For 115.2 kbps applications, the attack time constant should be set to 0.24 µs with a 0.001 µF ca-
pacitor to ground. (This adequately matches the peak detector decay time constant of 240 µs to the time constant
of the 0.0027 µF coupling capacitor C3.) A 10ꢀ ceramic capacitor should be used at this pin. The peak detector
is used to drive the “dB-below-peak” data slicer and the AGC release function. The AGC hold-in time can be ex-
tended beyond the peak detector decay time with the AGC capacitor, as discussed above. The peak detector ca-
pacitor is discharged in the transceiver power-down (sleep) mode and in the transmit modes. See the description
of Pin 3 below for further information.
2
PK DET
This pin is connected directly to the transceiver BBOUT pin. On the circuit board, BBOUT also drives the trans-
ceiver CMPIN pin through C3, a 0.0027 µF coupling capacitor (tBBC = 173 µs). RX BBO can also be used to drive
an external data recovery process (DSP, etc.). The nominal output impedance of this pin is 1 K. The RX BBO sig-
nal changes about 10 mV/dB, with a peak-to-peak signal level of up to 675 mV. The signal at RX BBO is riding on
a 1.1 Vdc value that varies somewhat with supply voltage and temperature, so it should be coupled through a ca-
pacitor to an external load. A load impedance of 50 K to 500 K in parallel with no more than 10 pF is recom-
mended. Note the AGC reset function is driven by the signal applied to CMPIN through C3. When the transceiver
is in power-down (sleep) or in a transmit mode, the output impedance of this pin becomes very high, preserving
the charge on the coupling capacitor(s). The value of C3 on the circuit board has been chosen to match typical
data encoding schemes at 115.2 kbps. If C3 is modified to support different data rates and/or encoding schemes,
make the value of the peak detector capacitor about 1/3 the value of C3.
3
4
5
RX BBO
RX DATA
TX IN
RX DATA is connected directly to the transceiver data output pin, RXDATA. This pin will drive a 10 pF, 500 K par-
allel load. The peak current available from this pin increases with the receiver low-pass filter cutoff frequency. In
the power-down (sleep) or transmit modes, this pin becomes high impedance. If required, a 1000 K pull-up or
pull-down resistor can be used to establish a definite logic state when this pin is high impedance (do not connect
the pull-up resistor to a supply voltage higher than 3.5 Vdc or the transceiver will be damaged). This pin must be
buffered to successfully drive low-impedance loads.
The TX IN pin is connected to the transceiver TXMOD pin through a 4.7 K resistor on the circuit board. Additional
series resistance will often be required between the modulation source and the TX IN pin, depending on the de-
sired output power and peak modulation voltage (4.3 K typical for a peak modulation voltage of 3 volts). Saturated
output power requires about 250 µA of drive current. Peak output power PO for a 3 Vdc supply is approximately:
P
O = 19.75*((VTXH – 0.9)/(RM + 4.7))2, where PO is in mW, peak modulation voltage VTXH is in volts and
external modulation resistor RM is in kilohms
This pin must be held low in the receive and sleep modes. Please refer to section 2.9 of the ASH Transceiver De-
signer’s Guide for additional information.
3
RFM [ RF MONOLITHICS, INC ]
