MC13156
CIRCUIT DESCRIPTION
General
The MC13156 is a low power single conversion wideband
amplitude. The RSSI current output is derived by summing
the currents from the IF and limiting amplifier stages. An
external resistor at Pin 20 sets the voltage range or swing of
the RSSI output voltage. Linearity of the RSSI is optimized by
using external ceramic or crystal bandpass filters which have
an insertion loss of 8.0 dB. The RSSI circuit is designed to
provide 70+ dB of dynamic range with temperature
compensation (see Figures 6 and 7 which show RSSI
responses of the IF and Limiter amplifiers). Variation in the
RSSI output current with supply voltage is small (see
Figure 11).
FM receiver incorporating a split IF. This device is designated
for use as the backend in digital FM systems such as CT–2
and wideband data links with data rates up to 500 kbaud. It
contains a mixer, oscillator, signal strength meter drive, IF
amplifier, limiting IF, quadrature detector and a data slicer
with a hold function (refer to Figure 8, Simplified Internal
Circuit Schematic).
Current Regulation
Temperature compensating voltage independent current
regulators are used throughout.
Carrier Detect
When the meter current flowing through the meter load
resistance reaches 1.2 Vdc above ground, the comparator
flips, causing the carrier detect output to go high. Hysteresis
can be accomplished by adding a very large resistor for
positive feedback between the output and the input of the
comparator.
Mixer
The mixer is a double–balanced four quadrant multiplier
and is designed to work up to 500 MHz. It can be used in
differential or in single–ended mode by connecting the other
input to the positive supply rail.
Figure 4 shows the mixer gain and saturated output
response as a function of input signal drive. The circuit used
to measure this is shown in Figure 1. The linear gain of the
mixer is approximately 22 dB. Figure 9 shows the mixer gain
versus the IF output frequency with the local oscillator of
150 MHz at 100 mVrms LO drive level. The RF frequency is
swept. The sensitivity of the IF output of the mixer is shown in
Figure 10 for an RF input drive of 10 mVrms at 140 MHz and
IF at 10 MHz.
IF Amplifier
The first IF amplifier section is composed of three
differential stages with the second and third stages
contributing to the RSSI. This section has internal dc
feedback and external input decoupling for improved
symmetry and stability. The total gain of the IF amplifier block
is approximately 39 dB at 10.7 MHz. Figure 5 shows the gain
and saturated output response of the IF amplifier over
temperature, while Figure 12 shows the IF amplifier gain as a
function of the IF frequency.
The fixed internal input impedance is 1.4 kΩ. It is designed
for applications where a 455 kHz ceramic filter is used and no
external output matching is necessary since the filter requires
a 1.4 kΩ source and load impedance.
The single–ended parallel equivalent input impedance of
the mixer is Rp ~ 1.0 kΩ and Cp ~ 4.0 pF (see Table 1 for
details). The buffered output of the mixer is internally loaded
resulting in an output impedance of 330 Ω.
Local Oscillator
The on–chip transistor operates with crystal and LC
resonant elements up to 220 MHz. Series resonant, overtone
crystals are used to achieve excellent local oscillator stability.
3rd overtone crystals are used through about 65 to 70 MHz.
Operation from 70 MHz up to 180 MHz is feasible using the
on–chip transistor with a 5th or 7th overtone crystal. To
enhance operation using an overtone crystal, the internal
transistor’s bias is increased by adding an external resistor
For 10.7 MHz ceramic filter applications, an external
430 Ω resistor must be added in parallel to provide the
equivalent load impedance of 330 Ω that is required by the
filter; however, no external matching is necessary at the input
since the mixer output matches the 330 Ω source impedance
of the filter. For 455 kHz applications, an external 1.1 kΩ
resistor must be added in series with the mixer output to
obtain the required matching impedance of 1.4 kΩ of the filter
input resistance. Overall RSSI linearity is dependent on
having total midband attenuation of 12 dB (6.0 dB insertion
loss plus 6.0 dB impedance matching loss) for the filter. The
output of the IF amplifier is buffered and the impedance is
290 Ω.
from Pin 23 to V . –10 dBm of local oscillator drive is
EE
needed to adequately drive the mixer (Figure 10).
The oscillator configurations specified above, and two
others using an external transistor, are described in the
application section:
1) A 133 MHz oscillator multiplier using a 3rd overtone
1) crystal, and
Limiter
The limiter section is similar to the IF amplifier section
except that four stages are used with the last three
contributing to the RSSI. The fixed internal input impedance
is 1.4 kΩ. The total gain of the limiting amplifier section is
approximately 55 dB. This IF limiting amplifier section
internally drives the quadrature detector section.
2) A 307.8 to 309.3 MHz manually tuned, varactor controlled
2) local oscillator.
RSSI
The Received Signal Strength Indicator (RSSI) output is a
current proportional to the log of the received signal
6
MOTOROLA WIRELESS SEMICONDUCTOR
SOLUTIONS – RF AND IF DEVICE DATA
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