Motorola Astro Digitalport Saber Detailed 68p81076c10 A Manual

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Notes 
						

							5-1
VHF/UHF Transceiver Board 
Detailed Theory of Operation
5
IntroductionThis section of the manual provides a detailed circuit description of an 
ASTRO Digital SABER VHF and UHF Transceiver Board. When reading 
the theory of operation, refer to your appropriate schematic and 
component location diagrams located in the back section of this 
manual. This detailed theory of operation will help isolate the 
problem to a particular component. However, first use the ASTRO 
Digital SABER Portable Radios Basic Service Manual to troubleshoot 
the problem to a particular board.
Frequency 
Generation Unit 
(FGU)The frequency generation unit (FGU) consists of three major sections: 
the high stability reference oscillator (U203), the fractional-N 
synthesizer (U204,) and the VCO buffer (U201). A 5V regulator (U202), 
supplies power to the FGU. The synthesizer receives the 5V REG at 
U204, and applies it to a filtering circuit within the module and 
capacitor C253. The well-filtered 5-volt output at U204 pin 19 is 
distributed to the TX and RX VCOs and the VCO buffer IC. The mixer 
LO injection signal and transmit frequency are generated by the RX 
VCO and TX VCO respectively. The RX VCO uses an external active 
device (Q202), whereas the VHF TX VCO active device is a transistor 
inside the VCO buffer. The UHF TX VCO uses two active devices, one 
external (Q203) and the other internal to the VCO buffer. The base and 
emitter connections of this internal transistor are pins 11 and 12 of 
U201.
The RX VCO is a Colpitts-type oscillator, with capacitors C235 and 
C236 providing feedback. The RX VCO transistor (Q202) is turned on 
when pin 38 of U204 switches from high to low. The RX VCO signal is 
received by the VCO buffer at U201 pin 9, where it is amplified by a 
buffer inside the IC. The amplified signal at pin 2 is routed through a 
low-pass filter (L201 and associated capacitors) and injected as the first 
LO signal into the mixer (U2 pin 8). In the VCO buffer, the RX VCO 
signal (or the TX VCO signal during transmit) is also routed to an 
internal prescaler buffer. The buffered output at U201 pin 16 is applied 
to a low-pass filter (L205 and associated capacitors). After filtering, the 
signal is routed to a prescaler divider in the synthesizer at U204 pin 21.
The divide ratios for the prescaler circuits are determined from 
information stored in a codeplug, which is part of the microcontrol 
unit (U204 on the VOCON board). The microprocessor extracts data 
for the division ratio as determined by the position of the channel-
select switch (S902), and busses the signal to a comparator in the 
synthesizer. A 16.8MHz reference oscillator, U203, applies the 
16.8MHz signal to the synthesizer at U204 pin 14. The oscillator signal  
						

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is divided into one of three pre-determined frequencies. A time-based 
algorithm is used to generate the fractional-N ratio.
If the two frequencies in the synthesizer’s comparator differ, a control 
(error) voltage is produced. The phase detector error voltage (V 
control) at pin 31 and 33 of U204, is applied to the loop filter 
consisting of resistors R211, R212, and R213, and capacitors C244, 
C246, C247 and C275. The filtered voltage alters the VCO frequency 
until the correct frequency is synthesized. The phase detector gain is 
set by components connected to U204 pins 28 and 29.
In the TX mode, U204 pin 38 goes high and U201 pin 14 goes low, 
which turns off transistor Q202 and turns on the internal TX VCO 
transistor in U204 and the external TX VCO buffer Q203 on the UHF 
circuit. The TX VCO feedback capacitors are C219 and C220. Varactor 
diode CR203 sets the TX frequency while varactor CR202 is the TX 
modulation varactor. The modulation of the carrier is achieved by 
using a 2-port modulation technique. The modulation of low 
frequency tones such as DPL/TPL is achieved by injecting the tones 
into the A/D section of the fractional-N synthesizer. The digitized 
signal is modulated by the fractional-N divider, generating the 
required deviation. Modulation of the high frequency audio signals is 
achieved by modulating the varactor (CR203) through a frequency 
compensation network. Resistors R207 and R208 form a potential 
divider for the higher frequency audio signals. 
 In order to cover the very wide bandwidths, positive and negative V-
control voltages are used. High control voltages are achieved using 
positive and negative multipliers. The positive voltage multiplier 
circuit consists of components CR204, C256, C257 and reservoir 
capacitor C258.The negative multiplier circuit consists of components 
CR205, CR206, C266, C267 and reservoir capacitor C254 in VHF and 
UHF radios. Out-of-phase clocks for the positive multiplier appear at 
U204 pins 9 and 10. Out-of-phase clocks for the negative multiplier 
appear at U204 pins 7 and 8, and only when the negative V-control is 
required (i.e., when the VCO frequency exceeds the crossover 
frequency). When the negative V-control is not required, transistor 
Q201 is turned on, and capacitor C259 discharges. The 13V supply 
generated by the positive multiplier is used to power-up the phase 
detector circuitry. The negative V-control is applied to the anodes of 
the VCO varactors. 
The TX VCO signal is amplified by an internal buffer in U201, routed 
through a low pass filter and routed to the TX PA module, U105 pin 1. 
The TX and RX VCOs and buffers are activated via a control signal 
from U204 pin 38. 
The reference oscillator supplies a 16.8MHz clock to the synthesizer 
where it is divided down to a 2.1MHz clock. This divided-down clock 
is fed to the ABACUS IC (U401), where it is further processed for 
internal use.  
						

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Antenna SwitchTwo antenna switches are part of the radio circuitry. One of the 
switches which is located in the radio casting is mechanical. It 
switches between the radio antenna and a remote antenna. Switching 
is accomplished by a plunger located on the accessory connector. With 
a remote antenna installed, continuity between the radio antenna and 
the RF input line is broken; continuity is made from the remote 
antenna to the radio RF line. The second switch is a current device. It 
is a pair of diodes (CR108/CR109) that electronically steer RF between 
the receiver and the transmitter. In the transmit mode, RF is routed 
through transmit switching diode CR108, and sent to the antenna. In 
the receive mode, RF is received from the antenna, routed through 
receive switching diode CR109, and applied to the RF amplifier, U1 
(UHF), Q1 (VHF). In transmit, bias current, sourced from U101 pin 21, 
is routed through L105, U104, CR108, and L122 in VHF and L105, 
CR108, and L122 in UHF. Sinking of the bias current is through the 
transmit ALC module, U101 pin 19. In the receive mode, bias current, 
sourced from SB+, is routed through Q107 (pin 3 to pin 2), L123 (UHF), 
L121, CR109, and L122. Sinking of the bias current is through the 5-
volt regulator, U106 pin 8.
Receiver Front EndThe RF signal is received by the antenna and coupled through the 
external RF switch. The UHF board applies the RF signal to a low-pass 
filter comprised of: L126, L127, L128, C149, C150, and C151. The VHF 
board bypasses the lowpass filter. The filtered RF signal is passed 
through the antenna switch (CR109) and applied to a bandpass filter 
comprised of: VHF; L11 through L14, CR1 through CR9, C4, C2, and 
C3, or UHF; L30, L31, L32, L34, L35, CR6 through CR9, C1, C2, and 
C3. The bandpass filter is tuned by applying a control voltage to the 
varactor diodes in the filter. (CR1-CR9 in VHF and CR6-CR9 in UHF.)
The bandpass filter is electronically tuned by the D/A IC (U102) which 
is controlled by the microcomputer.The D/A output range is extended 
through the use of a current mirror, transistor Q108 and associated 
resistors R115 and R116. When Q108 is turned on via R115, the D/A 
output is reduced due to the voltage drop across R116. Depending on 
the carrier frequency the microcomputer will turn on or off Q108. 
Wideband operation of the filter is achieved by retuning the bandpass 
filter across the band. 
The output of the bandpass filter is applied to a wideband GaAs RF 
amplifier IC, U1 (RF AMP) on the UHF transceiver board. The VHF 
board uses an active device for RF amplification (Q1). After being 
amplified by the RF AMP, the RF signal is further filtered by a 
second broad-band, fixed-tuned, bandpass filter consisting of C6, C7, 
C8, C80, C86, C87, C88, C97, C99, L3, L4, L5, and L30 (VHF); or C4 
through C7, C88 through C94, C99, and L11 through L15 (UHF) to 
improve the spurious rejection.
Via a broadband 50-ohm transformer, T1, the filtered RF signal is 
routed to the input of a broadband mixer/buffer (U2). Mixer U2 uses 
GaAs FETs, in a double-balanced Gilbert Cell configuration. The RF 
signal is applied to the mixer at U2 pins 1 and 15. An injection signal 
(1st LO) of about -10dBm, supplied by the FGU, is applied to U2 pin 8. 
Mixing of the RF and the 1st LO results in an output signal which is  
						

							5-4
the first IF frequency. The first IF frequency of VHF and UHF bands are 
45.15MHz and 73.35MHz respectively. The 1st LO signal for VHF is 
45.15MHz higher than the carrier frequency while that for the UHF is 
73.35MHz lower than the carrier frequency. The 1st IF signal output, 
at U2 pins 4 and 6, is routed through transformer T2 and impedance 
matching components, and applied to a 2-pole crystal filter (FL1), 
which is the final stage of the receiver front end. The 2-pole crystal 
filter removes unwanted mixer products. Impedance matching 
between the output of the transformer (T2) and the input of the filter 
(FL1) is accomplished by capacitor C605 and inductor L605 (VHF); or 
C611, C614 and L605 (UHF).
Receiver Back EndThe output of crystal filter FL1 is matched to the input of IF buffer 
amplifier transistor Q601 by components C610 and L604 (VHF) and 
C609, C610, and L600 (UHF). Transistor Q601 is biased by the 5V 
regulator (U202). The IF frequency on the collector of Q601 is applied 
to a second crystal filter through a matching circuit. The second crystal 
filter (FL2) input is matched by C604, C603, and L601 (VHF); or C604, 
L601, and L602 (UHF). The filter supplies further attenuation at the IF 
sidebands to increase the radios selectivity. The output of FL2 routed 
to pin 32 of U401 through a matching circuit which consists of L603, 
L606, and C608 (VHF); or L603, C606, and C605 (UHF).
In the ABACUS IC, (U401) the first IF frequency is amplified and then 
down converted to 450kHz, the second IF frequency. At this point, the 
analog signal is converted into two digital bit streams via a sigma-delta 
A/D converter. The bit streams are then digitally filtered and mixed 
down to baseband and filtered again. The differential output data 
stream is then sent to the ADSIC (U406) on the VOCON board where 
it is decoded to produce the recovered audio.
The ABACUS IC (U401) is electronically programmable, and the 
amount of filtering which is dependent on the radio channel spacing 
and signal type is controlled by the microcomputer. Additional 
filtering, which used to be provided externally by a conventional 
ceramic filter, is replaced by internal digital filters in the ABACUS IC. 
The ABACUS IC contains a feedback AGC circuit to expand the 
dynamic range of the sigma-delta converter. The differential output 
data contains the quadrature (I and Q) information in 16-bit words, 
the AGC information in a 9-bit word, imbedded word sync 
information and fill bits dependent on sampling speed. A fractional-n 
synthesizer is also incorporated on the ABACUS IC for 2nd LO 
generation.
The 2nd LO/VCO is a Colpitts oscillator built around transistor Q401 
(VHF) and Q1 (UHF). The VCO has a varactor diode, VR401 (VHF) and 
CR5 (UHF), to adjust the VCO frequency. The control signal for the 
varactor is derived from a loop filter consisting of C426, C428, and 
R413.  
						

							5-5
TransmitterThe transmitter consists of three major sections:
•Harmonic Filter
•RF Power Amplifier Module
•ALC Circuits
Harmonic FilterRF from the power amplifier (PA) module (U105) is routed through the 
coupler (U104), passed through the transmit antenna switch (CR108), 
and applied to a harmonic filtering network in UHF. In the case of a 
VHF transceiver board, RF from the PA module (U105) is routed also 
through the coupler (U104), then through the harmonic filtering 
network, and on to the antenna switch (CR108). The harmonic 
filtering circuit is comprised of the following components: L126, L127, 
L128, C149, C150, and C151 (for VHF models); or L126, L127, L128,  
C149, C150, and C151 (for UHF models). Resistor R128 (UHF) or R117 
(VHF) provides a current limited 5V to J2 for mobile ASTRO vehicular 
adapter (AVA) applications.
RF Power Amplifier 
ModuleThe RF power amplifier module (U105) is a wide-band multi-stage 
amplifier (3 stages for the VHF models and 4 stages for the UHF 
models). Nominal input and output impedance of U105 is 50 ohms. 
The dc bias for U105 is on pins 2, 4, 5. In the transmit mode, the 
voltage on U105 pins 2 and 4 (close to the B+ level) is obtained via 
switching transistor Q101. Transistor Q101 receives its control base 
signal as follows:
•the microcomputer keys the D/A IC to produce a ready signal at 
U102 pin 3
•the ready signal at U102 pin 3 is applied to the TX ALC IC at U101 
pin 14 (5V)
•the synthesizer sends a LOC signal to the TX ALC IC (U204 pin 40 
to U101 pin 16)
When the LOC signal and the ready signal are both received, the TX 
ALC IC (pin 13) sends a control signal to turn on transistor Q101.
ALC CircuitsCoupler module U104 samples the forward power and the reverse 
power of the PA output voltage. Reverse power is present when there 
is other than 50 ohms impedance at the antenna port. Sampling is 
achieved by coupling some of the forward and/or reverse power, and 
apply it to CR102(VHF) or CR101(UHF) and CR103 for rectification 
and summing. The resultant dc signal is then applied to the TX ALC 
IC (U101 pin 2) as RFDET to be used as an RF strength indicator. 
The transmit ALC circuit, built around U101, is the heart of the power 
control loop. Circuits in the TX ALC module compare the signals at 
U101 pins 2 and 7. The resultant signal, C BIAS, at U101 pin 4 is 
applied to the base of transistor Q110. In response to the base drive, 
transistor Q110 varies the dc control voltages applied to the RF PA at 
U105 pin 3, thus controlling the RF power of module (U105).
Thermistor RT101 senses the temperature of the TX ALC IC. If an 
abnormal operating condition exists, which causes the PA slab 
temperature to rise to an unacceptable level, the thermistor forces the 
ALC to reduce the set power. 
						

							5-6
Notes 
						

							6-1
800MHz Transceiver Board 
Detailed Theory of Operation
6
IntroductionThis section of the manual provides a detailed circuit description of an 
ASTRO Digital SABER 800MHz Transceiver Board. When reading the 
theory of operation, refer to your appropriate schematic and 
component location diagrams located in the back section of this 
manual. This detailed theory of operation will help isolate the 
problem to a particular component. However, first use the ASTRO 
Digital SABER Portable Radios Basic Service Manual to troubleshooting 
the problem to a particular board.
Frequency Synthesis The complete synthesizer subsystem consists of the reference oscillator 
(U304), the voltage controlled oscillator (VCO), U307, a buffer IC 
(U303), and the synthesizer (U302).
The reference oscillator contains a temperature-compensated 
16.8MHz crystal. This oscillator is digitally tuned and contains a 
temperature-referenced 5-bit analog-to-digital (A/D) converter. The 
output of the oscillator (pin 10 on U304) is applied to pin 14 (XTAL1) 
on U302 via capacitor C309 and resistor R306.
Module U307 is the voltage controlled oscillator, which is varactor 
tuned. That is, as the voltage (2-11V) being applied to pins 1 and 7 of 
the VCO varies, so does the varactor’s capacitance, thereby changing 
the VCO’s output frequency. The 800MHz VCO is a dual-range 
oscillator that covers the 806-825MHz and the 851-870MHz frequency 
bands. The low-band VCO (777-825MHz) provides the first LO 
injection frequencies (777-797MHz) that will be 73.35MHz below the 
carrier frequency. In addition, when the radio is operated through a 
repeater, the low-band VCO will generate the transmit frequencies 
(806-825MHz) that will be 45MHz below the receiver frequencies. The 
low-band VCO is selected by pulling pin 3 high and pin 8 low on 
U307. When radio-to-radio or talk-around operation is necessary, the 
high band VCO (851-870MHz) is selected. This is accomplished by 
pulling pin 3 low and pin 8 high on U307.
The buffer IC (U303) includes a TX, RX, and prescaler buffer whose 
main purpose is to individually maintain a constant output and 
provide isolation.The TX buffer is chosen by setting pin 7 of U303 
high; the RX buffer is chosen by setting pin 7 of U303 low. The 
prescaler buffer will always be on. In order to select the proper 
combination of VCO and buffer, the following conditions must be true 
at pin 6 of U303 (or pin 38 of U302) and pin 7 of U303 (or pin 39 of 
U302). For the first LO injection frequencies 777-797MHz, pins 6 and 
7 must both be low; for the TX repeater frequencies 806-825MHz pins  
						

							6-2
6 and 7 must both be high. For talkaround TX frequencies 851-
870MHz, pin 6 must be low while pin 7 must be high.
The synthesizer IC (U303) consists of a prescaler, a programmable loop 
divider, a divider control logic, a phase detector, a charge pump, an 
A/D converter for low-frequency digital modulation, a balance 
attenuator to balance the high frequency analog modulation to the 
low frequency digital modulation, a 13V positive-voltage multiplier, a 
serial interface for control, and finally a filter for the regulated five 
volts. This filtered five volts is present at pin 19 of U302, pin 9 of U307, 
and pins 2, 3, 4, and 15 of U303. It is also applied directly to resistors 
R309, R315, and R311. Additionally, the 13V, being generated by the 
positive voltage multiplier circuitry, should be present at pin 35 of 
U302. The serial interface (SRL) is connected to the microprocessor via 
the data line (pin 2 of U302), clock line (pin 3 of U302), and chip 
enable line (pin 4 of U302). 
The complete synthesizer subsystem works as follows. The output of 
the VCO, pin 4 on U307, is fed into the RF input port (pin 9) of U303. 
In the TX mode, the RF signal will be present at pin 4 of U303. On the 
other hand, in the RX mode, the RF signal will be present at pin 3 of 
U303. The output of the prescaler buffer, pin 15 on U303, is applied to 
the PREIN port (pin 21) of U302. The prescaler in U302 is a dual-
modulus type with selectable divider ratios. This divider ratio is 
controlled by the loop divider, which in turn receives its inputs via the 
SRL. The loop divider adds or subtracts phase to the prescaler divider 
by changing the divide ratio via the modulus control line. The output 
of the prescaler is then applied to the loop divider. The output of the 
loop divider is then applied to the phase detector. The phase detector 
will then compare the loop divider’s output signal with the signal from 
U304 (that is divided down after it is applied to pin 14 of U302). The 
result of the signal comparison is a pulsed dc signal which is applied 
to the charge pump. The charge pump outputs a current that will be 
present at pin 32 of U302. The loop filter (which consists of capacitors 
C322, C317, C318, C329, C324, and C315, and resistors R307, R305, 
and R314) will transform this current into a voltage that will be 
applied to pins 1 and 7 of U307, and alter the VCO’s output frequency.
In order to modulate the PLL, the two-port modulation method is 
utilized. The analog modulating signal is applied to the A/D converter 
as well as the balance attenuator, via U302 pin 5. The A/D converter 
converts the low-frequency analog modulating signal into a digital 
code that is applied to the loop divider, thereby causing the carrier to 
deviate. The balance attenuator is used to adjust the VCO’s deviation 
sensitivity to high-frequency modulating signals.
Antenna SwitchSwitching between the standard and external antenna ports is 
accomplished with the external mechanical switch which is actuated 
by a plunger located on the accessory connector.
An electronic PIN diode switch steers RF between the receiver and 
transmitter. The common node of the switch is at capacitor C101. In 
the transmit mode, RF is routed to the anode of diode CR104. In 
receive mode, RF is routed to pin 1 of U201. In transmit, bias current 
sourced from U504 pin 21, is routed through PIN diodes CR104 and  
						

							6-3
CR102, biasing them to a low-impedance state. Bias current returns to 
ground through U504 pin 20. In receive, U504 pin 21 is pulled down 
to ground and pin 20 is pulled up to B+, reverse biasing diodes CR104 
and CR102 to a high impedance.
Receiver Front EndFor the purposes of this discussion, the receiver front end is defined to 
be the circuitry from the antenna switch to the output of the IF crystal 
filter. The 800MHz front end is designed to convert the received RF 
signal to the 1st IF frequency of 73.35MHz, while at the same time 
providing for spurious immunity and adjacent channel selectivity. A 
review of the interstage components of the front end will now be 
presented with emphasis on troubleshooting considerations.
The received RF signal is passed through the antenna switch input 
matching components C101, L105, and C114 tank components C106 
and L103 (which are anti-resonant at the radios transmitter 
frequencies), and output matching components C103 and L104. Both 
pin diodes CR102 and CR104 must be back biased to properly route 
the received signal. 
The stage following the antenna switch is a 50-ohm, inter-digitated, 3-
pole, stripline preselector (U201). The preselector is positioned after 
the antenna switch to provide the receiver preamp some protection to 
strong signal, out-of-band signals.
After the preselector (U201), the received signal is processed through 
the receiver preamp, U202. The preamp is a dual-gate GaAs MESFET 
transistor which has been internally biased for optimum IM, NF, and 
gain performance. Components L201 and L202 match the input (gate 
1) of the amp to the first preselector, while at the same time 
connecting gate 1 to ground potential. The output (drain) of the amp 
is pin 3 and is matched to the subsequent receiver stage via 
components L204, C205 and C222. A supply voltage of 5Vdc is 
provided to pin 3 via an RF choke L203 and bypass C204. The 5 volt 
supply is also present at pin 4 which connects to a voltage divider 
network that biases gate 2 (pin 5) to a predefined quiescent voltage of 
1.2Vdc. Resistor R202 and capacitor C203 are connected to pin 5 to 
provide amp stability. The FET source (pin 7) is internally biased at 
0.55 to 0.7Vdc for proper operation with bypass capacitors C201 and 
C202 connected to the same node.
The output of the amp is matched to a second 3-pole preselector 
(U203) of the type previously discussed. The subsequent stage in the 
receiver chain is the 1st mixer U205, which uses low-side injection to 
convert the RF carrier to an intermediate frequency (IF) of 73.35MHz. 
Since low-side injection is used, the LO frequency is offset below the 
RF carrier by 73.35MHz, or Flo = Frf - 73.35MHz. The mixer utilizes 
GaAs FETs in a double balanced Gilbert Cell configuration. The LO 
port (pin 8) incorporates an internal buffer and a phase shift network 
to eliminate the need for a LO transformer. The LO buffer bypass 
capacitors (C208, C221, and C216) are connected to pin 10 of U205, 
and should exhibit a nominal dc voltage of 1.2 to 1.4Vdc. Pin 11 of 
U205 is LO buffer Vdd (5Vdc) with associated bypass capacitors C226 
and C209 connected to the same node. An internal voltage divider 
network within the LO buffer is bypassed to virtual ground at pin 12