Wood and Douglas SR500 Manual de usuario
SR500
RECEIVER
TECHNICAL MANUAL
(1891 1233)
©Copyright Wood & Douglas Ltd 1999
version: 1.0
issue: March 1999
LATTICE HOUSE
BAUGHURST ROAD
TADLEY
HAMPSHIRE
UK RG26 5LP
Tel: +44 (0)118 981 1444 Fax:+44 (0)118 981 1567
1.1/June 1999 1891 1233
Contents
Section 1
An overview .................................................... 5
Section 2
Operation ...................................................... 7
2.1 INSTALLATION ............................................... 7
2.2 CONNECTION
........................................................ 7
Section 3
Technical description ............................................ 9
3.1 POWER SUPPLY ............................................. 9
3.2 RF FRONT END & MIXER ....................................... 9
3.3 VCO & SYNTHESISER ......................................... 9
3.4 IF & AF STAGES .......................................... 10
Section 4
Frequency Programming ......................................... 11
4.1 INTRODUCTION ............................................. 11
4.2 RUNNING THE SOFTWARE .................................... 12
4.3 SERIAL CHANNEL SELECTION ................................. 13
4.4 PARALLEL CHANNEL SELECTION .............................. 13
4.5 PROGRAMMING RANDOM CHANNELS .......................... 13
4.6 PROGRAMMING SEQUENTIAL CHANNELS ....................... 14
4.7 PROGRAMMING FROM CUSTOMER EQUIPMENT ................. 14
Section 5
Technical specifications ......................................... 15
DRAWINGS
WD1660/XX1 CIRCUIT DIAGRAM FOR SR500
WD1660/XX2 COMPONENT OVERLAY FOR ST500 (Sheet 1 of 2)
WD1660/XX2 COMPONENT OVERLAY FOR ST500 (Sheet 2 of 2)
C WD1660 00 ITEM LIST FOR LEADED COMPONENTS FOR SR500
A SM1660 00 ITEM LIST FOR SURFACE MOUNT COMPONENTS FOR SR500
1891 1233 1.1/June 1999
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1.1/June 1999 1891 1233
Section 1
An overview
Wood & Douglas are an independent UK company dedicated to the design and manufacture
of high quality RF designs for the telemetry, security, broadcast and video markets world-
wide.
The information given in this manual is Copyright Wood & Douglas and is provided to the
original purchaser for the purposes of operation and routine maintenance. The contents of
this manual must not be disclosed to any third party, nor may the purchaser make additional
copiesofthemanualorstoretheinformationgivenonanyelectronicretrievalormassstorage
system.
The company reserves the right to amend or change specifications of its products without
prior notice in accordance with the company policy of continued product improvement.
The SR500 is a UHF Narrow band FM receiver intended for a variety of OEM applications.
It combines high performance with relatively low cost and small size and is intended for PCB
mounting within customer equipment.
NOTE:
This device complies with Part 15 of the FCC Rules.
Operation is subject to the condition that this device does not cause harmful
interference.
The unit is labelled with a serial number label as shown below:
ST500 31 980912345
FCC ID ABCSR500 CECE
where: 990912345 is the serial number of the unit containing the date code for
September 1998
ABC is the Wood & Douglas Ltd FCC code
The approval of the SR500 will still require that the system containing it will also require
approval in it’s own right.
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NOTES: Pin spacing on all connectors is 2.54mm (0.1")
All dimensions in millimetres
Section 2
Operation
2.1 INSTALLATION
2.2 CONNECTION
Connection to the SR500 receiver is via PL1, PL2 and PL3 which plug directly into the
user's own equipment. The location of these connectors is shown in the figure above and
detailed in the following tables.
PIN NAME FUNCTION REMARKS
10V 0 Volts Not Connected
on early units.
2RF I/P RF Input 50 ohms
3OV 0 Volts common ground
Connector PL1 pin detail
PIN NAME FUNCTION REMARKS
1+VIN positive
supply +6.0V to +15.0V
20V 0 volts common ground
Connector PL2 pin detail
PIN NAME FUNCTION REMARKS
1RSSI S meter output 0.5 - 2V signal strength indicator, 60dB range
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2SQF Squelch flag open collector output, ON (low) = no signal
(SR500C: ON = signal received)
3AUDIO AF output 250mV p-p baseband audio, no internal blocking
capacitor
(SR500C: 1.5V p-p audio, no audio muting)
4DATA Data output open collector, no pull-up
5+5V +5 volt supply
output 50mA maximum current drain
60V 0 volts common ground
7RS232
I/P serial
programming
input
if not used, leave not connected or ground
8RB1 parallel
frequency
select internal pull-up to +5V, active low
9RB2
10 RB3
Connector PL3 pin detail
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Section 3
Technical description
Refer to the circuit diagram WD1660 at the rear of this manual.
3.1 POWER SUPPLY
DC power is connected to the transmitter module via PL2. It is fed directly to regulator IC1.
This provides a stable output of nominally +5V which is used to power all the remaining
sections of the receiver either directly or via R-C decoupling networks. The 5V output from
IC1 is also available at PL3 for the powering of external equipment with a current
requirement of up to 50mA.
3.2 RF FRONT END & MIXER
The antenna is connected via PL3. The signal passes directly into helical filter F2 which is
tuned for the required frequency range. The use of a filter prior to the RF amplifier
provides a high level of immunity from overloading by strong out-of-band signals. The
signal then passes to a low noise RF amplifier, a bipolar design based around TR3. This
circuit is run at relatively high current in order to minimise the possibility of overloading by
strong signals, but to reduce overall current consumption it is “voltage stacked” with the
synthesiser and VCO in a current sharing arrangement. Thus only ~2V of the 5V supply is
dropped across the front end device. After the RF amplifier, the signal passes through a
second tuned helical filter F4. Along with F2, this provides sufficient selectivity to give
image rejection in excess of 70dB.
The signal then passes to TR1, a dual gate MOSFET which acts as the first mixer.
3.3 VCO & SYNTHESISER
A WD1647 VCO module (U1) is used to generate the first LO. This module is pre-tested
prior to assembly if the SR500 and provides an output of ~1mW at a frequency 45MHz
lower than the wanted frequency. The VCO is locked onto frequency by a PLL frequency
synthesiser based around IC4. X2, a 12.8 MHz TCXO oscillator provides a frequency
reference for the synthesiser. This is stable to better than 2.5ppm over the operating
temperature range.
The synthesiser is serially programmed by IC6, a PIC microcontroller. D2/C24 are used to
provide an out of lock indication from the synthesiser’s LD line. This is input to IC6 to
allow synthesiser reprogramming when an out of lock condition is detected. This has
been included to protect against malfunction of the unit due to corruption of the data held
in the synthesiser (for example during a power supply “brown-out”). The microcontroller
includes an internal EEPROM memory which is used to store operating frequency
information.
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3.4 IF & AF STAGES
From the first mixer TR1, the signal passes directly into F1, a 2 pole 45MHz crystal filter.
The output of this filter is buffered by TR2, then passed into a second 45MHz crystal filter
F3. The filter configuration used depends upon the channel spacing specified. For 25Khz
spacing both F1 & F3 are 15KHz wide parts, whereas for 12.5KHz spacing F1 is 15KHz
wide and F3 is 7.5Khz wide. This provides a good compromise between adjacent channel
rejection and filter loss.
IC3 is a complete NBFM IF chip. It takes an input at 45MHz which is mixed down on-chip
to 455KHz. The 2nd LO of 44.545MHz is provided by a 3rd overtone crystal X1, using IC3's
on-chip oscillator transistor. The 455KHz signal is filtered by ceramic filter F5. For 25KHz
radios this filter is 15KHz wide, with a narrower (8KHz) filter used for 12.5KHz units.
The 455KHZ signal is then limited and demodulated on-chip. An adjustable quadrature
coil is used (L3). The resulting audio output is low pass filtered by R32/C36 to remove any
high frequency components (including the 455KHz IF) before being buffered by IC5b and
output via PL3.
IC3 provides an RSSI output which is brought out of the SR500 via PL3 without any extra
buffering. This provides and output between ~0.5 and 2V for a signal level range of
~60dB. The SR500 also includes squelch circuitry to provide audio muting under no-
signal conditions and to drive an open collector “squelch flag” which is also brought out on
PL3 for customer use. The squelch circuitry is noise-derived as is normal with NBFM
equipment and the level at which it operates is controlled by RV1. Squelch triggering
circuitry is provided within IC3, but the actual muting of receive audio is done off-chip by
TR6, which effectively shorts out the audio input to IC5 when the squelch is closed.
IC5a acts as a data extractor, limiting the input audio such that simple FSK data may be
recovered from it. The output from IC5a drives into TR8 to provide an open collector
output on PL3. This can then be externally pulled up to a suitable logic rail to allow direct
interfacing to digital equipment.
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