Utah Amateur Radio Club
Farnsworth <-> Scott's Hill
Synchronous Repeater Project

Description of block/schematic diagrams

Drawing Reference:

farnsys1.gif    - System Diagram, Farnsworth Peak
scotsys1.gif    - System Diagram, Scott's Hill
rxdc1.gif       - Receive Downconverter
rxif1.gif       - Receive IF processor
txif1.gif       - Transmit IF processor
txuc1.gif       - Transmit Upconverter
clkrec1.gif     - Pilot Carrier Clock Recovery

System Diagrams for Farnsworth Peak and  Scott's Hill:

Farnsworth Peak System Block Diagram

Farnsworth Peak, where the current 146.62 repeater is, will be the hub/master site for the synchronous repeater system.  It is at Farnsworth that the master clock reference, modulator, and controller is located.  All other site(s) are slaved ultimately from Farnsworth.

Receive subsystem:  This is located remotely from the transmitter site, some 400 feet or so down the ridge to the north.  It is fed exclusively by transformer-coupled lines signal and power that provide isolation and protection from lightning-induced and other circulating currents.  Its remote location removes it from the majority of the background site noise that would otherwise reduce the effectiveness of the receive system.

The existing receive antenna, bandpass cavity, GaAsFET Preamplifier, and crystal bandpass filter will continue to be used in the system.  Note that the drawing shows that there is a 2-way splitter following the GaAsFET preamp:  The other output of the splitter feeds the 145.73 MHz receiver that is used in conjunction with the ATV repeater's receive subsystem.  Because it follows the preamp, it should have no effect on the effective sensitivity of the 146.02 receiver.

Following the splitter is the  RX Downconverter and 1st LO which bandpasses the receive signal to eliminate the image and mixes it down to 10.7 MHz.  This also contains the 1st LO which is phase-locked to the system reference, located at the transmitter site.  The output of this goes to the  Receive IF Processor which provides the bulk of the system bandpass filtering (using monolithic crystal filters) and performs limiting and conversion down to 40 KHz.  It should be noted that RSSI (Receive Signal Strength Indicator) voltages and demodulated audio are available at this point for monitoring.  The 10.66 MHz 2nd LO is in this unit, and it is locked to the system reference as well.

The  Lock Unit and Pilot Recovery module locks both the 1st and 2nd LOs from the RX downconverter and Receive IF Processor to the system reference.  It uses a 100 KHz signal sent down from the transmit site via twisted pair.

Finally, there are a number of transformer/receiver/drivers for the twisted pair lines that go between the receive and transmit sites on Farnsworth.  These are, by necessity, transformer-isolated and surge-protected.  The downconverted 40 KHz IF is sent up to the transmit site via one pair, and a 100 KHz reference from the transmit site site is sent down via another.  Additional control/status signals may be sent up to the transmit site as needed.

Transmit Subsystem:  The 40 KHz IF signal from the receive site is passed through a transformer/receiver/driver and then on to the  RX Demod and Squelch Unit.  The signals from both Scott's and Farnsworth are demodulated in this unit and it is here that squelch/quieting information is derived from the signals.  This information is used by the  TX Modulator and Controller to select which site (if any) has the best received signal, to key the transmitter, and other functions.  The best audio is then modulated onto a 40 KHz carrier and sent jointly to Scott's Hill and to the local  TX IF processor for upconversion.

The  TX IF processor filters the 40 KHz signal, converts it up to 10.7 MHz, and limits and filters it.  The 10.66 MHz LO in this unit is phase-locked to the  system reference which is located at this site.  The 10.7 MHz output is then passed to the  TX Upconverter and LO where it is mixed with the transmit LO, which is also locked to the  system reference, and converted to the final transmit frequency.  A 1 watt driver amplifier (which is keyed by the  TX Modulator and Controller) is used to drive the existing Vocom 100 watt amplifier.  The amplifier output is passed through the existing isolator, cavities, and onto the transmit antenna.

The  Lock Unit and Pilot Generator at Farnsworth provides the master system frequency reference to which all oscillators are referenced.  It also is used to lock both transmit LOs in the transmitter chain.  It produces a 100 KHz pilot signal which is sent over the 33cm microwave transmitter, along with the 40 KHz transmit IF, to Scott's Hill.  The 33cm transmitter output passes through an isolator, bandpass cavity, and then through a notch-duplexor before it is sent to the antenna.  The 33cm receive signal comes through the duplexor, a bandpass cavity, through a GaAsFET preamplifier, and then into the receiver.  The output of the receiver, which contains the 40 KHz receive IF from Scott's Hill, is sent to the  RX Demod and Squelch Unit for processing.
 

Scott's Hill System Block Diagram

The Scott's Hill site is slaved by the Farnsworth Peak site.  It receives control signals, frequency reference, and transmit IF signals from Farnsworth.  It is also capable of operating as a minimally-functioning standalone repeater in the event two separate repeaters are needed for some event or if the Farnsworth Peak master goes off the air and some repeater function is desired from this site.

Scott's Hill operates very similarly to Farnsworth Peak.  The signals from the receive antenna pass through a pass/notch cavity network and onto a well-filtered and protected GaAsFET preamplifier.  Like the Farnsworth Peak Site, it then goes to the  RX Downconverter and 1st LO unit followed by the Receive IF Processor.  Like Farnsworth, receive audio and RSSI information is available at this point.  It is this information that the local TX Modulator and Controller can use to provide the standalone repeater functions.  The 40 KHz signal from the Receive IF Processor is sent via microwave to Farnsworth Peak for processing.

The 40 KHz transmit IF signal from Farnsworth is received via the microwave receiver and sent to the  TX IF Processor where it is limited, filtered, and upconverted to 10.7 MHz.  It is then passed to the TX Upconverter and LO where it is mixed up to the transmit frequency and filtered.  The output of this goes to a driver amplifier, which is keyed, and onto a power amplifier.  The output of the transmit PA goes through an isolator, bandpass/notch cavities, and on to the transmit antenna.

Like Farnsworth, all LOs are referenced to a single source.  This arrives from Farnsworth as a 100 KHz subcarrier on the microwave receiver's baseband.  The Lock Unit and Pilot Recovery module reconstructs the pilot signal and locks all LOs to it.  It also provides the (selectable) 10-20 Hz frequency offset of the transmitter's frequency (compared to Farnsworth.)
 

Descriptions of various modules:

RX Downconverter and 1st LO:

The diagram shows first a 2-stage GaAsFET preamplifier.  This is required only at Scott's, since there is an existing preamp at Farnsworth.

The input signal passes through a 3-stage bandpass filter (for preselection and image rejection) and to a GaAsFET amplifier which is integrated into the filter.  A GaAsFET amplifier has much better strong-signal handling characteristics than either a FET or Bipolar amplifier.   Following the amplifier is another bandpass filter which eliminates image noise from the preamplifier and provides further image rejection.  This is followed by a high-level diode-ring mixer (DBM) which is capable of handling +13 dbm at its 1db compression point.  Following this is a diplexer (which provides proper termination of the mixer to reduce IMD) and a post-mixer amplifier.  At this point there is a broadly-filtered low-level 10.7 MHz IF signal to be sent to the next module.

This module also contains the first LO which operates at 135.22 MHz.  This is derived by an X9 multiplication from a 15.03555 MHz VCXO.  A sample of this VCXO from the first X3 stage is sent to the  Lock Unit for frequency control.

RX IF Processor:

The 10.7 MHz signal from the RX Downcoverter is first attenuated (to provide a consistent source impedance) and then matched with a 10.7 MHz IF transformer to the crystal impedance/reactance.  Two 2-pole monolithic filters are cascaded and the signal is sent to an NE-602.  This is a complete receiver-on-a-chip, but it is being used somewhat differently than normal.  The 10.7 MHz IF is sent directly into the IF amplifier chain instead of the mixer.  The output of the IF amplifier is filtered with a single 2-pole monolithic filter and sent to the limiter.  The output of the limiter is sent to two places:  Some of the signal is sent to the quadrature detector so that audio may be demodulated (for local test audio and/or so that Scott's Hill may function as a standalone repeater) and the rest of the signal is buffered and filtered by another 2-pole monolithic filter (to remove the spectral regrowth that resulted from limiting.)  RSSI (Received Signal Strength Indicator) voltage is available from this chip.  This output's current is proportional to the log of the incoming signal and it can cover up to 90db with good accuracy.

The again-filtered 10.7 signal is sent into the mixer section of the NE605 and it is downconverted to 40 KHz, filtered, and made available to the microwave transmitter (on Scott's Hill) or twisted-pair driver (on Farnsworth.)  The LO portion of the NE605 is configured as a VCXO and it is controlled by the Lock Unit.

RX Demodulator and Squelch Unit:

The RX Demod and Squelch unit  is to be found only at Farnsworth Peak.  This module consists of two identical demodulators that work as follows:

The 40 KHz input (via twisted pair the Farnsworth Receive site and Microwave from Scott's Hill) is lightly filtered at 40 KHz and then upconverted to 10.7 MHz using the mixer section of the NE605.  The LO is "borrowed" from the already phase-locked 10.66 MHz LO from the TX IF Processor.  The upconverted signal is then filtered with a single 10.7 MHz monolithic filter (which should be all that is necessary) and amplified, filtered by a 10.7 MHz ceramic filter (to get rid of any broadband noise that might be generated should the IF amplifier be driven into limiting) and then to the quadrature detector for audio demodulation.

The audio outputs of the identical demodulators are used to obtain squelch/quieting information.  A look at the  Demodulator and Modulator Diagram shows that the controller switches the ultrasonic squelch circuit between the two demodulators to determine which one has the best quieting and then selects the one with the best signal (if any) as the audio source.

TX IF Processor:

This unit is to be found only at Farnsworth peak.

The 40 KHz signal from the transmitter modulator is lightly bandpass filtered (to reduce harmonics) and passed through a quadrature network.  This network has two outputs which are 90 degrees in phase apart from each other.  These outputs are mixed (each with its own mixer) with a 10.660 MHz LO signal, also in quadrature.  The outputs of these mixers are summed to yield only the LO + IF signal at 10.7 MHz.  This is done to reduce the presence of the otherwise-present LO - IF signal that would appear at 10.62 MHz.

The signal is then buffered and filtered with two 2-pole monolithic filters to further reduce the image.  The signal is then limited with a CA3089 (to remove any AM components and eliminate any transmit power variations due to baseband injection levels) and then filtered with two more 2-pole monolithic filters.  The resulting 10.7 MHz signal is then made available to the TX Upconverter and LO.

This unit also contains a 10.66 MHz VCXO.  This is multiplied by 4 and then divided to digitally produce the quadrature LO signals.  A sample of this signal is sent to the Lock Unit for frequency/phase control.  In the case of Farnsworth, this LO is also used on the RX Demodulator and Squelch Unit to convert the 40 KHz receive IF to 10.7 MHz.  In the case of Scott's Hill, this signal is compared with the 10.66 MHz receive LO to provide the optional 10-20 Hz transmit frequency offset.

TX Upconverter and LO:

The 10.7 MHz IF signal from the TX IF processor is mixed with the 135.92 MHz transmit LO (which is phase-locked to the reference,) diplexed/filtered, and amplified.  It goes through several more stages of amplification and filtering to remove the conversion image and to bring it up to a power level of approximately +15 dbm.

In the case of both Scott's and Farnsworth, this module is followed by a low-power amplifier that is keyed by the controller and is used to drive the PA.

System Reference (Including Lock Unit and Pilot Recovery):

There are several parts under this heading.  The main frequency references for both sites are based on 20 MHz oscillators.  The one of Farnsworth is the master frequency reference for the system and the one on Scott's Hill is a VCXO that is locked to the Farnsworth reference via a 100Khz pilot signal that comes over the microwave.

The Pilot Recovery Circuit Diagram shows how the 100 Khz reference is extracted from the pilot signal that, in the case of Scott's Hill, comes over the microwave and, in the case of Farnsworth, gets to the receive site via twisted-pair.

The input signal is amplified/buffered and then broadly filtered at 100 KHz.  This signal is then applied to a high-gain JFET amplifier with a 100 KHz crystal in its drain lead.  On the other side of the crystal, there is a small trimmer capacitor (for peaking) and another JFET amplifier which serves as an amplifier/limiter.  In this amplifier there is also a 100 KHz crystal in the drain lead.  Again, this is followed by a JFET amplifier.  At this point, there is a constant, clean 100 KHz signal available for the phase/frequency detector.

The 20 MHz VCXO is used to provide several signals:  The 20 MHz itself is used in the downconversion of the 135 MHz local oscillators in the transmit and receive converters.  A 10 KHz signal is used for locking the 10.66 MHz transmit and receive IF processor LOs and the 40 KHz transmit modulator(s.)  A 13.333 KHz signal is used as the reference for the 135 MHz local oscillators.  Finally, a 100 Khz signal is used for comparison with the recovered 100 KHz signal.

In the event of signal loss, the PLL will lost lock and the VCXO can be switched to a "standalone" mode.  In this mode, a voltage is available that sets the VCXO to the correct frequency.  This lock signal is also used by the controller as in indication that something might be wrong.

TX Modulator and Controller:

This is another non-specific heading.  On the Farnsworth Site, it consists of the 40 KHz system modulator (parts of which may be seen on the Demodulator and Modulator Diagram.) This is the main modulator that supplies the carrier for all transmitters in the system.  On the Scott's Hill site, this unit exists soley so that the repeater is capable of operating in a standalone mode.

This heading also includes the site controller.  In the case of Farnsworth, it is the main controller that keeps track of everything that the system is doing and the status(es) of the various modules, as well as what is going on on Scott's Hill.  On Scott's Hill, the controller accepts commands from Farnsworth (keying, IDing, etc.) and allows it to operate as a standalone repeater as necessary.

END
***