Note:  Upon reading this page, it is assumed that you have already read The proposed UARC Synchronous and Voting linked repeater system.  That page explains the the basic theory of the proposed system.  This page deals more specifically with the technical details of the proposed system.  Also, see the  predicted signal contours for the transmitters.

Ver 1.06, 990327, KA7OEI

This document describes preliminary block diagrams of the 146.02/62 synchronous/voting repeater system.  This system is to consist of two sites:  The master site is on Farnsworth Peak in the Oquirrh mountains, and the slave site is on Scott's Hill in the Wasatch range, near Guardsman pass.

These two sites are to be linked via a 33cm full-duplex microwave link.  This link will primarily carry a 100 KHz pilot tone used to provide a frequency reference to the slave site, and downconverted versions of the transmit and receive signals both centered at 40 KHz.  This microwave link will also carry telemetry and control for the repeater system.  Additional subcarriers may be implemented on this link in the future.  This link will connect Farnsworth Peak and Scott's Hill and it will operate in the 33cm amateur band

REFERENCE LOCK CIRCUIT:  (Refer to Figure 1)

This circuit locks the 20 MHz master oscillator (a VCXO) at the slave site to the 100 KHz pilot carrier received on the microwave link.

A sample of the baseband is passed through a 100 KHz L/C bandpass filter and amplified/limited and passed to a crystal resonator.  This resonator, a quartz crystal, provides a high-Q element that will "ring" precisely on the pilot frequency and is resistant to noise that might appear under fade conditions.  The output of the crystal is amplified and limited, and fed to a phase-frequency detector.  The recovered 100 KHz signal is compared with a 100 KHz clock derived from the 20 MHz master oscillator, filtered, and used to lock it to the pilot (and thus the master site's master oscillator.)

The phase detector is monitored by the system controller and in the event of the loss of lock, the controller can switch to a free-running mode.  This status indication may be used to disable the slave repeater in the event that system operation has become non-synchronous.

The 20 MHz master oscillator is buffered and divided at various frequencies to provide references to lock other oscillators in the system to it.
 

LOCAL OSCILLATOR LOCK CONTROL:   (Refer to Figure 2)

This circuit locks the receive and transmit local oscillators to the master oscillator to guarantee frequency accuracy and stability.  This circuit gets its various reference frequencies from the REFERENCE LOCK CIRCUIT.  All PLLs provide their operational status to the system controller for monitoring.

Both the receiver and transmitter use low-side injection with 10.7 MHz first IFs and 40 KHz second IF.  The first local oscillators in both cases are multiplied by 9 from a 15 MHz oscillator (a VCXO.)  A sample of the 45 MHz signal from the first tripler (45.10667 MHz in the case of the receiver chain, and 45.30667 MHz in the case of the transmit chain) is mixed with the 20 MHz master oscillator with a frequency-doubling mixer, resulting in a 5 MHz signal (5.1067 MHz for the receiver, 5.3067 MHz for the transmitter.)  This is divided to 13.333 KHz and compared to a 13.333 KHz reference derived from the 20 MHz master reference oscillator and the result is used to lock the appropriate oscillator on frequency.

The local oscillators for the second conversion operate at 10.66 MHz to convert the 10.7 MHz first IF to the 40 KHz second IF.  The 10.66 MHz Receive local oscillator is sampled and divided down to 10 KHz and locked to the master reference oscillator.

In the case of the slave site, where it is desired that the transmit frequency differs from the master site by 10-20 Hz, the 10.66 MHz oscillator is offset slightly.  To do this, it is compared with the 10.66 MHz receive system local oscillator by the "OFFSET CONTROLLER."  This is a microcontroller-based circuit that first locks the two oscillators to the same frequency and then offsets the transmit oscillator in the desired direction.  It does this by operating charging/discharging an integrator in the loop filter while monitoring the difference frequency and direction by "looking" at the PCP and PCII lines on the 74HC4046.  This controller provides status, and is controlled by the system controller.
 

SYNCHRONOUS RECEIVE CONVERTER:   (Refer to Figure 3)

The received signal is first passed through a 146.020 MHz bandpass filter.  Following this is a low-noise GaAsFET preamplifier, another 146.020 MHz bandpass filter, a second GaAsFET amplifier (this one designed for dynamic range and to overcome mixer noise) and a final filter to attenuate the image noise from the conversion.  This amplified signal is applied to a high-level doubly-balanced mixer.  This mixer allows the receiver to handle very strong signals on adjacent frequencies with minimal degradation.  The local oscillator is a 15.03556 MHz VCXO that is multiplied by 9 (in two steps) to 135.320 MHz, 10.7 MHz below the receive frequency.

Following the mixer is a diplexer/filter to pre-filter the IF and properly terminate this port.  An IF amp follows this to boost the signal and to provide matching to the receive system IF filter.  It is this filter that sets the receiver's bandwidth.  The output of the crystal filter is sent to an NE-605.

The NE-605 is a complete receiver IC with RF amplifier, local oscillator, mixer, IF amplifiers and limiter, quadrature detector, and RSSI (Receive Strength Signal Indicator.)  The IF amplifier/limiter chain is used for additional amplification, and filtering.  The output of the limiter is bandpass filtered (to get rid of the residual sidebands created in the limiting process) sent to the mixer and, with a 10.66 MHz VCXO, is converted down to 40 KHz.  The output of the mixer is filtered, amplified, and made available to the 33cm transmitter.

It should be noted that the other features of the NE-605, while not necessarily used in the system, are implemented as needed.  The RSSI allows absolute signal strengths to be accurately measured, and the quadrature detector may be used to provide an audio output for local monitoring and to be used to allow the slave repeater to be operated in a "standalone" mode, possibly in case the "master" repeater is down.  Note that this audio output is recovered from the 10.7 MHz limiter/demodulator.

A sample of the 10.66 MHz local oscillator is sent to the LOCAL OSCILLATOR LOCK CONTROL for frequency locking.
 

SYNCHRONOUS TRANSMIT EXCITER:  (Refer to Figure 4)

The baseband is input and filtered and limited to remove any other baseband components and eliminate any AM, and sent to a broadband quadrature network.  The 10.66 MHz transmit local oscillator is also supplied in quadrature and these signals are mixed and summed to create a image-reject mixer (a "phasing" type of mixer) to cancel the difference signal.  The result is a 10.7 MHz output, with the 10.62 MHz image being attenuated by at least 30 db, easing filtering requirements somewhat.

The 10.7 MHz output is filtered, and buffered to further remove the image and to set the transmit path noise bandwidth.  It is then mixed with a 135.920 MHz local oscillator, the ninth harmonic of a 15.10222 MHz VCXO.  The mixer output is bandpass-filtered to remove the converted image and amplified to a level sufficient to drive the output power amplifier.
 

DEMODULATOR AND MODULATOR:   (Refer to Figure 5)

This module is present only at the master repeater site.  This takes several 40 KHz inputs from the various SYNCHRONOUS RECEIVE CONVERTER modules, one of which is from the microwave baseband.

The baseband inputs are filtered, and upconverted to 10.7 MHz by mixing them with a 10.66 MHz oscillator (obtained from the transmit exciter) which is locked to the master 20 MHz master reference oscillator.  This upconversion is done to permit sharp filtering of the received signals using stock filters.  Additionally, moving the demodulation frequency higher simplifies the design of an ultrasonic squelch circuit;  The output of a 40 KHz demodulator would have a very high carrier- frequency content in it that might be difficult to remove adequately.  The filtered signals are passed to a pair of identical 10.7 MHz demodulators based on the NE-605.

The outputs of the demodulators are sent to two switches.  One of these switches is used to select the audio source, and the other switch goes to a noise detector.  The system controller switches the noise detector switch between the two demodulators to determine which demodulator has the signal with the best quieting.  The result of this decision is used to select the which demodulator to use as the receive audio source, or whether to disable the audio (as in the case of the squelch being closed.)

The modulator consists of a 40 KHz VCO that is locked to the 20 MHz master reference oscillator.  The output of this VCO is bandpass filtered and made available to baseband.  The audio input to this circuit is low-pass filtered to keep ultrasonic noise from being modulated on the transmitted signal which would widen bandwidth and reduce performance under noisy-signal conditions.  Note that the audio going into this circuit is always either pre-emphasized or discriminator (which is, by definition, already pre-emphasized in a PM communications system.)
 

Several comments:

• While it has been discussed here, it is not strictly necessary to lock the first receiver LO to the reference.  Because all received signals are ultimately recovered as audio, AC coupling would remove the implied DC offset.  It might be worth locking those LOs, however, if precise frequency monitoring and "fingerprinting" is desired to maintain consistency throughout the system.  The possibility of using an interferemetric scheme for transmitter location also exists.
• The question of 'how to ID the system' has come up.  Certainly, this ID could be inserted at Farnsworth, as it will be the same for both sites.  It would be "nice" to know which system was hearing a signal, and which one was being heard.  To this end, I would propose a "double" courtesy beep at the end of a transmission.  The first beep would indicate the transmitter being heard, and the second would indicate the receiver that was receiving the signal that had just dropped.  These beeps might be exactly an octave apart for the benefit of those overlapping areas (although the beep would be so brief that it would likely not matter, anyway.)  The insertion of this beep could be done by modulating one of the transmit-chain VCXO's.

I welcome additional comments.

 Any questions or comments? Send me some email!

You may also be interested in the  Predicted Coverage Area of the system.  Maybe you or your club is interested in putting up a synchronous repeater?   Here is a brief FAQ that may (or may not) answer (or raise) some questions about doing so.  A more detailed (preliminary!) description of a block diagram/schematic (with links to the diagrams) can be found  here.

Go to the  Repeaters of the Utah Amateur Radio Club page.