HiFix Receiver
The HiFix receiver can perform two roles.As a user (field) receiver
The unit receives the RF signal from the master and slave transmitters and displays the phase difference between the master and slave 1, and between the master and slave 2. Using the trigger pulse radiated by the master station on the siganalling frequency, it synchronises a timer to run in step with that in the master drive unit at the master station. During the master pattern frequency pulse, a crystal oscillator is adjusted to maintain phase lock with the master station. During the slave transmission periods, the phase of the slave transmissions is compared with the oscillator and the difference is displayed to the operator on two mechanical digital lane counters, each displaying 5 digits: 3 digits before the decimal point which represent up to 999 whole lanes, and two digits after the decimal point which represent fractions of a lane down to 0.01 lane.
As part of a slave station
The unit receives the signal from the master station, and provides drive to the slave transmitter. Using the trigger pulse radiated by the master station on the siganalling frequency, it synchronises a timer to run in step with that in the master drive unit at the master station. In this way, the slave pulses are radiated at the correct time. It also receives the master pattern pulse and during this period a crystal oscillator is adjusted to maintain phase lock with the master station. The oscillator is used to generate the RF drive for the slave transmitter. This drive signal can be adjusted in phase to give the correct transmission phase for the slave station.
General description
The unit is a superheterodyne receiver employing miniature valves and both silicon and germanium transistors and diodes. The power input is 24V DC positive earth, and the HT supply and bias voltages for the valves are derived from a transistor invertor. In general, the RF signal stages and oscillators use valves while transistors are used in the timing and servo circuitry. The unit is constructed in a 4U high 19 inch rack and extensive use is made of plug-in modular circuitry. A crystal oven is used to maintain the oscillator crystals at a constant 70°C.
The antenna input is fed to an RF amplifier, which is followed by a mixer stage. A local oscillator consists of a crystal oscillator using one of four channel crystals, followed by bandpass amplifier stages. The IF signal from the mixer is at 132.840 kHz for the pattern frequency, and 132.900 kHz for the signalling frequency. Crystal IF filters are used to give an IF bandwidth of 100 Hz, with a response that encompasses both pattern and signalling channels. The 'composite' IF signal is amplified by three IF amplifier stages.
Signalling frequency channel
The signalling frequency is extracted from the composite IF signal and fed, via further amplification and a crystal filter to an envelope detector stage. This produces an output pulse whenever the trigger pulse from the master station is radiated. This pulse synchronises a ring-of-four oscillator which produces the timing signals required.
This oscillator consists of 4 stages connected in a ring. At each stage is a timing circuit, and each stage produces an output signal from a relay. The first stage time constant is 100mS and all others are 300mS- so essentially, the outputs correspond to the trigger and 3 pattern pulses.
Pattern frequency channel
The pattern frequency is likewise extracted from the composite IF signal and amplified further before application to one input of each of 3 identical discriminators. The first of these forms a phase locked loop that maintains a 132.840 kHz crystal in phase lock with the master transmission. The other two disrciminators are used together with goniometers as part of the display, one for pattern 1 (master - slave 1) and the other for pattern 2 (master - slave 2).
To properly understand the following description of how the HiFix receiver measures and displays phase differences, it is necessary to understand the operation of two essential components: the discriminator, and the goniometer.
Discriminator
In modern electronics we would undoubtedly call this stage a phase comparator. Essentially, it has two inputs and one output. When the two input signals are in phase, the output of the discriminator will be zero volts. When the two inputs are of different phase, the output will be a DC voltage whose magnitude depends on the amount of phase difference, and whose polarity will depend on the direction of phase difference.
Goniometer
The goniometer is a device that can produce any desired phase shift with respect to an input voltage. Since it may not be apparent how this can be acheived, the description below expands on this.
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In this diagram are a fixed coil A-B and an additional coil C-D which may be rotated about its centre with respect to the fixed coil. In the situation shown, if an RF voltage is applied across A-B then a current will flow in the coil, and by transformer action, a current will be induced in the rotatable coil C-D. The voltage produced across C-D will be in the same phase as the the voltage across A-B. |
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In this diagram, the moving coil has been rotated through 180 degrees. The voltage produced across C-D will now be 180 degrees out of phase with the voltage across A-B. By rotating the moving coil to other positions, corresponding changes in phase will result. |
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There is however a flaw in this theory, which is that when the two coils are at 90 degrees to each other, no magnetic coupling occurs and hence there will be no output when the moving coil is set as shown here. |
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To overcome this, an extra fixed winding is provided which is connected to the same voltage A-B but through a capacitor C1. This component is chosen so that the current in the horizontal coil is 90 degrees out of phase -in phase quadrature- to that in the vertical coil. Now, when the moving coil is rotated to the 90 degree position, an output results which is 90 degrees out of phase with the voltage A-B. In fact, any phase may now be obtained by positioning the moving coil appropriately with respect to the fixed coils. |
Let us assume for a moment that the IF input is in phase with the master oscillator and the goniometer is set for zero phase shift. Then the two inputs to the discriminator will be in phase, and its output at point A will by zero. The servo amplifier output will also be at zero and the motor will be at rest. Under these conditions, we can mechanically zero the display if we wish to read 000.00.
Now if the IF signal phase has changed, because we have moved with respect to the master and slave transmitters- then the phase of the two inputs to the discrimiator will be different, and correspondingly an ouput voltage will appear crresponding to the amount and direction of phase difference. The servo amplifier will now drive the motor in the appropriate direction, thus turning the goniometer, until the zero phase condition has been restored. Here's an example:
We have moved 0.1 of a lane away from the master and slave stations. This will correspond to a phase change of +36 degrees. The discriminator output will now be a small positive voltage, which will cause the servo amplifier to drive the motor in the direction needed to move the goniometer until its output is also at +36 degrees, i.e until the difference between the discriminator inputs is restored to zero. When this has been achieved and the motor is once again at rest, the mechanical gearing will have moved the goniometer by 0.1 turn, and the mechanical display counter will read 000.10.
There are of course two displays, one for each of the two patterns: master - slave 1, and master - slave 2.
Slave Drive channel and Automatic Phase Control (APC)
In addition to the pattern displays described above, the receiver provides an additional channel for use with a slave transmitter when the receiver is part of a slave station installation. The pattern frequency IF is mixed with the output of the four-channel oscillator to bring it back up to the correct RF frequency. A goniometer and associated servo operate to control the phase of the drive signal.
Alarm lamp
An alarm indicator is built in to the receiver. When a phase difference of greater than 90 degrees exists in one of the channels, the lamp is lit.
RF Attenuator
When part of a slave station installation, the receiver would be overloaded when the associated slave transmitter is keyed. To prevent this, an attenuator is switched in during the slave transmit period.
Photographs of HiFix receiver installations
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Last update 27/4/05