CommunicationsAudioInterface for Remote Operations

CAIRO Talk-Through Relays

CAIRO Relays

The simplicity and versatility of CAIRO-8 for Talk-Through Relay schemes stems from the availability of the extra PALS signals at the outlet of a CAIRO-8 host transceiver. In particular the Squelch output of any host Rx-Rig gives an unequivocal PTT input to the Tx-Rig, and the Audio-Line provides a fixed level (1Vpk) for audio coupling, via pre-set attenuation with isolation, to any CAIRO mike input. The d.c.-Power line is used to supply the TTU's active circuits.

With transceivers which only have a CAIRO interface, these signals are not available. Hence the Talk-Through scheme for CAIRO uses circuits which create these signals indirectly. Then, in all other respects, the resulting CAIRO TTUs have the same plugging conventions of the CAIRO-8 scheme (Fig. 24 B) and a pair of TTU modules may be used either for In-Band (IR) or Cross-Band (XR) Relays in equivalent configurations to those which are supported by CAIRO-8.

CR .1 PALS-Generation

At first, it may be helpful to think of this extra circuitry as if it was a separate module with a CAIRO Rx-Rig tail, to plug into the transceiver, and a CAIRO-8 socket to accept the previously described CAIRO-8 TTU module. This clarifies the role of the extra circuit as being the provider of PALS-like signals for the C8-TTU and also reveals how the same Relay configurations can be achieved, e.g. Cross-Band (XR) becomes a back-to-back plugging, Fig. 25, as before.

In particular, the extra circuit must detect an equivalent condition to that of real Squelch and must accept Speaker-level audio and re-present this at the fixed Audio-Line level : 1Vpk. This is done with active circuits, so the 'module' has a separate 12V Power-feed tail, both for its own power supply needs and also for the TTU section, when this is based on the *OPTO* technique.

Fig. 25

CAIRO to "CAIRO-8" Conversions within a CAIRO TTU

In reality, both the PALS 'generator' and the C8-TTU sub-system circuits will be housed together as one module - the C-TTU - with an Rx-Rig tail, a Tx-Link tail and a DIN-7 socket. This avoids the extra cost (and confusion) of the intermediate connections which, in any case, are not precise equivalents of the CAIRO-8 standard signals and could not be used in circumstances other than these Talk-Through systems.

Two alternative techniques for Squelch detection have been considered and subjected to extensive and rigourous evaluation with a wide range of Transceivers to find the better approach.

CR .2 Audio Gating

The first, though now discarded technique, may be described as Audio-Gating or "VOX". This involves a circuit which 'listens-to' the received audio from a using outstation and, when there is audio present (on the CAIRO pin-1 speaker line) in excess of a pre-set threshold, the outgoing PTT is keyed into the Tx-Rig. The pre-set threshold is chosen to be equivalent to the 1Vpk signal level from an adequate (i.e. not "scratchy") station. This simplifies initial set-up and subsequent operation because the unit will not key the PTT until there is a "good" signal AND the rig's receiver volume has been adjusted for the correct through-audio coupling level into fixed attenuation - either the *OPTO* or the *XFMR* isolation method. An LED is included with the module so that the installer performs a simple set-up procedure which almost exactly resembles that for setting-up a CAIRO-8 TNC with a CAIRO transceiver (see RP .7, steps 2-6).

In principle, this is a straightforward approach but it has a major operational drawback. Firstly, weak or distant stations are not relayed without frequent drop-out and, secondly, even strong stations exhibit drop-out during speech pauses - the "er factor". This effect invariably increases when outstation operators are working under the duress of full emergency conditions! The "er factor" can be reduced by including delays on the PTT release but these must be carefully and critically adjusted so that the turn-round time, in a Cross-Band (XR) configuration, is not excessively slow. Such adjustments and re-adjustments may well have to be made by the manning operator throughout the Talk-Through Relayed operation. This is an undesirably onerous task.

Therefore this method is not recommended, but it is useful to note the reasons why not.

CR .3 Noise-Gating

For reliable operations, without constant adjustment against variable factors, the Noise-Gate is preferable, despite some extra engineering considerations which must be included. In this approach, the transceiver is left with its squelch control turned fully OFF (i.e. 'zeroed') for the quiescent input to the TTU to be (loud) FM noise. The Noise-Gate discriminates between this and the somewhat quieter levels which occur when a carrier, with or without continuous speech, is received even if from a weak outstation which need not be a "fully quietening" signal. Having first been set for an adequate input level, the Noise-Gate generates a PTT output whenever this noise level reduces. In particular, it exhibits very fast 'attack' and 'release' characteristics just like a real squelch output. However, there are three snags to be overcome.

CR . 4 Logic States

Firstly, if a correctly adjusted TTU should become accidentally disconnected from the Rx-Rig, the noise supply will be lost. This will cause the Noise-Gate to key the PTT into the Tx-Rig and thereby 'jam' the using Net. To overcome this, the Noise-gate is enabled by the positive voltage which will be present on the Mic.-High line from the Rx-Rig. Being a fully CAIRO-adapted transceiver, its Mic.-High (DIN pin-3) input will have a voltage present for the electret biasing. This too will disappear during (accidental) unplugging and so will disable the Noise-Gate. (Please note now, that CAIRO TTUs will NOT operate with rigs which have no bias present in the Rx state.)

Secondly, the Cross-Band Relay involving two TTUs connected back-to-back, can enter a mutual lock-up condition. When one TTU keys its Tx-Rig the Rx section of that transceiver will become silent. This will cause the second TTU to key its Tx-Rig for the Rx section to go silent too. That will falsely maintain the condition which led the first TTU to initiate this lock-up chain. To prevent this, each Noise-Gate is designed to operate only when the PTT line which 'passes through' the module is NOT keyed. When it is keyed (metallic-contact to ground) the Noise-Gate is disabled.

Finally, the 'half'-Relay of a split-site In-Band configuration may use a CAIRO-adapted Dual-Band transceiver, just as in the CAIRO-8 plan. Here the CAIRO TTU module will be plugged into the CAIRO outlet of that transceiver and will have its Tx-Link tail connected in the loop-back scheme to its own socket. Consequently, the PTT line which 'passes through' the module in this configuration, will be its own Noise-Gate Squelch output and so it would lock itself out. To prevent this, the PTT inhibit is disabled whenever the Noise-Gate is asserting a Squelch output condition.

All these conditions may be summarised with a Boolean expression, thus;



NGO Noise-Gate Output = 1 to key Tx, else = 0 'Key' condition

MHV Mic.-High Volts = 1 if 'volts' > 0.8-0.9 volts, else = 0 Rig connected

RXQ Receiver Quiet = 1 if carrier present, else = 0 for FM-noise Squelch condition

PTT (passing thro') = 1 if not keyed, else = 0 when keyed for Tx other-TTU condition

Re-arranged for two-level NAND logic the above expression becomes;



and an elementary latch configuration emerges. This may be implemented with a triple of three-input CMOS NAND gates; i.e. one 4023 logic IC, as shown in detail in Fig. 28 B.

Each logic signal input is derived from a detector or buffer circuit as appropriate, Fig 26, while the logic output signal is buffered to drive the PTT relay and the set-up LED. Each of these four sub-systems uses one op-amp from a quad IC package, e.g. LM324. For overall simplicity, this linear device and the CMOS-IC are both powered from the 'raw' 12V supply but with zener-diode protection, e.g. 1N5352B [283-205], against overvoltage spikes on the CMOS.

Fig. 26


Block Schematic

CR .5 Sub-Systems

The Noise Detector may use any general-purpose NPN device, e.g. BC108, as an active 8-10KHz noise filter. The a.c.-coupled speaker (pin-1) input, has a preset 10K potentiometer (multiturn, PCB-mount) which is bench-adjusted for the detector to operate at an input threshold which is then equivalent to the 1Vpk level of through-audio into the isolator. When FM noise is present, the tuned amplification (22mH choke [228-359]) 'pumps' the diode rectification to a voltage which is just in excess of the diode reference for the RXQ output to be Low, Fig 27 A. Received audio will also generate a small voltage but this remains well below the diode threshold, even with strong ('loud') signals, for the RXQ output to stay at a steady High, as required.

(A 100R resistor - not shown - should be included as a minimum load on the incoming Speaker Audio line, before the feeds to the Noise Detector and isolated mike-coupling circuits.)

Fig. 27 A

Fig. 27 B

RXQ - Noise Detector

PTT and set-up LED Driver

A second op-amp is used as a non-loading buffer on the 'through'-PTT line, to generate a PTT-signal which is compatible with the CMOS, Fig. 28 A. A third op-amp is used in a similar arrangement (not shown in detail) as a non-loading detector for the bias on the Rx-Rig's Mic.-High for the MHV output to be High. In this case the output diode and CR pull-up are omitted. Lastly, the fourth op-amp of the IC package is used to source current into the PTT output relay, e.g. SIL [345-561], and set-up LED, e.g. [586-475], when the CMOS NGO output is High, Fig. 27 B.

Please note that for overall simplicity, all four op-amps (working as comparators) are referred to the same diode chain which is only shown explicitly (as 'V-comp') in Fig. 27 A.

Fig. 28 A

Fig. 28 B

PTT sensing buffer
(duplicated for the Mic-Bias detector)

NGO - Logic

CR .6 Anti-Hunting

The 47 uF capacitor at the output of the PTT op-amp, Fig. 28 A, is a 'slug' against hunting in cross-band configurations. Whenever two similar TTU modules are connected back-to-back, short quasi-interlocks could occur when the PTT is asserted into one rig while the other rig is still 'recovering' into its receiver FM-noise condition, following the release of its PTT. This capacitor and pull-up resistor is essential to delay the PTT inhibit release by about 400mS, to avoid such hunting in any back-to-back configuration. The delay is purposely quite long, to cover for the likely 'worst-case', but is still sufficiently short not to be noticeable by the using outstations of any cross-band Relay configuration which employs these CAIRO TTUs: "C-TTUs".

It is useful to note here that some dual-band transceivers, when converted to CAIRO-8, may also require a capacitor 'slug', at the internal Squelch (pin-8) relay drive transistor, to provide an equivalent 'release' delay to avoid this kind of hunting whenever a CAIRO-8 TTU is used in the 'self-loop' connection.

CR .7 CAIRO TTU Modules

The Noise-Gate circuits, together with either an *XFMR* or *OPTO* isolation, should be housed in a single module with the Rx-Rig tail emerging from one side. The DIN-7 socket and Link/Tx-Rig tail should be set at the opposite side to this, so that it is reasonably obvious to an installer which tail is which. To further clarify which is which, install the set-up LED near to the Rx-Rig tail, as this leads to the transceiver which must be adjusted for correct LED illumination. If the module includes a Set-Up/Talk-Through switch, install this near to the Tx-Link tail, since this leads to the transceiver which is controlled, in respect of its PTT, by the action of this switch.
(In CAIRO-8, the TTU tail cables are substantially different so there is no equivalent 'confusion'.)

It is particularly important to note that any Talk-Through Relay configuration involving two TTUs, may employ a CAIRO TTU in one 'direction' and a CAIRO-8 TTU, in the other 'direction', if the transceivers are of the appropriate kind.

Despite the manner in which the CAIRO TTU concept was introduced and explained (see CR .1 and Fig. 25), it is NOT recommended that the Noise-Gate and Isolation circuits are constructed as separate modules since the Noise-Gate outputs are not "real" CAIRO-8 signals.

In short, the Noise-Gate is NOT an alternative form of 'external' CAIRO-8 Rig-Converter.

Thus, it would be usual practice to implement the entire scheme of Fig. 26, as the circuits of Figs. 27 (A&B) and 28 (A&B), on a single PCB to ensure that all interconnections are kept short, consistent with minimising RF susceptibility, and housing the PCB in a diecast box.

CR .8 Bench Set-up Procedures

Initial set-up and commissioning of the C-TTU is straightforward and is, in effect, a 'reverse' sequence to that which is used for the operational set-up procedure. First, an audio input is established at the 1Vpk level so that through-audio passes correctly via the isolation coupling. Then the noise-detector sensitivity is adjusted for the Noise-Gate to operate at this threshold.

The method involves THREE transceivers, two of which must be CAIRO adapted. Tune the first CAIRO transceiver, as the Rx-Rig, to a strong station - perhaps a local Repeater output. Supply the TTU from this Rx-Rig and couple the isolated-audio output into a second transceiver, as a Tx-Rig, tuned to a different and 'silent' frequency, in the same band and working into a dummy-load on its lowest transmitter power output setting.

Use a 'hand-PTT' at this stage to key the Tx-Rig when required. Tune the third 'monitor' transceiver, e.g. a hand-held, to the Tx-Rig frequency but arrange for it to be easily switched to the Rx-Rig frequency as well. Monitor the Tx frequency and key the Tx-Rig while gradually increasing the volume control of the Rx-Rig. At a point where the monitored audio sounds to be "about right", switch the monitor transceiver to the Rx-Rig frequency and back again to the Tx frequency, adjusting the Rx-Rig volume until both sources appear to be indistinguishable. This has now established the 1Vpk-coupling so DO NOT adjust the Rx-Rig volume in the next step.

Re-tune the Rx-Rig to a 'silent' frequency. Reduce the Rx-Rig Squelch control to zero. Adjust the TTU input potentiometer, the 10K at the Noise-Detector, until the set-up LED is ON. Very gradually increase this potentiometer until the LED goes OFF and continue adjusting for a further half-turn, in the same direction. The C-TTU is now adjusted for use.

Set aside the 'hand-PTT' and now connect the relay output as the PTT control to the Tx-Rig. Double-check your bench set-up by undertaking the Operational set-up procedure, page 71 (select relevant instructions) together with the comparative listening between input (source) audio and output through-audio, as above. If there is a slight discrepancy, adjust the C-TTU potentiometer, but by no more than a quarter-turn at a time, and re-adjust the Rx-Rig volume to compensate.

Repeat the entire bench set-up procedure for the second CAIRO TTU of the pair. Please note that it is essential for each C-TTU module of a pair to be adjusted for its correct threshold, before 'closing the lid', so that subsequent Operational set-up procedures can be used by installers, who need not be familiar with the circuit details, to achieve a correct setting for reliable operations.

CR .9 Manning Operator Duties

Because the CAIRO TTU works with the absence of FM noise, two small hazards remain.

If the manning operator, of a Talk-Through Relay, was to inadvertently turn down the Rx-Rig's volume control, a C-TTU would key the Tx-Rig and so 'jam' the Net. This can be avoided by making the manning operators aware of this hazard and asking for their vigilance and remedial actions, throughout the operational period. Also, in the back-to-back coupling of two C-TTUs for the Cross-Band (XR) Relay, there is a very small, but nevertheless finite chance that two stations, one in each band, might call at exactly the same instant (after inactivity) causing both rigs to be 'cross-keyed' into transmit, before the 'through-PTT' sensing has formed the interlock. Should this occur, the remedial action is simply to unplug and immediately re-plug, one or other Link-Tail or Rig-Tail. Finally, the C-TTU uses a 'compromise' technique to re-generate squelch. Operators should ALWAYS check the performance of C-TTUs with particular rigs ("on the bench") to ensure compatibility; i.e. that the audio output has filtering which 'satisfies' the noise-gate.





Supplementary Material


Remote Operations

CAIRO Basics

CAIRO-8 Basics

Raynet Packet

CAIRO Installations

CAIRO-8 Relays

CAIRO Relays

Relay Installations

Summary Configuration

Data Card

Handy Hints