This page intends to explains a little bit about Audio connections and other considerations

• Is there anything one should think about, before starting the Link?

• Yes, there is actually quiet a bit to think about before setting up and starting a Simplex link. One of them is to consider the equipment that is available for a particular setup. If the link is built using older equipment, the question as to how to connect the AF to the Computer / Software, may arise. With newer equipment, there is another question that may come up.
• Lets have a look at an older Transceiver that we found on one of the swapmeets or had sitting around for some time. This Transceiver may only have a Speaker out (perhaps 2.5 Watt audio power max,) and as input, provides only a microphone input. There is no Carrier Detect also called, CAS, COR or COS. The PTT can be controlled through the microphone receptacle. At first it may appear to be useless as simplex link radio, since there is no COS (Carrier Operated Switch). Unfortunately, this transceiver has only CTCSS encode (transmitting PL) and its receiver will respond to anything on the air. But there is an LED in the Front Panel that illuminates when a signal is received. If the LED is a Green one you could be tapping this signal as COR. A green LED has a Voltage Drop of about 2.1-2.4 Volts. To drive the base of a Transistor you'll need at least 0.6 Volt or somewhat more to get a collector current to flow. A regular switching transistor has usually an amplification ratio of around 100 also referred to as hFE. This number is depending on other parameter too, but for switching the general hFE is enough. What it means is, that if the Collector current should be 10mA the base current can be 100 time less i.e.. 10mA / 100 = 100 Micro Amps. Only 100µA are necessary from the LED if you use a Transistor to provide a COR signal. To reliably switch this transistor we decide to drive the base at 1 mA current - 5V - 2.4V = 2.6V the 2.6volt / 1mA =  2600 Ohm but we'll either use a 3.3K or 2.2K. This has become more difficult with the newer Transceiver that use an LCD. Those LCD's are usually multiplexed and it is almost impossible to reliably pick off a signal for COR.



Fig.1  Addition - COR circuit from LED

• We now have a very simple COS circuit that puts 5 - 0.6 Volt to the COR Emitter whenever the LED illuminates. Since most COR input circuits are high impedance perhaps in the 10K order there is no need to limit the Collector Emitter current. As an added safety measure one could place a 47 Ohm resistor in series with the collector.  This will reduce the output voltage somewhat depending on load, however also will limit the current. The exact value would have to be determined by Collector Voltage / Current and the output load condition. The 2N2222 can handle a maximum current of 300mA but would significantly heat up the junction and is not suggested to run that high. Granted, one can simply use the VOX switch provide within the Echo-Link (Win and SVXLINK which can also provide CTCSS software detection) version, however the VOX is tricky to adjust and may open another can of worms in terms of additional delays and is not working well enough in crowded areas..
• The above will work fine in areas where the Echo-Link population is sparse and possible signals from other Links aren't existing. If however another node is within reach of your node, a new problem may arise. Your node will detect a signal anytime another RF signal is on the air that was not meant for your node. If there are control codes i.e. DTMF coming along, your node may respond to those commands. The solution is a PL tone decoder! Unfortunately the IC's to built a CTCSS decoder are rather expensive, but work extremely well. I have measured CTCSS decoder that will still decode down to 40 Hz deviation. A deviation higher than 200-250 Hz will produce audacity for the more sensitive ears. The good news is that an external CTCSS decoder can simply be inserted into the audio path and the digital signal can be used as COS. In such case your node will only get a COS signal if the PL tone is right.



Fig 2. Above schematic shows a programmable CTCSS Decoder / Encoder circuit

• With a circuit as above, you don't need a COR circuit or modify your transceiver. Simply route the Audio through this circuit and your node will only respond to signals that contain the right PL tone.
• The circuit above has another advantage, since the audio is routed through the chip a 300 Hz low pass filter is applied to the receive and transmit audio which does minimize the affects from TALK OVER.  In real life this means that false trigger generated by low frequency voice audio is minimized. As configured above you can expect a response time of approximately 120mS and a deresponse of approximately 150mS. By changing certain values one can extend these times in very crowded areas for a slower response.
• I am providing the Data sheet here for those that are interested in this solution.
• An other solution was to use an LM567 Low Power Tone Decoder. Unfortunately this IC is hard to find in low quantities and National Semi has stopped production. A good place to start looking is at Futurlec which does distribute electronics parts at reasonable price.



Fig. 3 Simple tone decoder, as configured F0 = 1050 Hz

• The above schematic shows a simple PL tone decoder that was meant to decode a 1050 Hz tone, but can be adjusted to other tones as well. The setback with this type of circuit is the rather wide bandwidth and false triggers are quiet possible.

Check the Data Sheet for the LM 567 to learn more abort its characteristics. The data sheet can be found here as PDF document.

The subject is continued here!

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