This board interfaces a receiver to a controller or transmitter, while it performs basic link or repeater functions, except for an ID'er. Emphasis on size, simple design, parts availability and easy modifications, limited only to your imagination. Depending on the application you can leave out some parts, while strapping for others, such as cor delay, and polarity on "COR" and PTT output. There are some extra pads on the board for this purpose. It's assumed you have a basic electronics background with some repeater building experience. Understanding schematic drawings is required. If you are new at the repeater operation you might want to check out additional technical books relevant to this documentation.
Changes for this VersionThe printed circuit board has no changes. There never was a 5.5 PCB produced. The only changes are in this document and how the board is used. This version is for a remote transmitter uplink. Therefore, circuitry for one timer has been left out. The audio section remains the same as the earlier version 5.3.
Definitions, terms acronyms and semantics
References can be expressed in a few acronyms. Test Tone level (TTL) into a two-way VHF/UHF transmitter or out of a VHF/UHF receiver is referenced to a test tone frequency of 1 KHz, of %100 system modulation. For this standard, that is +,- 5 KHz deviation. Other areas and services have different bandwidths, such as in P-25 systems. A Test Level Point, (TLP) refers to a measurement point, on equipment, for a system, in reference to Test Tone Level (TTL). TLP provides easy reference to any parts of the system for measurement and alignment. 0 dbm is referenced to 1 milliwatt at 600 ohm impedance. Therefore, a transmitter AF input with a TLP of 0 dbm, with a Test Tone Level of 0 dbm tone input, would fully modulate the system. A far end receiver with the same TLP would output a 0 dbm tone as well. A 6 db drop in (voltage) level would reduce the modulation in half, and so on. In general, for these standards, levels are stated in transmit-receive (Tx-Rx) order. Therefore, an audio (VF) "drop" TLP of 0/0 would mean a Tx TLP of 0dbm, Rx TLP of 0dbm. Sometimes operating levels are not at TLP. To avoid technician confusion two sets of numbers are sometimes used in diagrams and on the physical equipment. Figures in parenthesis are the TLPs. Non-parenthesis figures are operating levels, and, as mentioned, may be at a different levels from the TLPs. Most of the TLP's for this project are in the -10~0 dbm area, however, a resistor value change can be made for your system's requirements.
The term "COR" came from the old tube days of "Carrier Operated Relay", whereas, a tube receiver had a point, when its squelch opened, a tube (switch/valve) drew current through a relay's coil, to give some contact closure, to key the associated repeater's transmitter. As the solid state technology came in the later 1960's the term stayed with repeater operation, even though the Author saw no "relay" in most modern repeaters and felt the "relay" term should have been replaced with the term of "squelch", since it's the receiver's squelch that does the repeating. This would be called" COS", meaning a "Carrier Operated Suelch".
Both terms are true and this gets down to semantics. After careful consideration of modern technology used in the LMR field by Amateurs and professional alike, including recent repeater product terminology and to the fact that repeater stations in the early years were also called "Relays", whereas, the station would "relay" a signal rather than "repeat" a signal, the Author decided to stay with the majority's term of "COR", to avoid reader confusion. Therefore, this and other documentation will reflect this decision.
The term "PTT" will describe an active going "low" for DC functions, such as transmitter keying ("PTT Input"). It also will describe a receiver's COR line driving a NPN transistor, with the open collector being "Receiver PTT Out", or just "PTT Out". "PTT-1" will describe this function, however with a buffer, such as the output of the COR/AF board, which changes state for user signal change of status. This function would be used for audio switching, such as Auto-Patch audio routing. "PTT-2" will describe a buffered, and timed output of the COR/AF board, to keep a repeater's transmitter keyed up for normal back-and-forth conversations of the users of such system(s).
"COS" will be reserved to describe a "Carrier Squelch" as a part of a receiver. "CS" will be reserved to describe "Carrier Squelch" as a receiver's mode of operation, verses "TS", "PL" or "CTCSS" to describe a "Tone Squelch", "Private Line" or "Continuous Tone Coded Squelch System", respectively.
"PLI" means Private Line Indicator (or Input). It's also similar to an CTCSS line out of a tone decoder. "HUB" means Hang Up Box. Motorola's uses a "closed loop" and a HUB for mobiles and base station control. "AND squelch" means it takes both carrier + tone to activate a cor board, transmitter or system. AND squelch is also referred as a variable sensitivity squelch, whereas, the squelch setting affects activity. An "OR" squelch does not, whereas, it "bypasses" whatever squelch setting, using only tone to keep it active.
Setup for DC/Key outputs
For up link transmitter applications a lot of the parts can be left out, as mentioned in the beginning. Only the tail timer, U3 and its associated parts will be used. When the cor is active, U1 input translates polarity (depending on your jumper settings) and drives both the audio squelch and PTT circuit. When active, pin 8 goes low, turning off Q1 and letting the AF input through the two stages of equalization and amplification to the "AF OUT" to drive a transmitter or controller. Pin 8 also drives some logic which drives the open collector PTT output, active going low, to key a transmitter.
The standard configuration for this version is the repeat audio is carrier squelch, while the PTT is an AND squelch. The latter requires both carrier + PL decoder activity to produce a PTT (low going) output to key a (system) transmitter. U1, pins 12, 13 and 14 are set up as an AND gate to make this happen. This section of U1 is controlled by voltage dividers, for three possible conditions: standby, carrier and carrier+tone activity. In standby pin 13 is higher than pin 12 and therefore pin 14 is a "low". When cor causes pin 8 to go "low", pin 13 goes to 2.5 volts. If there is no PL activity pin 12 is at .6 which won't cause pin 14 to change. When PL is active, it causes pin 12 to rise to 3 volts. Since 13 is now at 2.5 volts (lower than pin 12) the output, pin 14 will now low "high". This turns on Q4 and keys the transmitter. If you need positive going PTT output move Q4 around so the emitter is the output and the collector is to the + line. There are extra pads on the printed circuit board for this application.
Start with the cor point. Study your receiver's schematic or documentation for the best point and make that connection. The board's cor input U1 buffer is high impedance, therefore, should not affect the squelch circuit of the receiver. It also can convert an "analog", varying voltage point to a logic level and if needed, invert the polarity. The cor input polarity jumper, JU1 can be set for inverted or straight through. The former can be identified on the schematic drawing by the "criss-cross" lines on the cor input buffer/driver, whereas, it's inverting the cor source logic state. There's a set of jumpers on the board for this purpose. They can be either the push-on (PC) type shorting bars, or just wire jumpers soldered in place.Here's a couple of examples of radios models and their two basic polarities:
Other (conventional) receivers:
Squelch modes (tone or carrier):
Previously discussed was the closed loop for the HUB. This feature can be used for either local or remote mode change. This mode change is handy for testing where you want both audio and PTT on carrier on a temporary basis. A mode switch on the front panel and it's contacts in series with the TS-32 can make this happen. With some thinking you can do this remotely with a (external) controller.
Load the board with all the components needed, depending on your application. You will need to connect at least ground, power, cor input and AF input for alignment and testing. The other connections can be made on final assembly. For your first board leave yourself enough wire length to work on the board as you will be experimenting with different component values. Later on future boards will go much easier so you can plan your fixed wire lengths to go into the radio on a permanent basis. Then power up the receiver and board. The green power led should be lit. Remember, it takes about 10 seconds for the board's audio circuits to stabilize on power up. Since repeater service normally is 24/7 on, this should not be an issue. Adjust VR1 for proper trigger level when the squelch is active. Give yourself a little "margin" with this trigger point, for component/aging variances. Use the yellow led to watch the transition.
Setup and theory for audio
Some older receivers are dual conversion, with the second IF of 455 KHz, and leak some of the IF out of it's discriminator. This was true with the older Motrac receivers, still in SRG service in 2009. When running a flat system this IF leakage will modulate the associated transmitter, as can be observed on a spectrum analyzer. To prevent this, the board's audio input has a tunable LC trap for that frequency. The tunable range is 420-800 KHz. For other low IFs you can change the L-C values and find the resonant point with by sweeping the trap. However, with most modern receivers such as the (single conversion) Motorola Micor, Mitrek and MX do not leak IF out it's discriminator at any noticeable level. If this is the case, you can leave these parts out and bypass with a jumper and install the DC blocking capacitor, as discussed earlier.
Most direct F.M. transmitters are flat in audio and sensitive enough so don't need further conditioning from this board's version. One except is the Micor transmitter's TLP is a +2.2 dbm. You have a couple options. You can bring up this board's output to match the higher TLP which deviates (no pun) from standard station TLP of 0 dbm. Or you can install a single transistor amp inside the (system) transmitter. Or you can build up a second cor board, only using the second audio section and its "AUX in" connected to the AF Out of this board. For simplicity the Author is considering the first option for SRG.
Inject a clean 1 KHz tone and turn up VR3 to just at clipping point observed on the board's "AF Out" with an oscilloscope. Tune the bias at pin 5 with VR2 for best even top and bottom clip on the output. Re-adjust VR3 as needed to fine tune VR2 adjustment. VR2 will be a one time alignment.
Some words about U1 abilities. With regulator, U2 as a 7808, the maximum unclipped output of U1 is about a +9 dbm (bridged). If you need higher output there is an option. You can operate U1 at higher voltages, say, with a 7812 as U2 which will drive U1 output near +14 dbm (bridged). U2 keeps out any small ripple that would be amplified on the system, so most any 78xx series will work, since the audio op amp U1 uses a single end supply with voltage dividers for the "+" reference. Just watch out for the maximum operating limits of U1, and that higher regulator values reduces ripple protection. As of 2003, Author's design settled on a the 7810 (+10v) for the best performance.
Another word about VR3 and the LM-324 Op Amp. The 5 Meg Burns pot might be hard to find. You can substitute with the 2M pot with some loss in gain of the second stage. 5 Meg was selected for a highest value. Anything much more would make the amp to go into the differential mode. Without the negative feed back resistance between pins 6 and 7, it's in the differential mode, which is used as a comparator, such as the cor input section. Voltage gain is the ratio of the negative feedback and input resistors, then you can make the Logarithmic conversion for a more realistic approach on levels. With R1 value and VR3 you can control the stage gain. Typical figures are in parenthesis on the schematic, (bridged) assuming the input TLP is -10 dbm.
Flat Audio setup
If you are not setting up a flat system you can skip this section. (Hopefully your are, so read on). Assuming you are using a conventional repeater controller, you will need to perform some modifications for it, such as pre-emphasis in the voice ID (if used) and auto patch and de-emphasis for the DTMF decoder and auto patch line driver (if used). This way the system should be transparent, while the internal parts will compensate for the user's pre-emped radios, all used in F.M. mode.
Most receivers have high end roll off. This is a conventional method for commercial systems. If you want your system to sound really good (flat) you can extend the system's frequency response. First, plot the receiver's response on a graph, from 10 Hz to 10 KHz. This sounds a little extreme, but this will show how you are progressing. You also will need a cap-res substitution box and a handful of various values of these components or access to such a source.
The board has two stages of equalization with amplification to bring the level back up to a usable level. The first stage will flatten out the upper end, say, above 2 KHz, and the second stage, for above 4 KHz. With some experimentation with different values you can extend it out to around 6 KHz +- 1 db. To cut some time and performance you can "plug in" some typical values sometimes found to work with the Motrac receiver. These values were found from early research. This vintage receiver is being phased out with later ones such as the Micor and Mitrek receivers. Another point to remember these latter receivers do not need IF filtering, however, do need a DC blocking capacitor as mentioned earlier. Therefore, the latter cor board versions audio input drawings will reflect these changes. As of publishing of this document research will progress (as time permits) on the "default" values for the latter receivers. Therefore, the only (Motrac) values are:
For the Micor receiver research is still under way, however, it's believed the typical values for that receiver are:
Re plot the receiver. If you wish for the highest performance use a resistance and capacitance substitution box for each of the stages and re plot as necessary to obtain a flat response curve. You may have to repeat this procedure several times. Remember for multiple links you need to get it really flat, since imperfections will add up at the far end.
Here's the same 2 charts as above, except with a "preview" as you can see. Click on the image for a larger window. Left is stock, right is equalized. Each chart has two lines, one for minor (1db) changes and the other for major (10db) changes in response. These were plotted in 1980 using a Motorola Motrac at the discriminator point. As of 2009 this receiver is still in service !
Assuming U2 is a 7808, here are the typical operational specs for the board;
some 324 IC's produce lower "and" gate voltages and still work fine, since it's the relative difference between the two input levels that determine the logic output.
|condition||U1, pin 13||U1, pin 12||=U1, pin 14||PTT 1 & 2|
|'cor' active "1"||+2.54v||+3.41v||"high"||active|
|system timed out||+2.54v||2.03v||"low"||standby|
|"con 1" pulled low||.||2.6v||"low"||standby|
|Point of measurement||Level||Remarks||Noise floor (s/n)|
|"rec discr." input||-10 dbm||test tone of 1 KHz||.|
|IF trap output||-10 dbm||.||.|
|1st eq stage output||-16 dbm||Junc of 68K & 15 K res||.|
|Squelch switch||-22 dbm||Q1 collector||-63|
|U1 input-first stage||-58 dbm||pin 2||.|
|U1 output-first stage||-1.5 dbm||pin 1||-62|
|2nd eq stage output||-20 dbm||Junc of 68k & 10 K res||.|
|U1-6 input||-59 dbm||mostly noise||.|
|U1-7 output||+7.2 dbm||VR3 and VR6 at maximum||-52|
TLP Chart: For levels other mentioned in the above chart, change R1 value per input TLP.
These are maximum TLP's; you can run lower levels and/or lower R1 values, if desired.
|Input TLP||R1 Value||Remarks||.|
|+5||470 K||Or lower value||.|
|0||820 K||Or lower value||.|
|-5||1.5 Meg||Or lower value||.|
|-10||2.1 Meg||Or lower value||.|
|-15||5.6 Meg||Or lower value||.|
|-20||9.3 Meg||Or lower value||.|
|1||IC, quad op amp, ln324||u1||511-lm324an||00.34|
|1||IC, +8v regulator, 1.5a 7808||u2||511-l7808cv||00.40|
|2||IC, timer, 555||u3,u4||511-ne555n||00.72|
|5||Transistor, npn, such as 2n3904||q1-6||625-2n3904||00.50|
|2||Resistor, 1 meg, 1/4w, 5%||one is "rx" value||291-1m||00.07|
|6||Resistor, 100 k, 1/4w, 5%||.||291-100k||00.42|
|2||Resistor, 68k, 1/4w, 5%||.||291-68k||00.14|
|1||Resistor, 33k, 1/4w, 5%||.||291-33k||00.07|
|1||Resistor, 15k, 1/4w, 5%||.||291-15k||00.07|
|13||Resistor, 10k, 1/4w, 5%||.||291-10k||00.91|
|1||Resistor, 2.2k, 1/4w, 5%||.||291-2.2k||00.07|
|9||Resistor, 1k, 1/4w, 5%||.||291-1k||00.63|
|1||Pot, trimmer, multi-turn, 5 meg, inline leads||vr 3||hosfelt #38-184||01.35|
|2||Pot, trimmer, multi-turn, 2 meg, inline leads||vr 4,5||594-64w205||04.00|
|3||Pot, trimmer, multi-turn, 10k, inline leads||vr 2,6||594-64w103||06.00|
|4||LED;diffused,red:vr,org:du,yel:yy,grn:mg||sub"xx" for color||592-slr56xx3||00.72|
|4||Diode, 1n4148 or 914 type,400piv,150ns||.||583-fr104||00.40|
|2||IC socket, 8 dip||tin/solder||571-26404633||00.16|
|1||IC socket 14 dip||tin/solder||571-26403573||00.08|
|1||Capacitor, tantalum, radial, 150uf/15v||hosfelt part #||15-238||01.75|
|3||capacitor, elect, radial, 100uf/25v||.||140-xrl25v100||00.21|
|2||Capacitor, elect, radial, 10uf/25v||.||140-xrl25v10||00.10|
|3||Capacitor, elect, radial, 1uf/25v||.||140-xrl25v1.0||00.15|
|1||Capacitor, mylar, radial, .0082uf/100v||*||140-pf2a822f||00.43|
|1||Capacitor, mylar or disc, 390pf/50v||*||140-50p2-391k||00.06|
|1||Capacitor, mylar, radial, .22uf||hosfelt #;for u2||15-315||00.18|
|1||Capacitor, trimmer, 20 pf||455 khz if trap||242-3810-23||00.88|
|2||Choke, radial, 4.7 mh,9mm.dia.||455 khz if trap||434-01-472j||02.28|
|1||Board, COR/Audio, ak2o||far circuits**||ver 5.3||06.00|
|11||PVC colored wire, "6 long, 22-24 gu.||various colors||see notes below||.|
|6||Bare wire around 22-24 gu||for board jumpers||.||.|
|.||Parts cort, less shipping, etc.||as of march,2000||total||$29.09|
Unless otherwise specified, resistor values are in ohms 1/4 w, 10%, chokes in milli-Henries, caps in Micro-Farads.
The color of wires: Black, red, white, green, yellow, orange, blue, brown, violet, pink, slate.
Refer to the schematic diagram or other charts for color assignments for the functions of the board.
Time: Allow 2 hours labor for building and 2 more for alignment; common tools and solder equipment.
For a simulcast system it's important to know the audio from "Discr." to "AF out" is non-inverting. That's because of the two stages of inverting amplifiers. Because of this, "AUX AF" (flat) input is inverted from the "AF out". Even thought you normally wouldn't use it for repeat audio it could be an ID'er input or some other alarm indicator input. If it's not used either ground the input or leave out the 1 Meg resistor to avoid noise being picked up and amplified.
This board was designed by Karl Shoemaker, AK2O for: Spokane Repeater Group
To purchase, contact the manufacturer, FAR Circuits at: 18 N. 640 Field Ct.,Dundee,IL,60118, or email at: email@example.com,
Or visit their web site. Fred Reimers, KF9GX, is your contact.
For alternative parts sources contact: Mouser Electronics (800) 346.6873 or Hosfelt Electronics (800) 264.6464
Schematic drawing Standard strapping.
Image support is provided for purchases of this product.Circuit Diagram For this version
This may be copied in complete form only for non-profit purposes, such as for the knowledge for the Amateur Radio Service, with AK2O credited as designer. For other arrangements please contact the author.
Copywrite: AK2O 2006~present viewing date