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.
This board design occurred in the early 1980's by the Author. Older methods were used in the circuitry such as a passive potentiometer array for equalization and the LM-386 for audio amplifiers. Some of these "old school" boards are still in service as of 2009. In the 1990's recent versions of 5.x utilized a better, two stage audio equalization, and the quieter, LM-324 for the audio amplifiers and other logic circuits. Multi-turn pots were also used for easy alignment (no backlash problem). Since then the Author analyzed these features for future SRG projects. Some of the changes were based on input from other techs as well.
After some consideration in 2008 the Author designed version 6.0, which has repeater functions, such as the timers, etc. with slightly better parts use efficiency. That version is documented separately and was not put into production. However, you are welcome to use the ideas and circuitry keeping the Author as designer. For receiver downlinks 6.0 was unnecessary, therefore a "cut down" version of 7.0 was designed (this one). Then a "mirror" version (7.1) was designed for mounting the board upside down in a receiver chassis. That version, too was not produced, but kept in reserve in case of. It's the bases of later versions documented separately. Then, version of 7.2 was designed, specifically sized for mounting in place of the PL deck of a Mitrek radio. There's separate documentation for that version.
Changes for this version of 7.0
The audio section is the same as earlier versions, however, the IC pin-outs are not. The cor input and bias pins are unique to this version; normal JU1 strapping puts the cor on pin 5 of U1. Pay close attention to avoid construction issues and system failure. Another change from earlier versions is the 1 uf cap was added on the AF input to address a discriminator that has DC component. Also, the audio or PTT (or both) can be setup either for carrier squelch or "AND" (carrier + tone). The cor input still has polarity and sensitivity selection. The PTT output is available for active going low (force collector) or active going high (force collector). This version also is sized differently to fit inside the chassis of a receiver.
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
When the cor is active, U1 input translates polarity (depending on your jumper settings) and drives both the audio squelch and PTT circuits. 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 logic and timers, which drive the open collector PTT outputs, active going low, to key a transmitter. PTT 1 is active for every squelch open, for example, controlling audio switching in a patch or to key a remote base. The PTT 2 should key most modern transmitters with timer control for normal repeater operation. Transmitter disable controls are active low inputs. You have some choices. CON 1 simulates a system time out, causing the tail to finish out its time, then drops the transmitter. This affects both PTT 1 and 2. CON 2 gives immediate transmitter drop out, but does not affect PTT 1, unless the timeout timer expires (if used). Or you can use both CONs for control, depending on your needs. If you need positive going PTT outputs move Q2 and/or 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. (JU3) CON 1 or 2 are still available for control in this case.
U1, pins 12,13 and 14 are set up as an AND gate, which require user activity, but not over activity. This is for the FCC requirement of automatic repeater control. This section of U1 is controlled by voltage dividers, for three possible conditions; standby, active and timeout. In standby, pin 13 is higher than 12 and therefore, pin 14 is a 'low'. When cor causes pin 8 to go 'low', pin 13 goes lower than pin 12, causing pin 14 to go 'high' and starts the tail and transmitter key up. If the cor stops, the tail finishes out its time, then drops the transmitter. If the cor stays active too long, (i.e. 3 minutes) U4 times out and U4-3 goes 'low'. Even though cor caused U1-13 to go lower, now pin 12 is even lower than 13; in this case, which causes pin 14 to return to a low and the tail finishes out its time and drops the transmitter and stays dropped as long as the cor is active. When cor stops, both timers reset, which keys the transmitter and finishes out with a tail. This way the others on the system know when the time-out has cleared. Since the timeout controls the tail timer, add both timers when figuring the system transmitter time out. Time out range is 0:10~3:52 with a 100uf cap and tail is 10~30 seconds with a 10uf cap. Part list shows for the 150uf for longer time out. Try experimenting.
For down link receiver applications the timers and a lot of their associated parts can be left out. For example, if you don't need a tail, leave out U3 and run a jumper from U1-14 to U3-3. If you don't need a time out, leave U4 out and run a jumper from pin 3 to 4. U1-14 is now an inverter. A link transmitter can be keyed with either PTT output. See the documentation on versions 5.4 and 5.5 for full details for on these configurations.
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.
The input TLP should be -20 dbm or higher. For different inputs change R1 value, per the TLP chart further into this documentation. If you don't need a squelch you can leave out Q1. 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. When properly set, you have the option (discussed below) of using it as a nice "IDC" (deviation limit) because it's linear up to that point, then just flat tops with further increase input. Most conventional IDC circuits use diodes, back to back, which start causing distortion before the actual clipping point of the industry standard of +- 5 KHz deviation. Since Amateur stations are not required to have as much splatter control with harmonics, as with commercial stations, this should not be a problem. However, you should be aware of any possible bandwidth limitations in your area, since there is a trade-off between bandwidth and system performance. This board was developed in the Pacific NorthWest were we are blessed with 20 KHz spacing for repeater pairs. In other parts of the country with narrower spacing, make your calculated changes as needed. Pin 3 doesn't need bias adjustment because it should be running well below clipping, if you follow the TLP chart to control it's gain.
Next, inject a test tone into the receiver this board is being set up for and adjust VR3 to just clipping point and then, VR6 for the TLP (test level point) to drive your transmitter, or controller input. This "IDC" mode, is useful for final/system transmitter inputs. For links, please continue reading the paragraphs, below.
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 are some options. You can leave out VR6, experiment with fixed resistors as a pad/voltage divider, or leave them out all together. VR6 or any pads were designed to further improve the S/N performance. With them out U1 typically is 52 db. If this is a stand alone station, this value is plenty, however, in multiple links (more than 3) noise can add up, therefore VR6 or a pad keeps the noise to a good level. There are some extra pads on the printed circuit board to do this kind of modification. 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.
For links, each time you limit deviation for each hop will add more distortion. In the past, this had been typical with both commercial and Amateur repeaters, which produces a less than optimal system. For superior audio it is highly recommended to run all your links in "passive" mode, only limit at the last point, such as the system's main transmitter. SRG (system) specification is to set the system output transmitter limit at 6 KHz and let the user's transmitters limit at 5 KHz deviation. This mode requires system management, technician maintenance discipline and user responsibility. This may require some enforcement on user's part. A circuit to "punish" over-deviated users is possible, however, is beyond the scope of this documentation. For this mode, install a pad in place of VR6 and use VR3 to control the AF output. The values will depend on the transmitter's TLP. One example is a 6 db pad and run the board's output at 0 dbm for a TLP.
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 !
Some commercial receivers either don't have an AGC (S) meter or their "M1" output will not properly drive a meter with a meaningfull scale. Most Amateur receivers do have an AGC meter, however typically give an "early" (generous) reading with weak signals. This is a waste of indication. FM receivers quite with signals, therefore, you can easily listen for these changes when checking performance. When the signal gets almost full-quiting is when you need a visual (meter) indication to observe signal strengh changes. This circuit will do just that in this version. U1 amplifies the receiver's AGC voltage, then with a strong signal will flaten out with no increase in output. This makes a handy logrimic voltage change, per RF input changes.
U1, pin 10 inputs a fairly wide range of receiver detected/IF amplifier DC meter function. Pin 9 sets the reference (bias) and pin 8 output drives most meters. The output has a resister in series to limit current to the ACG meter. The value can be changed to work with most any meter. Defualt for this version is 10K ohms.
Start by measuring the "M1" or AGC point of the receiver you are setting up. The circiut prefers to "see" around a tenth of a volt, DC, or less with no RF signal into the receiver. Adjust VR5 for this range. Then input a high RF signal into the receiver to cause a hard-limiting condition. Typically this would be in the -60 dbm range. Then adjust VR4 for a full scale meter reading. Then you can plot an AGC curve. If it's not a usable curve try different settings of the two adjustments just discribed.Test Level Points (Bridged and using a 7808 for U2). (will be updated later).
|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||00.68|
|1||IC, +10v Regulator, 1.5a 7810||U2||511-L7808CV||00.40|
|2||Transistor, NPN, such as 2N3904||Q1,Q3||625-2N3904||00.50|
|1||Transistor, PNP, such as 2N3906||Q2||625-2N3904||00.50|
|1||Resistor, 1 Meg, 1/4w, 5%||If "R1" is that value||291-1M||00.07|
|1||Resistor, 220 K, 1/4w, 5%||.||291-220K||00.42|
|6||Resistor, 100 K, 1/4w, 5%||Excluding cor pull-down||291-100K||00.42|
|2||Resistor, 68K, 1/4w, 5%||.||291-68K||00.14|
|1||Resistor, 15K, 1/4w, 5%||.||291-33K||00.07|
|5||Resistor, 10K, 1/4w, 5%||Including cor pull-up||291-10K||00.91|
|2||Resistor, 4.7K, 1/4w, 5%||.||291-4.7K||00.91|
|2||Resistor, 2.2K, 1/4w, 5%||.||291-2.2K||00.91|
|8||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|
|1||Pot, trimmer, multi-turn, 1 Meg, inline leads||VR 5||Hosfelt #38-183||01.35|
|3||Pot, trimmer, multi-turn, 10K, inline leads||VR 1,2,4||594-64W103||06.00|
|2||LED;Diffused,Yel:YY,Grn:MG||Sub"xx" for color||592-SLR56xx3||00.15|
|1||IC socket 14 DIP||tin/solder||571-26403573||00.08|
|2||Capacitor, Elect, radial, 100uf/25v||U2 filter||140-XRL25V100||00.21|
|1||Capacitor, Elect, radial, 10uf/25v||.||140-XRL25V10.0||00.15|
|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||Board, cor-audio, AK2O||FAR Circuits**||ver 7.0||unknown|
|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 count, less shipping, etc.||As of March,2000||Total||$20.00|
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 at: http://www.srgclub.org
This board was not produced. The design-ideas apply to other versions. Therefore, the accuracy of this version is not verified.
For alternative parts sources contact: Mouser Electronics (800) 346.6873 or Hosfelt Electronics (800) 264.6464
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