COR Board version 5.3



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. Most components are not labeled, therefore, you need to 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.

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 receiver's TLP for this project are in the -10~0 dbm area, however, a resistor value change can be made for your receiver.

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. "PTT-2" will describe a buffered, and timed output of the COR/AF board, to keep a repeater's transmitter keyed up (tail) 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. "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. For a though discussion on this subject the Micor squelch document can be accessed here.

"HUB" means Hang Up Box. Motorola's uses a "closed loop" and a HUB for mobiles and base station control. "CON" means control, for the PTT outputs of the cor board. There are two types, 1 and 2, which are discussed later.

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 time-out controls the tail timer, add both timers when figuring the system transmitter time-out. Time-out is adjusted with VR5, while the tail time is adjusted with VR4. Time-out range is about 10 seconds to 2 minutes, 30 seconds with a 68uf cap on U4, pin 6. This range can be increase to about 3 minutes, 48 seconds with a 100uf cap instead.

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 install a 1K resistor from pin 3 to 4. U1-14 now, just 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:

  • Non-inverting: For a cor that is high or higher on standby; then goes low or lower for active, jumper A-B and C-D . This would be in the case of the Mitrek's cor point " E" . Neither the pull or down resistors are used in this case.

  • Inverting: For a cor that is low or lower on standby; then goes high or higher for active, jumper A-D and C-B. This would be in the case of the Micor's shunt output or other conventional receivers. Use the pull-up resistor on the cor board, but not the pull-down in this case.

    COR points:


  • For "H" method, pin 9 would be the input, and pin 10 would be the bias. You set the bias less than the cor active high voltage. For this arrangement JU1 is set to inverted. Refer the schematic for clarification.

  • For "L" method, pin 10 would be the input, and pin 9 would be the bias. You set the bias less than the cor standby voltage, but more than the active low voltage. For this arrangement JU1 is set to non-inverted.

  • For "E" method, pins 10 and 9 are the same as "L". The difference with "E" is almost a 'logical' change with squelch, rather than a DC analog movement. This method is the most common used, including in the Super Consolette stock configuration. This method is used for SRG radio links.


  • The micor has an audio squelch board with a transistor collector type output called a "shunt" in its diagram. During standby it's "on", pulled to ground. During activity it relaxes. Therefore, a pull-up resistor is required to produce a voltage. Use the (optional) pull-up resistor shown, but not the pull-down. They are the grayed out components in the cor section of the COR board's schematic. If this is unclear (due to graphics) refer to the diagram for version 5.5, found on this site.

    Other (conventional) receivers:

  • Most other receivers may have a similar cor (open collector) output like the Micor receiver. If not, you would be "picking" off a voltage (change) point, similar to the Mitrek receiver configuration. In this case you would not use either the pull-up or down resistors on the COR board.

    Squelch modes (tone or carrier):

  • Connect the receiver's cor point to the cor input on the cor board. To make both the repeat audio and PTT outputs to an AND squelch connect tone decoder to PLI. The decoder line needs to be an active going relaxed output. A good choice is the Comm. Spec's TS-32 tone decoder, using it's "Out-2" with its "JU2" cut. If the TS-32 is unavailable the TS-64 will substitute. One disadvantage to this arrangement is the increase delay for an audio signal to get through the repeater. Quick user's first word may be cut off. This arrangement does have the most protection against RFI, however.

  • To make the repeat audio on carrier but the PTT outputs an AND squelch connect the TS-32 to CON 1 instead. For this mode the CON 1 is now being used as a PLI.

  • If you wish both the audio and PTT 2 outputs to be on carrier don't connect anything to CON 1 or PLI (just the cor input). For the Micor receiver the pull-up resistor would still apply.

  • Micor decoder: This cor board version needs to see a decoder's open collector going relaxed for activity. The stock PL deck does not have this feature, however, you have some options. You can modify the PLI. Reverse the diode and move the pull-up resistor to between the pull-down and the diode. Now the PL deck will provide the (divided) voltage for the AND gate's logic. Another option is to modify the PL deck to act like a conventional decoder (as described earlier). Another option is to make the micor (remote) receiver's both audio and PTT AND squelch. This is done by using the stock receiver/PL deck with an AND PTT output going to the cor board's cor input. In this case you would not use CON 1 (only the cor input). The same pro and cons exist as with any audio AND squelch as mentioned earlier.

    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 will need to have a good stock of resistor and capacitor values. It's very handy to have resistance and capacitance substitution boxes to test different values.

    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 on the value chart.

    Receiver TLPs (Discriminator output) and Eq starting values Chart: (will be updated as determined)

    Brand Model TLP R1 value EQ-1 Series resistor EQ-1 Series Cap. EQ-1 Shunt EQ-2 Series resistor EQ-2 Series Cap. EQ-2 Shunt
    Motorola Micor -5 1.8 M 68 K ohm 560 pf 15 K 82 K 220 pf 15 K
    Motorola Mitrek -5 1.8 M 68 K ohm .0082 uf 15 K 68 K 330 pf 10 K
    Motorola Motrac +2.6 560 K 68 K ohm .0082 uf 22 K 68 K 390 pf 10 K
    Johnson Fleetcomm II -12 3.9 M TBD TBD TBD TBD TBD TBD
    Midland 13-509 -8 3.9 M TBD TBD TBD TBD TBD TBD

    Re plot the receiver. 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.

    Another point to remember modern receivers probably 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.

  • Diagram COR board version 5.3
  • Frequency response Before (stock).
  • Frequency response After (with eq).
  • Large Photo of completed board , less the IF filter; 1280x960 pix
  • Smaller Photo of above 600x450 pix

    Image support is provided for purchases of this product.

    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 2011 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
    standby  +5.24v  +3.41v  "low"  standby 
    '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

    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 most receivers R1 can be 1 Meg Ohm. 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. Just keep pin 1 output below a +7 dbm (TLP)to prevent clipping. (clipps at +10 if U2 is a 7810)

    Input TLP R1 Value  Remarks .
    +10 270 K Ohm Or lower value .
    +5 560 K Ohm Or lower value .
    0 1 Meg Or lower value .
    -5 1.8 Meg Or lower value .
    -10 3.9 Meg Or lower value .
    -15 7.5 Meg Or lower value .
    -17 9.1 Meg Or lower value .

    COR board parts list:

    QYT Description: Notes: Part number: Approx cost:
    1 IC, quad op amp U1 511-LM324AN .50
    1 IC, +10v regulator U2 511-1781CV 1.50
    2 IC, timer, 555 U3, U4 511-NE555N .50
    5 Transistor, NPN Q1~6 625-2N3904N .50
    2 Resistor, 1 M ohm One is R1 value 291-1M .04
    6 Resistor, 100 K . 291-xx .12
    2 Resistor, 68 K . 291-1xx .04
    1 Resistor, 33 K . 291-xx .02
    1 Resistor, 15 K . 291-xx .02
    13 Resistor, 10 K . 291-1M .26
    1 Resistor, 2.2 K . 291-xx .02
    1 Pot, trimmer 5 Meg VR 3 TBD 2.00
    2 Pot, trimmer 2 Meg VR 4,5 291-xx 4.00
    3 Pot, trimmer 10 K VR 2,6 291-xx 6.00
    4 LED, diffused T1 3/4 G,R,Y,O TBD .40
    4 Diode, IN4148 . TBD .20
    2 IC socket, 8 DIP . TBD .30
    1 IC socket, 14 DIP . TBD .25
    1 Capacitor, electrolytic 100uf/20v TBD 2.00
    2 Capacitor, electrolytic 10uf/25v TBD .60
    1 Capacitor, electrolytic 4.7uf TBD .30
    2 Capacitor, electrolytic 1uf TBD .60
    1 Capcitor, mylar .0082uf TBD .30
    1 Capacitor, mylar 390pf TBD .30
    1 Capacitor, mylar .22uf TBD .30
    2 Capacitor, mylar .1uf TBD .60
    1 Capacitor, trimmer 20pf TBD 1.00
    2 Choke, radial 4.7mh TBD 1.00
    1 Header, double .100" spacing All DHS-6 .25
    2 Shorting jumper .100" spacing All SJ-1 .20
    1 Board, Cor Version 5.3 Far circuitsfar circuits**  8.00
    11 Wire, hookup 22 AWG various colors 1.00
    4 Wire, bare 22 AWG For jumpers .10


    Parts: unless otherwise specified, resistor values are in ohms 1/4 w, 10%, chokes in milli-Henries, caps in Micro-Farads, all in radial leads. The color some or all of the 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. First time: allow 4 hours labor for building and 4 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 though 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.

    In 2010 bypass capacitors were added to the circuity. The updated schematic drawing shows this. For old PCB just solder these caps on the solder side, near a ground. They can be .1~.01 uf range. If operating in high RF environments these may help, where the 100uf electrolytics won't.

    This board was designed by Karl Shoemaker, AK2O for: Spokane Repeater Group at: A high quality, hard copy of the schematic is available for a modest cost of shipping. (specify size desired) To purchase, contact the manufacturer, FAR Circuits at: 18 N. 640 Field Ct.,Dundee,IL,60118, or email at:, 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 Jameco Electronics (800) 831.4242

    Circuit Diagram For this version
    COR/AF board ver 5.3 PDF for 2016

    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

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