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Text File | 1988-07-15 | 25KB | 622 lines

2211 Modem Alignment Procedure Eric Gustafson, N7CL 2018 S. Avenida Planeta Tucson, AZ 85710 (602)-747-1410 (c) 17 JUN 88 Many thanks to Dan Morrison, KV7B, for his valuable time spent reviewing this document and his constructive criticism during the compilation of this procedure. ACCURATE HF MODEM ALIGNMENT PROCEDURE FOR TERMINAL NODE CONTROLLERS (TNC) USING THE EXAR 2211 / 2206 BASED AFSK MODEMS ********** This procedure, although primarily intended for TNC-2 and clones (including MFJ-1274), includes some information specific to the MFJ-1278. Some of the software calibration facilities present in the newer 1278 may not be available in the 1274 or TNC-2 (and clones). Specifically, references to RXCAL in the demodulator alignment procedure are for 1278 rev 7 board running VER 1.2 (ROM checksum ($A1)) or later firmware. The tune up principles presented here can be applied to any AFSK modem based on the Exar 2206 / 2211 ICs. This method consists essentially of using the modulator to send 50 percent duty cycle data to the demodulator for alignment purposes. This is not only the easiest but also the best, most consistent method for aligning this type of modem. Modems not attached to one of the TNC-2 variants can also be aligned using this method. All that is required is a source of 50 percent duty cycle square wave data (at a baud rate appropriate for the modem application) injected at the modulator data input. IMPORTANT! This procedure is a three step process. The three steps are: 1. Center the modulator tones over the required modem center frequency (Fc). 2. Align the demodulator center frequency. 3. Align the tuning indicator. ALL THREE STEPS SHOULD BE PERFORMED IN THE ABOVE ORDER! If they are not all done at the same time, or in the correct order, the modem may not be receiving and transmitting on the same frequency and the tuning indicator may not indicate properly. The third step, alignment of the tuning indicator, should only be used in conjunction with aligninment of the 300 baud 200 Hz shift demodulator. References to part or jumper numbers which are specific to a particular TNC are noted. Part or jumper numbers without specific reference to a particular model are the same for all models not specifically referenced. Some understanding of the use of ordinary test equipment is assumed. The 1200 baud modem in the TNC-2 and 1274, and the other AFSK modems available in the 1278 can be aligned using exactly the same steps presented here but substituting the appropriate part numbers for the adjustments. However, do NOT align the tuning indicator to anything but the 300 baud 200 Hz shift HF packet modem. COMPREHENSIVE ALIGNMENT PROCEDURE It is important that the tuning indicator alignment be optimized for the 300 baud 200 Hz shift HF packet modem. It will indicate correctly for all other modes when aligned for the HF packet modem. NOTE! Regardless of the type of modem, whether or not the modem has audio filtering built in, 300 baud AFSK modem performance on a High Frequency linear mode (SSB as opposed to NBFM) radio channel will NOT be optimum UNLESS a filter of approximately 500 Hz bandwidth is used in the radio IF strip. There are two reasons why this is the case. First, for the filtering to be fully effective, it has to preceed the first hard limiter in the system. This limiter is typically located in the first stage of the demodulator. Second, no filter at audio can prevent an off channel interfering signal from capturing the receiver AGC system and causing wide variation in the level of the audio presented to the demodulator. All demodulators are affected by audio level variations. If a narrow (approximately 500 Hz) filter is to be used in the radio for HF packet and RTTY operation (and this is STRONGLY recommended), it may be necessary to use a tone pair centered on the radio's filter rather than the "standard" 2120/2320 or 1600/1800 Hz pair. If the radio lacks IF shift capability, this will almost certainly be necessary. It will be necessary to determine the center frequency of the audio passed by the IF filter when the narrow filter is selected and the radio is in the LSB mode. NOTE! If the radio to be used DOES have IF shift capability, the modem can be aligned on one of the "standard" tone pairs and the IF shift control can be used to center the radio's filter over the modem center frequency. You may find it convenient, however, to do the alignment so that the IF shift control remains centered, or on its detent, and therefore doesn't require readjustment when switching operating modes. Some radios will require slight modification to allow selection of a narrow filter in SSB mode. Most radios which provide for direct FSK RTTY operation use the radio's narrow CW filter if one is installed. The Kenwood TS-820 is one example. Packet operation using direct FSK is a viable mode but extreme caution should be exercised to make sure TX and RX frequencies are identical and that the transmitted frequency pair is centered in the radio's 500 Hz filter passband. Once the required modem center frequency for the particular radio's FSK mode has been determined, the modem alignment procedure presented here may (should) be used to align the modem for this mode. A method for making the determination of transmitter FSK output frequencies and their relation to the LSB carrier oscillator and center of the 500 Hz filter is NOT presented here. Contact the manufacturer of the radio to obtain this information. Radios which have provision for an auxilliary "narrow SSB" filter can have the 500 Hz bandwidth filter installed instead of the approximately 1.8 KHz wide "narrow" SSB filter. This will allow direct selection of the narrow filter for AFSK work when in SSB mode. If the filter is also desired for CW operation and you don't want to invest in 2 identical filters, it is usually a relatively simple modification to cause the radio to select this filter when in CW mode too. In the TS-430, for example, this requires moving one end of one diode on the IF board. The 270 Hz filter can then be installed in the "normal" narrow CW filter position making two bandwidths of narrow filter available for CW operation with all filters selectable from the front panel. If the HF radio is to be dedicated to packet use, for instance as a BBS or for whatever reason (we really don't need any more privately run BBSs), The 500 Hz filter can be installed in place of the SSB filter. This is possible on ANY radio designed for SSB and CW use. Once the radio is configured with a narrow filter for AFSK work, one of the following 2 methods should be used to make the radio filter center frequency (Fc) determination. Both methods require access to a frequency counter capable of measuring audio frequencies to a resolution of 1 Hz. Almost any frequency counter should be capable of this. NOTE! The signal actually transmitted will cover a band of frequencies approximately 400 Hz wide and centered at the transmitter's indicated SSB carrier frequency (F(ind)) minus the modem center frequency (when using LOWER sideband for AFSK work). So use F(emission) = [ F(ind) - Fc ] to determine the actual operating frequency for band edge or netting purposes and remember to consider that you will be occupying a few hundred Hz on either side of F(emission). First method: NOISE AVERAGE FREQUENCY This method also requires an active noise source like a receiver noise bridge. I have been using a unit made by Palomar Engineering for this purpose. 1. Set the receiver to LSB mode with the 500 Hz filter selected. 2. Connect the noise source to the receiver input. Make sure there is no antenna connected to the system and that the receiver is tuned to a frequency which is free of coherent internally generated signals (birdies). 3. Set the noise source output for a reading of approximately S9 on the receiver S meter. 4. Connect the counter to the receiver audio output 5. Adjust the receiver output level for enough audio to reliably trigger the counter. 6. Make sure the radio's IF shift control, if one is present, is in its proper position (centered or on detent). 7. Record the frequency indicated by the counter. The counter should indicate the average frequency of the noise spectrum passed by the filter in the receiver and translated to audio by the product detector. This will be the frequency used for the modem center frequency (Fc). NOTE! If the counter is a phase locked loop (PLL) based prescaling type, its PLL may not lock properly to the noise signal. If this is the case, use method 2 below. If your counter can resolve 1 Hz with a counting gate time of less than 1 second, it is a PLL prescaling counter. Second method: FILTER SKIRT AVERAGE FREQUENCY 1. Set the receiver to LSB mode with the 500 Hz filter selected. 2. Make sure the radio's IF shift control, if one is present, is in its proper position (centered or on detent). 3. Using either a signal generator or a stable, relatively strong carrier from an AM broadcast transmission (preferably ground wave signal), tune the receiver so that the carrier falls near the center of the filter passband. Choose a signal level near S-9 for this test. If your rig has a built in calibration oscillator, this is a good source for this signal. 4. Slowly and carefully tune the receiver so that the tone frequency is decreasing. 5. Find the point where the signal is reduced by 1 S-UNIT from the peak value reached near the center of the filter passband. 6. Measure this tone frequency with the counter. Record this value as F(low). 7. Slowly and carefully tune the receiver so that the tone frequency is increasing. 8. Find the point where the signal is reduced by 1 S-UNIT from the peak value reached near the center of the filter passband. 9. Measure this tone frequency with the counter. Record this value as F(high). 10. Calculate the required modem center frequency as: Fc = [F(low)+F(high)]/2 Once the required center frequency has been determined, the modem calibration can be carried out. NOTE! It is essential that the modulator tones be properly aligned FIRST as they will be used to align the demodulator center frequency. Set the modulator tones to Fc MINUS 100 Hz for the low tone and Fc PLUS 100 Hz for the high tone using one of the following two procedures. SET MODULATOR TONES USING FREQUENCY COUNTER 1. Make sure that the modem and TNC are both configured for 200 Hz shift 300 baud operation. This is done by typing "MODE HP <CR>" from the command prompt on the 1278. On the 1274 this is accomplished by means of the HF/VHF pushbutton switch on the rear panel. On a TNC-2 or clone this is done by making sure the HF modem header parts are in place on the circuit board and that the rear panel radio port baud rate selector switch is set for 300 baud. 2. Install push on jumper at JMP-4. This is the watchdog timer defeat jumper. 3. Connect the counter input to JMP 9, pin 1. This is the TP 1 end of R61 in a TNC-2. 4. Command the TNC into calibrate mode by typing "CAL <CR>". 5. Command the TNC to key the modem by typing "K". 6. Select the low tone by pressing the space bar until a voltmeter connected to U16, pin 9 reads - 5 volts. This signal is more conveniently available at the junction of Q12 collector and R62. 7. Adjust R106 until the counter indicates the intended low tone frequency as determined above. This is R78 in a TNC-2. 8. Select the high tone by pressing the space bar once. 9. Adjust R105 until the counter indicates the intended high tone frequency as determined above. This is R77 in a TNC-2. 10. Return the TNC to command mode by typing "Q". This completes the alignment of the modulator tones using a frequency counter. SET MODULATOR TONES USING BUILT IN CALIBRATION SOFTWARE This method will be slightly less accurate than using a frequency counter but it should be possible to get within +/- 5 Hz using this method. 1. Make sure that the modem and TNC are both configured for 200 Hz shift 300 baud operation. This is done by typing "MODE HP <CR>" from the command prompt on the 1278. On the 1274 this is accomplished by means of the HF/VHF pushbutton switch on the rear panel. On a TNC-2 or clone this is done by making sure the HF modem header parts are in place on the circuit board and that the rear panel radio port baud rate selector switch is set for 300 baud. 2. Place a push on jumper at JMP 4 as in the above procedure. 3. Place a push on jumper at JMP 9, pins 1 and 2. This connects the modulator square wave output to the SIO chip so that the CPU can measure the tone frequency for you. In a TNC-2 this jumper goes on JMP 9, pins 1 and 6. 4. Type "CALSET n <CR>". Where n is a number determined by: n = INT [ 525000 / F(low) ] + 1 This tells the CPU what tone frequency you are trying to achieve. A table of CALSET numbers to use for the various "standard" modem tone frequencies will be included in an appendix at the end of this procedure. 5. Command the TNC into calibrate mode by typing "CAL <CR>". 6. Command the TNC to key the modem by typing "K". 7. Select the low tone by pressing the space bar until a voltmeter connected to U16, pin 9 reads - 5 volts. 8. Adjust R106 carefully until both the "CON" and "STA" LEDs on the front panel of the TNC are lit. This is R78 in a TNC-2. 9. Type a "Q". This exits the TNC from calibrate mode to command mode. 10. Type "CALSET n <CR>". Where n is a number determined by: n = INT [ 525000 / F(high) ] + 1 11. Command the TNC into calibrate mode by typing "CAL <CR>". 12. Command the TNC to key the modem by typing "K". 13. Select the high tone by pressing the space bar until a voltmeter connected to U16, pin 9 reads + 5 volts. 14. Adjust R105 carefully until both the "CON" and "STA" LEDs on the front panel of the TNC are lit. this is R77 in a TNC-2 15. Remove the jumper placed at JMP 9. This completes the modulator tone alignment using the built in calibration facility. Now that the modulator tones have been properly centered over the intended modem center frequency (Fc), the demodulator center frequency will be aligned using one of the following two methods. The second method using the built in calibration facility of the 1278 is preferred since it is as accurate as alignment with an oscilloscope but requires no digging around on the 1278's PC board looking for signals. NOTE! If a TNC-2 or clone (including the 1274) is to be used on HF packet behind a radio with a 500 Hz filter in it, it will be necessary to modify the demodulator Data Carrier Detector (DCD) circuit. The details of this modification are given in an appendix at the end of this procedure. The alignment procedure will work properly and result in correct alignment of the demodulator whether or not these modifications have been done. DEMODULATOR CENTER FREQUENCY ALIGNMENT USING AN OSCILLOSCOPE The oscilloscope used in this procedure will be used only to indicate when the output data stream from the demodulator has reached a duty cycle of exactly 50 percent. The 'scope can be used to do this in two ways. The first is on the basis of time and the second is by integrating the square wave for zero DC offset. The second method is the most accurate as it is insensitive to the normal jitter in the data introduced by the demodulation process. Both methods will be described in an appendix at the end of the procedure. NOTE! If you are performing this alignment on a 1274 or a TNC-2 clone, please make sure that JMP 8 is installed. This jumper is required for normal operation of the demodulator. 1. Place push on jumpers at JMP 4 and JMP 7 if they are not already in place. 2. Remove jumper at JMP 9 if it is still in place. 3. Connect the vertical input of the oscilloscope to the end of R68 which is connected to Q13's collector. This is the data stream coming out of the 2211 demodulator chip. 4. Command the TNC into calibrate mode by typing "CAL <CR>". 5. Command the TNC to key the modulator by typing "K". 6. Command the TNC to send a 50 percent duty cycle test data stream from the modulator by typing "D". 7. Adjust R113 carefully until the DCD LED on the front panel is fully lit. Continue adjusting R113 until the oscilloscope indicates that the square wave at Q13's collector has a duty cycle of exactly 50 percent. This is R79 in a TNC-2 NOTE! The threshold control on the 1278 must be set to a position which will allow the DCD circuit to function properly. A setting approximately 1/3 to 1/2 of the total range starting from maximum CCW should be adequate. 8. Return the TNC to the command mode by typing "Q". This completes the alignment of the demodulator center frequency using an oscilloscope. DEMODULATOR CENTER FREQUENCY ALIGNMENT USING BUILT IN CALIBRATION FACILITY NOTE! If you are performing this alignment on a 1274 or a TNC-2 clone, please make sure that JMP 8 is installed. This jumper is required for normal operation of the demodulator. NOTE! The RXCAL demodulator calibration routine used in this procedure is superior to the original TNC-2 and 1274 built in demodulator calibration routine. However, versions of the TNC-2 and 1274 firmware prior to 1.7 will NOT have the RXCAL feature available. I strongly recommend that you obtain firmware updates for your TNC-2 as soon as they become available. 1. Place push on jumpers at JMP 4 and JMP 7 if they are not already in place. 2. Remove jumper at JMP 9 if it is still in place. If you are calibrating a TNC-2 which has firmware updated to include RXCAL, place the jumper removed from JMP 9 pins 1 and 6 on JMP-9 pins 3 and 4. 3. Set the baud rate for use by RXCAL to 300 baud by typing "CALSET 32 <CR>". This will cause the modulator to send a square wave FSK signal when RXCAL is invoked. 4. Command the TNC to do a demodulator calibration by typing "RXCAL <CR>". 5. CAREFULLY adjust R113 until the DCD LED on the front panel is fully illuminated. This is R79 in a TNC-2 or clone. NOTE! The threshold control on the 1278 must be set to a position which will allow the DCD circuit to function properly. A setting approximately 1/3 to 1/2 of the total range starting from maximum CCW should be adequate. Continue to VERY CAREFULLY adjust R113 until the STA and CON LEDs on the front panel either change state very slowly (alternate which one is lit) or until both are simultaneously illuminated. NOTE! This is a very critical adjustment and it is unlikely that you will be able to cause both LEDs to be turned on simultaneously for longer than a very brief instant. 6. Return the TNC to command mode by typing a "Q". This completes demodulator center frequency alignment using the built in calibration facility. Now that the demodulator and modulator are properly aligned to one another, the tuning indicator can be set for proper center indication. NOTE! If you have a TNC-2 or clone which you intend to use on HF packet and it doesn't have a tuning indicator already installed, I STRONGLY recommend that you to obtain one of the TAPR tuning indicator kits or an exact clone of one if available from a separate source. Operation on HF packet without a tuning indicator will be very frustrating and contribute to much useless interference on already overcrowded pseudo CSMA packet channels. TUNING INDICATOR ALIGNMENT This procedure adjusts the tuning indicator so that it correctly indicates when a signal is properly tuned in relation to the demodulator center frequency. NOTE! The tuning indicator should ONLY be adjusted in reference to the 300 baud 200 Hz shift modem used for HF packet and RTTY. If adjusted for this modem, It will indicate with sufficient accuracy for all other modes. NOTE! The tuning indicator is NOT effective for tuning the 1278 on CW receive. On CW, it should be used only to get into the ballpark. Final tuning for CW mode should be based on the DCD LED. 1. If you have not just finished aligning the modem as per the above instructions, do so now. Otherwise you may be aligning the tuning indicator to an incorrect setting. 2. Install push on jumpers at JMP 4 and JMP 7 if they are not still in place from the modem alignment procedure. 3. Command the TNC into calibrate mode by typing "CAL <CR>". 4. Command the calibration routine to send 50 percent duty cycle square wave data from the modulator by typing "D". 5. Command the TNC to key the modem by typing "K". 6. While observing the LEDs in the tuning indicator, adjust R212 in the tuning indicator area so that either the 10th or 11th led from the left (or both) are illuminated. If you are using the add on tuning indicator, this is the only variable resistor on the board. 7. Return the TNC to command mode by typing "Q". 8. Remove jumpers at JMP 4 and JMP 7. This completes the tuning indicator alignment. Now the modulator, demodulator, and tuning indicator are all aligned to the same center frequency. If you notice that on the air reports suggest that you are transmitting and receiving on significantly different frequencies after successfully completing the above alignment procedure, it is possible that the radio needs realignment. Many different transcievers for HF are capable of being misaligned due to reference oscillator crystal aging or careless alignment by amounts exceeding 500 Hz. RIT inadvertently left on can also cause hard to detect problems in this area. Reliable HF packet communications requires that the frequency error presented to the demodulator be below 30 Hz. If several stations are to successfully share a single channel, transmitter / receiver offsets larger than this will be intolerable.