Technical Service Bulletin 030602

Technical Service Bulletin 030602 Adjustment of IOX LF Linearity, ICPM, (Intermodulation)

Precorrection Circuits

Procedure 030602: Adjustment of IOX Linearity, ICPM, and Intermodulation Precorrection Circuits

Applicability All IOX transmitters with common amplification (internally diplexed).

Prerequisites Correct setup of NTSC test equipment (030626). Correct video levels (030523). Correct IOT bias voltage / idle current (030525). Correct IOT filament voltage (030524). Correct IOT beam voltage (030611). Correct IOT tuning (030528). Correct visual power level (030530). AGC operational (030531). Correct aural power level (030601).

Equipment Required Consult Service Bulletin 030626.

Comments Adjustments are highly power level dependent and will affect power level. It is essential that all prerequisites (especially for proper AGC operation) be met before performing this procedure.

1. Apply modulated ramp video signal to transmitter. Poor transmitter (transfer curve)

linearity will appear as a bowing of ramp shape and fine scalloping superimposed on the chroma envelope.

NOTE: The possibility of intermodulation does not exist on those transmitters with separate amplification (externally diplexed). In such cases, the only requirement is to minimize low frequency linearity and ICPM, as described in steps 4 and 5.

2. Observe level of +920 kHz intermodulation product relative to peak visual carrier on

spectrum analyzer with a resolution bandwidth of 300kHz and the video filter deactivated. The +920 kHz intermodulation product should be at least 55dB below peak visual carrier reference level. If this is not the case, the linearity and ICPM correctors require readjustment.

REV B - 22 March 2004 1

Technical Service Bulletin 030602

3. Observe intermodulation level versus video amplitude by engaging differentiating filter on waveform monitor. The +920 kHz intermodulation product will appear as a fuzzy band on differentiated display. Intermodulation occurring at high power portions of amplifier transfer curve (black portion of ramp) appears at left of the screen. Intermodulation occurring at low power portions of amplifier transfer curve (white portion of ramp) appears at right of screen.

Unacceptable Acceptable

4. Intermodulation may be caused by poor amplitude linearity, phase linearity, or a combination of both. To obtain a rough indication of the source of the problem, apply monochromatic stairstep video signal to transmitter and measure low-frequency (LF) linearity with differentiating filter on waveform monitor.

REV B - 22 March 2004 2

Technical Service Bulletin 030602

To measure LF linearity, use vertical scale and vertical position functions on waveform monitor to establish a baseline at 0IRE (0%) and align highest differentiated spike at 100IRE (100%). Linearity performance is read directly from screen as the percentage difference between highest spike and lowest spike. As in the case of the differentiated ramp, linearity performance at high power portions of amplifier transfer curve is read from left of screen; low power linearity performance is read from right of screen.

Unacceptable (LF LIN = 19%) (compression at high power levels)

Acceptable (LF LIN < 10%)

5. To determine ICPM performance of transmitter, engage ICPM measurement function on

waveform monitor and measure ICPM level directly in degrees. To accurately perform this measurement, the zero carrier marker (chopper dot) must be activated on demodulator and be aligned to the "zero carrier" cross hairs appearing at top center of waveform monitor screen.

Unacceptable (ICPM = 4.7 deg) (phase lag at high power levels)

Acceptable (ICPM < 2 deg)

6. The results of the LF linearity and ICPM measurements will give some indications as to

which correctors on the exciter B1 module will be required to correct +920 kHz intermodulation products. Because the B1 module correctors act upon the complete

REV B - 22 March 2004 3

Technical Service Bulletin 030602

visual + aural IF envelope, the +920 kHz intermodulation product will be simultaneously minimized as the LF linearity and ICPM distortions are successfully corrected.

NOTE: Under certain circumstances, the point of perfect LF linearity and ICPM will NOT coincide with optimum +920 kHz intermodulation suppression. This is especially true with IOTs produced before approximately 1997. This was due to a low-frequency "video" bypassing problem in the tube input section, which has been largely eliminated in newer IOTs. In such circumstances, the correction should be set to minimize the +920kHz intermodulation level at the expense of LF linearity and ICPM performance. The residual ICPM level may be corrected with the B3 ICPM corrector described in Service Bulletin 030604.

The LF linearity and ICPM correctors both work on the principal of diode breakpoint correction. That is, a portion of the modulated IF signal envelope is selectively stretched in amplitude, or pulled in phase, according to DC-biased diodes that switch on at certain user-controlled signal threshold levels. The amount of predistortion applied by the correction circuits is user-controlled by means of a separate "slope" control for each diode breakpoint.

Predistorted signal measured at output of exciter. LF linearity corrector creates complementary intermodulation that cancels intermodulation occurring in IOT.

The exciter B1 module has three high-level and three low-level linearity correction breakpoints with individual threshold (B1 - 24 to 26, 14 to 16) and slope (B1 - 27 to 29, 17 to 19) adjustments. The high-level correctors perform "sync & black stretch" and

REV B - 22 March 2004 4

Technical Service Bulletin 030602

typically correct for amplifier saturation at the highest power levels. The low-level correctors perform "white stretch" and may be required to compensate for low-level "turn-on" type distortions due to the class AB transfer curve of the IOT. The exciter B1 module also has three high-level and one low-level ICPM correction breakpoints. The threshold adjustments for these breakpoints are B1 - 9 to 11 and B1 - 21, respectively. The slopes of the high-level ICPM corrections are determined by fixed resistors inside the B1 module and cannot be adjusted in the field. The slope of the low level ICPM correction is adjusted via exciter potentiometer B1 - 22. When dealing with this type of correction circuit, it is best to follow these simple rules: All correction thresholds should be dialed fully counterclockwise when not in use.

Never dial a correction threshold "all the way through" (until correction threshold

passes completely through and disappears out the opposite end of waveform). Avoid piling opposing correctors on top of each other (high-level corrector

nullifying action of low-level corrector). Always use the minimum amount of correction necessary to get the job done.

Use of excessive correction may result in a symmetrical hump (or dimple) appearing in the double sideband portion of the swept transmitter display.

7. Adjust LF linearity and ICPM correctors to minimize +920kHz intermodulation level on differentiated modulated ramp display. Use multiple diode breakpoints to eliminate distortion in different portions of differentiated ramp waveform.

REV B - 22 March 2004 5

Technical Service Bulletin 030602

Unacceptable Acceptable

NOTE: It is typical for the exciter output power level to shift as much as 30% to 40% as the linearity correctors are adjusted. For this reason it is highly advisable to have the AGC engaged during this process. Consult Service Bulletin 030531 for more details.

NOTE: In additional to the correction action provided by the dedicated ICPM correctors, the ICPM may also be modified through the use of individual phase trimmer capacitors on the high level correctors (B1 - C102 through 104). These trimmer capacitors determine the phase angle of the envelope stretching action of the high level (HL) correctors relative to the original, undistorted signal. If the stretching action is perfectly in phase, only LF linearity correction will result (no ICPM). If the stretching action is slightly out of phase, varying degrees of LF linearity and ICPM correction will result. These trimmer capacitors are normally deactivated when a B1 module ships from the factory, but may be activated by the field engineer during commissioning. To activate these controls, a small jumper must soldered across a pair of solder pads located directly behind each capacitor on the reverse side of the module PCB.

B1 module with cover removed

REV B - 22 March 2004 6

Technical Service Bulletin 030602

Some tips to remember: If a LF linearity corrector passes "through" a region of intermodulation fuzz without

having much or any effect, this is an indication that ICPM correction is probably required at this region…and vice versa.

If neither LF linearity nor ICPM correctors have any effect on well-defined "triangular flare" at the extreme left of the screen, the cause may be driver stage clipping. Reduce transmitter power 20% and see if effect disappears. If so, check sync compression with demodulator and waveform monitor at each stage of the drive chain using built-in -36dB directional couplers. Look for a stage where a large amount of sync amplitude disappears. Rectify cause of driver stage overload before proceeding.

8. Once good +920 kHz intermodulation suppression is obtained with differentiated modulated ramp display, measure final +920 kHz level on spectrum analyzer. The minimum acceptable level is -55dB relative to peak visual carrier, while -60dB is considered excellent.

Unacceptable (+920 kHz = -52dB) Acceptable (+920 kHz < -60dB)

9. Trim sync level to 40IRE with exciter potentiometer B2 - 6.

REV B - 22 March 2004 7

Technical Service Bulletin 030602

10. With a monochromatic stairstep video signal and waveform monitor ICPM display, center top-most stairstep dot on centerline at 100IRE with modulation depth (B2 - 30) and modulation balance (B1 - R14, inside B1 module) controls.

11. Readjust sync level to 40IRE with exciter potentiometer B2 - 6, as necessary. 12. Procedure complete.

Comark Communications LLC 104 Feeding Hills Road Southwick, MA 01077 U.S.A. (800) 345-9295 Attn: Customer Service

REV B - 22 March 2004 8