A new world of tuning a tube type transmitter running digital carriers has arrived. It is quite an experience. As I have an early HD transmitter I never received a manual other than a preliminary version. Nowhere within the preliminary version, based on the HT-35 transmitters, is there hints or suggestions on tuning for the linearity necessary for digital. Here I take the section regarding the initial turn-on of the HTHD+ and reproduce the tuning procedures. Most of this is from the current version of the HTHD+ manual found online on the Harris website. The only modifications to their documents is to proceed as this transmitter has already gone through the initial turn on phase.
IPA Power adjust potentiometer: As our transmitter already underwent the full initial turn on steps, I was directed to check and verify this potentiometer was set fully clockwise, or all the way up.
As in all tuning procedures, Harris reminds us to tune the grid and input match for minimum IPA reflected power. To improve the spectrum a minor adjustment to one side of the dip may be required.
Do the major tuning procedures with the RTAC bypassed! "On the FlexStar exciter, set the RTAC (real time adaptive correction) Non Lin FM+HD, linear FM+HD, and linear FM to bypass."
The following comes right out of the manual. It is found in Chapter 2, Section 188.8.131.52 Basic Readings. (Go figure. It is called basic readings as it is incorporated within the initial setup.)
37 Adjust the INPUT MATCH and GRID TUNING controls to obtain a minimum indication on the IPA REFLECTED POWER bar display.
38 If the multimeter screen voltage indication is different from value recorded on the factory final test data sheets, press and hold the power control board SCREEN RAISE or SCREEN LOWER switch until the screen voltage is the same as value recorded on the factory test data sheets.
A PLATE TUNING - Plate tuning is normally adjusted for a peak in power output. This may vary slightly with the power level, so if a power level adjustment is made, the plate tuning may also have to be adjusted. Plate tuning also affects synchronous AM noise. If the best synchronous noise does not coincide with the power peak, a slight compromise might be required.
B OUTPUT LOADING - As an initial setting, the power output may be peaked using the OUTPUT LOADING control. To achieve mask compliance, the OUTPUT LOADING control must be adjusted counterclockwise from this position. A half-turn off the peak will be close to the final adjustment.
C BIAS - PA tube bias affects the amount of drive required by changing the operating point of the PA grid. Lower bias (less negative) will reduce the amount of drive required, but also compromise PA efficiency and raises screen current. More bias improves the efficiency, but at the expense of reduced output and mask compliance. Use the factory test data sheets as a guide (approximately - 250 to -300 volts). Unlike a conventional FM transmitter, grid current will be typically only a few milliamperes.
PA efficiency is decreased as a consequence of adjusting the loading off the power peak. Therefore, the PA stack temperature must be checked while finalizing the adjustment. Use the factory test data sheet figure as a guide.
40 On the FlexStar exciter, set the RTAC non lin FM+HD to adapt. Let the transmitter run for a few minutes to allow the RTAC to initialize.
41 Finalizing the output loading requires use of a spectrum analyzer and allowing time for the adaptive correction to function. This is a matter of repetitively making minor adjustments, restarting the spectrum analyzer in the averaging mode, and watching the analyzer for results.
42 The Harris factory normally uses the Agilent E4402B spectrum analyzer for finalizing the tuning of the HT-HD+ transmitter, however any comparable analyzer will work. For any critical spectrum measurements, it is important that the measured signal be a margin of 10 dB or more above the noise floor of the instrument and setup.
43 Connect the spectrum analyzer to an RF sample port that is positioned after the transmitter low pass filter.
44 Adjust the center frequency of the analyzer to the transmitter carrier frequency and set the analyze to a span of 2 MHz.
45 Initially set the spectrum analyzer resolution bandwidth to 1 MHz. This will assist setting the carrier level to the top of the analyzer screen.
46 Attenuate the RF sample only as much as required to obtain a level that is near the maximum acceptable input level of the spectrum analyzer, such as 10 dBm. This will help separate the measurement above the noise floor of the instrument as much as possible to avoid erroneous results. A sample level that is too high can result in signal compression (distortion) in the spectrum analyzer that will produce.
47 Decrease the spectrum analyzer bandwidth to 1 kHz.
A check as to whether the spectrum analyzer is in compression is to temporarily broaden the sweep to include some harmonics, then insert an attenuator in the sample line. For example, if a 10 dB attenuator is inserted in the sample line and the signal drops 10 dB across the display, then the setup is probably correct. Harmonics dropping more than 10 dB in this example, this would indicate that the analyzer is in compression. Signals in the lower part of the display dropping less than 10 dB would indicate noise is adding to the signal under test.
48 Turn on the spectrum analyzer averaging function.
49 For a final measurement, at least 100 sweeps are required over a period of at least 30 seconds. (You should familiarize yourself with the iBiquity limits so you can quickly see if you make mask.)
50 Adjust the output loading control so that the output waveform is below the spectrum analyzer mask.
Always give the RTAC time to adjust to any loading changes. Patience is the key here.
We found that with our tube change that some of the adjustments were very far off. As I did not have the "tuning" instructions handy, the transmitter was running at peak FM only efficiency, but lacked in the digital spectrum. It took time with a Harris rep on the phone to make sure we were going in the right direction.
I was mildly disappointed with the support person as it seemed he was not listening to what our conditions were at start as other support reps had us in a very funky setup situation for troubleshooting. Once this hurdle was cleared we made progress. Again I stress patience with tuning your transmitter. Since this tuning procedure, I have gotten nearly 3 years of tube life. Your mileage may very.