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2.13 Search-tuning with IF-PLL.

Search-tuning usually tunes from low to high frequency. For all (non SECAM-L1) transmitters, enteringthe Picture Carrier in the SAW filter window from the lower side (sound shelf), gives the earliestpossibility for the IF-PLL to lock. Entering from the Nyquist slope side suppresses the Picture Carrierso much, that at weak transmitters the IF-PLL may not lock until the tuning has already passed thedesired IF frequency. Also the IF-PLL can then first give an unwanted lock on the Sound Carrier.

Tuning only in upward direction is the simplest to implement in software. The moment that backstepping is necessary, this will expand the search algorithm a lot. Memorizing tuning positions to avoid lock-up of the search algorithm costs RAM/ROM. Especially for VST this is a problem since there is no linear relation between tuning voltage and tuned frequency (How to limit back stepping to 5MHz” ?).

First tuning over a transmitter and then back causes large AGC jumps, because of the SAW filter: e.g. K2950= -50dB at 39.1 MHz. The total AGC response time to large signal jumps is 50..300 ms, so the search algorithm should avoid this. Fast settling of AGC means faster tuning.

The advised step size during rough scanning is 800kHz (VST: max. 800kHz). This choice determines the tuning speed and is dependent on the catching range of the TDA935x/6x/8x IF-PLL (= +/- 1MHz symmetrical). But above the specified IF frequency the SAW filter attenuates the signal a lot (e.g. - 50dB neighbour sound channel trap), that effectively we can not use that half of the catching range during search-tuning.

After each step (=disturbance), the tuning loop has to stabilise. The time constants in the loop are:

· VST: DAC pulse generation = 5.5ms ?????adapt the values in this section!!!!???

+ Non-linear integrator = 30ms

+ EMC filtering = 10ms

· PLL: stabilisation of tuner PLL loop = 20ms

· Tuner AGC (small step) = 20ms, 300ms when switching tuner bands

· IF-PLL cathing time = 20ms

· IF-AGC = 20ms

· AFC-detection (sampled during vertical retrace) = 20ms

In practice we don’t need to add all these time constants, but we find a practical “step-delay” value of 45ms. Do not try to step faster than this, or the search algorithm will start skipping (weak) transmitters.

The absolute minimum search tuning time for a 3-band tuner is calculated as follows:

VHF-I + S = 45 .. 175MHz = 130MHz

VHF-III = 175 .. 400MHz = 225MHz

UHF = 400 .. 845MHz = 445MHz

Total = 800MHz to search

For a PLL tuner we find: 800MHz / 800kHz/step x 45ms/step = 45 seconds. VST tuning does not have a constant frequency step size, but a variation of a factor 4. (= in areas with low steepness, tuning is 4 times slower). In practice the max. VST tuning speed is about 70 .. 100 seconds.
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