According to the service manual, the VCO in the ZPV-Z2 tuner is implemented as a current-controlled monoflop with feedback for input frequencies below 60 MHz, while above that frequency, a varactor-tuned L/C oscillator takes over. The manual, however, does not indicate the operating frequencies of either oscillator.

The table on this page lists the VCO frequencies measured for a number of inputs.

For each measurement, the ZPV was allowed to autorange until locked, then the autoranging was disabled because it keeps varying hte VCO frequency.

At an input of 50 MHz, the autorange circuit locked in the 60–100 MHz range, i.e. at this frequency and above, the L/C oscillator was active, which apparently has a rather small tuning range around 2.4 MHz.

Because of the sub-sampling measurement principle, the input frequency can mix with any multiple of the VCO frequency to result in the desired IF signal at 20 kHz. The control loop only reacts to the IF frequency, therefore the VCO may lock on any of a number of possible frequencies for a given input, as was observed on subsequent measurements at several of the input frequencies tested. For example, it can be seen that at 11 MHz, a VCO frequency of either 1.836667 MHz ×6 or 2.755000 MHz ×4 will result in a 20 kHz difference to the input signal.

Legend

Copyright (c) January 2014, Peter Holzleitner