thru zero VCO questions

[mbartkow at ET.PUT.Poznan.PL]

Dear all,

encouraged by Scott I propose to consider a relatively simple scheme
to use a PLL for the thru-zero frequency modulation.
As many of you know, PLLs allow to impose frequency modulation
onto an input signal of externaly determined (fixed) frequency.
As long as the modulation depth is kept inside the tracking range
of the loop, its own filter helps to keep the mean frequency of
the loop's VCO tied to the input frequency. If we assure the input
frequency to be kept within a relatively small (e.g. 100kHz-120kHz
range), the tracking conditions of the PLL are not very hard.

My idea is following:

Take the sinusoidal output of an ordinary synth VCO (NOT frequency
modulated), shift its spectrum by say 100kHz upwards using a reference
oscillator and a quadrature shifting network (easy suppression of the side 
bands here). The resulting spectrum should fit into 100kHz-120kHz range. 
Put the resulting signal into the phase detector of a PLL. The linear FM 
input signal is added to the output of the loop filter and drives the loop 
VCO. As a result, (after closing the loop by feeding the loop VCO output 
into the remaining input of the phase detector) the loop VCO follows the 
frequency of the synth VCO shifted by 100kHz upwards, but is additionally 
modulated by the FM input.
Note again, that the loop filter smoothes the frequency fluctuations and
keeps it swinging around the loop's input frequency (there are some limitations
here, more about them later). Since we want the signal in the baseband it
has to be spectrally shifted down by the same amount as it was shifted upwards,
so a second quadrature balanced modulator driven the same 100kHz is involved.

                                                      lin. FM input
                                                         |
  V/oct-->synth VCO-->MODULATOR-->PH.DET-->LOOP FILTER-->+-->loop VCO----->MODULATOR--> out
                          ^         ^                                   |      ^
                          |         |                                   |      |
                          |         \-----------------------------------/      |
                          |                                                    |
            OSC 100kHZ --------------------------------------------------------/ 


Note that there are no problems with frequency drifts and tracking, even if the
100kHz oscillator is not very stable since its absolute frequency does not matter
too much. The output is as stable as our synth VCO is.
The big advantage of this scheme is that probably extremely deep FM might be achieved.
The big disadvantage is related to the compromise that has to be set between the settle
time of the loop (we don't like permanent portamento in our synth) and the lowest frequency
of the FM input. To my guess it would be good to set the bandwidth of the loop filter to
about 50Hz. This gives an acceptable response time of the loop and attenuates modulation
signals slower than 50Hz. I would use 4046 for the PLL.

regards,

MB


--

Maciej Bartkowiak, PhD
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Institute of Electronics and Telecommunication     fax: (+48 61) 8782572
Poznan University of Technology          phone: (+48 61) 8791016 int.171
Piotrowo 3A                             email: mbartkow at et.put.poznan.pl
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