Spurious tones can be significantly reduced using the above approaches, but any open-loop technique will result in some residual spurious output because the circuits involved are operating from a periodic reference clock. The most effective techniques are sampling the loop filter, as it reduces any non-ideal and transient effects of prior circuitry to a single charge-transfer mechanism. Any remaining reduction efforts can then
concentrate on minimizing charge feed-through through the loop filter switch as well as reducing supply and substrate bounce.
Using any of the approaches, however, will lead to some residual spurious component that can vary over process, temperature and operating voltage, and no open- loop technique can fully cancel the spurious components of any periodically operating phase-locked loop.
In our work, we investigated a truly closed-loop technique that senses the spurious content of the VCO using the control voltage disturbance as a proxy. Active injection of small pulse-type waveforms is used to actively produce spurious output that counteracts the spurious output produced by the PLL. Our technique is general in that it can be operated either by itself or in parallel with other techniques previously presented.
Figure 2-13 illustrates our approach conceptually. In order to sense the spurious output of the PLL, the VCO control voltage is digitally sampled and the samples are
Figure 2-13: Conceptual illustration of the system-level closed-loop approach adopted.
Figure 2-14: Block Diagram of the implemented system
S
++ VCO
Uncorrected VCO control voltage time V o lt a g e 0 Tref Corrected, low-ripple control voltage waveform
Q-Pump H(s) V o lt a g e 0 Tref Added correction/spur cancellation signal time V o lt a g e 0 Tref time 1 N Sense DSP Actuate Q AD Q AD Q AD PFD Charge pump/ loop filter VCO Off-chip crystal reference S + + Detector (synchronous correlator) Correction signal generator
Digital closed-loop feedback control Harmonic Filter DSP ò x4 Q AD A D Divider state Divider state Divider state 1 N
processed in a DSP unit to reconstruct the control voltage waveform. The control voltage waveform serves as a proxy for the produced spurious output, and the voltage sampled is an amplified and band-pass filtered version of the control voltage referenced to the same supply rail as the VCO supply. In this way, supply bounce producing spurious can be taken into account as the control voltage bounce produces spurious output as a function of the VCO supply rail voltages.
The timing for the control voltage samples is generated from the states of the divider to divide the reference clock signal into N equally spaced time bins synchronous to the control voltage and any perturbation. Because the perturbation of the control voltage waveform and of the VCO itself is periodic, synchronously detecting the single tones allows for almost arbitrary sensitivity as longer integration times can be used to reduce the noise bandwidth similar to the operation of a lock-in amplifier or a spectrum analyzer. Because the VCO edge zero-crossings vary in time when spurious tones are present, using them to demodulate the FM signal directly is an alternative approach for detecting the spurious output of the VCO as will be discussed in more detail in the implementation section.
To inject an actuation signal, an error signal that consists of a series of timed pulses with controllable amplitude is used. This approach was chosen for a variety of reasons. First, the injected pulse waveform is similar in nature and shape to the waveform that is attempted to be cancelled. In this manner, using even a few pulses, the total spurious output power can be reduced more effectively as the power in several harmonics is reduced simultaneously. Secondly, the circuit overhead becomes very small,
effectively consisting of a capacitor that can be charged to a programmable value and whose charge is transferred at a programmable time instance to a loop filter. Thirdly, this approach is scalable, as several of these channels can be operated in parallel. An alternative approach could use single tone injections, where an arbitrary waveform is synthesized from (mostly) pure tones of the reference frequency and its harmonics. The difficulty with such an approach is that it requires generation of pure tones from a reference clock that is typically in digital form. Furthermore, it requires this production for all of the harmonics of interest. Our actuation circuit is more comparable to a poor man’s direct digital waveform synthesizer.
Using N injected pulses of controllable amplitude and phase provides
degrees of freedom that can be used to control the amplitude and phases of the first harmonics of the spurious tones produced. Higher frequency components are less important, as additional filtering can be added to the loop filter removing the harmonics more easily without unduly introducing phase-shift at the loop bandwidth, thus affecting loop stability. Furthermore, because a tone at a higher harmonic but of the same strength as a lower harmonic tone produces a lower spurious output as the effective FM modulation index is lower.
A complete block diagram of the proposed frequency synthesizer is shown in Figure 2-14. The control voltage is sampled using a subsampling correlator, and the time samples are used to reconstruct the control voltage signal. The analog-to-digital conversion is implemented off-chip in this test-chip, and the digital samples are read through a GPIB interface by a MATLAB program, which acts as a DSP end. The
program also generates programming values to control the timing and amplitude of the generated pulses in the correction signal generator, as well as programming the correlator to sequentially take samples (also providing software DC offset cancellation). The correction signal generator consists of four parallel channels of the programmable charge pulse generator discussed above. The pulses are injected into the control voltage loop, thus closing the feedback loop.
Figure 2-15: Detailed block diagram of the implemented PLL with all integrated components