Research Article  Open Access
AllPass Filter Based Linear Voltage Controlled Quadrature Oscillator
Abstract
A linear voltage controlled quadrature oscillator implemented from a firstorder electronically tunable allpass filter (ETAF) is presented. The active element is commercially available current feedback amplifier (AD844) in conjunction with the relatively new Multiplication Mode Current Conveyor (MMCC) device. Electronic tunability is obtained by the control node voltage () of the MMCC. Effects of the device nonidealities, namely, the parasitic capacitors and the rolloff poles of the porttransfer ratios of the device, are shown to be negligible, even though the usable highfrequency ranges are constrained by these imperfections. Subsequently the filter is looped with an electronically tunable integrator (ETI) to implement the quadrature oscillator (QO). Experimental responses on the voltage tunable phase of the filter and the lineartuning law of the quadrature oscillator up to 9.9 MHz at low THD are verified by simulation and hardware tests.
1. Introduction
Realization of firstorder allpass filters had been reported earlier using various types of active building blocks (ABBs), namely, VOA [1], Current Conveyor and its variants [2–5], DDA [6], DDCC [7], DVCC [8, 9], VDIBA [10], and OTA, with differential amplifier [11] as listed in Table 1; some of these building blocks are implemented with a basic device (OTA, CC, or DVCC) combined with some signal (current or voltage) differential unit. Recent literature suggests that electronic function circuits fabricated by commercially available IC modules are capable of providing desired results [12–16].
 
Notes. (a) Designs in [1, 2, 10], based on commercially available IC modules; (b) comprehensive listing of recent APFs using various ABBs presented in [5]. 
A varied genre of ABBs emerged during the recent past for realizing diverse functions towards signal processing/wave forming/filtering applications. Fabrication of internal design of such ABBs in CMOS technology leads to easy verification. However, recent studies address the issues of such approach concerning involvement of fabrication cost and complexity in their nodal relations with such wide variety of ABBs [12, 14, 16].
Use of readily available offtheshelf devices thus may be an alternate approach to obtaining satisfactory results. Some recent composite blocks yield quite useful results on signal processing applications wherein two types of basic commercially available chips are conjoined (e.g., DDCC, DVCC, and VDIBA). It has been suggested that composite blocks may provide better results in comparison to the constituent elements [17]. So for such ABBs requiring more than one commercially available IC, it is still economical and more convenient compared to chip fabrication [12, 16, 17].
The relatively new MMCC element [18] used in this work is thus configured using readily available CFA (AD844) and multiplier (AD835) [19] elements. The useful feature of the MMCC is that it has an inbuilt control voltage terminal () which is conveniently utilized for electronic tuning of a circuit parameter. Hence the conventional method of transconductance () to bias current () conversion is avoided; such conversion needs additional hardware complexity and involvement of thermal voltage () [3, 10, 16]. We now present here an electronically tunable allpass filter (ETAF) using the composite CFAMMCC block.
Next a linear voltage controlled quadrature oscillator (LVCQO) is implemented with the ETAF being looped in feedback with an electronically tunable integrator (ETI). Detailed analysis is carried out taking into account the device imperfections, namely, parasitic shunt components and the rolloff poles of the porttransfer ratios. Effects of these nonidealities are negligible but the higher range of usable frequencies is constrained. The oscillation frequency () is activeinsensitive relative to portmismatch error (). Experimental results on ETAF and quadrature oscillator responses are verified by PSPICE simulation and hardware test.
2. Analysis
The proposed ETAF is shown in Figure 1(a) where the nodal relations are as follows: CFA: , , and MMCC: , , and .The port tracking ratios are ideally unity but may be postulated by error coefficients () as , , and , where is the multiplication constant [20].
(a)
(b)
Analysis of Figure 1(a) yields the transfer function asparasitic elements, yielding a modified transfer where , , and .
In ideal devices port tracking ratios are all unity; assuming the realizability conditions as () for simplicity, we getwhich is the nonminimum phase allpass function, where transmission gain and phase () is tunable in a range , being variable by control voltage () as
3. Effects of Nonidealities
3.1. Parasitic Components
The parasitic components appear as shunt arms at current source nodes of the device; as per databook [21], the typical values are and ; ratio of circuit resistors (KΩ) relative to is assumed to be negligible. Reanalysis now with finite parasitic elements yields modified transfer elements in Figure 1(a) exhibiting a singlepole rolloff model [22], given bywhere
Then (4) simplifies to
The frequencydomain behavior may be estimated by writing in (6), given bywhere
With pF (measured) and KΩ, we estimated value of the parasitic pole frequency MHz. Hence for usable ranges of , effects of parasitic capacitors are negligible and ; therefore the nominal APF function is preserved with the limit .
3.2. RollOff Pole of DevicePort Transfer Ratios
At relatively highfrequency ranges, the porttransfer ratios , , and , , of the active components are as follows:
The dc values of all the products of port coefficients in (1) may be taken as unity; for example, since .
The modified APF function () is now written aswhereThe function relates to the MMCC block; it is decomposed as , where and
It has been observed that the 3 dB corner frequencies for all the porttransfer ratios appear at close proximity, and hence these may be assumed equal without loss of generality [22]. Therefore calculations for examining the effects of the rolloff poles become simpler if we write . Writing and solving for (11) we getwhich implies and . Similar results are checked for (12) to (14). For the effects of the MMCC rolloff, we put , where ; solving (15), we getwhich reduces to and . This analysis indicates that effects of rolloff poles of the active building blocks are quite negligible in the range of operating frequencies ().
4. LVCQO Implementation
Quadrature oscillators find diverse application in measurement systems [30–33] and in communication circuits, quadrature mixers, SSBmodulator, vector generator, and data conversion. A comprehensive summary of recent QO designs is listed in Table 2. It may be seen that even though a number of such oscillators were reported, very few are linearly tunable.

We now present the linear VCO as implemented by forming a regenerative feedback loop formed with the ETAF in Figure 1(a) and the ETI in Figure 2; transfer of ETI iswhere , , and ; hence . Grounding of the capacitor () facilitates absorbing in values at predesign level. Device imperfection relative to porttransfer rolloff pole frequencies () is estimated by writingwhere .
The deviation due to rolloff isAs in previous section if we assume and put , then reduces similar to (17) asNow implementation of oscillator is derived by equating which gives the characteristic equation (CE):
So tuned frequency ( in Hz) is linearly and directly variable by control voltage ()
The proposed method provides direct electronic tunability of oscillation frequency by the control voltage (). No additional current processing circuitry for to bias current () conversion is needed, as seen in the previous designs shown in Table 1, and thermal voltage () is not involved. It may be seen that ω_{o} is practically activeinsensitive, .
5. Experimental Results
The response of tunability of the proposed ETAF and LVCQO had been verified by PSPICE simulation [34] and hardware tests; these are shown in Figure 3. ETAF responses, along with its phase error () and THD (measured to 7th harmonic), are shown in Figures 3(a) and 3(b) which are relatively low compared to [10, 24, 25]. Some small phase error () on the experimentally generated sinusoid quadrature signals deviated around 90° was measured as shown in Figure 3(c). The oscillator spectral response in Figure 3(d) shows the LVCQO spectral response wherein phase noise measured is −107 dBc/Hz at an offset of 100 KHz from the tuned frequency of 10 MHz; this response is measured by Tektronix spectrum analyzer (RSA306B) [35].
(a)
(b)
(c)
(d)
(e)
(f)
This result is better as compared to that in [14]. A new method of examining the VCO tuning error (β%) is adopted by measuring the shift () in at phase crossover frequency. The tested value of β% in the overall tuning range of 1 MHz~10 MHz was observed to be less than 4.5%.
6. Conclusion
A new design of ETAF function and its subsequent application to the implementation of a VCQO with lineartuning law are presented. The active blocks are commercially available CFA (AD844) and fourquadrant multiplier (AD835) which are readily available IC modules. Responses of the function circuits are verified experimentally with PSPICE simulation and hardware tests. The quadrature oscillator had been tuned by control voltage () in a linear range (bandspread) of at low THD. The design does not need any additional current processing circuitry; hence there is no extra quiescent dissipation. Power dissipation of the proposed oscillator circuit is about 33 mW. The circuit is practically activeinsensitive relative to deviceport trackerror (). Analysis on the effects of device parasitic components indicates insignificant effect on design equations, even though parasitic capacitors tend to limit the usable highfrequency range. In this work, a relatively new building block, namely, the MMCC, had been utilized for the voltage () controlled linearly tunable quadrature oscilator design with 107 dBc/Hz phase noise at 100 KHz offset as shown in Figure 3(d). Thus a comparatively better quality result had been obtained as is shown in Table 2.
It may be seen in Tables 1 and 2 that in the cited designs active devices used are certain primary blocks with auxiliary voltage/current differencing unit (VDU/CDU) or differential amplifiers with their associated parasitics which tend to limit the desired range of frequency ( or ) [6]. Also, tuning by or needs additional current processing circuits with parasitic capacitors. The authors believe that, owing to such topological distinction, the frequency tuning ranges are somewhat lower. However in the case of design by VDIBA [10], the range improves owing to the fact that the active component OPA860 has itself a bandwidth (BW) of 470 MHz. In the proposed design, no such VDU/CDU and to conversion circuitry are used; this circuit is structured with commercially available AD844 (BW = 60 MHz) and AD835 (250 MHz) IC modules to achieve relatively higher frequency, wherein effects of portmismatch rolloff poles and parasitics had been included in support.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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Copyright
Copyright © 2017 Koushick Mathur et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.