## 1 Introduction  
Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a type of photoacoustic spectroscopy which employs a piezoelectric quartz tuning fork instead of a microphone as a transducer. Since the introduction of the QEPAS-technique by [PERSON] et al. in 2002 [1] it has emerged to be a very useful tool for photoacoustic detection of a manifold of gaseous substances. Further progress in QEPAS has been made in terms of using custom tuning forks with larger gaps and lower resonance frequencies to use light sources with larger focal diameters such as LEDs or THz-QCLs or by incorporating the photoacoustic cell and transducer, also called spectrophone, in a multipass cell to increase the power density and thus the sensitivity. A summary of achievements for different species and with respect to the used techniques can be found e.g., in the review articles by [PERSON] et al. [2, 3] and [PERSON] [4]. For a simple QEPAS setup only a bare quartz tuning fork is required; however, in most superphones the tuning fork is additionally equipped with acoustic resonators [5]. Their use can improve the S/N-ratio of a QEPAS spectrophone by a factor of up to 30 [6]. For a QEPAS system using the first overtone mode, an enhancement factor of even 50 as compared to the bare tuning fork was reported [7].  
On the other hand a strong coupling between the tuning fork and the acoustic resonator exists and the highest signals are achieved when the resonance frequencies of the tuning fork and the one of the acoustic resonator coincide. The resonance frequency of the acoustic resonator is essentially dependent on its length in relation to the velocity of sound. The most common approach is to use two cylindrical pipe elements sandwiching the tuning fork in a way that the radiation is aimed through both segments and between the prongs of the tuning fork. In this configuration best sensitivity is achieved when the length of a segment is between half and a quarter of the sound wavelength. However, since the tuning fork and the acoustic resonator react differently on changes in the environmental conditions the balanced condition is easily lost when the system is operated in real world scenarios.  
Provided that the excitation laser addresses multiple species, the large dynamic range of photoacoustic spectroscopy enables the measurement of both, main components as well as trace gases of a gas mixture. This is one demand for biogas analysis. Therefore, we demonstrated that the system is able to measure methane in a carbon dioxide atmosphere (main components) as well as ammonia in small concentrations as present within the biogas.  
Besides the compact size and the small measurement volume of only a few cubic centimeters the high Q-factor of the tuning fork is usually highlighted as an advantage of the QEPAS-technique in contrast to conventional photoacoustic spectroscopy which gives rise to the high sensitivity of a QEPAS-spectrophone. However, at the same time, the high Q-factor results in a sharp resonance curve and thus a careful setting of the laser's modulation frequency is required. Since the resonance frequency of the spectrophotometer is affected by environmental conditions such as pressure, temperature, and gas composition related density and viscosity, changes of these parameters can easily cause measurement errors. In this paper a new detection scheme for using both electrical and photoacoustic driving of the tuning fork and phase modulation of these two driving forces is discussed.