Filters / Discriminators
Applications for Filters/ Discriminators
KVG provides a wide spectrum of standard and custom specific filters for telecommunication, navigation, avionics and precise measurement for filters in the frequency range of 1 MHz to 200 MHz. In house filter design and crystal manufacturing combined with new technologies like HFF-crystals gives a maximum of flexibility to the customer.
Most of the applications imply an exact specification of the filter. They can be summarized as follows:
- Size, case
- Filter frequency
- Bandwidth, stopband attenuation, ripple, group delay, intermodulation etc.
Depending on the applied technology it is common to divide filters into five main types:
- Monolithic crystal filters
- Discrete crystal filters
- Linear phase filters
- Front end filterss
A very similar product is a crystal disriminator, which can be used for FM-demodulation and for measurement applications.
Monolithic crystal filters
Monolithic crystal filters are mostly used in IF -stages of fixed and mobile radios.
Discrete crystal filters
The crystal filters supplied by KVG are mainly band-pass filters with Chebyshev-characteristic (theoretical ripple 0.1 dB). Depending on the selection in the stop-band range and the ripple in the pass-band range the filter design can be chosen between Chebyshev-design and Butterworth design (theoretical ripple 0 dB). Some of the filters give the option of internal matching networks.
Linear phase filters
In order to get a low distortion transmission of signals and pulses (digital signals), crystal filters with a linear phase response or low group delay distortion are necessary. However, these linear phase filters (Gaussian or Bessel characteristic) have low selection characteristics. By several transfer functions (Gauss 6 dB, Gauss 12 dB, EQR) between linear phase and selection filters, better selectivity can be achieved without essentially changing the group delay in the pass band range.
Antenna filters, used as preselection or as front-end filters in the VHF-range, suppress unwanted signals. The most important characteristics of these filters are low insertion loss and good intermodulation characteristics.
Currently KVG produces crystal discriminators in the frequency range of 1 MHz to 35 MHz (priority from 9 MHz to 25 MHz) for linear FM-demodulation and for measurement applications. Within a certain frequency range, crystal discriminators produce a DC-voltage proportional to the input frequency. The linearity and the temperature characteristic of the slope is determined by the electrical parameters as well as by the temperature characteristics of the crystal.
LC-filters are a combination of coils and capacitors instead of using a crystal. These filters provide a much wider passband with nearly the same shape-factor as crystal filters. Another advantage is the lower spurious response of a LC-filter. A drawback is the higher temperature response of LC-filters because of the temperature dependent elements coils and capacitors.
Definition of electrical parameters
A) Insertion loss In order to measure the insertion loss, the test adapter is short circuited and the imaginary impedances are compensated at the corresponding centre frequency. The resulting attenuation value is the corresponding reference point 0. When the filters are inserted at the test adapter, the insertion loss is the difference between the minimum attenuation value of the filter and the reference point.
B) The passband range between two frequencies (f1, f2), where the attenuation should be equal to or higher than a specific value. The passband range is mostly related to the 3 dB or 6 dB points.
C) The passband ripple is the difference between the maximum and the minimum attenuation in the passband range, or in a specific range of the passband.
D) Range in which the ripple is specified. KVG specifies this range with 80% of the 3 dBbandwidth.
E) The stopband is a band of frequencies in which the relative attenuation is equal to, or greater than the specified value. In the diagram the specified stopband attenuation values are f3 and f4. In many cases the shape factor (SF) is also specified.
F) The ultimate attenuation is the difference between the attenuation in a given frequency range and the minimum attenuation in the passband.
G) Spurious responses are various resonances caused by crystals which do not correspond to the normal crystal frequencies of the filter design.
H) The group delay distortion is the difference between the minimum and the maximum value of the group delay in a specific frequency range.
Measurement technique of crystal filters
Calibration and final control of the filters are carried out by means of a selective level measurement or with a network analyzer, depending on the requirements. The different filter impedances are adapted to the measurement system impedances by transformer circuits which are integrated in test adapters.
Attenuation, phase and group delay distortion measurement for filter terminating impedances RF // CF with RF < 1 kOhm and CF > 5 pF. The matching pads can be calculated as follows:
In the following table the resistances of some typical filter impedances are listed:
For terminating impedances above 1 kOhm, additional attenuation due to the matching network is extremely high. This result in problems with the ultimate measurement. In this case the filters are adapted with LC-transformers in accordance to the next figure:
The following relations are valid for the reactances X1 and X2 depending on the circuit
The impedances of the test adapters are adjusted at the centre frequency of the crystal filters using a vector impedance meter. In order to avoid a cross-talk between input and output, the input and output have to be carefully shielded and the filter case has to be taken to ground. For measuring the ultimate attenuation, the attenuation of the test adapter has to be better by 10 dB than the specified ultimate attenuation of the test unit. KVG offers corresponding test adapters for every filter.
The centre frequency f0 of symmetrical bandpass filters is calculated as follows:
Measurement technique of crystal discriminators
Test circuit for crystal discriminators The specified data of the crystal discriminators are valid for a constant input voltage U0=1V or P0=+13dBm(50 Ohm) at the input of the test jig.
The non-linearity (l) of the voltage-frequencycharacteristic is the percentage deviation of the output voltage U2 from a linear shape. Monolithic crystal discriminators are available for narrow-band FM combined with FM-demodulators. The slope is defined as: