The notch filter can be used to measure sideband noise at offsets greater than 2KHz. The 14.31818MHz crystals have an average Q of 90000 and a motional resistance of 5.2Ω. The toroid inductors used in the 1/4λ lumped circuits are rotated 90° each to minimize leak through. Ultimate stop-band at 14.313734MHz is measured at -111dB, which is remarkable because no extra metal shielding between the 5 sections has been used. The performance of this filter is shown in the following network analyzer picture: All parallel shunt capacitors such as the crystals parallel capacitance and adjacent LC-π capacitors are collapsed in single capacitors optimizing and simplifying the design.įT50-10 toroids are used together with polystyrene capacitors to obtain a high-Q circuit.
#CRYSTAL FILTER DESIGN ON LINE SERIES#
X c and X l are dimensioned to 200Ω at the crystal series resonance. The 5 crystals are isolated from each other with 1/4λ lumped circuits, which are LC-π low pass networks. The impedance is raised to 200Ω with transmission line transformers on small FT37-43 ferrite toroids to increase the notch depth.ĥ crystals are used as shunt series resonators to produce the notch effect. The filter is based on the following elements: The block diagram of this measurement method is shown below:įollowing W7ZOI's recipe, a 5-pole crystal notch filter is designed using 5 of the remaining 14.31818MHz "AQ" crystals of the batch also used for the crystal filter with the reciprocal mixing method.
My spectrum analyzer can be used to inspect the remaining noise sidebands up to -140dBm at best at its smallest RBW (3Hz), so at least an additional 40dB of amplification is needed to further amplify the noise sidebands behind the notch filter to be able to measure all the way down to the thermal noise floor at -174dBm. The idea behind it is to suppress the carrier with a very selective crystal notch filter, leaving only the noise sidebands to be inspected with a spectrum analyzer without overloading problems. This method is described in detail by Wes Hayward, W7ZOI, in the July/August 2008 issue of QEX: "Oscillator Noise Evaluation with a Crystal Notch Filter". Measuring sideband noise using a crystal notch filter Local Oscillator - Noise Measurement - Crystal Notch