With the rapidly growing interest in home brew and kit HF transceivers there has been a corresponding interest in the design of crystal filters. In order to design a crystal filter one must first extract the crystal motional parameters, create a design and then sort candidate crystals by frequency.
This simple board allows two different methods of deriving crystal motional parameters. The first is through the G3UUR method of measuring the resonant frequency shift of the crystal by inserting and removing a small capacitance in series with the crystal. The second is by simply connecting the crystal in series with a frequency sweeper and detector or a spectrum analyzer with a tracking generator and measuring the series resonant peak and parallel resonant dip frequencies. The latter is the method I normally use.
This board is just a modified and prettier version of Vasily Ivaneko’s test circuit shown here. This board has a 3dB pad on the input and output and a pair of 4:1 transformers surrounding the crystal to provide a very low impedance crystal source and load. In this case 50/(4^2) = 3.125 Ohms as opposed to his 1.811 Ohm circuit. The wideband coilcraft RF transformers are good over a range of 150kHz to 500MHz. Input and output connections are SMA. The series capacitor on the board is approximately 27pF which, after PCB strays comes out close to 30pF.
The board supports standard HC49 leaded crystals and also has a footpring for HC49/S surface mount variants.
Shown below is a typical crystal response for a 20MHz crystal using the test board and a Siglent SSA3021X spectrum analyzer with tracking generator.
|Dimensions||6 × 6 × 1 in|