Reference signalsource for 136kHz
With a few components a 5mV (S9+40dB) reference signalsource can be made with an accuracy better than 1dB.
With the appropriate attenuators any other (lower) voltage can be generated.
*1 : R1 = 10k and1k2 metalfilm resistors (1%) in series
|IC1 : 74HCT4060||IC2 : 78L05||R1 : 11k2 (*1)||R2 : 50 (*2)||R3 : 1M|
|R4 : 2k2||C1 : 33p||C2 : 100p||C3 : 10µ/15V||C4 : 100n|
*2 : R2 = 2 times 100 Ohm metalfilm resistors (1%) in parallel
X1 is a crystal at 8, 16, 32 or 64 times the wanted output frequency in or near the 136kHz amateur band. Dependent on the crystal frequency the appropriate divider output must be selected :
IC1 is a 14-stage binary devider with oscillator, the output at the deviders is a 5V 'rail to rail' square wave. To get the proper 'rail to rail' signal (ie. exactly 5V) it is important to use a 74HCT4060 and not a CMOS4060.
The harmonic content of a square wave signal is given by :
With a 5V square wave (= 2.5V amplitude) the fundamental signal is 3.18V peak or 2.25V effective. With the voltage divider at the output (R1/R2) the signal is reduced to 5mV effective into a 50 Ohm load. It is not recommended to try to get a smaller signal by increasing the R1/R2 ratio, because due to stray capacitances the accuracy will be insufficient. A better (more accurate) way is to add one or more 50 Ohm attenuators. At 136kHz these can be easily made for values up to 40dB :
With a 'chain' of 7 attenuators any voltage between 5mV (S9+40dB) and 0.1µV (less than S1) can be selected in 1dB steps :
In order to get an good 'all-over' accuracy the use of 1% metalfilm resistors is recommended. The practical values are based on the available values for 1% metalfilm resistors (0.6W) of the Philips MRS25 series.
|Calculated :||Practical :|
|dB||R1 = R3||R2||R1 = R3||R2||dB|
If a high frequency stability is not required the crystal oscillator can be replaced by a RC oscillator :
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