For some while I was looking for an easy way to calibrate the s-meter of my
receiver.
Sometimes the solution is that simple that it takes some time to find it :
with a handfull of common parts it is possible to make a 5mV into 50 Ohms
(S9+40dB) source that needs no calibration. I did built it 5 times with
random parts and alway came to 5mV within 0.5dB (checked with a Rohde &
Schwartz spectrumanalyzer at work) !
The principle is rather easy : with a 74HCT4060 a 5V square wave close to
136kHz is generated. The advantage of the HCT types is that they are very
good 'rail to rail', so if you feed them with 5V you have an almost perfect
5V square wave. With the knowledge Mr. Fourier left us we know that a 5V
square wave has a fundamental content of 2.25Veff. With a 2 resistor
divider (of 1% metalfilm resistors) the signal is reduced to 5mVeff.
(S9+40dB) into 50 Ohm.
With the more classic 50 Ohms attenuators the signal can be further reduced
to any wanted level.
More informantion can be found at http://www.qsl.net/on7yd/ref136.htm
73, Rik ON7YD
| 