17 January 2012

Single frequency calibrated signal generator

This piece of lab instrument was built after the curiosity to measure (... estimate ...) the sensitivity of an homebrew receiver. As described in a previous post about measuing RX sensitivity with simple tools, I tried to reproduce the Elecraft XG2.

The local shop did not have the 1.22V zener diode and a couple of 1% resistors were not available, but in the end I could get it working with a LM317T regulator at minimum voltage, giving about 1.268V. Having an higher supply voltage means that the generated signal is slightly more powerful than advertised (according to my calculations the difference lies within 1dB).

I used a single 14.000 MHz XTAL which resonates steadily at about 14.0045 MHz.

The completed circuit on a piece of recycled copper-clad board was powered with two exhausted CR2032 button cells (theoretically 3+3 = 6V). Current drain from the battery measured 4.98mA load independent, which is 20x higher than XG2's 250microA but still acceptable for portable use (just in case).

When feeding my commercial receivers with the generator output I could read the S9 signal, a bit on the high side, FT817 indicating S+ few times. Is that caused by the 48mV higher supply voltage, some "hand" effect increasing the output coupling or a calibration error on the receiver's meter?

I took only one test at S1 level and my FT817 went down to S0: need to cross check the result with other receivers.

Another measurement done was to check the signal level on 14 MHz harmonics, which I will publish in a future post. With the 14 MHz XTAL the S9 signal is readable up to the 4th harmonic at 56 MHz

NB: FT-817 conditions were IPO ON (no antenna preamplifier), ATTenuator OFF and NARrow filter OFF.

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