A LVDS TO SINGLE ENDED OUTPUT TRANSFORMER FOR THE SI570 CHIP

ON THE RTW-SDR-LOCALOSC BOARD (2014-01-16)

Preface

Si570 chip is a DSPLL PLL clock generator that is manufactured by Silicon Labs in several versions. The Si-570 chip version that I use on my RTW-SDR-LOCALOSCILLATOR BOARD has LVDS output. The chip supply voltage is 3.3 V. Si-570 has two output pins labeled as CLK+ and CLK-. This pins provide a differential output signal at the programmed frequency. Frequency range is 10-280 MHz. People have checked that the chip can go down to 10 MHz with no problems.

Schematic

FIGURE0: SCHEMATIC.

I had in my junk box a BN-43-302 binocular core from Micrometals. Also had a BN-43-2402 core but appeared to be very small for a trifilar winding. Having in mind that the transformer should have enough reactance to work at 3.5 MHz, I choosed the lowest number of turns that generated a reactance of at least 4 times the system impedance. I needed the required number of turns to have a reactance of at least 200 ohms. We have an useful WEB toroid calculator at http://toroids.info/BN-43-302.php

The core has an AL=1.28 uH per square turn. Three turns on the core has an inductance of 3^2*1.280 = 11.52 uH

At 3.5 MHz, the reactance is X = 2*pi*3.5e6*11.52e-6 = 253.34 ohms.

FIGURE1: RTW-SDR-LOCALOSC BOARD WITH TRIFILAR OUTPUT TRANSFORMER TO A BNC CONNECTOR.

FIGURE2: OSCILLOSCOPE TRACE AT 3.5 MHz Z=50 OHMS.

FIGURE3: OSCILLOSCOPE TRACE AT 10 MHz Z=50 OHMS.

FIGURE4: OSCILLOSCOPE TRACE AT 50 MHz Z=50 OHMS.

FIGURE5: OSCILLOSCOPE TRACE AT 100 MHz Z=50 OHMS.

FIGURE6: OSCILLOSCOPE TRACE AT 280 MHz Z=50 OHMS.

My oscilloscope has a 50 MHz bandwidth. The 100 MHz signal has a false sinusoidal appearance due to the limited bandwidth. I can see the signal at the maximum 280 MHz frequency but strongly attenuated, as you see in the oscilloscope screenshots.

Next

It would be interesting to measure output signal using my diode RF probe that sure will have a better bandwidth. Also exploring the spectrum analysis capabilities of my VNWA3 network analyzer would be interesting. Many things to do, limited time... :-(

Bibliography

Si-570 datasheet.
Kits And Parts Dot Com. Toroid calculator.‎
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