Monday, December 01, 2014

Silicon Labs Si5351A loaded output

I'm really considering this device (Si5351A) to be the VFO and BFO for my new transceiver (Trevo).

I implemented some code, on the Arduino, to change frequency depending on the position of a potentiometer since I'm waiting some rotary encoders to arrive.

While I don't proceed with more code, started to investigate the best way to interface the output to an ADE-1 mixer since I'm using them on the "Trevo".
 Saying that: connected the oscilloscope probe to one of the Si5353 outputs and started measuring:

Unload (or for that mather loaded with the scope probe) I got around +19dbm.... _if_ the voltage was applied on a 50 Ohm load (measured 2 V on the scope).... that looks promising...., let's try loaded... and it drops to the following values and matching load (a carbon resistor was used):

47 Ohm: 316 mV
100 Ohm: 700 mV
220 Ohm: 1V

That sayd then the output impedance is 220 Ohm, not even a calculator is needed...

The stated datasheet impedance, for the Si5351, is 85 Ohm, I might be doing something wrong or not taking in account something else... I might try again with the probe in 10x to raise the scope loaded impedance.

 Anyhow loaded with around 50 Ohm (47) that's still +3 dBm, enough for some more testing ...
 Test frequency was 19.100 Mhz,

Have a nice week!


YO9IRF said...

Hi Ricardo,
the output dBm level is a power level (dB relative to 1mW), therefore 1V into 220 ohm = 4.5mW (6.5dBm) and 0.7V into 100ohm = 4.9mW (6.9dBm), wich is about in line with the manufacturer's figures. Keep in mind the ADE1 mixer is designed with 50ohm impedance in mind (although it might have lower losses in a higher impedance system). It would be interesting to see how the frequency response looks like, how much the output level decays when you go up in frequency.

73 de YO9IRF (Razvan)

Ricardo - CT2GQV said...

Hello Razvan,

Exactly, that's why I placed the "_if_" the voltage was applied to 50 Ohm, in the article. On 47 Ohm is 3.2 dBm, I only made that calculation.

For the impedance, i have first to rule out the scope impedance effect since the "measured" output impedance (220) it's way out from the manufacturer datasheet (85 on the recommended output configuration).