LO2 Isolation Amp installed

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LO2 Isolation Amp installed

Post by Sam » Wed Mar 02, 2011 10:45 pm

I have installed the LO2 isolation amp in the line from PLO2 to Mixer 3 to prevent leakage of LO3 into PLO2. The results are impressive and are shown in my album:

http://groups.yahoo.com/group/spectruma ... 8/pic/list

There are two photos of the Zero-Response, one with and one without the buffer amp. At 100 kHz the level dropped from -75 dBm down to -95 dBm, and by 500 kHz it is into the -120 dBm noise floor. This greatly expands the dynamic range of the MSA at low frequencies, especially below 1 MHz. This is especially nice for the Component Meter function, which prefers to measure large capacitors at 100 kHz. I haven't tested it, but this expanded range conceivably could increase the maximum measurable capacitance by a factor of 10.

The zero response is in the noise by 500 kHz, but it is still there. There is a trick that can be used to detect it, as I showed in a document I posted somewhere, but can't remember where. If you do a zero-width Transmission mode scan at 1 MHz, with a 50-ohm termination on the TG and the MSA input connected to a 1 MHz signal source, you would expect to get a straight line graph. However, the phase of the signal source will drift with respect to that of the zero response, and the result will be a sine wave, if the zero response is strong enough to show itself.

At 1 MHz, with a -100 dBm input signal, I got a sine wave with amplitude about 1 dB, meaning the zero response was causing +/- 0.5 dB of error. Based on the table in my "Effects of Interference" paper, this suggests the zero response was about 26 dB below the input signal, or -126 dBm.

At 2 MHz, the zero response was just barely detectable as a wavy line with input of -105 dBm, suggesting it was below -140 dBm. This is important because 2 MHz is a nice frequency to use for path calibration. I previously suggested shifting to 5 MHz for low level calibration, but that is not necessary with the LO2 isolation amp.

Sam W.

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