Program and Abstract Book - SRON

19 th International Symposium on Space Terahertz Technology
P7-3
Sideband Noise Screening of Multiplier-Based Sub-Millimeter LO Chains
using a WR-10 Schottky Mixer
Eric Bryerton 1 and Jeffrey Hesler 2
The local oscillators for the ALMA band 9 receivers cover the frequency range from 610-712
GHz. They are built using a YIG tuned oscillator as the fundamental source at 22.6-26.4
GHz, then tripled and amplified in two ambient temperature modules before being multiplied
again by a VDI (Virginia Diodes Inc.) cryogenic nonupler. Further descriptions of the LO
chains and of their contributions to receiver noise are summarized in [1]. Recent receiver
noise measurements using the ALMA band 9 SIS receiver show excess noise contributed by
the LO at certain frequencies. Since the LO and the SIS receiver are developed and produced
at different institutions on different continents, it is highly desirable to have the ability to
diagnose and debug these noise problems before the LO is delivered. It is even more
desirable to make these measurements without a cryogenic sub-millimeter receiver. Using a
similar setup as Erickson [2] used to demonstrate SNR improvements from saturated
amplifiers in LO chains, we use a VDI WR-10 single-ended Schottky mixer to measure the
noise before the cryogenic multiplier, at 66-82 GHz.
The 4-12 GHz IF from the Schottky mixer is amplified by approximately 50dB and measured
by a spectrum analyzer. We see that the IF output measured when a Gunn oscillator is
driving the WR-10 mixer is identical to the IF spectrum with no LO, i.e. only the IF noise is
present. This shows that the Gunn is not adding any excess noise over that of the IF noise
(and also that the mixer noise is dominated by the IF amplifiers in this case). With the
ALMA LO in the same setup, we typically see a very small increase in noise (less than
1K/uW), except for certain LO frequencies, where the measured IF spectrum is much higher
for some portion of the IF (typically below 6 GHz). These noisy LO frequencies correspond
precisely to the noisy LO frequencies seen with the ALMA band 9 SIS mixer at nine times
the frequency. By debugging the problem with a room-temperature Schottky mixer at this
much lower frequency, we were able to quickly identify a likely solution to mitigate the
excess noise. For this specific case, we find that using narrower bandpass filters in the active
multiplier chain reduces the excess noise. The LO unit is presently being shipped back to
SRON for verification with the SIS mixer. At the conference poster session, we will show
SIS measurements before and after, as well as Schottky measurements before and after, to
show that this screening technique can be used to predict excess noise from an amplifiermultiplier
based local oscillator chain.
[1] E. Bryerton, M. Morgan, D. Thacker, and K. Saini, “Maximizing SNR in LO chains for ALMA singleended
mixers,” in Proc. of the 18 th Intl. Symp. on Space THz Tech., Pasadena, CA, April 2007.
[2] N. Erickson, “AM noise in drivers for frequency multiplied local oscillators,” in Proc. of the 15 th Intl. Symp.
on Space THz Tech., Northampton, MA, April 2004.
1 - National Radio Astronomy Observatory, Charlottesville, VA, USA
2 - Virginia Diodes Inc., Charlottesville, VA, USA
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