In the ALMA LO, for reasons of simplicity and reliability it has been specified that mechanical tuners will not be employed. This is a departure from designs traditionally used in total power radio telescopes. As a paradigm, consider the NRAO12m. Since the demise of Klystrons, the ultimate LO source was a Gunn diode, probably made by John Carlstrom. This device could be tuned, rejecting noise in the LO signal to a very high degree. At higher frequencies, the Gunn was located in series with multipliers, also tunable, sometimes vexingly so. John Payne estimates: " It is instructive to calculate both the intensity and stability of any spurious signals that may accompany the LO. A few assumptions are needed. Make the following: the minimum bandwidth is 100 KHz: the position switching time is 30 secs: the LO power required is 0.5 micro Watts: the bandwidth of the LO signal is 1 Hz and any spurious signals in the bandpass are stable to better than one part in one thousand over one minute( the usual position switching cycle). This latter point establishes the residual amplitude of any spurious signals in the I.F. passband to be around one Kelvin. Of course a crucial point here is the bandwidth of the spurious signal. If the same as the LO its intensity will be lessened by a factor of 10E5 by the resolution of the spectrometer. The strength of the LO signal in Kelvin is around 10E16 so any spurious signal for this very simple scenario should be around 160dB down on the LO signal." Now consider ALMA. There is no mechanical tuning of the YIG oscillator, so noise in the LO signal is not rejected but passes to the amplifiers and multiplier(s). Thacker notes that the YIG itself is very clean but operates at intrinsically lower frequencies than does a Gunn. Owing to this, more amplification and multiplication must be included to reach the very high ALMA frequencies. Within the YIG, all those heavy nuclei precessing in lockstep make a very good resonator which has the advantage of being electrically tunable but still a tuned resonator which rejects noise just as the mechanical resonator in the Gunn. This ~15GHz signal goes through amplification and multiplication. The chain needs to be bandpass filtered as one doesn't want spurious signals which might show up, mix in other signals, or otherwise perturb the system. The multiplier is not tunable either, and passes noise not only in the desired harmonic but perhaps in others as well, From spectrum analyzer observations, it is known that this noise is present and it can be roughly characterized in bandwidth and strength but not as well in temporal stability. If this noise is injected early in the signal chain, it is subject to further instability in any amplification as even a perfectly stable signal will have superposed upon it gain fluctuations from the amplifying device. The same gain fluctuations are affecting the system noise and the astronomical signal, so the spur does not necessarily make things any worse, as Larry notes. How are we to know whether the ALMA total power system will be capable of the science tasks assigned to it? It is planned to test this system as completely as possible using an ALMA Front End in conjunction with the Green Bank Spectrometer. This test will take place in late May or early June and involve many of the elements of a complete ALMA system. Skip Thacker will carry out this test, accompanied by someone from the science group (AW). This test will not include a laser synthesizer; the 100 GHz signal will come from a harmonic multiplier as traditionally provided as a clean test is needed. Skip plans to set the system up and integrate for several hours to reach levels of several mK with spectral resolution provided by the spectrometer in its widest spectral mode, a few hundred kHz. The system will be switched, so a non-stable signal will show up as a spectral feature within the bandpass. The result of the test will be a demonstration that the ALMA LO can provide stable clean signals down to a level which will be determined. If the system provides a clean bandpass down to the several mK level, it is still possible that the output of the multiplier produces power at multiples of the desired frequency. This should be characterized also, by a future test. A further worry is that the LO from the WVR, at 183 GHz, will also provide an unwanted harmonic at 366... GHz which may affect some receivers. This will also be characterized when the system becomes available. Now...the actual test. Setup for the test will occur 27-28 May in GB. The tests will be run the week of 2-6 June in Green Bank. The ALMA LO plate will send LO into a 3mm SIS mixer looking at a 300K load. This will provide Tsys ~ 400K or so. The YIG chain for the LO will add a few degrees of noise to this. On ALMA Tsys=60K, with the same few degrees contribution from the LO chain. Skip will measure the phase noise of the LO plate. This plate involves a YIG but tests will also be done with a Gunn (needs lock system) for comparison. Some training in He transfer and other items will be necessary also. Pan will oversee these items. We plan to observe the whole receiver IF if possible. The spectrometer is capable of 4 x 800 MHz at a time, so three settings will be necessary. LO frequencies should be chosen over a range, say 92 GHz, 100 GHz, 108 GHz. Best if we put the Receiver output right straight into spectrometer IF rack at 1-8 GHz band level. But the rx output is 4-12 GHz and so a downconverter will be needed. Webber suggests putting a small offset into the system to resolve ambiguities. Or other solution? Need image rejection >40 db or so in conversion. The GBT Spectrometer has a converter rack, which has as an input the 1-8 GHz from the Spectrometer. This then gets filtered and mixed and output as 8.5-10.3 GHz before further downconversion to 1 GHz or so. Ron thought that it might be possible to somehow deal with the upper part of the ALMA receiver IF band by tapping into the converter rack after the first mix. I hope to get some idea of whether this can happen from Galen Watts, who is in charge of the converter rack. Galen Watts responded: "I think tapping into the Converter Module as Ron suggested is feasible. It will require some other LO generator than the GBT system uses but we have those. It would mean doing only two IF's in the 8-12 range at a time unless I can come up with more generators, a possibility. Roger Norrod mentioned that he'd talked about these tests with Skip Thacker a few weeks ago and Skip might have been building a converter for the 8-12 GHz range, do you know any more about that? I'll try to talk with Skip later today." On the phone, Galen mentioned that he had talked to Skip and that Skip had a backup plan. Switching. How do we get the data calibrated? How do we look at it? Calibrating IF bandpass shapes. can switch in and out test tones etc. 800 and 1600 MHz IF for spectrometer has a noise source built in so a sig ref calibration of bandpass shape could be put in. Lacasse has built this into the spectrometer racks. We can use Lacasse's built in sig and ref. Hot cold loads are possible. Does the receiver setup allow this? I would imagine so but I am unsure. Can take one scan, subtract another scan and divide by it. Can then write out and sum in ascii. It can also average scans together, then write out sums in ascii. Involving normal data reduction (aips++) would require a filler; we will not attempt this. Don't want data glut, so say 30s on 30s off is probably fair. If IF noise is 1-2% of Tsys, integrate down to 0.1% of Tsys or few tens of mK. The maximum bandwidth for 800 MHz 8 samplers is 391,000 Hz resolution; one sampler 48,400 Hz. So 2 (Tsys)^2 time = ----------- = 15 minutes or so, say ~20 on-off pairs. 10^5Hz (60mK) This would be a reasonable amount to keep track of with the SPECTROMETER program. This would be run at several LO settings, as above, and probably on different days we would use YIG, Gunn, or other variation on the system. The "spectrometer" program can 1) read all GBT binary fits files 2) can average any two sets of spectra (ie define any set as Signal and another set of scans as reference) BUT only if the data are taken with the same number of lags/channels and spectral resolution and number of bits. This should be no problem, as we want 800 MHz which only has 2 bit/3 level sampling. The ascii data are always written out every time a plot is made. The "spectrometer" program works exactly correct in TOPCENTRIC coordinates. This is probably what we want anyway. GB should check the observing setup in 4 times 800 MHz band, dual polarization mode before we get there.