ALMA Frequency Switching

Larry D'Addario describes the state of frequency switching capabilities in ALMA as follows:

Frequency switching is certainly possible, but we need to consider the parameters: maximum switching speed and maximum frequency change between signal and reference. An old specification requires that a change of .03% of the sky frequency be accomplished within 10 msec (indeed, this was item #431 in 'System Technical Requirements' 80.04.00.00-004-D, now obsolete), but it applies only in single dish mode, so that phase tracking across the frequency step is not necessary. We expect to be able to achieve this, and the actual time should be much less than 10 msec. Originally it was assumed that this must be done in the 1st LO, but that becomes difficult in the high bands (.03% of the sky frequency reaches 284 MHz in band 10 (sky frequency = 950 GHz)) because the 1st LO cannot re-acquire lock that fast. However, it can still be done in the 2nd LO. It needs to be understood that because of tuning resolution constraints, the frequency step size cannot be freely choosen (except at the lower bands, where it can be done in the FTS). The spec is understood to require at least .03% of the sky frequency. The actual step size may be rounded up from that requested to the next larger available value, so it might be more much more than .03% of the sky frequency. (The tuning of the 2nd LO is not continuous, but has gaps of up to 40 MHz.)

Outstanding Questions

  • Is 0.03% of the sky frequency the frequency throw or the total switch range (i.e. is it +-0.03% or a total range of 0.03%)? Answer: It is the throw. -- JeffMangum - 09 Aug 2004

-- JeffMangum - 25 Jul 2004


Chip Scott's comments:

I agree with what Larry is saying with some minor clarifications.

The YIG tuning speed is roughly 2ms per 1 GHz step. This will be the slowest tuning. The FTS tunes even faster but the frequency change takes place on a timing event. The overall tuning speed will depend on whether phase lock is lost or not.

I think the subtle point everyone misses is that there is no way the system can verify successful tuning for anything under 48 ms or one timing event. It is possible the tuning is taking more than 10 ms, all the way up to close to 48 ms. On the next timing event, the PLL will show lock but when was lock achieved or when did the tuning event settle out?

It might help if I understood this particular requirement better.

Chip

-- JeffMangum - 01 Aug 2004
System Requirements Summary

Based on previous single dish (140ft, MWO, 12m) frequency switch capabilities, FS on ALMA should:

  • Switch by at least +-0.03% of the sky frequency
  • Switch frequency and lock LO on a timescale not more than 10ms
  • Integrate at a given frequency switch state for not less than 1TE (48ms)

Dick Sramek suggested the following elaboration on these general specs:

  • The 2nd LO shall be able to switch frequency by at least 285 MHz (0.03% of sky frequency at 950 GHz) and establish phase lock in 10 ms;
  • The change in frequency should only be initiated at a timing event, TE (48 msec). This implies that the duration in any frequency state will be a multiple of 48 msec;
  • Since frequency switching will only be used in total power mode, the requirement that the 2nd LO be phase continuous after a frequency switching cycle, is not needed while frequency switching;
  • Continuous selection of LO frequency within the 285 MHz range is not required; 35% of the band must be available for use, but gaps up to 41 MHz are permitted;
  • After a frequency change, a monitor bit should be set if lock is not achieved within 10 msec. This monitor point would be read after each command for a frequency change.

-- JeffMangum - 07 Sep 2004
Discussion from Darrel

Here are some thoughts on the frequency switching issue. There are separate things: what we'd really like, what compromises we've already agreed to, and what we'll actually get.

First what we'd like: obviously this is only relevant to single dish spectroscopy, not to any form of interferometric observing.

IN AN IDEAL WORLD:

1. Frequency switching is the most efficient single dish spectroscopic observing mode, since the "Off" position that all observing modes require is simultaneously looking "On" source. So, we would prefer not to compromise this important observing mode too much.

2. If there were no other constraints, we would insist on the 1st LO being switched. The whole idea of frequency switching is to keep everything in the receiver constant between the two switch cycles, so all the slope and ripples in the rx passband, due to the rx itself, cancel as perfectly as possible. By switching the 2nd LO instead, we are effectively moving the IF filter response of the first IF back and forth on the sky. This has two bad effects: (a) the IF filters and other circuitry will not have a perfectly flat frequency response. When the two phases of the 2nd LO switching cycle are subtracted, we'll be subtracting a shifted version of the 1st IF response from itself. As it's shifted, it won't cancel completely. In other words, by switching the 2nd LO rather than the first LO, we'll get worse baselines, even in the total absence of atmospheric effects. (b) the total usable spectrum width is reduced by the switching range; the edge of the spectrum is going to fall outside the IF filters in one phase of the switching cycle. If we're trying to look at a spectrum that's more than 2 GHz wide, we're going to have to arrange the different 2-GHz 2nd IF bands to overlap by more than the switching distance.

3. Switching offset precision. What doesn't cancel in frequency switching is the frequency response of the antenna itself. This is usually dominated by a standing wave ripple, typically coming from coherent noise being radiated out of the receiver feed horn and reflecting back into the feed after bouncing off the subreflector, feed struts or whatever. This causes a variation in the rx input noise, typically dominated by a sinusoid corresponding to the path length involved in the strongest reflection. With frequency switching, if the precise switching frequency interval is made to match an integer number of periods of this dominant sine wave, the sinusoidal ripple is effectively filtered out, and the spectral baseline is very much better. So, in an ideal world, we'd like to match the frequency switching interval to the standing wave. The period is likely to be around 25 MHz (say, a 12-meter round trip reflection from the rx to the subreflector), and we'd probably like to match a multiple of that within, say, 1(?) per cent - that means the switching distance should ideally be settable to within 0.25 MHz.

4. Frequency switching cycle time: If we want to do frequency switching OTF observing, the cycle time obviously needs to be very fast - less than a millisecond probably. However, I don't think we're planning to do rapid-scanning-OTF observing of frequency switched mode (although I don't know why not). This is probably just one of those things we compromised on years ago. At the Kitt Peak 12 Meter, we typically used a 1-second switch cycle (but not with rapid OTF scanning).

WHAT WILL ALMA ACTUALLY PROVIDE?

I remember these discussions taking place years ago, and we had to accept a compromise. That battle has been lost, so it probably shouldn't be brought up again. However, we should just try to approach the ideal (i.e. first LO switching, with complete choice of frequency switching interval, and a reasonable fast switching cycle) so far as the existing hardware plans and agreements allow it.

This doesn't answer Clint's questions, but it might nevertheless be worth pointing out what we'd really like if there were no constraints.

-- JeffMangum - 07 Sep 2004

Comments from Al Wootten to Clint Janes regarding a document written by Chip Scott describing his understanding of the frequency switching science requirements.

You asked me to comment on a document you and Scott wrote.

I think your document makes sense to me. The switch of .03% is always about 90 km/s. For frequencies above 3mm that will always exceed 25 MHz which will make matching the FS interval to the antenna period of 25 MHz achievable under the 'at least 0.03%' understanding until perhaps when one gets to the lower bands. Your summary:

  1. 1st LO can't do it, but that needs to be verified.
  2. 2nd LO can do it without too much grief if the minimum integration time (observing time on the sky for a single frequency) is at least 48 ms.
  3. 2nd LO can still do it for shorter integration times, but with a ~$30k redesign of the 2nd LO. BE requests a CRE in this case in order to document the funding request.

You asked three questions. Would you please verify:

  1. Our understanding of the frequency switch,
  2. Advise if the minimum integration time on a single frequency is 48ms, and
  3. Verify that 1st LO cannot reasonably perform the frequency switch?

My answers are:

  1. I think your understanding is good. Scientifically one would want to use FS when one wanted to observe an atmospheric line (position switching would cancel out the signal from the large source) or a bright continuum source (position switching switches between two signals with a large signal difference while frequency switching does not, making this preferable for some experiments).

  1. I don't think there are compelling reasons to switch on time intervals less than 48 ms. At the 12m the time was 1s; at MWO we used a 5 Hz switch. 20 Hz will be fine I think.

  1. Well it would be best for the first LO to do it. I think at one point Shillue told me what you wrote in your memo--that the laser couldn't switch and grab lock in 10ms for the 0.03%. But is there a frequency interval for which it CAN grab and lock in 10ms? Or is there a time interval for which it CAN grab and lock at 300 MHz shifts? It seems to me this is what your conclusion is driving at, and it would be good to know the answer.

Clear skies, Al

-- JeffMangum - 07 Sep 2004 -- Since frequency switching came up in our discussion yesterday, here are a few comments on the observing technique.

Although the need to support frequency switching for single dish spectral line observations is in the ALMA requirements, I'm not sure that the importance of frequency switching being carried out only in the FIRST local oscillator has been appreciated.

First, to reiterate the importance of frequency switching as a single dish observing mode: all SD observations require switching against something, whether that's blank sky, a cold load or whatever. Frequency switching, where the frequency throw is small compared to the total rx passband, has the advantage that both the "OFF-souce reference" and the "ON-source" signal are observed simultaneously for 100% of the time. It's like an increase of a factor of 2 in observing time, making frequency switching one of the most efficient SD observing modes there is. Further, for observing terrestrial atmospheric lines - which are of interest for atmospheric physics, but also for some calibration issues - frequency switching is the best way to go; you can't position-switch against blank sky, because the atmosphere is more or less the same in all directions.

Why does the switching have to be in the first LO? When you switch the first LO, everything in the rx chain beyond the first mixer stays constant, so will cancel nicely when you subtract 2 phases of LO frequencies, "ON-source" and "OFF-source." The only thing that doesn't cancel is the noise spectrum before the mixer, which includes antenna spillover noise, atmospheric noise, and the desired astronomical signal. This allows you to observe the astronomical signal, while canceling out the strong ripples, slopes and drifts occuring in anything in the receiver system after the first mixer.

If, instead of switching in the first LO, you chose to switch just the second LO, the results will be very poor. The first IF amplifiers and filters will inevitably have slopes and ripples across the passband. By switching the 2nd LO it won't be possible to distinguish between spectral features in the first IF amplifiers and filters, and astronomical spectral features. Remember, these IF slopes and ripples are multiplicative factors on the entire system noise. If you're trying to observe a milliKelvin astronomical signal, which will often be the case, it will have to be done in the presence of ripples which may be effectively hundreds of degrees in amplitude - say, 40 dB to 50 dB stronger than the signal you're trying to see. Results will be very poor indeed.

The ripples in the frequency response of the feed horn, and the antenna itself, of course do not cancel with any form of frequency switching. However, they will normally be very small indeed, being just a multiplicative factor on the background (cosmic and atmospheric) and spillover noise received at the feed. This compares to the ripples in the first IF, which are multiplicative factors applied to the entire system noise.

So, frequency switching should be supported in the first LO. Frequency switching applied to the 2nd LO doesn't make much sense.

Cheers, Darrel. -- The 12 M at Kitt Peak used to do lots of detection experiments in beam-switched mode. With only 4 nutators, that's not going to be a very sensitive mode on ALMA. However, for a detection experiment on extended objects, frequency switching may be the only mode possible. If any source is sufficiently extended (i.e. more than a primary beam width) than it won't be visible on any interferometric baseline. The most sensitive prospect of detecting a weak signal under these circumstances is to average the power spectra from 64 single dishes. You have to be switching something for observations like that, and frequency switching is going to be the only thing that may be fast enough. Position switching, slewing the entire antenna back and forth, may simply not be fast enough to take out the atmosphere and still allow good spectral baselines. Frequency switching, since we can't nutate the S/R on 64 antennas, is likely to be the only way to go.

So, weak detections of extended sources will require frequency switching.

Cheers, Darrel.

-- AlWootten - 29 Apr 2005
Topic revision: r5 - 2005-04-29, AlWootten
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