Index of sections

A New RF System For the Upgrade

The Upgraded 40 Meter will have an entirely new configuration of the RF system. That is mostly because:

1. the length of the short cavities will change (getting longer);

2. the interferometer will be set in a broadband sensing configuration and a new locking scheme and signals will be necessary.

This is how the new RF system will look like.

A PDF version of the diagram can be found [attachement:RFsystem_plant_VISIO.pdf here].

[https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_rev6.html Here's an older version of the plan made with yEd.] Descriptions of the single components in the diagram are shown just by passing over them with the mouse. Clicking on them links to the correspondent data sheet. The numbers included in the diagrams describe the limit case of the highest modulation depth that we may use at the 40m (gamma=0.3).

Requirements

1. Need control of the modulation depths from the control room. Either we keep the current RFAM Stab. boxes or we introduce our own remote control attenuators.
2. The system needs to preserve the low phase noise of the Wenzel crystal and multiplier (~ -160dBc/Hz).
3. Care should be taken to avoid reflections. There should be an amplifier at each split to avoid reflections between the loads from cross-talking.
4. Wherever we have high level signals, we need to use the Heliax cable. Each Heliax cable must be strain releieved at each end. The connection between the Heliax and the components shall be the same semi-rigid RG-174 as we have now or some equally low radiation type.
5. All connections shall be SMA or type N. Absolutely no BNC allowed.

Main Features of the new RF Scheme

The change of the length of the recycling cavities will impose different sideband frequencies. (It is actually the other way around, since we're changing the cavity lengths in order to accommodate lower modulation frequencies).The frequency f1 of the first sideband will be now about 11 MHz (11.065399MHz), ad the second one will be 5xf1=55 MHz.

Sideband generation

The modulations will be generated by a single broadband EOM, thus ending the current configuration with the two EOMs in parallel in the Mach-Zehnder.

Also there will be a single main oscillator to generate the main 11 MHz frequency. All the other signals, with frequencies multiple of the main one, 2x, 3x, 5x, 10x, 15x respectively, will be generated starting from it, rather than by independent oscillators.

f1 will be generated by a crystal oscillator; f2 will be obtained by f1 by means of a 5-time frequency multiplier. The two signals will then be combined into one which will modulate the EOM driver.

The designed modulation depth for the Upgrade will range somewhere in between 0.1 and 0.3. The New Focus KTP 4064 boradband EOM has an efficiency factor of 13mrad/V. That means that the driving voltage should range between 8 and 23 V, that is between 30 dBm and 40 dBm. The frequency multiplier will output 20 dBm, so 20 dBm will have to be gained by the signals before the EOM in order to match the requirements on the modulation depth. The step will be covered in part by the EOM driver in part by an low noise, high power amplifier placed in between.

The requirements on this amplifier depend on the gain achieved by the EOM driver, which is still unknown for sure at the moment.

After the multiplier and before the driver, the f5 signal will be have to be amplified

Demodulation

The plan under study also uses the same signal from the main oscillator for the demodulation of the signals from the RF photodiodes.

The demodulators will be the same demod boards of the old 40m: [attachment:LIGOD990511.pdf LIGO D990511].

In each board we are going to install surface mount low-pass filters in corrispondence of U5 (as in the schematic):

We are also going to attach coax high pass filters to the RFPD input of the demod boxes:

Frequency Generation and Distrubution

This page describes the frequency generation box.

This page describes the frequency distribution box.

Hartmut's Suggestions

• Grounding inside the frequency generation box. If we choose to mount the single components inside the box on to a conducting surface, they should make good contact with it. Bad contact can come from oxide forming on the surface, or by mere unevenness of the surface. for this reason at GEO, to make sure they wouldn't incur into this issue, they mounted the components on a plastic [http://www.williams-sonoma.com/products/7256886/popup/view-larger.html?zoom=1 breadboard].

• Testing the system. Bad connectors, bad cables, bad soldering can all deteriorate the modulation/demodulation signals and eventually produce phase noise that goes into the demod mixers. One way to measure the presence of phase noise is to pick off with a coupler a demod signals from just before the modualtions' combiner and use it as a local oscillator in the demod boxes. Then we can connect audio earphones to the demodualted output and listen for phase noise. That would simulate the 'closed loop' made by the IFO and the actual phase nosie present in the modulation/demodulation chain. We could test single connections or cables, inside the frequency generation box or in the frequency distribution board by touching/shaking them and listening to the earphones.

Modulation/demodulation system's SNR calculation

Plan reviews

In addition to the requirements listed above, Rana suggested the following changes/additions to plan B:

• replace the directional coupler with splitters

• replace LIGO LSC Frequency Distribution splitters with 8-way power splitters from Mini-Circuit

• include cable power losses in the calculations

• abandon the 2 Omega I and Q Demodulator Board

• buy a voltage controlled attenuator to test it

• check what's the modulation depth we need for the mode cleaner and evaluate whether we need a high-power amplifier for the 29MHz as well as for the 11 and 55MHz

• redesign the cabling: check out Intra-Flex for custom made cables

• a band-pass filter before each demodulator board

RF Photodetectors

This page follows the state of the work in progress on the RF photodiodes upgrade

LSC Signals

For locking, the baseline plan is to use the same error signals of AdvLIGO:

In case that 3f signals will be used for lock acquisition, as a result of Optickle simulations, we propose the following sensing matrix:

Photodiode properties

[http://www.ligo.caltech.edu/docs/D/D000455-00.pdf The LIGO document D000455-00-C is the one that resembles the most the board inside the "golden boxes" resembles].

[attachment:C3064XGH.pdf Here's the datasheet of the photodiode.]

[attachment:cat_photodetection.pdf More specs for the diodes (as diode shunt resistance) can be found inthis].

[attachment:AbbottC30642.pdf This document by Rich Abbott contains lots of measurements of PD capacitance and resistance.]

We're going to adapt the current photodetectors for the new signals as described in the following table:

Purchase List

A list of the components that we have to buy can be found here.

Tables and diagrams

[attachment:40mUpgrade_PDlists.pdf Table of old and new PDs plus diagram of signal extraction scheme]

[attachment:40mPDs.pdf Table of PDs and Demodulators]

Upgrade Schedule

As of Sunday July 25th, the remainder of the RF System Upgrade is organized in blocks as below.

The daily schedule can be found in this [http://www.google.com/calendar/embed?src=0606mdhpbl7m9m4ctue2po80ls%40group.calendar.google.com&ctz=America/Los_Angeles RF System Upgrade Google Calendar]

1. (2 weeks: Sun Jul 25 -> Fri Aug 6): RF Generation

2. (2 weeks: Mon Aug 9 -> Fri Aug 20): RF Distribution

3. (3.5 weeks: Sun Aug 22 -> Wed Sep 8th): LSC PDs

4. (2 weeks: Thu Sept 16 -> Fri Oct 1st): Commissioning

Coax Cables

Coax cables are available online either as assemblies (already with end connectors) or in bulk coils.

We are using two kind of cables: RG405(0.086") semiflexible for the internal connections; RG402(0.141") for the external interconnections

Here are two manufacturers that I found online.

[http://www.crossrf.com/rf/cable-assemblies Crossrf]. These guys make (pricy) assemblies. They have several kind of cables: flexible, semi-flexible, semi-rigid, rigid.

[http://www.pasternack.com/product-RG405U-BULK-COAXIAL-CABLE-MIN.-50-FT-COILS-RG405U-73123.html Pasternack]. They sell in bulk.