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.

Requirements

1. Need control of the modulation depths from the control room. This makes us either keep the current RFAM Stab. boxes or we put in 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. The frequency f1 of the first sideband will be now about 11 MHz, 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. Although we can expect two possible scenarios:

1. (very) pessimistic: the driver won't be able to provide a positive gain
2. realistic: the driver will give us about 10 dB
3. optimistic: the driver will provide 20 dB as in theory

In either case the choice of the amplifier will be different. For instance, the following could be two possible candidates:

1. +20 dB gain by [http://www.ophirrf.com/files/products/5303055.pdf Ophir Solid State Broadband High Power RF Amplifier 5303055]

2. +10 dB gain: this will be hard, because we will need something that makes the step from +20 dBm to +30 dBm which is not a lot in terms of gain (+10 dB) but the input and output power are rather high; unless we ask for less than +20 dBm at the output of the multiplier. If, for instance we had 8-10 dBm coming from the multiplier, then this [http://www.cougarcorp.com/databasePDF_Files/A2CP2596.pdf A2CP2596] by Teledyne Cougar Corp. could be a candidate.

3. no amplifier will be needed (?)

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

RF photodiodes demodulation

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

Plan drafts

I've laid down a few slightly different layouts for the Upgrade RF System. 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).

(You can read the description of the single components in the diagram just by passing over them with the mouse. Clicking on them links to the correspondent data sheet)

1. [https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_without_Coupler.html The first option] maintains the same RF AM Stabilizers used at the present to tune the modulation depth

2. [https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_with_Coupler.html The second option] totally abandons the old RF AM stabilizers (I like this more)

3. [https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_hybrid_Coupler.html A variation] of the second plan which still uses the AM stabilizer but only for the Mode Cleaner

Rana's Suggested Changes

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
• ask Rich to order 2 11MHz crystals and 2 29.5MHz crystals

[https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_with_Coupler_rev2.html Here's a diagram which includes the main changes suggested by Rana]. The calculations of the power along the chain still does not include the cable losses. That is what is going to follow very soon.

SNR Calculations

Here are the diagrams showing the [https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_with_Coupler_rev2_phase Noise.html Noise Power] and the [https://nodus.ligo.caltech.edu:30889/UpgradeRFplan/yEd_RFplan_with_Coupler_rev2_SNR.html SNR] in the RF System, respectively.

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]