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Entry  Fri May 6 01:27:12 2011, Koji, Summary, General, aLIGO BBPD / Green PD investigation PD_response.pdfFFD-100_AP389_MCL.pdf
    Reply  Sat May 7 12:57:54 2011, Koji, Summary, General, aLIGO BBPD / Green PD investigation PD_response.pdf
       Reply  Tue May 10 09:05:49 2011, Koji, Update, General, aLIGO BBPD / Green PD investigation PD_response.pdf
          Reply  Wed Jun 1 21:56:43 2011, Koji, Update, General, aLIGO BBPD / Green PD final result PD_response.pdflinearity_GALI_6.pdf
Message ID: 4649     Entry time: Fri May 6 01:27:12 2011     Reply to this: 4658
Author: Koji 
Type: Summary 
Category: General 
Subject: aLIGO BBPD / Green PD investigation 

Minicircuits ERA-5SM was used for the RF amp of the BBPD. This amp is promising as a replacement of Teledyne Cougar AP389
as ERA-5SM gave us the best performance so far among the BBPDs I have ever tested for the aLIGO BBPD/Green.

The -3dB bandwidth of ~200MHz and the noise floor at the shotnoise level of 0.7mA DC current were obtained.

1. Introduction

The aLIGO BBPD candidate (LIGO Document D1002969-v7) employs Teledyne Cougar AP389 as an RF amplifier.
This PD design utilizes the 50Ohm termination of the RF amp as a transimpedance resistance at RF freq.

However, it turned out that the bandwidth of the transimpedance gets rather low when we use AP389, as seen in the attachment2.
The amplifier itself is broadband upto 250MHz (the transfer function was confirmed with 50Ohm source).
The reason is not understood but AP389 seems dislike current source. Rich suggested use of S-parameter measurement
to construct better model of the curcuit.

On the other hand, the RF amplifiers from Minicircuits (coaxial type like ZFL-1000LN+), in general, exhibit better compatibility with PDs.
If you open the amplifier case, you find ERA or MAR type monolithic amplifiers are used.

So the question is if we can replace AP389 by any of ERA or MAR.

2. Requirement for the RF amp

- The large gain of the RF amp is preffered as far as the output does not get saturated.

- The amplifier should be low noise so that we can detect shot noise (~1mA).

- The freq range of the useful signal is from 9MHz to 160MHz.

The advanced LIGO BBPD is supposed to be able to receive 50mW of IR or 15mW of 532nm. This approximately corresponds to
5mA of DC photocurrent if we assume FFD-100 for the photodiode. At the best (or worst) case, this 5mA has 100% intensity modulation.
If this current is converted to the votage through the 50Ohm input termination of the RF amp, we receive -2dBm of RF signal at maximum.

This gives us a dilemma. if the amp is low noise but the maximum output power is small, we can not put large amount of light
on the PD. If the amp has a high max output power (and a high gain), but the amp is not low noise, the PD has narrow power range
where we can observe the shotnoise above the electronics noise.

What we need is powerful, high gain, and low noise RF amplifier!

Teledyne Cougar AP389 was almost an ideal candidate before it shows unideal behavior with the PD.
Among Minicircuits ERA and MAR series, ERA-5 (or ERA-5SM) is the most compatible amplifier.

AP389 ERA-5
Freq Range 10-250MHz DC-4GHz
Gain >24.5dB 20dBtyp
Output power (1dB compression)
23dBm 18.4dBm
IP3 36dBm 33dBm
Noise Figure <3.3dB 3.5dB

Considering the difference of the gain, they are quite similar for our purpose. Both can handle upto -2dBm,
which is just the right amount for the possible maximum power we get from the 5mA of photocurrent.

3. Test circuit with ERA-5SM

A test circuit has been built (p.1 attachment #1) on a single sided prototype board. 

First, the transfer function was measured with FFD-100. With the bias 100V (max) the -3dB bandwidth of ~200MHz was observed.
This decreases down to 75MHz if the bias is 25V, which is the voltage supplied by the aLIGO BBPD circuit. The transimpedance
at the plateau was ~400Ohm.

Next, S3399 was tested with the circuit. With the bias 25V and 30V (max) the -3dB bandwidth of ~200MHz was obtained although
the responsivity of S3399 (i.e. A/W) at 1064nm is about factor of 2 smaller than that of FFD-100.

The noise levels were measured. There are many sprious peaks (possibly by unideal hand made board and insufficient power supply bypassing?).
Othewise, the floor level shows 0.7mA shotnoise level.

Attachment 1: PD_response.pdf  172 kB  Uploaded Fri May 6 02:30:12 2011  | Hide | Hide all
PD_response.pdf PD_response.pdf PD_response.pdf PD_response.pdf
Attachment 2: FFD-100_AP389_MCL.pdf  105 kB  Uploaded Fri May 6 02:36:37 2011  | Hide | Hide all
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