This entry summarized the status of REFLAIR_B signals. REFLAIR_B is the broadband photodetector, D1002969, mounted in reflection of the interferometer which is responsible for measuring the 3f signals at 27 MHz and 136 MHz in DRMI. The 3f signals are relatively weak, especially at low modulation depth. This has lead to a low signal-to-noise ratio at sub milliampere photocurrent and distortion problems above a milliampere. The problem of saturation by out-of-band intermodulation products was recognized early on a LLO and fixed with a diplexer amplifier, D1300989.
The situation at LHO and LLO are essentially the same.
- LLO operates with 0.2 mA of photocurrent and can lock both the DRMI and the full interferometer. The signal-to-noise is not great and sensor noise dominates the 3f locking signals above 20 Hz. The modulation depth is three times the design value and will generate some excess light at the antisymmetric port.
- LHO operates around 4 mA of photocurrent and can lock DRMI with a better signal-to-noise ratio in the 3f locking signals. However, the signals have a strong dependency on the incident power, see here.
Koji has recently measured the second order intermodulation products of the RF chain in the broadband photodetector and found the second order intercept point to be rather low, around 30 dBm. This is maybe not too surprising considering these RF amplifiers are single transistors or Darlington configurations operated around a fixed DC working point. No specification of the IP2 is given in the datasheet.
This leads to the conclusion that 3f signals at LHO are predominantly due to intermodulation distortion in the RF amplifier chain and not due to the optical signals. This obviously works just fine for the DRMI, but probably doesn't give the required immunity to the carrier mode for full interferometer locking.
Possible solutions:
- Try to operate LHO at low photocurrent and re-optimize the DRMI locking signals.
- Removed the first amplifier stage from the broadband photodetector. This will remove 22 dB of gain and hopefully greatly reduce the RF distortion. The dark noise of the photodetector will increase by about 3 dB. At 27 MHz we should be shot noise limited with 0.2 mA of photocurrent. So, this should have little effect on the sensor noise. However, at 136 MHz the shot noise limited sensitivity is around 2 mA, so we should see a ~1 dB increase in the noise level.
- Get the 3f photodetector which is based on a tuned design into fabrication.
Increasing the modulation depth and reducing the photocurrent at the same time will not improve the situation, since the distortion depends on the absolute signal strengths of the individual RF lines. However, removing the first amplifier stage should give us enough room to further increase the modulation depth and improve the signal-to-noise ratio. Using a high modulation depth during locking may require an adjustable EOM driver, such as the, D0900760.
Summary
- Follow up measurement for the alog above was done.
- It was confirmed that the first preamp (MAR-6SM) is creating the domnant intermodulation and we will be able to improve it
by removing this first preamp as suggested (by costing some noise increase).
- It may become overkill if we are going to apply notch filters as being tested at LLO. Therefore it is also planned to test other amplifiers
that are similarly low noise to MAR-6SM, and are located between MAR-6SM and GALI-6 in terms of the intermodulation performance.
2nd-order & 3rd-order intercept points (IP2/IP3)
To quantitatively confirm Daniel's expectation above, I took measurements of the amplifier IP2/IP3.
IP2 and IP3 for an amplifier are defined by from the amount of harmonic distortions as
P2 [dBm] = P1 [dBm] x 2 - IP2 [dBm]P3 [dBm] = P1 [dBm] x 3 - IP3 [dBm]
Here, P1 is the power of the linear output, and P2/P3 are the power of the 2nd/3rd harmonics.
When P1 reaches IPn, Pn becomes equal to P1. i.e. the output starts to be dominated by the n-th order.
Of course, we usually can't drive the amplifier at that level, this is purely a mathematical way to quantify nonlinearlity of the amplifier.
Basically the power of the bilinear intermodulation can also be estimated with IP2 in the same way as above.
Just replace P1 with the total power of two signals into the formula for P2. There may be some factors like 3dB, but just forget about it for now.
TEST1: Nominal configuration (MAR-6SM + GALI-6)
In order to measure IP2/IP3 of the nominal amplifier configuration of the BBPD, the input power was swept from -60dBm to -20dBm.
The input frequencies of 9MHz and 45MHz was used in order to check the frequency dependence. In fact, there was no significant
frequency dependence as we'll see in the result. Therefore only the input frequency of 45MHz was used in the other measurements.
Attachment 1 shows the relationship between the amplifier input power and the output power at the fundamental, 2nd harmonic,
and 3rd harmonic frequencies. The lines were manually applied to illustrate IP2/IP3. From the line for the linear power (red), the gain of
the amp chain was determined to be 32dB. In this configuration, IP2 and IP3 were 35dBm and 31.5dBm, respectively.
Practically, we want to know how much intermodulation (IMD) we produce when the amplifier is connected to the IFO.
I gazed Evan's measurement (14807) again and determied the combined power for 9MHz+36MHz, and 45MHz+91MHz to be
-0.5dBm (-32.5dBm at the input) and -11.9dBm (-43.9dBm at the input), respectively. These are indicated as the vertical black lines in the figure.
We expect to have -0.5*2-35 = -35.5dBm of IMD for 27MHz, and -11.9x2-35 = -58.8dBm of IMD for 135MHz. That is not too far from what we see
from Evan's meausrment. (Sanity check)
TEST2: The 1st preamp only (MAR-6SM)
Attachment 2 shows the same measurement only with the first preamp (MAR-6SM)
Roughtly to say, IP2 of MAR-6SM is reduced by a factor of 14.5dB, which is close to the gain of the second amp (13dB).
This means that the IMD performance of the chain is limited by this amp. Minicircuits show IP3 only in the spec sheet.
The measured value (18.5dBm) is close to the spec (18.1dBm). (I'm not insane)
TEST3: The 2nd preamp only (GALI-6)
Attachment 3 shows the same measurement only with the second preamp (GALI-6)
This amplifier has much better IP2/IP3 than the 1st one. Again the measured IP3 (38dBm) is close to the spec (35.5dBm)
This measuerement indicates that we'll have the IMD of -70dB and <-80dB relative to the source of the IMD when the first amp is removed.
Drawback & some other possibilities
As Daniel pointed out, the second preamp has worse Noise Figure than the first one. So we expect to have worse noise level in terms of the shotnoise intercept photocurrent.
Also Matt is testing on-board notch filters at LLO. If we consider to apply some notching, this GALI-6 could become overkill.
I ordered some other amplifiers like GALI-39, GALI-52 (Daniel's pick), and MAR_8A. They are similarly low noise to MAR-6SM, compatible packages
to the PCB, and located between MAR-6SM and GALI-6. Once they arrive, I'll carry out the same tests.