Hi Guys,
I have two SR560 Low noise amplifiers. Different series number.
After some teardown work, I found the difference.
JFET of first amp stage on new one is LSK389B, the older one is NPD5564.
I tested the noise floor with them by APX515.
(Of cause CMMR and OFFSET adjustments is done according the service manual.)
For the older one, the output noise of about 4.0nV√Hz at 1KHz (SR560 Gain is x1000). That is make sense since there is a NPD5564 JFET inside. The Noise of NPD5564 is about 3.5nV√Hz at 1KHz.
But for the new one, the output noise of about 3.5nV√Hz at 1KHz (SR560 Gain is x1000). I am confused, It is a LSK389B inside. The noise of LSK389B is about 1.6nV√Hz at 1KHz at all. The Noise of this unit (with LSK389B) should be close to 1.6nV√Hz, but it is not. : -(
(1) Noise calculation:
I tested most major DC value of components of SR560, make sure it’s working in good condition.
The gain of JFET stage is about x10(~20ms(JFET gfs) x 499Ω( R131 ). The input noise of OP37 is about 3nV√Hz, so the equivalent noise of OP37 at input is about 3nV√Hz / 10 = ~0.3nV√Hz
Looking at 499 Ω resistor R131(or R117), it’s the load of JFET, the noise of 499Ω is about 1.8 nV√Hz, so the equivalent noise of R131 at input is about 1.8nV√Hz / 10 = ~0.18nV√Hz
Summing LSK389 noise and R131 and OP37 noise together.
Total noise at input = √ 1.6 x 1.6 + 0.18 x 0.18 + 0.3 * 0.3 = 1.64 nV√Hz.
(2) 2nd amp stage
The 2nd amp stage is LT1028.
I found that the U101A Pin2 and Pin3 is shorted when setting the SR560 Gain to x50000.
After setting Gain to x50000, it’s possible to reduce the noise downgrade by divider (R108,R109,R110).
Test the noise from SR560 50 Ω output BNC connector is show below.
The noise as show in pic is 138.8 nV, so the equivalent noise at SR560 input A is about
138.8 uV/ Gain ( 50000) = 2.78 nV√Hz。
It is closer to the calculation 1.64 nV√Hz, but still not corrective.
Is there something wrong or something I miss?
I am so confused.
From my test, That’s mean,to enhance noise performance of SR560, changing NPD5564 JFET to a LSK389B only, it will not work at all.
Hei guys, it's appreciated let me know if you have any idea.
Roger.