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SR560 Low noise amp -- Noise Calculation
sky2city:
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.
moffy:
I believe that the gain of the JFET stage, differential, is less than 5, (500+500)/(110+110) plus whatever the effective JFET source resistance added to the 110 source resistors. The quick LTSpice simulation I did got a gain of about 3 to 3.5 but my bias point might be a little off.
Kleinstein:
As an additional noise source there is the noise of R113+R144, that should be about as much or a little more than the noise if the LSK389.
Changing this would need a much more dramatic change to the amplifier, as R113 and R114 are also setting the gain.
The gain of the JFET stage is indeed quite low, which is also not ideal, but could be still OK.
sky2city:
To Improve noise performance, changed the Gain of first amp. Stage.
As show below, add 2 parallel 10R resisters (R113,R114). The gain change to x110.
Now the noise performance seems to better. About 2.7nV√Hz @ 1KHz.
And then I try to replace NP5564 with LSK389, JFE2140,2SK389.
Yeah, the noise is better and better.
Looking at 1/f noise, it seems to be another problem and it’s confused me again.
The noise of JFE2140 and LSK389 are:
at 1Hz is about 8nV.
at 10Hz is about 3nV/√Hz.
As my measure data show, it’s 65 nV/√Hz@1Hz.
The spec. of SR560 at 1Hz is 30nV/√Hz.
Where is this 1/f noise come from? :-//
Kleinstein:
For the 1/f noise, there is a chance that the LF411 in the current source contributes. Ideally there should not be much sensitivity to the amplifier current, but the LF411 has lot's of 1/f noise and the suppresion from symmetry is limited. Exchanging the LF411 for something less noisy would be relatively easy.
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