Electronics > Metrology

HP437B with a new microcontroller system

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diodak:
Thank you for your tips  :)

C6 and C7 are selected according to the service manual and may not be there, or even up to 10pF there may be one of them. I currently don't have these capacitors. Soldering a 4-14pF trimmer for C7 allows you to reduce the noise to -65dBm, but zeroing results in an error very quickly (twice as fast as without C7). C11 is actually currently 100nF X7R. Its short circuit also allows the noise to go below -70dBm.

Or maybe the gate current is important in this system? The currently tested JFETs had a max of 1nA and the MMBFJ201 has a max of -100pA.

Kleinstein:
The gate leakage specs are more like test limits. The typical gate leakage is much lower, often less than 10 pA if the temperature is not high.
I would be more worried about the base current of the BJT Q3. The transistor choice looks odd. I would have expects a high gain audio part, not an RF part with relatively low gain.


If I understand the circuit right the 2 JFETs work as switches for a chopper amplifier with low input impedance. C11 and the 1.33 K in parallel provide quite some load to the rectified part.
This different from a normal voltage input chopper amplifier which wants high impedance and low capacitance after the chopper part.

diodak:
Before further testing, I decided to change the PCB a bit. I added protection circuits around the gates. I also changed the shape to fit the Gainta G0123 aluminum housing.

Whether this will be better I don't know. With all this learning, an extra 5USD doesn't make a difference anymore and I hope it will bring something.

diodak:
The next version of the probe has arrived, the housing is ready. It remains to test the modified system :)

diodak:
Some time has passed, but the work has not stopped. The new, next version of the PCB did not offer much new information. The background level is approximately -49dBm.

Suspecting that maybe there was something wrong with the element values, I decided to simulate the system. This showed that the choice of NPN transistor is not really sensitive. However, in the sampling system, the best choice will be MMBFJ113 or MMBFJ309. Having already soldered the MMBFJ113, I decided to investigate the issue of grounding. And here I noticed great sensitivity. However, such conclusions only came to mind when I routed the 220Hz signal not via a cable to the probe, but completely separately. The cable currently used is 10x0.14mm^2 with a screen. I ordered a special cable with 3 pairs of 0.14mm^2 each in the screen and additionally 3x0.14mm^2. I'll check it out when I get it. I don't know what is in the original E9288A cable. Some diagrams suggest two beams in the shields, one for the amplifier input/output and the analog ground and zero level. The second one is for the 220Hz signal. I still have the option of testing the Ethernet cable. Perhaps running a 220Hz signal through a twisted pair will help.

In any case, everything from the measurements indicates that the problem is signal penetration from the chopper amplifier. Therefore, the background/noise level is too high and without an RF signal, actually the entire HP437 amplifier block is close to overdrive (at the most sensitive range - when the signal attenuators do not work).

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