HP Agilent 8753ES VNA repair [Help]
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Hi everyone,
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I am looking for some help on the repair of my 8753 VNA.?
I took some time trying to find the problem it myself with the service manual without success.? The unit was working great and failed during my last filter mesurement. The signal is now noisy on port 1 and 2 (in S11,S22 ans S21), even if I do a new calibration.? I have no error on power on. Have you ever entered this type of failure ? I was first thinking of an ESD failure but it do not seems to be the case.? The two input couplers mesure OK. I have 16 dB of coupling loss on port 1 and port 2. Visual inspection do not show any damage on the RF patch.? The solid state switch also seems to be OK. I have 2 dB of loss between the input and the two output of the SP2T.?
In span zero mode I mesure a RF signal at the right frequency in the 1 MHz- 6 GHz band. Output power is also OK. The stepped attenuator seems to be Ok, signal source is there.?
The only problem I can see here is when I look at the RF out signal on a spectrum analyser, the RF signal do not seems to be locked. ?
When I reduce the SPAN on the VNA, I can see a lot of spike on the response.?
From what I understand, I now need to check the ?A7 Pulse generator diode stage and the A11 Phase lock board. The red LED on the A11 board is blinking but do not stay ON.
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Could you help me from this point on this repair ? Thank you ??
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S11 signal, port 1 open SPAN 6 GHz:
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S11 signal, port 1 50 Ohms load, ?SPAN 6 GHz:
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S11 signal OPEN SPAN 1 MHz
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S11 Signal SPAN 6GHz with 50 ohms load
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开云体育Hi, ? To make a first comparison, here is the output of my 8753ES at 1GHz, CW mode, 0dBm output, Trigger HOLD, measured on a Spectrum. ? If you have a similar image, the PLL A11 would be correct. You do not have an UNLOCK message, this means that you have enough signal to maintain synchronization. ? If the signal going to the sampler R was attenuated, the average during an S11 measurement would give an average signal offset from the reference (when nothing is connected to port #1). But looking at your image, test S11, nothing connected to port #1, apart from the noise everything seems correct. ? Do a test from 30kHz to 250kHz, the A11 module is bypassed, if the problem persists, A11 is not the cause. ? Either the problem comes from A7, or it is further away like on the IF DIGITAL. ? Make measurements on the samplers separately, MEAS A, B and R. Do you have the noise problem on all samplers. ? I will wait for your answer, and I will be able to make comparative measurements on my VNA 8753ES. ? Yves ? ? ? ? |
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Yves,?
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OK. I just did a test with a SPAN from 30 KHz to 250 KHz and I have exactly the same problem.
A11 do not seems to be the problem on this unit.? ?
30 KHz-250 KHz S11 response:
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Here is two pictures of the output of my 8753ES. One at 1 GHz and one ar 3 GHz with the same parameters (CW mode, 0dBm output, Trigger HOLD, measured on a Spectrum with 1 MHz SPAN, same RBW). The result is not as clean as your measurement. What is your conclusion about it ?
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1 GHz, 0 dBm, SPAN 0 mode, PORT 1 output: ?
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3 GHz, 0 dBm, SPAN 0 mode, PORT 1 output: ?
Here is a measurement of the R chanel output, I have about -16 dBm of output power. ?Same as your measurement.
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R output power verification at 1GHZ, SPAN 0 mode :
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Measurement of the second LO on B sampler :?
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Measurement of the IF out on B sampler with a cable between port 1 and 2 :
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The output of B sampler seems to be KO (should be about -15 dBm at 37 MHz). Is my B sampler dead ?
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Measurement of the A5 sampler (middle one, with nothing connected on port 1 and port 2):
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I have about -8 dBm, this sampler seems to be OK.?
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Measurement of Sampler R (with R cable connected on A channel):
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I measure about -18 dBm on the output of R sampler. It seems to be OK.
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From what I see here, the problem is from my A6/B sampler ? What do you think about the result ?
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开云体育Hi, ? To make a short summary: You have tested several RF paths, <300kHz, <16 MHz, CW 1GHz, sweep up to 6GHz. In all cases the signal is noisy for all S parameters, but always PHASE LOCKED. You have done tests on samplers A, B and R. Even if B seems weak according to the measurement you have done, this would not affect the other possibilities. The only common point between all these paths is the DIGITAL IF. ? Do the oscilloscope tests for channels R, A and B on the DIGITAL IF. Here again there are separate paths for these three signals, but they have common power supplies and a switch leading to the ADC. ? Out of curiosity do TEST 17 ADC LIN. This test activates a precision ramp at low frequency and tests the ADC. ? Here is a picture of a DIGITAL IF PCB, does your PCB look like this version. If so, you have the schematics available for that, here is the DIGITAL IF section. ? ---------------- CW mode 1GHz, test on DIGITAL board. ? ? ? ----------------------------- ? 1 GHz, 0 dBm, SPAN 0 mode, PORT 1 output:??? your signal is OK, with same setup, similar result (I have 1D5 option, high stability ref). ? ? R output power verification at 1GHZ, SPAN 0 mode :? nominal -16dBm, it’s OK ? ------------------ ? ? |
开云体育If the three signals are absent, there would have to be a common point between them, so there is the local power supply. ? Signal A, B and R on DIGITAL IF come from of the 3 outputs of samplers at 4kHz via motherboard. Check TP12, TP10 and TP11 connected to input lines from mother board. ? If there are no signal at theses points, problem is near samplers? ? ? |
开云体育Sorry, test point TP12, TP10 and TP11 are ground from sampler…no signal on that. Input signals are on TP16, Tp18 and TP20. Output R, A and B are -6dB lower than input (TP 9,8 and 7). ? Signal at TP16 R: 1.18Vpp Signal at TP18 A: 3.75Vpp (no loop cable) Signal at TP20 B: 3.51Vpp (with loop cable) ? PRESET SWEEP SETUP CW FREQ (defaut 1GHz) SWEEP TIME 10sec ? ? ? ? De?: [email protected] <[email protected]> De la part de Yves Tardif via groups.io ? If the three signals are absent, there would have to be a common point between them, so there is the local power supply. ? Signal A, B and R on DIGITAL IF come from of the 3 outputs of samplers at 4kHz via motherboard. Check TP12, TP10 and TP11 connected to input lines from mother board. ? If there are no signal at theses points, problem is near samplers? ? ? |
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Hello Yves,
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Thank you for your screenshoot. Here is what I got on my board.
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TP20 B (with look cable between port 1 and port 2):?
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TP18 A (OPEN loop between port 1 and port 2):
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TP16 R:
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I check the TP1 5V and the voltage is OK here.?
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Two points here :
- The IF frequency seems to be off, I have 22 KHz instead of 4 KHz. Do I have a problem with one of the final OL ? Perhaps it is why I have nothing on TP7,8,9. There is some LC filter in the board. ? - I had to put the probe level in x2 mode in my scope to measure the same voltage than you in peak to peak mode. The voltage I measure initially was divided by two. I don't think this is important.?
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开云体育Hi, ? This means that one of the PLLs does not have the right frequency. The frequency near 22.5kHz will not pass through the filters which are for 4kHz at the DIGITAL IF. ? You should check the second LO, it arrives through the motherboard, not easy to access. Connect a test wire as in the picture to measure the output of the second LO on A12 PCB. ? For a frequency below 16MHz, the frequency will be CW FREQ+4kHz, for example, for 100kHz output Port#1, you will have 104kHz. For frequencies of 16MHz and above, you will have 996kHz. ? Yves ? ? De?: [email protected] <[email protected]> De la part de MattSNE via groups.io ? Hello Yves, ? Thank you for your screenshoot. Here is what I got on my board. ? TP20 B (with look cable between port 1 and port 2):? ? ? TP18 A (OPEN loop between port 1 and port 2): ? ? TP16 R: ? ? I check the TP1 5V and the voltage is OK here.? ? Two points here : - I had to put the probe level in x2 mode in my scope to measure the same voltage than you in peak to peak mode. The voltage I measure initially was divided by two. I don't think this is important.? ? ? |
开云体育If we focus on the high band for now, 3 sources are involved: the 10 MHz reference, the 40 MHz VCXO and a VCO near 40 MHz (39.996 MHz). ? Check your 10 MHz reference. Measure the TP VCO TUNE PRESET START 15MHz STOP ?16 MHz ? I'll look for places to plug in to measure the 40 MHz VCXO output and the VCO. The cause could be a faulty divider. ? ? ? ? _._,_._,_ |
MattSNE
