HP8714ET FRAC-N unit problem - Page 1
Forum section: electronics / testgear

author: mrf245
author_rank: Regular Contributor
timestamp: June 03, 2016, 02:04:16 am
content: Hello I wonder if anyone use or ever repaired HP8714ET VNA.My VNA have some problem that relate to output frequency.When set frequency down to 350MHZ,output freq is not equal set,and impedance measure result is chaos.I used SA to check FRAC-N unit FM out port,when VNA freq set to 350MHZ,FRAC-N output is not stable and not like normal output spectrum.I also checked analog bus test on item 18(VCO Voltage Drive) reault,like below:center freq votage result100MHZ -5.7V300MHZ -5.72V350MHZ -5.5V500MHZ -4.2v1GHZ 9.3V1.2GHZ 12.5V1.5GHZ 12.5V1.8GHZ 12.5V2GHZ 1.1VAccording to srevice manual,this votage is not correct when freq down to 350MHZ and up tp 1.2GHZ(normal is -5.5 to 8V)I consider there are some issue in FRAC-N unit.But I have no found this unit datasheet(08714-60171).I'm so appreciate if anyone can give some good idea.

author: rf+tech
author_rank: Frequent Contributor
timestamp: June 03, 2016, 05:22:14 am
content: Hello mrf245,Page 6-4 in this manual:http://exodus.poly.edu/~kurt/manuals/manuals/HP%20Agilent/HP%208711C,%208712C,%208713C,%208714C,%208730A%20Service%20Guide.pdfdescribes the A3 Fractional-N/Reference assembly and the A4 Source assembly. The A3 consists of two boards - the Fractional-N board synthesizes a 30 to 60 MHz signal that is used by the A4, and the Reference board generates three reference signals of 10 MHz, 5 MHz and 100 kHz.As a starting point, follow the troubleshooting procedures on pages 4-17 and 4-18. Also, measure the voltage on TP11 of the A3 when the source frequency is set to 1.82 GHz. This should be –1.17 +/- 0.03 volts.Please report your results and I will try to assist you further. Also, I cannot find what page shows the "analog bus test on item 18...". Perhaps you have a different manual? If so, please provide the link that I may download the same which you are working from.RF+ Tech

author: mrf245
author_rank: Regular Contributor
timestamp: June 03, 2016, 06:24:04 am
content: Quote from: rf+tech on June 03, 2016, 05:22:14 amThanks for your reply!8714ET sofrware is different than 8714C,please refer this manual:http://cp.literature.agilent.com/litweb/pdf/08714-90017.pdfYou can find page 5-17 (A3 Frac-N Assembly Nodes),item 18. In this item it will checking Frac-N VCO votage.My Frac-N unit is different than previous 871X series.Tonight I will take some photos for my board and other which you need.

author: mrf245
author_rank: Regular Contributor
timestamp: June 04, 2016, 02:31:10 am
content: This is my RFAC-N unit board,I removed RF shield cover

author: rf+tech
author_rank: Frequent Contributor
timestamp: June 04, 2016, 04:35:39 am
content: Okay, this gives a good first look at the layout. As I do not know if you are familiar with the fractional-n synthesizer concept, this HP Bench Brief explains the concept in a simple to understand manner:http://www.hparchive.com/Bench_Briefs/HP-Bench-Briefs-1977-05-08.pdfAs you have learned, HP/Agilent did not make schematics available for this N/A, but the above file mentions that the HP 3325A was one of the first instruments to incorporate a fractional-n synthesizer. The schematics for the 3325 can serve as a point of reference so we are not completely in the dark:http://www.ko4bb.com/getsimple/index.php?id=download&file=HP_Agilent/HP_3325A_Synthesizer/HP_3325A_Operating_Service_manual.pdfSome thoughts about where to begin: I would suggest checking every yellow tantalum capacitor for potentially shorted, while the board is open. A few more photos, very closeup that clearly show the IC part numbers will help to identify the functional blocks.The performance test PT01 on page 1-24 should be performed to determine the amount of error observed in the CW accuracy.Some method of monitoring the 30 to 60 MHz output is needed, to identify where the locked/unlocked boundary occurs. When unlocked, does the condition persist all the way to the 30 MHz end of the tuning range? Your previous data appears to indicate this to be true.Once the boundary is located, is it possible to approach and pass the point by making very small frequency steps, or is it like hitting a wall?Can you make a table of VCO tuning voltage from 30 to 60 MHz in 5 MHz steps? Your previous data seems to show a "binary" effect with the tuning voltage going rapidly from negative to positive near the center of the tuning range. I am interested in seeing where this transition occurs between 30 and 60 MHz. At first glance, the effect in the data looks like an open feedback loop.This is going to be a good mix of challengeand entertainment.RF+ Tech

author: mrf245
author_rank: Regular Contributor
timestamp: June 04, 2016, 06:58:02 am
content: OK rf+tech,I tell some news I found today.First I show you some detial photos about my FRAC-N board and I drawed a diagram(as I analyzed) of this unit.When VNA power down for long time and power again,at first 10 minutes it work FINE,every test result is OK! But after about 10 minutes,issue coming out.Now I consider the PLL part is OK,because PLL can locked on some part of output freq as set but it has unusual boundary.I found a OP amp IC AD829JR(U304) is VERY HOT! Follow PC board track to analyze this IC is use for amplify tune control votage.As AD829 datasheet shows it supply current is small(Quiescent Current is 5mA),so very hot surface I think is unnormal.How do you think this?

author: rf+tech
author_rank: Frequent Contributor
timestamp: June 04, 2016, 12:32:38 pm
content: Excellent progress, mrf245 and great photos. The two best pieces of test equipment are fingertips and eyes.Is it possible to lift U304 pin 6 and isolate this from the board? This should tell us if U304 is defective or if it is sourcing/sinking excess current.RF+ Tech

author: mrf245
author_rank: Regular Contributor
timestamp: June 04, 2016, 03:30:13 pm
content: Hello rf+tech,Follow you suggest,I lift U304 PIN6 form circuit,but U304 still very hot.I guess we already find out the answer.I ordered 2 pieces new AD829JR and tomorrow it can arrive to my house.

author: rafka
author_rank: Newbie
timestamp: April 10, 2018, 07:23:11 am
content: dear friend, do you have manual guide service 8714ET?http://cp.literature.agilent.com/litweb/pdf/08714-90017.pdf(i can't open it)regards