When working on RF circuits only a proper RF active probe such as 85024A (ideal) or 1152A or 54701A or etc....will work
forget about passive probes unless accurate amplitude measurement is not your concern and you just want to see the signal
even for that purpose sometimes passive probes load your oscillator (depending on where you probe it) such that it stops oscillating!
The total system bandwidth (probe + scope or SA) is the key parameter.
Anyways, how do you measure the 600MHz? Do you have the proper connector (SMB or SMC i forgot) and do you measure it with 50 Ohm SA?
There are a couple of MMIC ampifiers in the final stages of the 600MHz REF, I would check them for DC bias voltage, DC current draw and? temperature to see if they are dead
of course with an active RF probe you could check their gain.
I would still focus on the mixers, for now you can just inject a 600MHz into the second converter
because if the low band mixer or the second converter is dead they are very expensive to repair or replace specially the second converter used to cost a fortune as I recall
and you might find it beyond economic repair. the 600MHz ref is easy to repair and its cheap. those MMICs can be found on ebay and they are cheap
or you might conclude that the 1st and 2nd mixers are fine and there is a problem in the IF and log amp, that is usually easy and not expensive to repair.
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On Fri, Apr 12, 2019 at 08:11 PM, Dave Ireland wrote:
Amir
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I think you are correct about the 100Mhz. I have become obsessed with the level without a proper method of measuring it. All the 300Mhz stuff following is working so can't be much wrong with it.
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I am still however worried about the 600Mz level as this is being measured at the proper alignment point but another day dawns here in Australia and I have to leave it for a while.
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Thanks for your help to date.
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Dave
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Sent from my Samsung GALAXY S5
-------- Original message --------
From: amirb <amir.borji@...>
Date: 13/04/2019 00:00 (GMT+08:00)
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP8560E Low Level
I still doubt that there is anything wrong with the ref osc and 600MHz lo, really. it could be just a measurement trap.
how are you measuring those 9dBm and 17dBm in-circuit amplitudes? remember that only and only a proper RF active probe?
with careful grounding can give you the correct value. Any passive probe even the best 500MHz passive probe will give you totally out of whack values. I have been there...
even with an active probe (cap <1-2pF) still the precise point at which you measure it, is very critical. If the 100MHz is like 10dB low I am sure
you would be getting errors because the PFD will probably fail to work and the loop becomes unlocked.?
Moreover, as you explained, even when you inject 600MHz Lo from the outside your second IF is still 24dB low. That has nothing to do?
with the ref osc anymore.
another thing that puzzled me is that when you inject 310.7MHz to the third converter you get a trace at -32dBm? it should be much higher, close to -10dBm I think.
(adjust your atten to 10dB and ref level to 0dBm) How is the level of 300MHz third converter LO drive??
your 'default' noise floor is too low (it must be around -65 to -70dB as I recall from the units that I had in the past)
usually that means a mixer is not working but it can have other reasons further down the signal path
On Fri, Apr 12, 2019 at 11:08 AM, Dave Ireland wrote:
John
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Just some feed back. Tracked down the manufacturer of the crystal, Croven Crystals in Canada who are very helpful, and found the replacement crystal is the exact unit and even has the the HP number 0410-4065 lasered on the side where I didn't notice it.
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Might just go away now and brush up on my oscillator theory to try to get more level so I can drive U700 with +9dBm instead of the -2dBm I have now.
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Thanks for you assistance.
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Dave
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On 12/04/2019 5:27 pm, Dave Ireland wrote:
John
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I am still back at U700 which seems to be working but I can't get enough drive into it.
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The oscillator circuit is not tuning properly and I suspect it is the crystal. It has been changed sometime in the past and I not sure characteristics of the crystal are the same. I will see if I can trace? details of what has been put in it.
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Thanks
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Dave
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On 12/04/2019 1:55 pm, johncharlesgord via Groups.Io wrote:
Dave,
I think I had an HP856x with a similar 600 MHz problem.? It turned out to be a failed RF amp on A15. (U501?? U701?? U702?)
--John Gord
On Thu, Apr 11, 2019 at 06:16 PM, Dave Ireland wrote:
Amir
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A lesson in the need for methodical fault finding techniques methinks.
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Checking out the RF section block diagram, trying to follow your advice, I noticed the 600Mhz drive into the Second Converter. The 600Mhz is derived from the 100MHz VCXO that the regulator transistor had failed on. The 600MHz out of A15 is about 15dB low and when I injected 600Mhz at the right level into the second converter the level has come right up. Still about 20dB down but more promising.
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I will go back now and go through the A15 board and get the outputs correct and then after that maybe check out A14 for the mixer and converter bias volts.
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Thanks
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Dave
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On 11/04/2019 10:29 pm, amirb wrote:
I forgot to ask, what is your noise floor level at full span?
On Thu, Apr 11, 2019 at 10:27 AM, amirb wrote:
I assume there are no error messages appearing anymore, right? Any error message about Log amp or step gain amplifiers?
if there is no error and you are sure the attenuator and the input AC/DC coupling are ok (you can even check with oscilloscope by injecting a signal)
then the best and perhaps only way to isolate the problem is by injecting signal at various points and measuring power levels according to the service manual
I dont think this is due to low LO amplitude in any of the converters. otherwise you would get an error message and besides
90dB down is way too much for that. I suspect this is due to a fault in one of the 3 converters or in the Log amp (but not in any of the LO drives)
since you dont have a second spectrum analyzer, I suggest you inject a signal at say -20 or -21dB to the first mixer (make sure span is set to zero)
and then measure the 310.7MHz second IF with an oscilloscope (I hope you have a 400-500MHz scope?) or even a power meter.
if the amplitude is way off then the problem is either the first mixer or the second converter. By the way, you can check the mixer bias voltages coming from A14
In some versions even the first mixer has bias voltage.
Next you can inject a clean 310.7MHz at around -35 or -36dBm to the third IF input (you might have it on the front panel) (you must choose it first in the menus and also choose zero span)
and then you can measure the 10.7 Mhz final IF with an oscilloscope it must be around -15 to -17dBm I think.
On Thu, Apr 11, 2019 at 09:44 AM, Dave Ireland wrote:
I am repairing a HP8560E and until now have been doing well.
The unit on delivery had the classic distorted display and no apparent response to any input though it did sweep it was very slow. 12 Alarms were reported.
Choose the "600 MHz Reference Oscillator Unlocked" first and replacing failed 2N2222A regulator on the 100MHz oscillator which bought the display back and all the alarms disappeared. Best 40c I've ever spent.
Was it completely fixed? No unfortunately not, the input reads about 90dB low. Every thing else looks fine, the resolution band width, span and frequency accuracy all look good. Checked the attenuator and the filter and even went straight into the RF port on the first low band mixer but still 90dB down.
I am at an impass now until I can borrow a decent SA as I do not have anything to measure 3-6GHz.
I did the LO feed through test in the service manual ie centre freq 0Hz, Span 1MHz, input attenuator 0dB and it says the feed through should be between -6 and -30dBm. It says if it is inside this range the RF path following the Low band mixer is operating properly.
Mine is -26dBm, so in the range.
My question is does that test mean that the the Yig, the LO distribution amp, the first mixer LO and IF ports and all the following mixers are operating correctly albeit not 90dB out? If so, the only thing I can think is a faulty RF port on the mixer (sounds unlikely). Also, I suppose if the LO drive was low, maybe 10 dB or so, there would still be feed through but may be not enough level to bias the mixer to mix.
Any though while I wait for a better spec an?
Dave
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