Tuesday, June 16, 2015

Anritsu MF76A Repair - Part I

I recently acquired a nice Anritsu MF76A 18GHz frequency counter in a non-working state off of eBay. While older, this is still a very usable counter that goes for quite a bit of money when working. This one sold for cheap so I figured why not, I'm sure I'll be able to get it working. I didn't really need it as I have a good EIP 545A 18GHz counter, but this one looked fun to fix. The images of it indicated that it was powering up and had the cpu portions functional as the display went through its power up cycle and displayed all zeros on the display. Typically with frequency counters in this state and not counting, you either have a failed 10MHz reference or a damaged front end from over-driving the input. I was hoping for the former as I already have a stable rubidium reference I use with all of my gear.

Anritsu equipment is notoriously difficult to repair due to the lack of service manual availability. I ran into the same issue with my second MT8801C communication analyzer / spectrum analyzer that I had purchased in a non-booting state. I was ultimately able to acquire a service manual for it but it took some serious searching and asking around. Once I obtained it, it was nothing like the older HP and Tektronix manuals I was used to. This manual went through a basic troubleshooting process to identify the issue to a board level, no further schematics or information was available (in the manual I had anyway).

Upon arriving, I powered this MF76A up on the bench, it was in good overall condition and did power up as it was supposed to. Upon applying a RF source to either the 200MHz or 18GHz inputs resulted in no counting. In a way this was good, it could indicate an issue further down then the first converter stages. The back of the unit has a switch to either output the internal 10MHz reference or accept an external one. Checking this with a counter, the internal reference is working as expected. Damn. Time to open it up.

The back of the unit had the calibration stickers broken which was expected, this came from a calibration house / test equipment reseller and I'm sure someone mucked around inside with some attempt to repair it. With covers off, everything looked at least complete, this unit was constructed very nicely.

The power supply is tucked into the back of the unit, not the easiest to reach to check voltages, but the power comes off of it and is transferred to the backplane which has wider traces for increased current making it easy to identify. There are no voltage markings or test points on the boards but it was pretty easy to determine which were power. Probing voltages, everything looked fine except for one strange voltage reading which was around +6.8 volts. Also noticing that a -12V rail was present but no +12, I was betting that something simple like a bad cap was holding the +12V rail down to +6.8 volts, very similar to my EIP 545A repair.

Pulling all the boards and reinserting them to locate the source of the issue resulted in the +12V voltage drop being caused by the input IF brick at the front of the unit. This was fed directly from the 18GHz input, the 200MHz input bypassed it and went right to one of the digital boards. As a test I left the 18GHz section unplugged and ran a 50MHz source into the other input. At first there was no response but I realized that one of the coax connectors coming off of the 200MHz input was not connected in the correct location. Whomever had been in this unit messing around didn't put it back together correctly. Once it was where it was supposed to be, success. 50MHz displaying correctly.

So the counter itself is working fine, just the 18GHz converter is causing problems. This was always a concern, you see this often from someone providing too much input and damaging the input section. So while 200MHz is great, 18GHz is why I wanted this counter. At this point I pulled the input brick out and started taking the covers off of it. The cables for this are long enough to allow you to work on it outside of the unit, I so love designs that allow for repairs.

Each side of this brick was made up of multiple boards all of which actually had labeled power input pins. This made locating the problem board very easy. Measuring +12V to ground I was reading roughly 1.2K ohms through all boards. Through process of elimination I was able to isolate the boards that were sinking the +12V down to two boards, both exactly the same. Pulling the +12V cable off of each board returned the rail to normal levels. Both boards were labeled '342U7363', each containing a handful of passives, some transistors and MMICs, along with what looked like a large thick-film precision resistor.

By following the traces on the board and divide and conquering by cutting some traces, I discovered the +12V sinking was actually coming from this large mystery device package. Looking it up, it is an NEC MC-5156 broadband amplifier in a sip package and was nearly shorted from vcc to ground.

Removing these two amplifiers from each board returned the +12V rail to normal, so these two parts were at least indicating to be bad. Was anything else bad? Maybe, it is hard to tell for sure and without a full schematic it is really difficult to trace the signal paths between these boards to guess what else may be blown. Luckily these old amplifiers are still available as NOS from some suppliers, so with two ordered we shall see. Right now with these two amplifiers removed, the unit powers up and counts correctly on the 200MHz input. I am also able to see some RF on the input to each now missing amplifier, so RF is at least getting that far. Is this the only problem? Maybe, maybe not as this would be too easy. I'm betting there are more issues than just this but I can't do any more at the moment until the replacement parts arrive.

Monday, June 15, 2015

Decreasing Phase Noise with Low Noise Regulators

I have posted several times in the past on the PLL frequency synthesizer I designed and built based on the Analog Devices ADF4107. The overall design is a platform for a fractional PLL frequency synthesizer for any frequency range up to about 5GHz.  A single frequency or range could be generated simply by changing out the VCO and loop filter and reprogramming the ADF4107s registers. The design overall has worked very well, I have used it as a LO for a 1.42GHz hydrogen line radio telescope, a 2556GHz LO for my 10GHz ham station, and a 5.4GHz LO for some specific satellite downlinks.

One element of the design that has been less than ideal was the devices phase noise. My specific PLL was on average about 10dB to 20dB under spec of what the documented phase noise should be for similar designs using the ADF4107 and Z-Comm VCOs. After reading to the application notes some more and a recommendation via twitter from Tony (KC6QHP) who suggested looking into using very low noise regulators for the design, I decided to make the change.

Searching regulator semiconductor manufacturers for very low noise versions is not an easy task, often, the noise levels are not available in any parametric search. So to keep things simple, I just went with the ADP150 which is what Analog Devices recommends for their own designs including the ADF4107. Now this is definitely something I should have considered to begin with in the design, but it was my fault for not reading the docs and assuming the basic ST Micro KFNN regulators which I often use would be suitable for a project like this. Looking at the datasheets, the stated noise levels of each are quite a bit apart:


OUTPUT NOISE  10 Hz to 100 KHz 50 µV rms


OUTPUT NOISE  10 Hz to 100 KHz  9µV rms

The issue I now have is I had designed the board for standard DPAK package regulators, the ADP150 used tiny TSOT packages. Because of this I would have to be creative in mounting the devices in the DPAK footprints. This turned out to not be too bad of a task although not the most elegant solution.

The results speak for themselves, after replacing the regulators with the ADP150s, phase noise has considerably decreased. I have already started on a version 2 of this synthesizer and I will be definitely switching to these regulators for all future versions.

Standard KF33 and 7805 regulators on the left, low noise ADP150 regulators on the right.