ADF4252 confused

[baxterdmutt]

I have a transmitter that uses the ADF4252 as part of the whole VCO. The one that was on the board was shot. Testing to make sure all my pins connected right I noticed that the Vp1 pin goes to ground. I’m wondering if I could get advice. Is there a configuration where that would make sense, or is it possible that the ground pad under the chip leaked some solder out. I don’t think it did. This chip is tiny and I know all my other connections are correct. It’s a more complex chip that a hobbyist like me is used to working on. An opinion about that Vp1 is appreciated

https://www.analog.com/media/en/technical-documentation/data-sheets/ADF4252.pdf

 

[tumbleweed]

Looking at the datasheet, Vp1 and Vp2 both have a min value of VDD1, and VDD1 has a min value of 2.7V, so I'd say if you're measuring continuity to GND on Vp1 that's not going to work.

 

[JimB]

Looking at the datasheet, the ADF4252 has the circuitry for two phase locked loops, which Analog Devices refer to as "RF" and "IF".
RF can be used up to 3 GHz.
IF can be used up to 1.2 GHz.
The two phase locked loops are independent of each other, and the chip can be set to use both PLLs or only one of the two PLLs.

If your radio is using only the IF PLL, then Vp1 (the supply for the VCO charge pump for RF VCO) is not required.

So in answer to your question:

I noticed that the Vp1 pin goes to ground. I’m wondering if I could get advice. Is there a configuration where that would make sense,

is yes, there is a configuration where this makes sense.

However, whether this is appropriate we do not know as we have no information about your transmitter.

JimB

 

[baxterdmutt]

…
However, whether this is appropriate we do not know as we have no information about your transmitter.

JimB

Thanks Jim. Unfortunately I have no info about the transmitter either because it’s a GE MASTR III repeater. There is datasheet for the old RX Synth module but the ADF4252 is I a new version of the RX Synth and no schematics ever made it out of GE HARRIS.
I guess my best bet it to remove the chip (to make sure solder spillover isn’t the problem and then try tracking the short to ground.

 

[baxterdmutt]

Ok. I took the chip back off of the board and the short to ground still exists. Does anyone have any shortcuts to tracking such a short without a schematic.

 

[JimB]

Does anyone have any shortcuts to tracking such a short without a schematic.

The only thing that I can suggest is to get an understanding of how the board works, ie to try and work out the circuit.
However if the circuit board is multi-layer rather than just single sided, tracing the circuit could be a bit difficult.

Can you take a good quality photograph of the board and post it here?
It is difficult to make any suggestion while knowing nothing about the board.

JimB

 

[Nigel Goodwin]

As a 'last resort' method, you could apply a current limited power supply to ground, and the PCB point that is shorted to ground - gradually turn the current up, and look for what smokes or gets hot. If you're lucky a component will get hot, and that's likely to be what's at fault - if you're unlucky the PCB itself will get hot, and it's likely an internal short on a multi-layer board.

But before you get to the 'last resort' try measuring the actual resistance of the short with a low-ohm meter, and trace along the track till you find the lowest resistance point. With this technique you can tell which end of a track is closest to the short - and also test directly from a component pin, if it's a transistor, and the shorted pin read 50milliohms, and the other pins read higher than that, then obviously it's not the transistor at fault. You can also use the same technique to actually remove components, or lift connections.

I purchased a low-ohm meter specifically for that use - and actually had occasion to use it a few weeks ago. It was on a board I'd designed, and had just populated one that didn't work properly - and a few tests showed the MAX7219 wasn't working correctly (two of the outputs were shorted together - and yes, I tried changing the chip before going further). So I dug the low-ohm meter out, measured along the tracks and was able to locate the area where the short was - and using a magnifier headset I was able to see a hairline of copper between tracks that hadn't etched away. A quick cut with a sharp knife, job done

But without circuits, and full service manuals, it's usually a pretty difficult job

 

[baxterdmutt]

Thanks all. It’s only a back and front layered board. I did find a small piece of solder that made the short elsewhere on the board. So that’s fixed. Something still isn’t right so I’ll just keep chipping away at it. Before I replace the ADF4252 the oscilloscope showed no activity in many of the pins. I now have wave activity but it still won’t create a VCO lock. I’ll keep chipping away. About all I can do is follow pins to a component and see what the datasheet suggests I should find. It’s not an overly complex board and I’m thinking the problem might be with the multivibrator or one of the AND OR gates. There is only 5 chips of that type. I’ll see if they show anything.