IC1 74AC574 issue.

[G4EWJ]

A problem on a local DigiLite has highlighted an issue with IC1, the 20 pin chip on the underside of the pcb. This is an 8 bit latch / driver. It may well be just one chip, but it's worth mentioning the implications. If your DigiLite is working, then you almost certainly don't have a problem.

IC1 latches the digital I/Q signals from the dsPIC and acts as a driver into the modulator section. On this chip, I and Q each have 3 inputs connected together and the corresponding 3 outputs are connected together to increase the drive. The problem unit was showing lots of noise on the power supply rails and on the spectrum analyser. It turned out to be caused by not all of the 3 tied outputs changing at the same time, which meant that a large amount of current was being drawn because of an output of 5v being connected to an output of 0v.

As the design has progressed from "Poor Man's DATV" to DigiLite, various parts of the circuit have been run at various voltages. On DigiLite, the digital section runs at 3.3v and the modulator section at 5v. The 74AC574 has a 5v supply, so each output is at either at 5v (high) or 0v (low), depending on the input. The inputs are driven by the outputs of the dsPIC, which are either at 3.3v (high) or 0v (low). The minimum guaranteed input threshold of an AC574 to give a high output is 3.5v, so the high from the dsPIC is below the threshold. In this case, it's possible that the 3.3v from the dsPIC was right on the threshold and it was high enough for some of the 3 inputs, but not all of them.

Chips are designed to work over a wide temperature range, so there is usually quite a bit of leeway. A better chip to use for IC1 is the 74ACT574. The 'T' version has an input threshold of 2v, so the 3.3v output from the dsPIC is well above it.

You can check for the problem if you have a scope. Locate R9 and R10, south west of the I level pot. On the left side of these are the I and Q outputs from the 574. Check that the squarish wave levels are either 5v or 0v, with no other steps visible in between. In Phase test mode should make it easier to see. If there are no steps, then the problem is not there.

To summarise, this may be an isolated incident with one chip. if your DigiLite is working then there is almost certainly no problem. If the problem is there, or you haven't yet bought a 74AC574, use a 74ACT574. e.g. Farnell 959-0412. I'm now using one of these on my DigiLite.

Brian

[M0DTS]

Thanks for that Brian,
Possibly this has an impact on the noise some of us have seen on the output either side of the RF, If the output is not switching hard on then the output level may vary generating noise...
My main tx with the old seperates system using your first pcb layout is by far the best here, it's output is very clean and uses a leaded 74AC574 which may have slightly different performance to the SMD one.

I'll change mine soon and post the results.

Cheers

Rob
M0DTS

[G4EWJ]

I'm not convinced that's the cause of the general noise. I took the AC574 off my board and put it on the problem board and put an ACT574 on mine. I had a quick look at mine on the analyser and it didn't seem any different.

When I asked the board owner where his AC574 came from, he said "You gave it to me". I'd forgotten that I ordered two at the same time, probably from Farnell. It looks like a TI chip and the code on it is: 04ALR8K. Chances are that my original, which showed no problems, has the same markings, but I don't have it any more.

I don't think characteristics would vary with package. Several packages are often listed at the top of data sheets.

Brian

[F1FCO]

hello everybody !
I am a french hamradio, F1FCO, Pierre
new to Digilite

is it possible to use an 74HC574 ? or only an 74AC574 ?

because I own a 74AC574...

thank you fon answer

73s from F1FCO

[G4EWJ]

Hi Pierre,

The 74HC574 should work, but the recommended chip for the green v5.6 and v5.7 pcbs is the 74ACT574. The ACT is better than the AC.

The recommended chip for the homebrew DTU1141 serialiser pcb is the 74AC574. The ACT will not work on that pcb.

Brian

[F1FCO]

thank you for answer
I own a v5.6 PCB
no risk with my 74HC574, I order an 74ACT574

is it Farnell reference 1741102
here : http://fr.farnell.com/texas-instruments ... dp/1741102

F1FCO

[G4EWJ]

It is the same for v5.6 and v5.7 pcbs. I have edited my previous message now.

The chip number on your link to Farnell is the wrong shape. It is a TSSOP. "Too Small to Solder, On Purpose".

This is the correct chip, which is a SOIC shape.

http://fr.farnell.com/texas-instruments ... t=959-0412

Brian

[F1FCO]

ok for pack SOIC
thank you again Brian

F1FCO, Pierre