RF Switch Problems

Discussion about this major DATV Project. See https://wiki.batc.org.uk/The_Portsdown_Transmitter
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G4uvz
Posts: 219
Joined: Sat Dec 09, 2017 10:30 am

RF Switch Problems

Post by G4uvz » Sun Oct 28, 2018 1:07 pm

I have been chasing my tail recently with the attenuator reducing outputs disproportionately through the RF switch
Having tested the output directly from the attenuator levels change as expected.....However I am getting very high losses through the RF Switch..... Levels have always been kept at or below 10mW ..but ghuess that something has died!

Nick
Posts: 121
Joined: Sat Oct 03, 2015 10:09 am

Re: RF Switch Problems

Post by Nick » Sun Oct 28, 2018 1:52 pm

Hi,

I had problems with my RF Output Switch a while back.

It was the ADG904 that had died - replaced and all OK. See viewtopic.php?f=103&t=5446

I did have an earlier issue with the RF Output Switch - that turned out to be a resistor that was not soldered on one side. See viewtopic.php?f=103&t=5350&start=10

Gegards,

Nick - G4NKV

G4uvz
Posts: 219
Joined: Sat Dec 09, 2017 10:30 am

Re: RF Switch Problems

Post by G4uvz » Sun Oct 28, 2018 2:01 pm

Thanks Nick for the info...
I will strip out RF switch and do some more tests...

If it is the chip which has failed then that is a pain as the P&P from the States is probably more than the cost of the chip!

Adrian

Nick
Posts: 121
Joined: Sat Oct 03, 2015 10:09 am

Re: RF Switch Problems

Post by Nick » Sun Oct 28, 2018 7:02 pm

Hi Adrian,

Farnell Element 14 have them in stock at the moment - see https://uk.farnell.com/analog-devices/a ... ?st=adg904

Nick - G4NKV

G4uvz
Posts: 219
Joined: Sat Dec 09, 2017 10:30 am

Re: RF Switch Problems

Post by G4uvz » Sun Oct 28, 2018 8:17 pm

Thanks Nick!

G4uvz
Posts: 219
Joined: Sat Dec 09, 2017 10:30 am

Re: RF Switch Problems

Post by G4uvz » Mon Oct 29, 2018 10:22 pm

Have now checked the RF switch and all ports are very hi attn... I am guessing that I have gone too close to the 16dBm max figure. Problem I had was withe the programable attenuator and the RF switch I was running out of steam on a couple of the band ..so had to add an amplifier post a attenuator ...problem is getting sufficient drive at one band withour over dosing on the next! ..I am thinking that I may now go down the conventional SMA relay route to steer the signals

G4uvz
Posts: 219
Joined: Sat Dec 09, 2017 10:30 am

Re: RF Switch Problems

Post by G4uvz » Mon Oct 29, 2018 10:36 pm

Just reading the spec sheet of the ADG904..and noted the bit about bias ...close inspection of my board reveals that the earth end of RI ...1K bias resistor is not connected to ground!
Which migh explain why the device went pop!

A

g0mjw
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Joined: Sat Sep 20, 2014 9:15 am

Re: RF Switch Problems

Post by g0mjw » Tue Oct 30, 2018 8:58 am

The ADG904 inputs have an absolute maximum limit of -0.5 to VDD + 0.3. Exceeding this could damage the chip.

Assuming a 2.75V supply (the maximum) that's 3.5V or a little over 1V RMS assuming the pin is biased at half VDD. In terms of power 13dBm. Should be fine.

In the BATC rf switch, the input pin is biased to 0.5V, as per the app note. That means to not exceed the -0.5V limit we need to keep the RF input voltage to under 1V peak to peak. That's about 4 dBm, not fine as the filter modulator board is higher, but 100s of them seem to work fine and the data sheet is a bit odd here as it quotes 16 dBm in this setup. In your case, with R1 disconnected, the bias would be 2.75V so you would be exceeding the maximum input (VDD + 0.3V) with more than 0.6V peak to peak, 0 dBm. That could well be the cause of the damage.

If there is no power on the chip, say the supply is disconnected, then VDD = 0, the maximum input is -0.5V to 0.3V In that case 0 dBm is exceeding the limit. I would be surprised if that caused damage with the device powered down, but it might. Something to watch.

Why doesn't this impact the LO filter board which also uses an ADG904? There is no biasing on this board and the maximum output from the ADF4351 is only 5 dBm and more likey, 0 dBm which after the splitter and filter losses its likely to be within the limits. However, I am puzzled that there is a modamp an ADL5602 with 20 dB of gain following, which seems a bit high given the input requirements for the modulator board but I am sure it has been optimised.


Mike

G4uvz
Posts: 219
Joined: Sat Dec 09, 2017 10:30 am

Re: RF Switch Problems

Post by G4uvz » Thu Nov 01, 2018 5:43 pm

Chip now replaced and all working to spec ..I am limiting input to switch at a max of 15dBm

Plan now is to house the filters in the Portsdown so O/P from RF Switch will go direct to filters then the O/P from the filters will connect to another unpopulated RF Switch board mounted on the rear panel with miniature coax soldered to the board.

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