RF Output Switch Problem

Discussion about this major DATV Project. See also https://wiki.batc.org.uk/The_Portsdown_Transmitter
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RF Output Switch Problem

Post by Nick » Sun Apr 15, 2018 10:11 pm

I have noticed some strange behaviour with my RF switch.

Firstly, I noticed that any signal passed through it is attenuated by 5-6dB. This seems quite high. For instance, the 2Mtr output from the FM board is 11.88dBm, and the output from the RF switch is only 6.8dBm. The datasheet shows a insertion loss of less than 1dB at this frequency.

Then I noticed that the signal level at the output of any band that is not selected is quite high. For instance, if 2Mtr is selected the output on 2Mtr is 6.8dBm, but the 2Mtr output on sockets for other bands is -16dBm. This seems high based on the datasheet figure of -37dBm for isolation.

Then, the most strange behaviour was noted when I checked the other bands. When the 4Mtr, 70cm or 23cm bands are selected, as well as the low outputs noted above, when output on the 2Mtr socket is checked it is nearly as high as the output on the band selected, being only about 1.5dB lower than the output on the band selected.

What sort of outputs should I get and what level of isolation should I expect from the RF switch ?

Could it be a faulty ADG904 or some other reason ?


Nick - G4NKV

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Re: RF Output Switch Problem

Post by MN-tech » Mon Apr 16, 2018 12:01 am

The ADG904 has 2.75 volts power in and two selector lines. Check the voltage and look at the those three lines with a scope for noise before condemning the ADG904. Most likely they are OK but ADG904's aren't cheap so eliminate the less costly possibilities.
Just a thought, but is the ADG904 mounted correctly?

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Re: RF Output Switch Problem

Post by G8GKQ » Mon Apr 16, 2018 2:12 pm

Hi Nick

It does sound like you have a problem there. The loss at 146 MHz should be less than 2 dB and the isolation should be at least 20 dB.

You might want to check that every one of the ADG904 pins is properly connected. The symptoms you describe sound like a poor solder joint on a ground pin. Use a test meter between the pin (close to the IC body) and the PCB track; visual inspection won't necessarily find these faults.


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Re: RF Output Switch Problem

Post by Nick » Mon Apr 16, 2018 6:31 pm

Hi Dave,

It looks like the chip has failed. I removed the board and carried out some resistance checks on it. RF1 to ground measured about 78ohms. RF3 measured 285ohms. RF2 and RF4 measured around 1Mohm.

I have removed the chip and get the same readings. Also measured RF1 to RFC - 17ohms. RF3 to RFC 195oms. RF2 and RF4 to RFC 0.8Mohms.

It was working OK, but I have since been testing a 2Mtr amp (RA60H1317M1A) I have built so maybe something here has caused the problem ?

I was wondering why I did not have to attenuate the signal from Portsdown and still getting no spectral regrowth - it looks like is was because the signal was well down anyway.

Just how sturdy is the ADG904 - are they easily damaged ?

Has anyone else blown one of these chips ?

Got to find my spare now and get it installed. If I have any more issues I may try and use a relay RF switch - got some SMD relays that would be suitable.


Nick - G4NKV.

Posts: 47
Joined: Sat Oct 03, 2015 10:09 am

Re: RF Output Switch Problem

Post by Nick » Mon Apr 16, 2018 11:08 pm

ADG904 replaced and RF switch now working OK. Less than 1.5dB attenuation on 2Mtr, and 3dB loss on 23cm. Isolation greater than 50dB on other outputs when 2Mtr output selected.

Don't know why the original failed - hope it was not something to do with 2Mtr amplifier I am setting up. Will find out when I get back on it.

Nick - G4NKV

Posts: 45
Joined: Thu Jul 16, 2015 1:36 pm

Re: RF Output Switch Problem

Post by g3zgz » Tue Apr 17, 2018 7:32 am

Hi Nick,

Probably not related to your chip failure, but...

I was setting up my version of the RD06 RD15 2m amplifier and using the Portsdown to drive it.

Suddenly my PA gave about 12 watts output for a while and then stopped grafting.
I thought one of the RD's had gone to meet it's maker, but not so.

The Portsdown was giving very low output but the signal was still being modulated.

Turned out that the MMIC after the modulator chip had gone S/C and burned out the RFC in it's supply circuit.

I can only think that the PA taking off fed back a fair amount of RF to the Portsdown and the MIMIC "didn't like it up em".

Replaced the MMIC and choke and all was well.

Could also be something like static with your chip failure!

Hope to work your side of the Pennines on digital sometime soon. We have 5 ATVers on the Fylde and can operate 2m, 70cm, 23cm and soon 3cm digital plus analogue on 23cm, 9cm, 6cm and 3cm.

73 Dave G3ZGZ.

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