LO Filter test

Discussion about this major DATV Project. See also https://wiki.batc.tv/The_Portsdown_Transmitter
G3UEQ
Posts: 78
Joined: Sun Jul 26, 2015 10:28 am

LO Filter test

Post by G3UEQ » Thu Nov 29, 2018 9:22 am

Testing my LO filter and RF output switch on spectrum analyser I see on 71MHz output a spurious at around 42.6MHz that is same amplitude as wanted o/p but pulses on and off every few seconds. the main o/p stays stable. Is this normal?

Also. on 2M o/p the level is around 20dB higher than all the other bands, does this indicate fault in filter, for these tests not going through RF switch.
73
Andy

G8GKQ
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Re: LO Filter test

Post by G8GKQ » Thu Nov 29, 2018 9:38 am

Hi Andy

The pulsed spurious at 42.6 (71 * (6 / 10)) does not seem like is should be there. I have never noticed it. Is it there on the output from your ADF4351, or only after the LO filter?

Stupid question - please confirm that you have put the LO filter between the ADF4351 and the F-M board? Sorry to ask the obvious, but I have seen this error!

Where are you measuring the power difference for 146 MHz? At the output of the F-M board or before the F-M board?

Dave

G3UEQ
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Joined: Sun Jul 26, 2015 10:28 am

Re: LO Filter test

Post by G3UEQ » Thu Nov 29, 2018 10:10 am

hi Dave
Thanks for coming back quickly. The filter does go after the LO and before modulator board. I am now going to bypass filter and check o/p of LO.

The power difference is similar after the RF switch and the o/p of the modulator. The RF switch appears to be OK. All other functions on TX work fine, frequency selection etc. So I assume there are no errors on the LSB/MSB bus.
73
Andy

G8GKQ
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Re: LO Filter test

Post by G8GKQ » Thu Nov 29, 2018 10:14 am

Hi Andy

Sounds like you do have an issue with the output level. Here are the typical output levels: https://wiki.batc.org.uk/Portsdown_Output_Levels

Dave

G3UEQ
Posts: 78
Joined: Sun Jul 26, 2015 10:28 am

Re: LO Filter test

Post by G3UEQ » Thu Nov 29, 2018 11:04 am

Dave
It looks like duff filter board, 71MHz o/p has now died, 2m and 1.3GHz o/ps reasonably OK, lower o/p on 437MHz. Bypassing the filter showed no o/p at 42.6MHz!
So will order new PCB and set of parts from Digikey. That was my first go at SMD and not pretty so maybe s/c somewhere. RF switch was quite neat attempt.

Cheers
Andy

G3UEQ
Posts: 78
Joined: Sun Jul 26, 2015 10:28 am

Re: LO Filter test

Post by G3UEQ » Thu Nov 29, 2018 4:12 pm

Right, for the moment bypassing the filter board and have good levels out on all bands. However despite good lock/timing and all green lights, cannot get any video through on 71, 146 and 437MHz, perfect on 1255.
Looking on spectrum analyser everything looks OK, FM board functioning correctly, tried Mpeg, H264, Low and High SRs and ticked low SR box when relevant. Camera and test card as well. mmmmm?

Andy

radiogareth
Posts: 155
Joined: Wed Jan 06, 2016 9:46 am

Re: LO Filter test

Post by radiogareth » Thu Nov 29, 2018 4:50 pm

You are not alone with suffering RF-aggravation! What was working fine this morning (I was using it to drive amp experiments on 146) now seems to have suffered an expired the DDS, along with my little eBay power meter which now reads 246mV regardless of what gets put in :-( Just left with SDR based spectrum analyser which led me to no DDS output.
My afternoon spent building my second filter board (having expired the previous) and all ready to test - nowt coming out.
Back up the chain disconnecting stuff and nothing emerging from either DDS sockets.
Ugly mode works well though, so I'll use that and the pi-cam for demo purposes Monday night at my annual club construction contest.

Ordered parts from digikey Sunday, they arrived today. Good service.

Gareth.

G3UEQ
Posts: 78
Joined: Sun Jul 26, 2015 10:28 am

Re: LO Filter test

Post by G3UEQ » Thu Nov 29, 2018 9:20 pm

Sorry to hear that Gareth
Yes, Digikey are very good, most probably get mine by Saturday or even Friday.
I assume on your DDS board the Lock LED is not illuminating on TX, or if it is then the o/p stage has failed.
Commiserations!

Andy

G3UEQ
Posts: 78
Joined: Sun Jul 26, 2015 10:28 am

Re: LO Filter test

Post by G3UEQ » Thu Nov 29, 2018 9:24 pm

Update on filter board, 1 o/c 15nH and 1 o/c 39nH, two dry joints on 4m i/p, did order spares on Digikey for new board, so hopefully will have back up available just in case. My SMD work is much better now !!!
That was an expensive exercise.

Andy

radiogareth
Posts: 155
Joined: Wed Jan 06, 2016 9:46 am

Re: LO Filter test

Post by radiogareth » Fri Nov 30, 2018 8:27 am

Andy, the lock LED is alive and well so I'm a bit puzzled as to what/why I'm getting no output. Yes, second time round was easier, flux pen is indispensable as is solder wick. Before I fitted the 4 way chip I checked DC continuity on everything - paranoia!!
I've had a close look at my original board, seems OK but in extracting the dead switch I've lost tracks 1&2, DC power. Easy enough to run a couple of wires.....
Onwards....
Gareth

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