DATV on 70MHz.

The place to discuss reduced bandwidth fast scan amateur television on 70cms and below
G1BVI
Posts: 155
Joined: Wed Aug 02, 2017 7:32 am

Re: DATV on 70MHz.and 70 MHz amp module

Post by G1BVI » Sat Sep 15, 2018 3:56 pm

For anyone intererested Anglia have 2 modules left at the time of writing this..

http://www.anglia-live.com/Products/275 ... Z-30W-125V

73

Nick

radiogareth
Posts: 356
Joined: Wed Jan 06, 2016 9:46 am

Re: DATV on 70MHz.

Post by radiogareth » Tue Sep 03, 2019 9:45 am

Finally, I'm building up my Module amp for 4. using the RA30H0680M module. Just wondering how much drive I'll need for maxing it out on DATV? So I googled it...
http://www.mitsubishielectric.com/semic ... 0608m1.pdf which shows that 50 mW should fully drive it, so for DATV probably not much more than half that. Single stage amp after my portsdown/limeSDR/pluto should do that then :-)
Gareth

radiogareth
Posts: 356
Joined: Wed Jan 06, 2016 9:46 am

Re: DATV on 70MHz.

Post by radiogareth » Mon Sep 09, 2019 3:57 pm

The Amp module passes all its tests and delivers an indicated 50 watts at least on carrier only. Suitable attenuation (-11dB on my Portsdown modulator) then into a +20dB modamp seemed to drive it without the shoulders getting worse.

Adding a decent power lead(longer) seems to have induced a pulsing instability. Current draw drops from 5 amps or so, to 0.9 amps.

Its uncased at the moment. Should I add ferrite filtering on the DC power leads, and more 10/100nf decoupling capacitors where DC feeds in.

Gareth

radiogareth
Posts: 356
Joined: Wed Jan 06, 2016 9:46 am

Re: DATV on 70MHz.

Post by radiogareth » Wed Nov 27, 2019 4:12 pm

With my clubs annual construction competition looming I decided to properly case and finish my amp. Its seemingly well-behaved now but I'm wondering how best to measure the true DVBS RF power its delivering. I have a reasonable (?) quality (?) power meter (Welz SP-220) and I can of course set my Portsdown to 'Carrier' which I presume gives a reasonable reading of RF power into a 50R load. Switching to DVBS produces a reduced reading on the same Lime Gain setting (accepted that the WELZ not capable of reading the 'peak' power of DVBS) BUT does it correlate in some way?
I've read that this module (RA30H0608M) will deliver 60 watts of FM and 30 watts of DVBS, but how can I measure this with any degree of accuracy? I'm trying to set up the required 'gain block' to provide the required level of drive from both a Pluto (more gain needed) and a LimeUSB/Portsdown.
Thanks
Gareth G4XAT
4_M_Amp.jpg
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g0mjw
Posts: 958
Joined: Sat Sep 20, 2014 9:15 am

Re: DATV on 70MHz.

Post by g0mjw » Wed Nov 27, 2019 4:53 pm

The module is good for 5W with reasonable regrowth. The power meter should be reading correctly the average power. The only real way to measure is with a power meter that measures power and not voltages. E.g. a Bolometer head. However the Welz meter is probably close enough. The reason CW is more than DATV is DATV is filtered and not actually constant envelope due to that filtering, the average is a little less.

Mike

radiogareth
Posts: 356
Joined: Wed Jan 06, 2016 9:46 am

Re: DATV on 70MHz.

Post by radiogareth » Wed Nov 27, 2019 4:59 pm

OK, Thanks Mike. As an example, I'm driving a small 2-stage amp from the lime at gain=83 and with 'carrier' selected' on Portsdown I get 50 watts on the meter from the brick. If I switch to DVBS I get about 2.5 watts, signal looks OK on my analyser and decodes fine on LeanDVB nearby.
I was hoping for a bit more really, time to build a MOSFET amp for the band maybe.....

Gareth

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

Re: DATV on 70MHz.

Post by g0mjw » Wed Nov 27, 2019 5:15 pm

The amount you get out depends on the level of the shoulders - in the 71 MHz band we need to be really careful to stay in band so that's why the level is lower for -50dB shoulders. You generally get 10-20% of the peak output but it depends on bias. You might get more but would need to check with a spectrum analyser that it's OK. You are limited by the EIRP on the band anyway so you can't use a lot more than 10W to a 3 or 4 element yagi with 10 dBd gain.

Mike

G8GKQ
Site Admin
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Re: DATV on 70MHz.

Post by G8GKQ » Sat Nov 30, 2019 11:07 am

Hi Gareth

Unlike with the filter-modulator board, the power measured in "Carrier" mode with the Lime does not have a consistent relationship with the DATV power. This is because the Lime sets a lot of internal settings itself that Portsdown does not have control of.

However, in my tests with DVB-S2 power measurement I have rarely found more than 20% difference between a diode-based power meter (Bird Thruline) and a thermal power meter (HP432A with power attenuator). So I would suggest that you just use the meter readingd that you have.

As for how much a specific PA will deliver, you can only tell by setting yourself a shoulder power limit (I would suggest -30dB in-band, -50dB out-of-band) and then winding up the power looking at a spectrum analyser. Do remember that if you are running 250KS, the first shoulders may be in-band (so -30dB) and only the second shoulders need to be below -50 dB.

Dave, G8GKQ

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

Re: DATV on 70MHz.

Post by g0mjw » Sat Nov 30, 2019 11:38 am

G8GKQ wrote:
Sat Nov 30, 2019 11:07 am
As for how much a specific PA will deliver, you can only tell by setting yourself a shoulder power limit (I would suggest -30dB in-band, -50dB out-of-band) and then winding up the power looking at a spectrum analyser. Do remember that if you are running 250KS, the first shoulders may be in-band (so -30dB) and only the second shoulders need to be below -50 dB.

Dave, G8GKQ
This is a good point by Dave - and for contests 125ks and even 66ks are quite usable to send the contest exchange. So, you gain 3dB or 6dB from less bandwidth and possibly a few dB more from the PA. Overall, we have at least 10 dB more capability than we had when trying to break 250km on 2m a few years back.

Mike

radiogareth
Posts: 356
Joined: Wed Jan 06, 2016 9:46 am

Re: DATV on 70MHz.

Post by radiogareth » Sat Nov 30, 2019 12:24 pm

Thanks both, good pointers for me to go back and check on :-)

Gareth

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