Acceptable tx intermod level ?

[G4WIM]

Just been playing around with the DTX1 and it generates a nice clean signal, but as expected by the time it's amplified up to say 2 watts output from a tx chain capable of 18 watts the spectrum re-growth shows to be about 30dB down on the main signal.

What is considered acceptable ?

FYI I plan on installing a filter on the PA output to limit the signal to 20MHz bw.

73 Tim

[g4wvu]

Hi Tim,
I reckon acceptable regrowth rather depends on the band in use.
2MS on 437mhz ....regrowth/shoulders need to be low for obvious reasons. (don't really want -25 shoulders either side - unsocial & could be out of band even)
2MS on 1248mhz with -25dB however isn't such a problem as the first shoulder will extend down to approximately 1244.4mhz & up to 1251.6mhz. - It's narrower than an analogue FM signal & well within band. (also much less potential for upsetting other services).

Eutelsat / Astra etc. specify -26dBc total - including through the transponder as worst case acceptable but this wouldn't be clever on 70cm !

70cm - I aim for below -35dB or better. You need good amplifier linearity to get this & or some pretty tight filters on the output (e.g. 3mhz passband with steep sides)
Some Ham 70cm "linears" I have tried are abysmal (-20db!) - with the majority of amplifiers you need to back off output by around 4-5 db (1/3 average power) to prevent the amplitude component causing significant regrowth / intermod .....why didn't they design DVB-S using OQPSK !

Good luck with the amplifiers. (the DTX1 shoulders are typically -60 or better at 70cm - very good)

Mark
G4WVU

[oe7dbh]

higher power = less shoulder distance = bad MER ........ = poorer receiving + unsuitable antenna = even poorer receiving


Hi all .....

23cm PA.jpg (94.82 KiB) Viewed 5871 times

70cmPA.jpg (91.65 KiB) Viewed 5871 times

.... i have suitable power amplifier for 70cm and 23cm Band.

if you have a signal with output power between -12 dBm and + 3 dBm, shoulder distance -60dB ....... then you're in the right place !
with 70cm power amplifier can get 11Watt dvb-t Output & -40dB shoulder.... or 13Watt dvb-s Output & -40db shoulder
with 23cm power amplifier can get 5Watt dvb-t Output & -40dB shoulder.... or 6Watt dvb-s Output & -40dB shoulder

both amplifier more than 50dB gain ( Typ. 53dB )

http://www.oe7forum.at/viewtopic.php?f=15&t=448#p1392

right now with weak euro, particularly cheap, availability check... oe7dbh@tirol.com

[G4WIM]

Thanks for the info guys - I think I should be able to hit those numbers without too much trouble if all goes according to plan.

Aiming for 60 to 70 watts out on both 437 and 1248MHz from a couple of amps which are capable of 200 watts PEP when running from 28V in SSB mode.

Looking forward to getting active on DATV - already had a couple of 24cm FM TV contacts, so looking good.

Although I suspect working GB3TM might be a stretch at 69 miles over a sea path - but a worthy challenge.

73 Tim

[G4WIM]

So I've been testing my 24cm DTX transmit chain.

The driver is producing 1watt with shoulders at -32dB.

This is from a M57762 PA brick which can produce 18W

The final PA is producing 70 watts with shoulders at -25dB, this is from a pair of MRF286's driver by another MRF286 run at 28V DC.

I was aiming for shoulders of -30dB.

I'm wondering what others manage at these power levels - 1W and 70 watts.

73 Tim