Tuesday, November 18, 2008

33 cm amp arrived

The 900 MHz amp I got the sweetheart deal on has arrived.

It's actually two amps, in fact.

The big one only has about 37 dB of gain. My thinking at the time was that its output at maximum input would not be sufficient for my needs. But now, my thinking has changed. If I feed the thing the maximum power from the exciter of 12 dBm average, the output power would be 79 watts average. Well, as we now know, the peak-to-average ratio of ATSC is 5 dB (at least it is 98% of the time), so that's actually 250 watts peak, which is probably too much (we'll have to get the thing on a scope to see for sure).

I bought that amp, plus a companion amp designed to boost the signal from -2 dBm to up to 5 watts. But I think, in retrospect, that given what I've learned that I'll probably just run the big amp by itself.

The first stage amp does have the advantage of having a power control pin that can be used to reduce the gain, but the exciter also has software output power control.

One thing that the big amp lacks is a PTT line (that is, something that can key the bias voltage for the modules on and off). This is something I'll likely need to add myself. The kind folks at the factory said they'd be happy to help with that when I got to that point.

It also lacks a cover. I'll need to get some sheet metal bent somewhere to fix that.

First step, however, will be trying it out on the air. But that will have to wait for a 28 volt power supply.

I'd like to publicly thank Ken and Lance - you two know who you are. I'd mention the company name, but I am not sure whether they would appreciate the publicity or not. If they would like a plug, I'll edit this post and add one happily. They are very nice people to deal with.


N6GKJ said...


Congratulations!!! You are the only one in the US that I know of who is building an ATSC based system. I am building DVB-S based system here in the Valley, but anything on the air is a good thing.

To all the people that put out there crap of ATSC vs DVB-S....what are they doing? I hear it all the time. There are pros and cons to both systems. It comes down to personal preference and availabilty. You keep going....if you need anything in the way of testing or parts, let me know. I dont know how well you will be sucessfull on 910 because of the WiFi there, but hey maybe you will chase them off!!!

Well done keep up the good work.

You can follow my progress at http://www.lodiamateurradioclub.org/datv.htm

N6GKJ - Ron

Nick said...

Thanks, Ron. It's not the WiFi on the 902 MHz band so much as the part 15 stuff - cordless phones, baby monitors and the like - and the M-LMS systems. Officially, the part 15 devices aren't supposed to interfere with licensed users, but we all know that enforcement of that is impossible in practical terms. In the mean time, M-LMS licensees are primary in the band, so we have to stay out of their way. But fortunately, they are (at least currently) limited to sub-bands which appear to leave enough room for D-ATV in the middle of the band (either 909-915 or 910-916 as channels).

We'll see.

N6GKJ said...

Hey Nick,

Over here in the valley, there are alot of illeagal 900 mhz high power WiFi systems that use 902.300 to 923.5 finding usable repeater inputs is getting harder all the time.

I will be happy to drive over to your part of the world to do am air test once your up and running.

I was thinking about 910 over here, as I have some very cool high power 900 mhz vey linear amps. They do about 800 watts in CW!!
28V at 40+ amps!!!

Call me sometime at 209 570-0878