Saturday, October 10, 2009

Downconverters for receiving

The PC Electronics TVC-9S is a very nice downconverter, and I'm glad I bought one, but it is a bit expensive. To really make it easy to get started, I think it would be great if something simpler were available.

North Country Radio make a VCO controlled 33 cm downconverter. I haven't tried it, though, and my one worry is that without a frequency counter, it might be tough to tune. It depends on how much tuning slop your ATSC tuner will allow. But on the other hand, their downconverter is a third the price of the PC Electronics one.

One thing I think that could be changed is that these downconverters are designed to shift 33 cm down to VHF low. That means their mix frequency must be very high - in the 800 MHz or so range.

For DTV, I don't think it's necessary to chose such a low IF. If you picked a mix frequency of, say, 421 MHz, you'd wind up with 909 MHz being output on TV channel 17. That would be ideal for around here, because channels 15 through 18 are unused (since 16 and 17 are used for land-mobile in San Francisco).

So, dear readers, if you can design a cheap crystalized or PLL controlled downconverter to go from the middle of 33 cm to the lower end of the UHF TV band, please let me know - I think you could make a lot of hams very happy.

3 comments:

Kevin said...

I would suggest going to TV channel 52-69 as those channels are now out of band, but all digital ATSC tuners will still tune them. 15-18 may not be in use in your area, but are in others (WQOW-DT-15, KTCI-DT-16, WLAX-DT-17, and KYIN-DT-18 are in use in my area) and 52-69 are not in use in any area for DTV since June 12th. I was quite intrigued by your project and was talking with a ham buddy of mine (NT0Z) about this down-convert problem and he said it would be quite simple to do for much cheaper than what they are selling them for.

-- KD0GTJ

Nick said...

The reason to pick an unused channel is so that your downconverter, coax and receiver don't have to be tightly shielded against a competing signal. 52-69 are not going to be used for TV, but they're certainly going to be used for something - they were just auctioned off for billions.

Hopefully, whatever someone can design will operate anywhere in the low end of UHF, say channels 14-23 (470-530 MHz) without too much fiddling. Whoever's building it will be able, hopefully, to pick a crystal to match the available channels in his area and then tune the circuit once it's built.

N6GKJ, RON SIMPSON said...

Nick,

We have much experiance with the North Country Downconverters. Not only are they much cheaper, they out perform PC Electronics. NC Downconverters typically provide us about 40db gain verses about 15db on the PC Electronics units. As far as the tuning goes, I remote tune my downconverter, I have my downconverter in an aluminum box at the antenna with my t/r relay. I run my power up the coax for the tuner and 24vac to the box, and rectify, filter and regulate that 24vac for my 28vdc relays. I am more than 50 miles from Mt. Diablo and with the exception of local 900 Mhz intereferance, I am getting a consistant P5 picture on my NC downconverter and a P1 at best on a PC Electronics downconverter in the same configuration.

Your idea of MC145151 or MC145152 those chips are getting hard to find, the National LX series is current and simple to use. There are alot of hams using those chips and there are many PIC designs to program those synthesizers. One thing that you are not looking at is the complex aspect of your loop filter....do some reading....

The National chip has a little ap on the website you can download that will design a proper loop filter for their synthesizer...

Good Luck and keep up the good work.

My DVB-S system has been operational now for several months low level here in Lodi and works great. I have experimental DTV working form the the shack using a webcam, a usb transimitter interface and some coold softeware written in Europe...Will keep you posted....

I am using a Harmonic exciter, and a Divicom MV10 MPEG2 encoder. That exciter upconverts 10 1255 and I am running 65 watts with very good results. My repeater reciever is an FTA reciever tuned to the input. I also have FM inputs as well....all is working and progress comes in small steps

Ron Simpson, N6GKJ