VK4ZXI

Modifying cavity filters for DATV TX or for repeaters Introduction I am currently doing further work on using notch cavity filters for DATV DVB-T transmitters. My earlier efforts were with what I had at hand and not knowing the solution; I (re)discovered that notch filters clean up DVB-T TX very well. However, it was at low power, 10 W, and high losses, >6 db because of the six cavities in a mobile duplexer. Here, I will report on modifying high power >100 W individual filters. In the next post I will report on using them and determining is just one pair are sufficient. The other goal of this post is to show how easy it is to modify older commercial filters for DATV or repeater use. Modifying cavity filters Old commercial filters are relatively easy to modify as the only thing that changes is the coupling loop, provided they are on frequency (not too hard to change that too!). Notch filters are the simplest as they use a single simple coupling, just a loop of metal. Old co...

Commercial DVB-T Amplifiers and filters and implications for DATV (first draft) Introduction Commercial solid-state DVB-T amplifiers use about 250 W "pallet" amplifiers; a pair of  amplifiers, each using a pair of LMDOS transistors in a single package. The circuit boards are mounted on a piece of thick copper then on a heat sink. To get higher power, 1 to 50 KW, many pallet amplifiers are used in parallel with a system of splitters and combiners. The amplifier output passes through a series of filters, to stay within a standardized spectrum mask to limit adjacent channel interference. To conclude I note some implications for DATV. Pallet amplifers A typical pallet amplifier is pictured below (bought on eBay). The gold rectangle on the left splits the input to a pair of Doherty amplifiers, while the one on the right combines the amplified signal to the output. The combiner uses a type of circulator to dump RF to a dummy load, the black rectangles on either side of the ...

DVB-T TX Filter: 23 cm filter using 70 cm duplexer Introduction I came across a commercial UHF bandpass cavity filter that was as long as a VHF cavity. I removed a coupling to see inside. The probe was nearly 500 mm long, rather than the 170 mm in most UHF filters. They were using the probe as a three quarter length, rather than the typical quarter wave length. I checked the cavity at VHF and, sure enough, there was another peak. The obvious question then was whether the 70 cm duplexer filter would have another bandpass at three times the frequency, about 23 cm? It does! With careful tuning it should be possible to use the ubiquitous 70 cm duplexer at 23 cm. Analysis Raw response With the filter set to 7 MHz at 446.5 MHz, I had scanned three times that frequency and a very wide span. Sure enough, there was a passband, although at a different frequency and three times wider. The frequency of these cavity tuners are determined by a fixed length probe (tube) and a capac...

Duplexer DVB-T TX filter: Technical details Introduction Analysis Bandpass is addition of two notches  Three cavities RX to ANT Reverseded notch TX to ANT Sum of two notches with six cavities RX to TX Two left hand couplings are conventional, the two right hand couplings, with plastic spacers are unusual, give reverse notch (not sure how). The image shows internal construction of 4 cavity duplexer. Tuning is achieved by slug into the top of the probes, like a capacitive hat on an antenna. Tubes or probes are slightly less than a quarter wave length. Basically antennas in a box with couplings in and out. Ratio of tube diameter to outside diameter determines impedence. One to three gives about 50 Ohm. SWR of Duplexer filter 7 MHz on 70 cm, not great but probably affected by tuning, possibly by coupling design. Conclusion Coupling for reversed notch needs further investigation

A 70cm DATV DVB-T 7 MHz band-pass filter using a cheap Chinese duplexer Introduction DATV transmitters for DVB-T are notorious for "spread" outside the channel, to the point that keeping it 30 dB or more below the signal becomes a limit for power output, typically 10 W out of a 70 W module amplifier. Even with -30 dB spread, it is desirable to have a band-pass filter before further amplification or transmission. Usually an interdigital filter is used, but they are either expensive to buy or a bit difficult to build. VK4JVC suggested using a cavity filter duplexer instead. I tried a four cavity notch duplexer, but the pass-band losses were too high, more than 20 dB. I had bought a cheap, ~A$100, Chinese Jiesai  duplexer, but had put it aside as the response looked bad. After try other duplexers (notch and pass-reject types), I tried the Chinese one again, this time successfully. The result is that the Chinese filter seems to provide a good pass-band for the 7 ...

200W DATV power amplifier- 50V 20A power supply Introduction I am builder a DATV power amplifier, about 200W maximum. The power supply is unusually powerful as the amplifiers are only bout 25 per cent efficient. Commissioning such a power supply is not simple. DATV power amplifier I discovered through one of the Yahoo groups that it was possible to buy UHF DVB-T pallet amplifiers, broad band from 470 MHz to 900 MHz. I bought one through the Italian eBay for 400 Euro. It has a pair of BLF888A  LDMOS transistors. The pallet is a pair of amplifiers with a splitter and combiner to allow single in and out. The BLF888A are each a match pair of transistors in push-pull. Power supply The power requirements are about 48V at 20A as the amplifiers are only about 25 percent with the ultra linear DVB-T. I remembered that a lot of telecommunications equipment use 48V, so had a look on eBay. I managed to buy a new  ELTEK Flatpack2 2000W 48V HE Power Supply...

BlackMagic Design ATEM TV Studio problems with 1080p; a hardware limitation? In this post, I think I have discovered the problem of the ATEM TV Studio not accepting 1080p input, a hardware limitation. Alternatives are then considered. BlackMagic Design ATEM TV Studio As noted in earlier posts, I use BlackMagic Design ATEM TV Studio as the main component of my DATV studio to connect multiple cameras and other video sources. Overall it performs brilliantly well allowing for most features needed in a professional TV switcher, and with nothing else even remotely in the price range; $1000. http://vk4zxi.blogspot.com.au/2014/06/datv-dvb-t-blackmagic-designs-atem-tv.html     No 1080p input, despite being common format for cameras However, it has one very annoying problem, it will only accept 1080i input, not 1080p, which to a point is reasonable as 1080i is the highest resolution for broadcast TV. The big problem is that almost all cameras...

HD DVB-T HiDes HV-202E ATEM TV Studio DVB-T DATV all working Finally, I have all the pieces connected for a high-quality, live DVB-T TV studio and TX. My interest has been in establishing a high quality, video and audio, DATV system. This post covers the full working system, albeit small-signal. The details of each of the components are covered in earlier posts. The main components of the system are: HiDes HV-202E, self contained DVB-T TX with HDMI input: http://vk4zxi.blogspot.com.au/2014/07/hides-hv-202e-dvb-t-self-contained.html BlackMagic Designs ATEM TV Studio: http://vk4zxi.blogspot.com.au/2014/06/datv-dvb-t-blackmagic-designs-atem-tv.html High-definition digital SLR and CCTV video cameras: http://vk4zxi.blogspot.com.au/2014/06/dslr-cameras-for-live-tv-not-so-simple.html , http://vk4zxi.blogspot.com.au/2014/06/hd-cctv-cameras-cheap-quality-live.html The front of the operator console. The ATEM TV Studio is PC-based and is mounted in a small stand, just viable behind the...

HiDes HV-202E DVB-T self-contained transmitter: Quality all digital live DATV from DSLR camera at last! The HiDes HV-102E DVB-T self-contained transmitter has arrived at US$660 delivered. I ordered the USB version of this professional HDMI/HD-SDI 4 band (100 MHz - 2.5GHz) DVB-T TX originally, but upgraded to the stand-alone box instead. (see why latter). It works perfectly out of the box and is easily configurable for any modulation or media parameters. I had a good experience with the HiDes DVB-T HD CCTV camera transmitter; see earlier post. As such I thought I would try their HDMI input DVB-T TX. Surprising similar, as will be explained. The impressive specifications per HiDes: There isn't much this box can't do! Any frequency (up to 2.5 GHz!), any band-width, any media modulation parameter. There isn't anything that comes close, at any cost. I set it up on a channel my little 16" TV could receive (by cable with an attenuator) and connected...

DVB-T, UT-100C new drivers and software, DVB-T Repeater UT-100C new drivers and software There are new drivers and software for the UT-100 series devices from Hides/ITE: https://www.dropbox.com/sh/r80tjnkapkgzeg9/_3yj1s4t9 - I have yet to try them, but they are meant to fix compatibilities with Windows 8 and 8.1, possibly the unsigned driver problem, and more fixes to the PC2TV. I have yet to try them out. Blackmagic Design Intensity Pro capture card I have had trouble getting PC2TV to work but have been concentrating on the video source end. I want to be able to transmit digital video, not analogue (composite or S-video). PC2TV worked with my laptop webcam straight off, so I have been trying to get HDMI video to work. I want to be able to do amateur QSOs with real-time video and audio at high definition; the state of the art. As is noted later in this post, a repeater will assist this. I bought a Blackmagic Design Intensity Pro PCI-E capture card (AU$249)...

I have had success with transmitting video files, both test and HDMI ones I have captured off my camcorder via a Avermedia DarkCrystal HD Capture Pro PCI-E card. It all works as intended at full frame rate. The UT100C works well with files, even Full HD. Dongle at 1mW to rabbit's ears antenna, received off main house TV antenna. There is new software for the UT100C at   https://www.dropbox.com/sh/dvshv74hvjkau3a/uifV9Ve95l I was using Windows 8 but swapped back to Windows 7. Not sure it makes much difference as there is now a signed Windows 8 driver. Some receiver screenshots: The image and channel properties at 16QAM. TV receiver software is ProgDVB. It works, its free, its been around a long time. The 16QAM constellation diagram from Crazyscan2 with TBS 6220.  (I think this is very neat! All praise crazycat69!) Re-transmitted at 64QAM as per Australian DVB-T standard. The band scan showing my strong signal and the FTA stations further up...