VK4ZXI

DIY 2m single connector pass-reject coupling Introduction In my last post I describe the repair and tuning of a high performance 2 m duplexer that uses an unusual single connector pass-reject coupling. In this post I describe how to make one for about $30 that achieves the same level of performance. First, I explain what seems to be the theory of the pass-reject coupling as a parallel tuned circuit, using the coax as the inductor and part of the capacitor. The coax conductor is the other part. The variable piston capacitor is in parallel with the coax capacitor to allow tuning. It also gives the necessary mirroring for RX and TX responses. Then I describe how I made one that achieves about the same performance as the original. I am very pleased as I have been working on DIY pass-reject couplers for some time. It allows the construction of a complete high performance, six cavity VHF duplexer for about $300, less the cost of connectors and cabling. Theory of the tuned cou...

2m Duplexer design repair tune pager-reject Introduction My local radio club's main 2m duplexer had an intermittent fault. The duplexer was fixed and retuned to a new channel, 1600, not 600 kHz split, partly to move the RX further from a Pager TX. The Telewave TPRD-1556 Pass-Reject duplexer has an unusual coupler that I had not seen before that works very well, 45 db reject each. They should be possible to DIY. The fault was a defective piston capacitor used to tune a cavity. An unexpected benefit of the pass-reject RX cavities is a significant 72 db reject of the 148 MHz pager TX. More is possible, if needed from notch cavities at 25 db reject each. Tuning high performance cavities, 45 db each, highlights the limitations in instrumentation, particularly the dynamic range, 85 db, with the reject response lost in the noise floor. Design of the Telewave TPRD-1556 Pass-Reject duplexer  The Telewave TPRD-1556 Pass-Reject duplexer looks like any other six cavity pass-r...

High power UHF DVB-T amplifier, filters and testing- very draft I have a used 150 W pallet amplifier from a scrapped DVB-T transmitter via eBay. It is bolted to a heat sink from a satellite transmitter. I am basically following the 1 kW CW UHF amplifier from W6PQL. I have the low pass filter for the amplifier. I temporarily soldered some SMA connectors to test its frequency response. Down about 1 db at 500 MHz, but -40 db at the third harmonic; very good. It has been suggested to use a pass band cavity filter duplexer. I had one on hand that I had just tuned for a repeater, trying to get a narrow pass-band. Just using three cavities, I varied two of the three cavities to try to get a wider response. No joy, little wider, but more importantly 10 db loss. To be continued...

Polyphase harmonic rejection mixer: AirSpy HF+ Introduction Can you get excited about a new mixer, usually boring devices that haven't changed in decades? Yes, the new polyphase harmonic rejection mixer in the AirSpy HF+ is almost as revolutionary as SDRs and will have a major influence on their design. The big advantage of a polyphase harmonic rejection mixer is that it acts as a RF filter for the selected signal, as well as suppressing harmonics and other aliases of the mixing process and local oscillator. It means that the mixer can virtually be connected to the antenna. Typically, a polyphase harmonic rejection mixer converts down to an ADC at base-band. It seems they can be used for both RX and TX. The post covers how the AirSpy HF+ works, and gives references to what I have been able to find out about polyphase harmonic rejection mixers. They are new and still covered by recent patents. A link to a PowerPoint gives general technical details of the mixer...

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...

LimeSDR running DATV Express DVB-S TX software (1st draft) With the MiniTioune DVB-S RX, I have begun trying different TX using DATV Express software under Windows 10. The logical first hardware would be the DATV Express hardware TX, but having shifted rooms in the house, I have not been able to find; I know exactly where it was in the other room! The LimeSDR is a popular recent SDR dual duplex transceiver by Lime Microsystems using a new version of their own chip. Cost is about US$250, but they have just announced a mini version for about US$150. It replaces the popular BladeRF; I sold mine to by the new model. DATV Express TX software is available for the LimeSDR; https://discourse.myriadrf.org/t/windows-based-dvb-s-s2-t-transmitter-for-limesdr/1348 (https://wiki.myriadrf.org/LimeSDR-USB) . It worked well without any hitches on 23 cm, with both TX and RX running on the same computer. I am currently using my main PC, but will move it to my fast Dell laptop for project work. Wi...

Decontis dtvtools DVB-T/S measurement, analysis and monitoring software (draft)  Introduction There has been a lack of good DVB-T monitoring software for both TX monitoring and RX measurement, unlike DVB-S that has Tutioune. I came across a commercial grade package from decontis that is relative inexpensive and uses a cheap USB-T dongle. While comprehensive, it is not particularly easy to use, but is network-based. I have managed to get it going and plan to use it for TX power amplifier modification and monitoring. My favorite element is a proper constellation chart. Other software and hardware The available DVB-T measurement,   analysis and monitoring  software is limited. CrazyScan2 for terrestrial/cable DVB-tuners https://sourceforge.net/p/crazyscan/wiki/Info/ uses PCTV USB tuner. The other alternative is to use a TV tuner, which gives MER and BER, but not constellation diagrams. A standalone TV installer instrument can be used, but good ones are expensive...