VK4ZXI

Testing a cheap Chinese duplexer as a 70cm DATV DVB-T 7 MHz TX filter- Wow! Introduction In my last post I described a cheap Chinese duplexer re-tuned as 70 cm DATV DVB-T 7 MHz TX filter. The duplexer uses notch cavity filters, six in all. The notch filters have a much sharper edge, compared to a band-pass filter. The sharp notch seems suited to the vertical edges of a DVB-T signal. I initially check the signal source, a HiDes camera with direct DVB-T output at 1080P. I was a little surprised at the spread, but the filter cleaned it up well. This would indicate the need for a filter before the main power amplifier. I pressed on with just one filter and tried it at the output of the amplifier, a 10 W device, from Darko OE7DBH, using a RA60H4047M1 60 W module. Even with the indifferent input, the filter was able to reduce the spread to -60 dB and give a clean 10 W output. The notch duplexer/filter seems to overcome some of the major hurdles with DVB-T amplifiers and warrants ...

Airspy, SpectrumSpy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer? Introducton A basic spectrum analyzer/tracking generator for less than $250? Yes. Can it be used to do a demanding task like tuning a UHF cavity filter from a repeater? Seems so. The "proof of concept", spectrum analyzer software, SpectrumSpy, can be used with the Airspy SDR and a noise source to show the characteristics of a pass-reject UHF cavity filter. SpectrumSpy and Airspy SpectrumSpy, "proof of concept", spectrum analyzer software is a new addition to the SDR# download for use with the Airspy SDR. It has the potential for a new direction with low cost SDRs, spectrum analyzers.  Spectrum analyzers are expensive; $1500 then skyward. SpectrumSpy: http://airspy.com/download/ (separate executable in SDR# folder). Airspy has a 24 – 1800 MHz native RX range, but down to DC with the SpyVerter option. http://airspy.com/ $199  and US$59 An earlier post is...

Low cost spectrum analyser/scanner software for the Airspy and RTL-SDR Introduction It is not often I am amazed at new technology, especially for free, but the Spectrum Spy software, a spectrum analyser/scanner for the Airspy SDR, impressed me. It is a poor man's spectrum analyser. However, it is preceded by at least two spectrum scanners for the RTL-SDR hardware; rtl_power and RTLSDR Scanner . This post will compare the two devices and three software packages, scanning the entire FM band and the 100 MHz of the local TV band. The software The three programs all run under Windows, Windows 10 in my case. All three installed and ran without much difficulty. Spectrum Spy is part of the SDR# software package. It is a separate program to SDR#, but in the same folder. Spectrum Spy has a spectrum and a waterfall. It updates every few milliseconds, depending on the span. Spectrum Spy only works with the US$199 Airspy. I have a V1 Airspy. RTLSDR-Scanner is a stand-alone prog...

Tuning the receiver front-end of a Kenwood TKR-750 repeater Introduction I bought a second-hand Kenwood TKR-750-1 VHF repeater. However it was not clear what band the receiver was tuned to. The actual receiver and transmit frequencies can be programmed with a computer. However the the receiver band width is tuned with a spectrum analyser and tracking generator. This post describes how to re-tune the receiver front end. The band pass filters for a 2m repeater RX are quite critical as they are the only selectivity. With a 600 kHz spacing, the cavity filters are usually just isolating the TX and RX when sharing the same antenna, but providing no bandpass to strong nearby signals. Starting point and instrumentation I have an old HP8591A spectrum analyser and tracking generator, circa the 1990s, but still a capable instrument. The basic procedure is to connect the tracking generator output to the RX antenna input and the spectrum analyser input to a test point after the RX bandp...

Tuning a commercial UHF duplexer with RigExpert antenna analyser The club is assembling a 70 cm repeater with some equipment kindly loaned to us to use; a Kenwood TKR-820 repeater and a good quality commercial six cavity band-pass duplexer. The tuning was done with a RigExpert antenna analyser that gives a plot of SWR swept across a frequency range. The analyser was able to tune each cavity, but the overall response, while good for separation was compromised, possibly by incorrect inter-connecting cable lengths. Tuning the duplexer The duplexer is a band-pass type, with six cavities, three each for RX and TX with a "T" combiner. The cavities use N connectors and thick LMR400 type cable, and are about twenty years old. The probes are set for maximum coupling, which is desirable for selectivity but for some transmission loss. As a band-pass cavity filter, each cavity has a N connector in and out, making tuning each cavity quite easy; put a 50 Ohm termination on ...

Red Pitaya: arrived and working My Red Pitaya has arrived and was working with a minimum of fuss. Simply type in the device's unique MAC address on the connect page http://discovery.redpitaya.com/  and hit connect. The instrument functions are web applications. Just click which instrument you want and it comes up in a web page. I was curious how they could get it to work on any computer, tablet or operating system, but the web access is the answer. I am not sure what consequences that has for performance, but it works and it is still early days. The Red Pipaya connected to give some idea of size. It gets pretty hot, maybe too hot for a warm climate like northern Australia; we shall see. The device is working as an oscilloscope and signal generator, with an output connected to an input. A sine wave at 20 MHz. Many of the adjustments are manual and take a bit of getting used to. However, it works. Not the cleanest sine wave; not sure if it is the signal generator or os...

Summary of BladeRF SDR TRX BladeRF is a high performance SDR transceiver made by a small start-up company, Nuand http://www.nuand.com/bladeRF . Currently only mainboard is available for US$420, with a HF/VHF transverter due late November to give coverage down to 10 MHz. For receive only, an up-converter for RTL-SDR dongles could be used to go lower. Technical Specifications: •Fully bus-powered USB 3.0 SuperSpeed Software Defined Radio •Portable, handheld form factor: 5" by 3.5" •Extensible gold plated RF SMA connectors •300MHz - 3.8GHz RF frequency range •Independent RX/TX 12-bit 40MSPS quadrature sampling:  LMS6002D is a field programmable RF  transceiver http://www.limemicro.com/products/LMS6002D.php     •Capable of achieving full-duplex 28MHz channels •16-bit DAC factory calibrated 38.4MHz +/-1ppm VCTCXO •On-board 200MHz ARM9 SOC with ...