VK4ZXI

Rohde & Schwarz CMU200 Universal Radio Communication Tester resources Introduction This post is a collection of information for the Rohde & Schwarz CMU200 Universal Radio Communication Tester that I have purchased. They can be bought on eBay and other places often for a very reasonable sum. In its day it was an expensive but capable instrument. While the CMU200 is primarily designed for testing now obsolete mobile phone equipment, it can be used for working with analog radio. It has a spectrum analyser, RF generator, RF power measurement and with the option, an audio test set. While not directly having a tracking generator function, there are two PC programs that allow it to be used for testing filters. It can also be done with a noise source or an external tracking generator. The CMU200 uses an embedded Celeron or similar AMD processor running MS-DOS. It has an internal IDE HDD that is wise to replace as the instrument can do tens of thousands of hours. A...

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...

Using notch filters for Rx and TX and extending the range of a notch duplexer (Draft) I have a six cavity notch duplexer for 70 cm. At the original frequencies, outside the amateur band, the RX and TX notch responses are a mirror of the other, as notch filter responses are asymmetrical, compared to symetrical for band pass.. The mirroring of the responses are achieved by adding a quarter wave line to each cavity in the TX half. Being a quarter wave length, it changes with frequency when the duplexer was tuned about 20 MHz lower from its original frequencies. This was enough to stop the mirroring, making the RX and TX curves much the same; high losses for TX. The desired TX shape can be restored by adding about 10 mm to the quarter wave lines on the TX side. The length was calculated as the difference in quater wave between the two TX frequencies, in this case about 8 mm. For this duplexer, it was quite easy as the join to the cavity was soldered. The cable was lengthened by addi...

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

Duplexer DVB-T TX filter; Low bandwidths: 2 and 1 MHz. Introduction The duplexer DVB-T TX filter was adjusted to see if it would work at low bandwidths, specifically 2 and 1 MHz at 70 cm. It achieved this easily, but with a small increase in losses. Testing The duplexer was original adjusted for a 7 MHz bandwidth with a spectrum analyser and tracking generator. I only moved the lower frequency notch, one side of the duplexer. At the request of a USA operator, wanting to work DX, I readjusted the filter for a 2 MHz bandwidth. This was possible, with with little effect on losses. For interest, I adjusted it to a 1 MHz bandwidth, again possible, but with slight losses. Discussion The filter works surprising well at the lower bandwidths. I thought losses may have been higher. The losses are not a major issue as the DVB-T amplifier can be driven a little harder to make up for them, and possible even more because of the filter. Notch cavity filters could be ...

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 ...

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 ...