Friday, 30 September 2016

Duplexer DVB-T TX filter: Low bandwidths

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 used, in principle, for DVB-T in other bands as such filters are relatively common, although less so with the notch going both ways. I will discuss this in another post.



Tuesday, 27 September 2016

Testing a duplexer as a DVB-T TX filter-Wow!

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 further investigation.

I have not investigated the effects of the filter and different power levels on signal quality at the receiver. I have limited instrumentation, but will report my findings in the next post.

Instrumentation

A 20 dB directional coupler, plus an additional 30 dB of attenuation, was used to tap a signal from the transmit path to a HP 8591A spectrum analyser. A cheap SWR/Power meter was used in line to give some idea of output.

Filter before the amplifier

The duplexer/filter works remarkably well. In setting up to test the amplifier, I checked the source from a HiDes camera with direct DVB-T output; 7MHz channel, centered on 446.5 MHz, just to check it was clean. It wasn't too good, acceptable maybe at -40 dB, but with quite a spread.




Inserting the duplexer/filter cleaned it up almost perfectly!



Now I need another duplexer/filter to put after the amplifier.

Preliminary tests with just output filter

With the amount of gear needed to test a DVB-T amplifier, I thought I might see how the filter works, even with the less than perfect signal from the camera source.

With no filter and adjusting the input to keep spread at about -30 dB gives about 6 W, but it is not pretty.




Filter after the amplifier

With the filter after the amplfier, the results are surprising; with adjustment, a clean 10 W signal. The spread is 60 dB down. Magic! Drawing about 8 A at 13.8 V.




The power meter is showing 10 W. However I am not sure that is the full envelope power of a 7 MHz wide DVB-T signal. Cheap meters are for measuring low bandwidth CW and SSB signals, not complex ones. (I would like this clarified/explained by someone in the Yahoo group).



For amusement, I tried the direct signal input, thus over-driving the amplifier and putting out a few extra Watts of power.

The result shows the notch-nature of the cavities and duplexer. There is some rubbish, still -40 dB, above and below the filter's two notches.



Taking out the filter, and about 20 W output, with terrible spread. It sure does some clean-up job!




Conclusion

The notch duplexer/filter seems to overcome many of the problems of amplifiers for DVB-T.

A filter seems to be needed both before and after the main power amplifier.

There is still probably a need for a lowpass filter to stop harmonics.

The received signal quality needs to be checked.

This is a preliminary study and needs to be tried by others.

I again thank Martin VK4JVC for suggesting a duplexer rather than building an interdigital filter.

Monday, 26 September 2016

A 70cm DATV TX filter using a cheap Chinese duplexer

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 MHz DVB-T DATV signal with acceptable pass-band losses and steep skirts. The next test is to try it with my 10 W amplifier from Darko in Austria.

The filter

The duplexer is a typical mobile device available on eBay for about A$100 delivered, taking a week or so to Australia. The store insisted I supply some tuning data, even though I was immediately going to change it. I specified 440.5 and 446.5 MHz to keep them happy. It came with notches at those frequencies, but had not been well tuned.



The filter has three square cavities for each of RX and TX. The only adjustment is a screw at the top that capacitively alters the cavity's resonate frequency. There are no other adjustments. Each cavity has an cable in and out, but they seem to be notch filters rather than pass-band.

I am not sure what the power handling capacity is, 25 W, from memory. That would make it an adequate final TX filter for most DATV applications.

Tuning

The Australian 70 cm DATV band is 7 MHz wide centered on 446.5 MHz, with edges at 443 and 450 MHz. I use the whole 7 MHz as it makes reception on conventional TVs easier and I want high quality 1080p.

Using a HP 8591A spectrum analyser and tracking generator, re-tuning is quite easy. Three cavities at a time first (RX-ANT, TX-ANT), then checking all six with the input and output through the RX and TX connectors, ignoring the antenna connector.

I have tuned the cavities to about -3 dB at the channel edges. The loss through all six cavities is about 2 dB which indicates quite reasonable construction. The side slopes are quite steep.



I am not sure this tune will be adequate to suppress the channel spread, but I will re-tune to find an acceptable compromise. I may need to narrow the width, but without affecting the TX signal.

Conclusion

A cheap Chinese duplexer has bee re-tuned to produce what appears to be an acceptable TX band-pass filter for a 7 MHz wide channel on 70 cm.

Despite their reputation, the Chinese duplexer seems of reasonable quality and has typical characteristics for this type of device. The original tuning was indifferent, so should always be checked.