BATC Forum

The Knucker is a fantastic receiver for DVB-T DATV. However, it is a bit of a black box. It either decodes and displays a signal or it doesn’t. If it doesn’t you are in the dark about the reason. Especially because it is also rather sensitive to frequency - a 20 kHz difference in frequency could mean the difference between no decode or perfect decode with lower bandwidths. Frank, DD0CW and I were searching for a method to obtain spectrum information from the Knucker. And a constellation would also be nice, as that would indicate the signal is decodable. After opening up the NIM in the Knucker, Frank found two pads marked IF-n and IF-p, coming from the MxL608 DVB-T tuner chip. After soldering a 220 pF and a short coax cable to one of those pads we found a 5 MHz IF signal there. The signal is inverted, as it moves down when the transmitted frequency moves up. Routing it through 45 dB of attenuation to an RSP1(A) we were able to get both spectrum and constellation in GNURadio. For the correct constellation we had to swap I/Q in GNURadio. It also decoded perfect in GNURadio. Based on those results I modified an old RTL-SDR dongle. The traces from the R820T tuner chip to the RTL2832 were cut just before the capacitors C33 and C34. Also the trace coming from the antenne socket to the R820T was cut. A short length of coax connects C34 with the antenne socket, leaving the ESD-diode in place. With the Knucker IF connected to the RTL-SDR antenne input it was now also possible to get the IF spectrum and constellation in GNURadio. For this low frequency the RTL-SDR needs to be in direct conversion mode. Therefore add direct_samp=1 to the device parameter (rtl=0,direct_samp=1) in osmocom source. If you would have used pin 4 or 5 of the RTL2832, direct_samp must be 2.

When you see a constellation, decoding in GNUradio is possible. However, that doesn’t mean the Knucker can decode it. That is because the IF filter is probably wider then the signal bandwidth. So the signal you see can be anywhere within that bandwidth, but it needs to be more or less exactly at the right frequency for your Knucker. This is a problem of local oscillators, as it seems that different Knuckers have different exact IF frequencies. The LO of the Knucker starts with a 28 MHz crystal, but for a 436 MHz signal must be synthesized to 441 MHz. So if the 28 MHz is 20 kHz of, at 441MHz it could be 300 kHz off. However, you can try to calibrate your best Knucker receive frequency.

For these tests I used a python program based on the Ryde software, especially CombiTunerExpress (work in progress). That makes it possible to work with a GUI on an RPI4 (RPI400 in my case). One problem remains: with the current software it is impossible to quickly finetune the frequency. After each adjustment, the knucker is initialized again. The whole procedure takes 10 to 30 seconds, way to long for contests for instance. Maybe it is possible to update this software to only change the frequency if all else stays the same? I’ll be glad to help if manpower is a problem. By the way, you can get a similar result without reverting to your soldering iron. Just put a splitter behind the LNB or preamp, connect the Knucker to one branch and an SDR to the other. Then you only have to do the calibration between the Knucker and the SDR and you are in bussiness.


73, Poll PA3BYV

Excellent work Poll - just what was needed as our experience with the Knucker has been that it is very slow to lock and I suspect that the frequency error was the problem. How quickly do you find it locks now?

Dave or Charles G4GUO may be able to comment re tuning with reinitialization but I'm sure they will appreciate your help if changes are possible.

Is it possible to make applications/batch files from GNUradio that can be run as a standalone instance?

Also, we are just finalising the next CQ-TV - if there is room is it OK if we publish this forum post?

73

Noel - G8GTZ

The tuner portion is controlled via I2C and is independent of the demodulator.
So there is nothing in principle stopping external frequency control being done separately.

I specifically asked the manufacturer whether it would be possible to route IQ information from the
demodulator over the data port and was told their chip does not support that feature.

- Charles

If you like you can use the second F-Socket which is not in use as an IF outlet. Just disconnect the F Socket from the PCB and connect it direct with a cable to the IF out according to Poll's description.

Reinhard

The second socket is for DVB-S/S2/C input (however, it is not supported by BATC software).

- Charles

Hi Charles,
If there would be a possibility in the channel setting to tell the RX which FEC, guard and modulation pos/neg will be used on the TX, the frequency locking of the RX would be much faster. The more DVB-T parameter the RX has to find out by itself the longer it takes to lock. As you are familiar with the chipset do you know if it is possible to control those parameter?
The DVB-T info we exchange typically in the chat when we use GNU radio for DVB-T.

73 Reinhard

Hello Reinhard,
Those parameters are transmitted in the pilot tones in a manner which is more robust than the payload data.
It is not possible to control them in software.

I suspect it is the frequency error that is the problem. From memory, the receiver scans across the reception
band in a zigzag pattern looking for a valid signal. It may be possible to narrow the search range.

- Charles

https://drive.google.com/drive/folders/ ... sp=sharing

Here you can download an .GRC file for the RTL-Stick or one for the SDRplay RSP1 for the Knucker IF 5Mhz.
Download also the OFDM input file for the "constellations view kick starter". And put it in the same directory like the .GRC file.
For other RSPx swap the SDR source block.

A very good and cheap RSP1 clone comes from China for about 20 €:

Search in EBAY for "Simple SDR Receiver Compatible RSP".

Vy 73 de Frank ( DDØCW ).

Sorry to be a bit late with my reply, but we had a ATV-contest going on this weekend.
Re Noel G8GTZ: of course, if you want to publish it in CQ-TV go ahead. Please let me know if you need pictures in better quality.

Re Charles G4GUO: it would be nice if CombiTuner Express would have a seperate command for frequency change. I could possibly try to figure out how that command should be shaped from the open source driver, but a quick look didn't quit enlighten me as how to do that. Thank you for the explanation about the parameters transmitted via the R signal.

Re Reinhard DL4FBN: smart idea to use the second F-connector. I tried to get the CombiTuner to work with CombiTunerExpress on DVB-S/S2 but didn't succeed. So better use what is already there...

Re Frank DD0CW: Thank you for sharing the GNURadio script. As you know I used that as demodulator for my standalone Python application to control the Knucker.

Going forward, I think it is most interresting to find a way to do the frequency control without a complete reset of the CombiTuner. Together with the IF-output it should be much quicker to find and lock on the signal. Of course, demodulation can also be done with GNURadio, but that has its own quirks. So simply tuning until you see a constellation appear should make the whole process much easier and faster.

73
Poll

All this development and testing is great. I have been using both Portsdown and the latest DATV Easy 2.12 today with my Knucker and also across town transmitting and receiving with Justin G8YTZ. The Knucker won't decode below 150kS so I'm looking for lower SR's to help with multi-pathing on 29MHz
I'm interested in how low a SR you are using with your systems and also hoping that a DVB-T demodulator might be written for SDRAngel which works very well with DVBS at just 18kS.
As I have a spare RTL and a RSP-1 I will also try these modifications on my Knucker.
Gareth