DATV modulator

PA0RJV’s set-up PCB for DATV on a RaspberryPi using UglyDATV from Evariste F5OEO.

By Robert Janus PA0RJV / F4VSG


First Gnu radio IQ tests were made in 2012/2013 with Simon PA9TV generating FM – SSB – and DATV. We tested at 144, 432 and 23 cm with nice results.

Later DATV experiments were made using BATC Digilite and altered Nyquist filtering.

Iq modulator met nyquist filter.jpg

On this small PCB I and Q signals from GNU radio and Local oscillator were used as input. The output came direct from the IQ-modulator chip without any amplification.

After this setup a more sophisticated setup was made in order to have more proper impedance from a PC’s soundcard.

SAM 6159.JPG

In basic, this setup is identical to the previous PCB, but a dual low noise impedance buffer OP-AMP was placed in between the sound-card output and the both I and Q inputs.

Start of testing with Digilite and later UglyDATV.

After Evariste’s UglyDATV was there, existing pcb’s were modified to fit on an RaspberryPi. 2014/2015 Early setup experiments:

SAM 6065.JPG

On this picture you find a combination of the I/Q modulator connected to a Nyquist filter as used in DIGILITE This setup was used in combination with Digilite’s I/Q output.

We were happy with these results so a new PCB was made:

Pcb layout IQmodulator met nyquist zonder filter amp.png

And here this test PCB:

SAM 6154.JPG

This is the top-side of the PCB showing main components and Nyquist filtering

SAM 6155.JPG

This is the bottom side of the PCB with dip switches to chance SR-filter settings. The two multi-turn trimmer potentiometers are used to get the proper constellation set-point.

Nyquist filtering.

For Nyquist filter calculations the free software Elsie from Tonne software was used.

On the set-up DATV board I use two filters (I and Q) for the symbol-rate. 250 KS and 2000KS (2083 works also OK)

We are working on a user selectable wide-band I/Q SR-filter, but on this 2015 DATV set-up PCB the present Nyquist filter works OK for this moment.

Tests have been made using additional LC filtering but we kept to the ‘old’ Digilite idea, not ‘state of the art’ but OK for us HAM’s !

Nyquist filter tonne software.jpg

At the -3 dB point of this setup you see a 250 KS filter curve

The used filter has 100 Ohm input and output impedance.

In the previous (test) PCB no additional buffer OP-AMP was used.

It was more difficult to get a proper setpoint for the DATV constellation.

In the present setup of the DATV PCB a proper fast and low noise op-amp is used for both I and Q channels.

Present setup schematic diagram with 250 & 2000 Ks Symbol-rate filtering:

Schematic diagram present DATV setup PCB.jpg

Circuit description:

Connections are made to the Raspberry-Pi GPIO bus on the left. Internally the Pi works at 3,3 Volt.

Input buffering and shifting to 5 Volt is made so a proper Nyquist filtering could be made with 100 Ohm input and output impedance.

After the I and Q Nyquist filtering two fast low-noise pre-amps were used as buffer to give the I and Q differential op-amps their proper input levels.

Finally differential I and Q signals are connected to the IQ modulator chip.

For 70 and 23 cm we use the AD8345. For 23 & 13 cm the AD8349 is used.

The RF output is connected to a wide-band RF-amp.

On 70 cm we have an output level of >100 mW,    23 cm ~50-70 mW.

An RF input a level of -6dBm is used. Both top and bottom side of the PCB are used. The PCB is powered from 12 Volt only.

DATV picture.jpg

A nice MER in the test setup.

There is a protective diode against polarity mistake. Furthermore the negative voltage for op-amps comes from a DC/DC converter on the PCB..

Thanks to Tutioune and UglyDATV we now have a great working DATV system.

Tests in the Netherlands show that distances of over 10 km. is possible with < 20 mW. Furthermore tests were made on 437 MHz with a Symbol-rate of 250.

After a initial higher RF power level the level could be changed up to 4 mW. We are talking of a distance of 15 kilometer with a path over a large town.

Both stations used yagi antenna’s with proper coaxial cabling and good antenna height.

Recent tests (Ham-Radio 2016) between the approx. 1000 meter Pfänder (Bregenz) and the 830 meter high mountain Am Höchsten (Bodensee) (54 km link) shows that less than 100 mW power with nice DATV results was OK.

Also links to nearby ATV/DATV repeater(s) in the Netherlands are working fine.

The DATV picture remains until I go lower than 1,5 mW @ PI6ZDM or < 2,5 mW @ PI6HLM. Both tests at 23 cm. Distances of approx. 10 and 4 kilometers.

These results after a proper connection is made and power level is lowered for that test.

The present DATV setup PCB to be connected with a RaspberryPi using Evariste’s UglyDATV:

Top DATV pcb.jpg
the top of the present pcb.

Bottom DATV PCB.jpg
the bottom of the present pcb.

And the overview of the combination in my combined 70 & 23 cm DATV transmitter system.

DATV PCB in system.jpg

Here a RaspberryPi connected with the DATV setup PCB.

Local oscillator:

Local oscillator input: the approx. -6 dBm RF level is connected into a wide band balun on the PCB.

In 2014 a nice design was made for a nice working local oscillator. (see under un text the additional information)

Joop PE2JKO made a nice controller and software with the following functionality:

Selection of band 144, 432, 1260 and 2300 MHz

Selectable raster and power level. This is made using the ADF4351 35 – 4400 MHz PLL chip.

Furthermore two 1>4 rf switches are used to have multiple RF outputs. Joop’s controller connects to this PLL oscillator PCB.

But now we are some years later, on E-bay or AliExpress you can buy ready-made PLL oscillators on a PCB round 35 $. [[1]]

You need to make your own controller, or you can buy a PLL oscillator in connection with an LCD display /control PCB and make even that a simple combination.

(by pushing the touch-screen you can set your desired frequency)

Hard to compete. But another proper RF signal could also be used!

Local Oscillator.jpg

Here the local oscillator pcb. See also the detailed info on this site with info from Joop PE2JKO.

Visit the PG540 site with useful info about Joop’s approach of DATV interfacing.[2]

And more additional info about past local oscillator and future idea’s.[3]

(at present only in Dutch language, but we will fix that in a later stage !)

Frequency filtering:

It is essential to use a proper band-pass filter behind this DATV- PCB. When you use some extra amplification filtering can be placed behind the final AMP. But a good idea to have also some extra filtering before your RF-PA.

Starting your own DATV station:

Visit VivaDATV.org [4]

Subscribe yourself (for free) so you can download the UglyDATV program made by Evariste F5OEO.

How to get the above described DATV setup-PCB : You can order a ready-made PCB, tested, aligned and with an individual test-report by me.

UPDATE spring 2019

Since BATC’s Portsdowm are the I and Q outputs changed on the RaspberryPI connector.

I changed my PCB, and working again!!

73’s Robert