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Old 13th Jul 2020, 11:34 am   #1
German Dalek
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Question How to build a RF-converter?

Hi,

I have to build a RF converter for the old belgian standard!
It has to be constructed without SMD junk.

Thats given:
From Scart to antenna-in, channel "Brussels" video: 210,25 MHz,
sound: 215,75 MHz (5,5MHz), AM sound and positive video
modulation.

Any ideas?
Many thanks in advance!

German Dalek
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Old 13th Jul 2020, 1:07 pm   #2
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Default Re: How to build a RF-converter?

Two crystal oscillators, each with a multiplier chain, one arriving at 210.25MHz, the other at 215.75MHz So your crystals need to be at a non-prime integer submultiple of those frequencies.

US amateurs have VHF band near there so you can nick designs for multiplier chains from period ARRL handbooks etc.

Two diode-ring double balanced mixers EG from Minicircuits. Oscillator chain into the LO port, audio plus some DC into the IF port, RF output comes from the RF port.

Second mixer does the video. Video plus some DC into IF port, video carrier osc chain into LO port and modulated RF comes out of the RF port.

You'll need a basic bandpass filter on the output and a resistive combiner.

The DC levels added to the baseband video set the carrier level.

You'll need about 10mW to drive the mixer LO ports.

Yes, I know the DC offset for AM uses a double balanced mixer as a single balanced mixer, but DBMs are off the shelf or on the amateur surplus market.

David
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Old 13th Jul 2020, 2:58 pm   #3
German Dalek
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Default Re: How to build a RF-converter?

Hi David!

That sounds great!
Please add now the schematic (for a better understanding)

Thanks in advance!

Best regards,
German Dalek
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Old 13th Jul 2020, 3:31 pm   #4
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Default Re: How to build a RF-converter?

I'm looking for a new job (Got a redundancy letter a week ago.... yes really!)

I'll provide advice and steering if you want, but not the full design job. If I did that, you wouldn't learn as much.

How you approach this sort of job depends partially on what parts you have available, partially on what parts turn up when you start looking and partially on how you feel like doing it. Valves, discrete transistors and ICs are all possibilities here at the beginning.

Crystals may be difficult, but then if you did a frequency synthesiser it would need programming at switch on.

Have a look at the amateur radio stuff for their 220MHz band and see what you think.

Can you source crystals.


Right at the beginning of a design project you have more freedom to make choices than you will ever have again in that project... sometimes too much freedom!

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Old 13th Jul 2020, 3:40 pm   #5
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Default Re: How to build a RF-converter?

Off topic, I know, but just wanted to say sorry about the redundancy. I've had that t shirt twice, once at 37 and again at 48. Good luck.

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Old 13th Jul 2020, 4:56 pm   #6
G6ONEDave
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Default Re: How to build a RF-converter?

If memory serves, I think that Pye/Labgear produced a pattern generator that had VHF output many years ago. Might be worth looking at what they used in their modulator.
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Old 14th Jul 2020, 6:31 am   #7
German Dalek
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Default Re: How to build a RF-converter?

I just find that, a converter for the french 819 lines television.
What I have to change is the frequency of the set.
However it is not far away from the Brussels frequency.

The big problem in that case is, that only a few belgian TV sets
survive, which cannot operate with CCIR.
That means that there is no deaper interest to construct something
like that.
For 405 lines televison is a real big market to bring them back to life.

You can call these few belgian survivors "lemons" because their builders
didn`t add the possibility to receive dutch and german television.
They were build as 2 standard (belgian 625 & 819 lines) or 3 standard
(incl. french 819 lines) sets, were of course cheaper and minor
complicated in their design (3 times AM-sound).

http://819lignes.free.fr/Realisation...r_VHF_F8A.html

Best regards,
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Old 14th Jul 2020, 6:56 am   #8
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Default Re: How to build a RF-converter?

David, best of luck mate, Been there a couple of times at least, maybe looking at again shortly as well if rumours for my position currently are correct, have to say though, what a fantastic skill set and knowledge David has,
Regards, Alan
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Old 14th Jul 2020, 8:18 am   #9
peter_scott
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Default Re: How to build a RF-converter?

Quote:
Originally Posted by Aub View Post
Off topic, I know, but just wanted to say sorry about the redundancy. Good luck.

Aub
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Old 15th Jul 2020, 11:56 am   #10
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Default Re: How to build a RF-converter?

Quote:
Originally Posted by Radio Wrangler View Post
How you approach this sort of job depends partially on what parts you have available, partially on what parts turn up when you start looking and partially on how you feel like doing it. Valves, discrete transistors and ICs are all possibilities here at the beginning.

Crystals may be difficult, but then if you did a frequency synthesiser it would need programming at switch on.

Have a look at the amateur radio stuff for their 220MHz band and see what you think.

Can you source crystals.


Right at the beginning of a design project you have more freedom to make choices than you will ever have again in that project... sometimes too much freedom!

David
There are a number of DDS synthesisers which will power up in an already programmed condition, and in fact you could generate the required carriers directly. Unfortunately they are all "SMD Junk"....
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