amplifying RF modulator output

[mdanh2002]

Hi,

I salvaged an RF modulator module from an old VCR. The modulator is outputting PAL on Channel 3/4 VHF (selected via a switch) and the video/audio can be viewed clearly on my TV. However, I wish to amplify the output to make a short-range TV transmitter. Here is what I have tried.

1. Feeding the output of the RF modulator into an antenna. With no obstacles between the TV antenna and the RF modulator antenna, the best range I can get is 2-3 meters before reception is lost.

2. Feeding the output of the RF modulator to the last stage (Q3 transistor, RF amplifier stage) of audioguru FM transmitter (Mod 4, 1 Mar 2005 version), and then use a 30cm antenna for the output. To my big disappointment, the TV can't receive anything at all even if the circuit and the TV are very close (worse then just feeding the output to an antenna directly). I successfully built audioguru FM transmitter before and achieved quite a good range with it. As I understand transitor Q2 in Audio Guru circuit is for the oscillator and Q3 is for the RF amplifier. When I built it the circuit worked fine with just Q2 but at a much shorter range. With Q3 the range is extended. So should it achieve the same effect in my approach?

3. Feeding the RF modulator output to this simple single transistor amplifier circuit, https://www.instructables.com/id/Re-Transmitter-1/. The author stated he successfully amplified the low power output of his car FM transmitter and achieved a range of 3 metres. Some very grainy images can be seen with this approach but otherwise, it's not working.

Can anyone advise me what is wrong here? As I understand Channel 3/4 VHF frequency is just around FM frequency, so why can't the approach work? Is it to do with impedance matching, or should I use a 2N3866 and try again?

Thanks in advance.

 

[JimB]

1. Feeding the output of the RF modulator into an antenna. With no obstacles between the TV antenna and the RF modulator antenna, the best range I can get is 2-3 meters before reception is lost.

The output of the modulator is probably only a few milli-volts.
The modulator output is normally connected directly to the input of a tv receiver, so there is no need for a big signal, there is usually an attenuator on the output of the modulator.

Feeding the output of the RF modulator to the last stage (Q3 transistor, RF amplifier stage) of audioguru FM transmitter (Mod 4, 1 Mar 2005 version), and then use a 30cm antenna for the output. To my big disappointment, the TV can't receive anything at all even if the circuit and the TV are very close (worse then just feeding the output to an antenna directly).

The low output of the modulator is probably insufficient to drive the RF amp of the toy Tx.
In this state the amp will give even greater attenuation to the signal from the modulator.
Did you re tune the Tx output stage from Band2 (88 to 108Mhz) to Band 1 (50 to 70Mhz (ish))?

Feeding the RF modulator output to this simple single transistor amplifier circuit, **broken link removed**.

Good Grief! That circuit is a joke, how does "Instructables" get away with publishing such rubbish?
Why is it so bad?
It is untuned, at best has a gain of 1, the 10nF capacitor gives it a gain of about zero at any frequency above DC.

As I understand Channel 3/4 VHF frequency is just around FM frequency,

No it is not.
The old UK Band1 channel 3 frequencies were 56.75Mhz (vision) and 53.25Mhz (sound), channel 4 was 61.75 and 58.25Mhz. In the UK, television on these frequencies was abandoned about 30 years ago.

so why can't the approach work?

Apart from everything which I have said already, the vision carrier is ampltude modulated, a non linear amplifier will at best distort the signal and maybe remove the modulation altogether due to limiting action.

JimB

 

[mdanh2002]

Hi JimB,

Thanks very much for the detailed fast reply!

Yes, I did try to retune the Tx output stage by adjusting the trimmer capacitor. Try various other things such as adding/removing turns on the inductor to change the peak RF amplifier frequency but nothing changes,

I measured the output of this modulator on a 150MHz oscilloscope - around 8mV at 62MHz - approximately the frequency for channel 4 VHF here (61.75MHz) in Singapore (still using analog television) like what you mention. I also disassembled the modulator trying to locate the attenuation resistors - some sites say I can increase the output power by removing/reducing these resistors. Couldn't locate anything that is feasible to change - many components on the board are surface-mounted except for some air core inductors. I also tried probing around the points in the PCB too to hopefully locate the signal before being attenuated but could not find any.

For the Instructables circuit - it looks a bit simple to me too but I tried it nevertheless. As I could probably have realized, it didn't work.

What do you think about this circuit?

**broken link removed**

Is it going to work? I understand from your post that a non-linear amplifier will not work well because the video signal is AM (while the audio is FM) but there's no mention in the post whether the amplifier is going to work with a TV signal. I am going to try this

Many thanks. And I hope you don't mind my "beginner" questions - I am an electronics hobbyist who loves building transmitter (among other things) and likes to learn by trial and error.

 

[JimB]

What do you think about this circuit?
https://www.eeweb.com/blog/circuit_projects/wide-band-rf-amplifier-with-input-range-10mhz-500mhz
Is it going to work?

That would probably be OK, but to make things very easy have a look here:
**broken link removed**
And the look at the MAR1+ here:
**broken link removed**
Wide range RF amplifiers dont get much simpler than that!
That will give you good gain for not much complexity.
The output power will not be much, about 2mW max, but it will get you going.

JimB

 

[mdanh2002]

That would probably be OK, but to make things very easy have a look here:
**broken link removed**
And the look at the MAR1+ here:
**broken link removed**
Wide range RF amplifiers dont get much simpler than that!
That will give you good gain for not much complexity.
The output power will not be much, about 2mW max, but it will get you going.

JimB

Thanks for the very fast reply! Now let me see if I can get a few samples of the MAR-01+ from minicircuits.com to play around with...

While waiting for that I will attempt the circuit I mentioned above from eeweb. Currently do not have the exact transistor but I do have the BF199, 2SC3355 and the J310 RF transistor. Hope it'll be able to give me something.