Need more info on a simple UHF oscillator

[neazoi]

Hello I have found this **broken link removed**
I have also managed to find an altered version of it from an old magazine which I attach.

Does anyone have more info on the original circuit? Where it has been published, or any construction details?

Also what if someone wants to video modulate it, where should the video signal be fed?

 

[jiripolivka]

Hello I have found this **broken link removed**
I have also managed to find an altered version of it from an old magazine which I attach.

Does anyone have more info on the original circuit? Where it has been published, or any construction details?

Also what if someone wants to video modulate it, where should the video signal be fed?

This type of UHF oscillator mostly depends on transistor parameters. I built many similar oscillator with various UHF as well as audio transistors (like 2N2222). They oscillate after a careful adjustment of collector current and sometimes need a feedback. A 1 or 2 pF capacitor between e and c is good; I preferred to use a coaxial resonator and an emitter loop feedback, one turn coupled to the coax. I used such oscillators in UHF TV converters covering 470-900 MHz as well as below, from ~50 to 500 MHz, with various LC and coaxial resonators.

I disagree with the statement that the output power is 50-100 mW. A typical oscillator like this generates < 10 mW. Best to use is 5-12 VDC, typical current is ~ 5-10 mA.

Using a stripline loop gives a poor frequency stability. You can use one-two turn coil made of 1 mm dia. copper wire, and tune it by the trimmer where you need the frequency. The easiest way to see if such oscillator oscillates is to use an analog milliammeter, 0-10 or -20 mA to monitor collector current. Then vary the base bias (instead of two resistors use a potentiometer like 10 k) and observe the current. Putting your finger on the collector forces the current up or down if the thing oscillates. Try to prevent too much current, many transistors do die fast.
Use the shortest connections around the transistor (5 mm can be too much) and try your skill in experiments. No simulation can replace the hands-on experience.
Get a good frequency meter tunable over your frequencies of interest. Or make one from an air trimmer and a coaxial resonator , with a detector coupled to it, feeding a microammeter. Calibrate it by a signal generator.
To modulate the oscillator by a video signal, base is usually the best point. The amplitude must be adjusted carefully, to achieve a good AM. If you wanr FM, the best way is to use a varicap diode connected via a 1-2 pF to the collector. Again adjust the signal amplitude for the varicap while monitoring the signal by a receiver with a TV monitor.

 

[neazoi]

This type of UHF oscillator mostly depends on transistor parameters. I built many similar oscillator with various UHF as well as audio transistors (like 2N2222). They oscillate after a careful adjustment of collector current and sometimes need a feedback. A 1 or 2 pF capacitor between e and c is good; I preferred to use a coaxial resonator and an emitter loop feedback, one turn coupled to the coax. I used such oscillators in UHF TV converters covering 470-900 MHz as well as below, from ~50 to 500 MHz, with various LC and coaxial resonators.

I disagree with the statement that the output power is 50-100 mW. A typical oscillator like this generates < 10 mW. Best to use is 5-12 VDC, typical current is ~ 5-10 mA.

Using a stripline loop gives a poor frequency stability. You can use one-two turn coil made of 1 mm dia. copper wire, and tune it by the trimmer where you need the frequency. The easiest way to see if such oscillator oscillates is to use an analog milliammeter, 0-10 or -20 mA to monitor collector current. Then vary the base bias (instead of two resistors use a potentiometer like 10 k) and observe the current. Putting your finger on the collector forces the current up or down if the thing oscillates. Try to prevent too much current, many transistors do die fast.
Use the shortest connections around the transistor (5 mm can be too much) and try your skill in experiments. No simulation can replace the hands-on experience.
Get a good frequency meter tunable over your frequencies of interest. Or make one from an air trimmer and a coaxial resonator , with a detector coupled to it, feeding a microammeter. Calibrate it by a signal generator.
To modulate the oscillator by a video signal, base is usually the best point. The amplitude must be adjusted carefully, to achieve a good AM. If you wanr FM, the best way is to use a varicap diode connected via a 1-2 pF to the collector. Again adjust the signal amplitude for the varicap while monitoring the signal by a receiver with a TV monitor.

Thank you for the information!
Hm.. it seems quite complex, maybe I should better build another copy of the elektor harmonics modulator. I have build it at the past and it is not critical at all... https://petlibrary.tripod.com/rfmod.htm I just liked the stripline because it would be a coilless modulator..

 

[Abhishekabs]

According to my knowledge the PCB trace which they are calling inductor is an antenna.
This looks like some sort of short range transmitter device to me.

I will not disagree to statement of 55 to 100mW of power
it is only 50 Ohm power he is talking about which he measured on SA.

Actual transmitting power may be say 10~ 20 mW.


If you do not mind I would like to ask you what are you looking for?
This oscillator shown is very power consuming & will empty your battery like anything.

What do you want to build?

 

[neazoi]

According to my knowledge the PCB trace which they are calling inductor is an antenna.
This looks like some sort of short range transmitter device to me.

I will not disagree to statement of 55 to 100mW of power
it is only 50 Ohm power he is talking about which he measured on SA.

Actual transmitting power may be say 10~ 20 mW.


If you do not mind I would like to ask you what are you looking for?
This oscillator shown is very power consuming & will empty your battery like anything.

What do you want to build?

20-30mW is still very good output power for me.
I am trying to embed this PCB on a vintage computer, that has composite video output, to modulate it with the video signal (fed probably on the base), to make a uhf coil-less modulator.

it is for one of these projects
**broken link removed**
**broken link removed**

 

[neazoi]

Ok, sorry to open the topic again but I have just finished the construction of it.
I have not used the varicap portion, I have made it just LC tuned by connecting the capacitor to the ground on one end.
It works fine, frequency stability is fine and I do not have used the optional feedback capacitors. Consumption is about 10mA at 12v.
Indeed when placing the internal antenna of my portable tv 10-20cm away, the unmodulated signal is crystal clear on the screen.
I am satisfied, comparing the simplicity of it. No coil to build, no cable to connect to tv if tv is near, simple circuit.

Nevertheless, when I apply a video signal on the transistor base as well as the r8-r11 point, video comes highly distorted. A potentiometer in voltage divider configuration for the video stops the flickering of the display but the video is very blur.

Do you think I need a video preamplifier or is it an oscillator problem?

 

[jiripolivka]

Ok, sorry to open the topic again but I have just finished the construction of it.
I have not used the varicap portion, I have made it just LC tuned by connecting the capacitor to the ground on one end.
It works fine, frequency stability is fine and I do not have used the optional feedback capacitors. Consumption is about 10mA at 12v.
Indeed when placing the internal antenna of my portable tv 10-20cm away, the unmodulated signal is crystal clear on the screen.
I am satisfied, comparing the simplicity of it. No coil to build, no cable to connect to tv if tv is near, simple circuit.

Nevertheless, when I apply a video signal on the transistor base as well as the r8-r11 point, video comes highly distorted. A potentiometer in voltage divider configuration for the video stops the flickering of the display but the video is very blur.

Do you think I need a video preamplifier or is it an oscillator problem?

The problem is in your SYSTEM CONCEPT.

A TV receiver you use for the signal generated by your UHF oscillator needs a VSB-AM signal. You
should modulate the signal amplitude and filter out one of the sidebands.
By applying the video signal in transistor base bias circuit, the signal modulation is a mix of AM and FM due to the use of UHF. Commercial UHF and 2.45 GHz TV transmitters and receivers use FM.
To improve your signal modulation, try the following:
1. Use a very low TV signal amplitude, < 0.1V p-p.
2. Use a "fine" potentiometer to adjust the base bias voltage for a "linear" response.
3. Monitor transistor current while adjusting base bias. An optimum DC collector current may be within 3-10 mA. Do not exceed ~15 mA, you may kill the transistor.
4. Make sure another person can fine-tune the TV receiver during adjustments. Base bias does pull the oscillator frequency.