27 MHz Amplification Query

[betwixt]

Actually, none of them are regenerative receivers! - they are SUPERregerative receivers. Similar circuit but a different way of operating.

The first link dates back to the 1960s and uses vacuum tubes and germanium transistors (silicon transistors were hard to find back then!). The other links are more 'recent' but seem to be based on the parts available at the time rather than being designed for repeatability. The description of how they operate is misleading at best, a superegerative receiver is a an oscillator running at two frequencies, one the signal frequency and the other the 'squegging' frequency which is usually just above audio range. You will note that on all of them the antenna is connected directly to the tuned circuit which has two drawbacks, first is it radiates some of the oscillation and therefore causes inteference nearby, second is the proximity of the antenna to other things nearby causes a tuning shift.

The base capacitor serves two purposes, it has to be big enough to ground the base to RF and it also has to set the time constant for the squegging. I suspect in the designs with electrolytic base capacitors are quite unstable at RF!

Brian.

 

[RMMK]

I am a bit confused with the pi-network..... I couldnt find variable caps for this large value in nanofarads.... only found trimmers at certain pico's.... The pi-network calculation require the output impedance of the amplifier to match it with my 50-ohm impedance at the end.....
I still havent been able to clearly know how to find analytically or numerically or some other way (even practically because now i have a working circuit with me) the output impedance for a tuned amplifier???
Plz help me in the design of the pi-network.....

 

[davenn]

I am a bit confused with the pi-network..... I couldnt find variable caps for this large value in nanofarads.... only found trimmers at certain pico's.... The pi-network calculation require the output impedance of the amplifier to match it with my 50-ohm impedance at the end.....

The pi-network is just a 27MHz low pass filter. exactness isn't critical you shouldn't need trimmers in that stage

 

[chuckey]

"The pi-network is just a 27MHz low pass filter", no its not, its a resonant circuit, it is tuned to 27 MHZ and the ratios of the capacitors step/down the aerial impedance to match the output impedance of the PA transistor.
Frank

 

[davenn]

it will have that effect, but its still just a standard LPF that and decent person would put on the output of a transmitter

 

[RMMK]

the thing is that i am getting a peak of 27MHz at output (collector of last transistor) without the pi-network but after the pi-network gets connected to circuit and when i place the probe of on the output of this pi-network is at 6MHz...... why is it so?? is it really that the 1.2nF is wrongly selected by me??
Also using the rf probe i get a read of 15.69Volts without pi-network with the probe placed on the collector capacitor of the last stage. But when i connect the pi-network and place the probe on the output of the pi-network it reads some 0.4Volts.......
whats happening? plz explain!

I am also in the process of building the receiver and will come with the results as soon as possible!

 

[chuckey]

As I said, measure the CURRENT the kit is taking from its power supply. As the circuits are tuned correctly, the current will at its maximum. Ensure that you can go through a current peak with every tuned circuit. You should be able to get a low wattage bulb to glow from this transmitter. ( 2 V .1 A)
Frank

 

[RMMK]

the transmitter circuit is consuming some 0.25Amps at 12Volts.......

 

[RMMK]

As I said, measure the CURRENT the kit is taking from its power supply. As the circuits are tuned correctly, the current will at its maximum. Ensure that you can go through a current peak with every tuned circuit. You should be able to get a low wattage bulb to glow from this transmitter. ( 2 V .1 A)
Frank

I measured a consumption of around 3Watts (0.25Amps @ 12Volts) by the transmitter. the led lightened up quite brightly when i placed it across the pi-network output......

 

[betwixt]

DO NOT USE A LED!!! use a flashlight bulb instead. A LED has a high capacitance of it own and may be damaged by the reverse voltage from the AC signal. There is also a risk, albeit a small one, that the LED will unbalance the AC by drawing current on half-cycles and leave residual DC at the output.

Brian.

 

[RMMK]

okay i used the flashlight bulb and it lit up quite nicely.... I will build the receiver in a week or so and share the results and probably would ask for help as well :twisted:; the delay is because of my in-session exams.....

 

[RMMK]

I made the below layout for the receiver..... do you think its okay?

 

[betwixt]

I would think it highly unlikely that will work properly but over the last 232 messages I've lost the one with the schematic!

For all RF work you need to keep the wiring as short as possible and ideally use a ground plane as well. Try to keep components as close to the transistors as possible so the track lengths are minimized. Remember that at RF, a copper track looks just like an inductor so your present layout effectively has lots of extra coils in series with the components. A ground plane is easy to use if you have a double sided board, just keep one side (the ground) covering the whole side and solder ground connections on top and bottom sides. If you have to use single sided board, "flood fill" it with ground areas, basically extend ground connections so they fill the gaps between other tracks.

Brian.

 

[chuckey]

Was that track diagram computer generated? Its not like any thing I have ever seen before. I would get a print out of it (as large as possible), then trace the layout out on to tracing paper with a pencil. When you have a physical piece of paper you then go around and sort the layout out.
Take for instance the area around R1 and C5 there are clearly extra long leads, if C5 was moved slightly, a much better layout would result.
A better way to start would be, to draw out the circuit and replace each component with its actual physical size. Then just squeeze the components together and get cross overs in the track to run under components. At least the finished PCB would have a roughly logical signal flow fro left to right.
Frank

 

[RMMK]

yeah it was computer generated....