Golborne Vintage Radio

Hello forum

I'm new to RF and currently working on trying to design and build my first receiver.
I'm aiming for TRF 118.5MHz AM airband, to listen to the planes coming in over the park

This is not my first attempt, my latest design suffers from self-oscillation. The oscillations are in fact the tank-frequency resonance frequency..

I've uploaded my front-end design...
The IDSS of the MPF102 FET was calculated to be 10mA, so I set the Id current to 5mA, thinking it is good practice to be near the middle... the output impedance is 135 ohm so I designed my transistor stage to have that inpout impedance. I added the 3uH RFC because the output impedance is quite close to my 150 ohm resistor... so I thought I needed to rise the impedance at RF.

Below that is the circuit I'm getting inspiration from. The same transistor (with unknown VGoff and IDSS) , but assuming I built the circuit with my MPF102 the Id current would only be 0.7mA, and the output impedance 538 ohm... this doesn't match the input impedance of the SA602 mixer which is 1.5K

now my questions:
1. when designing a common-drain for an RF front end, what parameters determine the desired drain current??
does a higher current have more risk of self-oscillation?
are we attempting to be near the middle of the range?
are we trying to achieve a specific output impedance?

 2. how to kill self-oscillation?
I tried adding capacitors to the power supply, as close to D as possible.
Tried shielding, tried replacing the inductors with toroids, even tried shielding the FET itself.
on one post I read that a 'limiting resistor'  could be placed between the tank circuit and the G, the value as a rule of thum should be between a few hundred ohm and a k. Ideally I'd like to understand if this is the solution .. WHY ? and how to optimize this resistor value, i.e. what am I trying to achieve with this value... my LC tank has a 87k (calculated) equivalent parallel R. I thought I should form a voltage divider for the signal coming back from the gate, so I selected this to be 10 times my parallel R... at 800k I thought relative to the gate (open circuit) this should not attenuate the signal at all, but signals coming back would be 10 times smaller by the time they get back to the tank... (?)
in practice the oscillations stopped, but I had no sensitivity at all.

my next idea is to put a 1M pot (dotted lines) to feed back some inverted signal, (need to add an R to the D), this negative feedback should kill oscillations and lower the gain a little...

is this the correct approach?

I've been experimenting for a long time (over three years) and have not managed a working radio yet.... I think just experimenting trying things out, is a dead end and is taking the joy out of what should be a hobby..

if anyone can please offer some advice, I am putting in the effort, that I guarantee, but sometimes it's just a matter of being told and guided and having someone to talk to.

Thanks for your help
Ryan

(06-10-2018, 11:11 AM)BusyBee Wrote: [ -> ]I was at the point of replying when I noticed where the post was as I couldn't readily repond.  In looking at the content (without seeingor needing to see the diagram) I felt there were a number of basic questions that could be asked of someone apparently without great experience.

Tracy

Tracy,
I attached the project data. He made the one on the board first and then the one on the coffee can lid. The second does function but breaks into oscillation when tuned across the band. He has put in the time to learn the math as you can see. Needs help with the physical layout ( I think)?
Greg
[attachment=18062]
[attachment=18063]
[attachment=18064]
[attachment=18065]

Hello Invercargill,

https://golbornevintageradio.co.uk/forum...p?tid=7116

Welcome to the forum. The vero layout is unlikely to work predictably above a few MHz. The use of a good ground plane as in the can lid is the way to go. It's not clear from the schematic or the photo if there is any supply rail decoupling. If not I would suggest a ceramic 0.01uF with short leads between rail and ground and possibly a larger electrolytic in parallel with it.

HTH

Peter

Apart from Peter's comment about the use of Veroboard, I'm fascinated bu the way it has been used 'upside down' with the components on the printed wiring side instead of on the other side of the board!

I am confused, the person making this project seems to have reset his forum name and transported from New Zealand to the USA, and changed his name to Greg.

Mike

(06-10-2018, 06:08 PM)Crackle Wrote: [ -> ]I am confused, the person making this project seems to have reset his forum name and transported from New Zealand to the USA, and changed his name to Greg.

Mike

Sorry, my bad. I was trying to get the data posted for your review. I referred Ryan here because I know you can help him.
Ryan the builder.
Greg the referrer.
When his attachment didn't show I posted what I had.
Now that I can see his attachment it seems I did not post the right one.
I will be watching and learning.
Greg

Hi
Sorry Greg, I was probably being a bit dim at the time, after reading it again later I sort of realised..
I dont think I can help though, design is not my thing. I will just be following it, so long as it does not get too technical.


Mike

If I wrestle with RF in these modern times the starting point would be a plane sheet of copper clad.
I would first cut a strip off one edge and put it to one side and with the main piece cut the gaps between the intended tracks with a small model drill or even a knife. I would leave as much copper at practical as a ground plane.
Then I would fit surface mount components for the gain stage.
Then I would cut a notch in the edge of the strip I cut off for the transistor and solder it to form a barrier to stop feedback from the input to output.
I would never use strip board because the strips would just form a ladder of capacitors like in a phase shift oscillator.
Using diode tuning will keep the amount of RF components physically small and make the design more predictable.

Hi.
I think refugee has nailed it with his method. One other thing to add is the use of ferrite beads, they do help massively with spurious oscillations.
Again forget veroboard above a couple of MHz.

Hi Trevor, I hope you make sure that those beads really are made of ferrite!

https://www.radios-tv.co.uk/community/bl...post-18951