Tuesday, March 30, 2010
Not so super-regenerative
The success of my project to make an FM version of G3XBM's Fredbox was always going to depend on the receiver. The Fredbox receiver is a simple three transistor super-regenerative design. This is the only practical way to make a receiver small enough to fit in a hand-held case. It is also the only cost-effective way to do it, because making a conventional double-conversion receiver with crystal control just wouldn't be worth it for such a project.
What I didn't know were a) would the super-regenerative receiver work with FM signals (the original Fredbox was designed for AM) and b) would it be sufficiently stable to stay on frequency without the benefit of crystal control? However, what I didn't give too much thought to was the possibility that I wouldn't be able to get it to work at all.
Because this is a VHF circuit I didn't think I could try it out on the breadboard so I made it up directly on to Veroboard. The layout is pretty similar to Roger's original layout from what I can tell from the pictures on his website, although I think he made an etched circuit board, something I'm not able to do.
I drew up the layout using a bitmap editor. This shows the components from the top view. The large outline components are the Toko S18 inductors, and the red lines are wire links below the board connecting the ground traces. The schematic for the receiver part of the Fredbox is shown below. As with the Nano-40 I used an MPS13 in the audio stage in the hope of getting a bit more audio gain.
I applied power and expected to hear a hissing sound in the earpiece which stops when a signal is tuned in. However I heard nothing at all. The audio stage appears to be working, as I can hear a faint buzz if I touch something metal to the input.
Alan, VK2ZAY had a lot of trouble getting the receiver to work as well, and he is an experienced constructor who knows what he is doing. I suppose this should have warned me off trying this project. Alan replaced the axial choke for L3 with half a dozen turns of wire on a ferrite bead, and I did the same, but to no avail. I also substituted a variable trimpot for R2 which in many other super-regenerative receiver designs is a variable regeneration control. But no matter what the setting of the pot I could not get any regeneration to occur at all.
I think my FM Fredbox is about to become yet another abandoned project. :(
What I didn't know were a) would the super-regenerative receiver work with FM signals (the original Fredbox was designed for AM) and b) would it be sufficiently stable to stay on frequency without the benefit of crystal control? However, what I didn't give too much thought to was the possibility that I wouldn't be able to get it to work at all.
Because this is a VHF circuit I didn't think I could try it out on the breadboard so I made it up directly on to Veroboard. The layout is pretty similar to Roger's original layout from what I can tell from the pictures on his website, although I think he made an etched circuit board, something I'm not able to do.
I drew up the layout using a bitmap editor. This shows the components from the top view. The large outline components are the Toko S18 inductors, and the red lines are wire links below the board connecting the ground traces. The schematic for the receiver part of the Fredbox is shown below. As with the Nano-40 I used an MPS13 in the audio stage in the hope of getting a bit more audio gain.
I applied power and expected to hear a hissing sound in the earpiece which stops when a signal is tuned in. However I heard nothing at all. The audio stage appears to be working, as I can hear a faint buzz if I touch something metal to the input.Alan, VK2ZAY had a lot of trouble getting the receiver to work as well, and he is an experienced constructor who knows what he is doing. I suppose this should have warned me off trying this project. Alan replaced the axial choke for L3 with half a dozen turns of wire on a ferrite bead, and I did the same, but to no avail. I also substituted a variable trimpot for R2 which in many other super-regenerative receiver designs is a variable regeneration control. But no matter what the setting of the pot I could not get any regeneration to occur at all.
I think my FM Fredbox is about to become yet another abandoned project. :(
Labels: Construction, VHF
Comments:
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Julian
Could the problem be the FET. This type of FET varies enormously in characteristics from one to another. I went though a period where I almost stopped using FET's for years because of the huge variation in performance. My suggestion is that before you give up totally try some other FET's for the main transistor.
Good luck I know how you can feel after putting in quite a bit of work and then not finding why something does not work.
73's Mark VK6WV
Could the problem be the FET. This type of FET varies enormously in characteristics from one to another. I went though a period where I almost stopped using FET's for years because of the huge variation in performance. My suggestion is that before you give up totally try some other FET's for the main transistor.
Good luck I know how you can feel after putting in quite a bit of work and then not finding why something does not work.
73's Mark VK6WV
Thanks for the suggestion, Mark. I do have several more BF245 and I have some alternatives like 2N3819, J310 that I could try. I don't unfortunately have a FET tester that would allow me to look at their characteristics.
It is a really simple circuit and it is hard to see what could be wrong. Normally things oscillate when you don't want them to...
It is a really simple circuit and it is hard to see what could be wrong. Normally things oscillate when you don't want them to...
Hi Julian
Generally speaking tracked Veroboard, whilst fine for audio and digital applications, causes problems in RF projects due to stray capacitance between traces. I would expect this effect to be particularly acute at VHF where even very small stray capacitances can alter circuit (non)performance significantly.
Before you consign it to the 'failed projects bin', why not try building it 'ugly style' over a groundplane?
Good Luck
73 de Richard F5VJD
Generally speaking tracked Veroboard, whilst fine for audio and digital applications, causes problems in RF projects due to stray capacitance between traces. I would expect this effect to be particularly acute at VHF where even very small stray capacitances can alter circuit (non)performance significantly.
Before you consign it to the 'failed projects bin', why not try building it 'ugly style' over a groundplane?
Good Luck
73 de Richard F5VJD
Julian - since posting my last comment I have looked at the article on VK2ZAY's site and I noted with interest that he cites specifically having a number of problems with stray capacitances. This reinforces my view that this is where your problem might lie - that and getting the coupling right between L1 & L2, which also seems to be a critical area.
If it was easy, it wouldn't be any fun....!!
73 de Richard
If it was easy, it wouldn't be any fun....!!
73 de Richard
I've rebuilt it now on a piece of plain copper board as a ground plane, minus the RF stage (to rule out over-coupling as an issue) and it still won't regenerate. Apart from trying another FET in case the one I used is no good I really am out of ideas on this now. I think this is one project it really would be best just to forget about.
Julian - I've been messing around with the VK2ZAY circuit shown and demonstrated at http://www.youtube.com/watch?v=DG_lh_x92e0 Worked first time and very sensitive. it's not a lot different to the G3XBM circuit.
Though I found that it was easier to get best sensitivity without the RF preamp and with the antenna applied via 5pF to the source.
73, Peter VK3YE
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Though I found that it was easier to get best sensitivity without the RF preamp and with the antenna applied via 5pF to the source.
73, Peter VK3YE
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