8 Feb 2010

VK2ZAY's version of the XBM80-2 transceiver

A few more people have been making the XBM80-2 transceiver. One is Alan Yates VK2ZAY who has made a de-luxe version with some refinements and additions. See it on his blog at http://www.vk2zay.net/article/238


g4ilo said...

Thanks for the link to this, Roger. I'll try a few of his ideas in my version. Interesting that Alan shows AF decoupling capacitors on the power rail, the absence of which in your circuit diagram was pointed out to me by Maurice G4DVM in an email. Adding some did improve the LF audio response a bit. I'm awaiting a 3.560 crystal to try the rig on 80m.

Alan Yates said...

Julian/Roger, I added the large electrolytic when I noticed crackles and pops in the audio (from what turned out to be a noisy pot in my bench supply). The PSU noise rejection is quite poor, the emitter sits at almost the raw rail voltage during RX. It might be worth adding an active decoupler into the collector or fixing the DC base bias to prevent saturation if you are using noisy supplies - I just replaced the scratchy pot in my PSU and all was good. The unit runs well off batteries which are generally pretty quiet.

The audio amplifier is a bit noisy too, and probably affects the MDS (you need a quiet room to hear the hiss), I was going to try a lower noise transistor, but it is likely that distributing the gain in several stages would be a better approach.

Combining the fixed base bias with DC coupling to the AF amplifier could give the receiver response to essentially DC and automatically cut off the AF amplifier on TX. It would also be easy to use this DC coupled approach to put Sallen-Key filters in the audio path and finish up with an emitter follower to drive low impedance headphones. Limiter diodes wouldn't hurt either, the circuit can produce very loud sounds if like me you accidentally blast it with a big signal from the generator. I want to try Michael's method of putting an AF transformer in instead of an AF amplifier at all, simplifying the transceiver even more. That is the wonderful thing about this radio - for which Roger should be commended - almost endless experimental options!

Unfortunately most of these "improvements" ruin the charm of the simplicity of this little radio. The basic rev E design is about as simple as it can get while still being usable.

Roger G3XBM said...

Lots of things can be improved. I was aiming for the "most with the least". No supply decoupling was deliberate (saved one part) as most rechargeable batteries will offer a low impedance. It will be interesting to see how the performance per component ratio can be improved further. Mine has 14 parts and it would be great to see better performance with , say, 10 parts total.

g4ilo said...

Yes I can see that improving the circuit destroys some of its simplicity. By using an extra transistor you lose the bragging rights of having a two transistor transceiver.

On the other hand I think the changes to reduce the harmonic output are essential if one is going to build it into a nice box - which implies you'll be using it again - instead of just making one contact to prove it can be done.

With or without the mods this is a wonderful little circuit that anyone can make and experience for themselves the fun of making and receiving radio waves using something they built, which is what this hobby is all about. Thank you, Roger, for developing it.

Anonymous said...


Have seen G3RJV'S version of the
Pebbel Crusher in the March Edition
of practical Wireless.
The first stage of the Tx is the Mighty Mite.I like the harmonic filtering design by W1FB (Doug DeMaw)and including 33 Ohm resistor in the base of TR1 to reduce the risk of parasitic oscillation.
I like the version of the XBM80-2
by Alan.You have generated a lot of interest in experimenting with the bare minimum of components.
I sure the Quotation in carrying on the practical way applies to you Roger.

Kind Regards Tony G4LLW