Regenerative receiver thoughts

[WB6BYU]

I've taken on the task of building a radio receiver to help further a interest in radio
and electronics.  There are a number of quirks to the requirements:

1) Must have low current drain.  Their electricity supply is sometimes uncertain,
so it needs to be able to run on batteries, or solar cells, or a wind generator, etc.

2) Has to be fairly simple to build and use.  True, I have a very well stocked junkbox,
and access to additional parts as needed, but I want it to be understandable, and
easy to modify and/or duplicate depending on interest.

3) Should cover the AM BC band (MF), and some SW BC and/or ham bands as well.


My current plan is a regenerative set similar to this one:

http://www.qsl.net/7n3wvm/regen.html

with separate regenerative stage and detector.  I'd probably look for a dual-gang
variable capacitor, such as those used for AM/FM radios, with one large section
(for MF) and a smaller one (for SW).  Probably at least two SW bands, covering
something like 5 - 10 and 11-16 MHz, depending on the bandspread available
from the capacitor.  (Might just be multiple taps on the input coil, allowing a wide
range of frequencies to be covered.)

I'm looking at using a band switch rather than plug-in coils because then there
are no parts to loose, but it may require several sets of contacts, especially
to change between MF and SW.

Using a regenerative receiver avoids the requirement for a dual-gang variable
as would be required in a super-het.  And adds to the "retro" feel - apparently
they like that sort of thing, so I might look for an old ARC-5 case or similar
to build it in, otherwise I'd use a piece of double sided circuit board for the
front panel with the circuit on the inside and the outside polished to look nice.

Best life from a 9V battery would be to design the circuit to operate from
+5V and use a low dropout regulator - that works down to about 5.5V.
I'd also provide for an external DC input (up to +28V?), and/or a larger holder
for AA, C or D cells for extended use.  AC power would be via a wall wart,
and I might even put a full-wave bridge rectifier internal to the set so that
just about any type could be pressed into service - even if the output was
AC instead of DC, or if the polarity were reversed.  Once I have a sense of
the current draw I can see if I can fit enough solar cells along the top of
the case to power it that way.

At 5V, I'm probably better off using bipolar transistors rather than FETs
(though I have some SMD units that would work well at that voltage.)

Audio output would need to be able to drive headphones or a small speaker.
An LM386 might be a good compromise here for low current draw, though
discrete transistors wouldn't be a bad choice if I can manage it with a
2N3904/2N3906 or similar pair to keep the quiescent current low.  Output
should be able to drive 16 ohms.

So, while I have a number of sample circuits already, I though I would
see if anyone has any special favorite circuits to try, or other thoughts
on the design of such a receiver.

How would you go about designing a receiver to meet these needs?

[K8AXW]

Perhaps you should reconsider your requirements for versatility.....you've added quite a few things to make the "simple" receiver much more complicated and more difficult to replicate.

SMD parts don't lend themselves well for easy replication unless those who are going to build it are quite adept at soldering small parts.

Availability of the parts is a prime consideration.  For example, if it's necessary to obtain parts from more than one or two sources you're defeating your "simple" criteria because of the ordering and shipping costs.

Just some thoughts.

[W5FYI]

Charles Kitchin has done a lot of good experimentation on regenerative receivers. Take a look at some of his designs.

[JAHAM2BE]

If you're just looking for something to casually listen to, the 7n3wvm circuit should probably be OK, though I suspect the base bias method of controlling regeneration will lead to significant frequency shift as regeneration is adjusted, especially at SW frequencies, due to changing internal capacitances in the transistor as base bias is varied. Using UHF transistors, with very little inter-junction capacitance, may mitigate this.

If you're looking for a regenerative receiver that behaves "well" (smooth threshold behavior, no hysteresis, low drift) then I would second the suggestion to look at Kitchin's circuits, which take these little details into account. My favorite is here: http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm

If you're looking for an ultra-low-power design, this one is interesting: http://theradioboard.com/rb/viewtopic.php?p=28151#28151 .

Let us know what you come up with.

Edit: one more thing - bandswitching will make the coil leads longer, which is almost certain to reduce performance and may even cause outright misbehavior (squealing, squegging, hysteresis, etc.). But if you're not concerned with highest possible performance it might not matter. One way to achieve bandswitching is to short out a portion of the coil. Example: http://pa2ohh.com/08regrx.htm. Shorting turns though probably kills the coil Q, with corresponding poorer overall receiver performance.