I'm trying to build the simple AM receiver shown he

http://www.electronics-lab.com/projects/rf/006/

The only modifications I've made are changing the 200uH inductor to a
100uH (ferrite rod) inductor. I'm using a 10-120pF variable capacitor
(http://murata.com/catalog/t13/t13_09e.pdf), so I believe I should be
able to pick up the higher AM frequencies (correct?). I haven't been
able to get it to work yet (usually silence or static, sometimes a
high-pitched buzz), and have a few questions:

1. One of the caps is labeled 100n. This would be the same as the
other caps (.1uF). Is this a typo, or is the diagram just not
consistant?

2. The first three voltage checks I'm getting withing a couple
hundreths of a volt. The Q3(b) voltage (supposed to be 0.62V) is
reading 7.06V. And the Q3(c) (supposed to be 3.87V) I'm getting 1.07V.
Can anyone spot a connection error I might have made that would cause
this?

Thanks in advance for any help on either question!
Dave

Something very fishy there with respect to the voltages at Q3. Notice
that Q3-b is fed from Q3-c, so the potential there should be lower than
Q3-c in any event. And if Q3 is OK and wired in correctly, since Q3-e
is grounded, you can't reasonably have Q3-b much more than 0.7V above
ground. It's just a forward-biased junction. So as a start, look for
problems around Q3! Remove Q3 from the circuit, and check the voltage
at the bottom of the 10k resistor...should be nearly the same as the
battery voltage. And at the junction of the 560k and the 100nF, should
be just slightly lower. Then if you ground the bottom of the 10k, the
junction of the 560k and the 100nF should be zero volts. If it's not,
perhaps the 100nF is bad (leaky or shorted). I'd say the 100nF (which
is indeed 0.1uF) is way larger than needed in this circuit--100pF would
likely be large enough.

The description says it's a "regenerative" circuit, but I'm not seeing
any significant regeneration! Looks like a simple RF amplifier
followed by detection in Q3's base-emitter junction, and Q3 amplifying
the audio.

100uH and 10pF would tune about 5MHz, though stray capacitance and
parasitic capacitance in the coil will make the net capacitance
somewhat higher. 100uH and 120pF would tune about 1.45MHz, just the
upper end of the medium wave broadcast band. I'd suggest you try for
more inductance and/or more capacitance in the tuning system. For a
quick trial, just parallel in 100pF or so; the tuning range will be
limited but at least you'll be in the MW broadcast band more solidly.

Cheers,
Tom

K7ITM wrote:
Something very fishy there with respect to the voltages at Q3. Notice
that Q3-b is fed from Q3-c, so the potential there should be lower than
Q3-c in any event. And if Q3 is OK and wired in correctly, since Q3-e
is grounded, you can't reasonably have Q3-b much more than 0.7V above
ground. It's just a forward-biased junction. So as a start, look for
problems around Q3! Remove Q3 from the circuit, and check the voltage
at the bottom of the 10k resistor...should be nearly the same as the
battery voltage. And at the junction of the 560k and the 100nF, should
be just slightly lower. Then if you ground the bottom of the 10k, the
junction of the 560k and the 100nF should be zero volts. If it's not,
perhaps the 100nF is bad (leaky or shorted). I'd say the 100nF (which
is indeed 0.1uF) is way larger than needed in this circuit--100pF would
likely be large enough.

The description says it's a "regenerative" circuit, but I'm not seeing
any significant regeneration! Looks like a simple RF amplifier
followed by detection in Q3's base-emitter junction, and Q3 amplifying
the audio.

100uH and 10pF would tune about 5MHz, though stray capacitance and
parasitic capacitance in the coil will make the net capacitance
somewhat higher. 100uH and 120pF would tune about 1.45MHz, just the
upper end of the medium wave broadcast band. I'd suggest you try for
more inductance and/or more capacitance in the tuning system. For a
quick trial, just parallel in 100pF or so; the tuning range will be
limited but at least you'll be in the MW broadcast band more solidly.

Cheers,
Tom

Tom,
Fantastic, thanks for the help. I realized that one of the transistors
was in backward, so that was one problem. The voltages are closer to
those reported, although off by a couple tenths of a volt. I also put
another 120pF variable cap in parallel with the first, so I have a
range of 20 to 240pF. After some tinkering, I've managed to pick up a
station. However, it's pretty poor quality (I can make out the words
despite a constant, high pitched whine). I also used a 12k resistor in
place of the 10k (as I'm out of 10k). So I'm led to a couple more
questions:

1. The current circuit doesn't have an antenna. How could I attach
one to this circuit? And what would be the best way to do so? (single
wire, x" long wire, etc?)
2. How much would attaching the circuit ground to a fence (or
something) help?
3. What might be the cause of the nearly constant background whine?
Is there anything I can try to fix it?

Thanks again for the help!
Dave

Dave,

Assuming the 100uH is a standard ferrite rod type antenna, you can
likely give it a boost by winding just a couple turns (experiment: try
from one to five turns, for example) of wire around it, grounding one
end of that wire and extending the other end into a wire antenna
several feet long, up in the air as best you can. I'd suggest ten feet
minimum length, on up to maybe a hundred feet, but just do what you
can. You can connect the circuit common to a "ground" made up of a
ground rod driven into the earth, or just a wire on or close to the
earth. Experiment! You're not likely to hurt anything, so long as you
keep it away from power lines.

Dunno for sure about the background whine. Is it independent of
tuning, or is it only there when listening to the station? Does the
pitch change with changes in the tuning? Can you describe it better?
It could be an oscillation in your circuit. Substituting the 12k for
10k should not be a problem. But I'd add a bypass capacitor across the
battery voltage (and keep all the leads reasonably short) I'd use
maybe 10uF at 16V or more, be sure the polarity matches the battery,
and put it physically near Q3.

In troubleshooting things like the whine when you have limited test
equipment, you can just try things like opening connections in the
circuit to see which part of the circuit it's in. For example,
disconnecting the 100n capacitor should tell you if it's a problem in
the input section or output.

Good luck, and don't be afraid to experiment.

Cheers,
Tom

K7ITM wrote:
Dave,

Assuming the 100uH is a standard ferrite rod type antenna, you can
likely give it a boost by winding just a couple turns (experiment: try
from one to five turns, for example) of wire around it, grounding one
end of that wire and extending the other end into a wire antenna
several feet long, up in the air as best you can. I'd suggest ten feet
minimum length, on up to maybe a hundred feet, but just do what you
can. You can connect the circuit common to a "ground" made up of a
ground rod driven into the earth, or just a wire on or close to the
earth. Experiment! You're not likely to hurt anything, so long as you
keep it away from power lines.


Tom,
Thanks again for the advice. I originally attached a 10ft wire
directly to the L-C loop, and that helped a lot. But I've read
somewhere that it's best to magnetically couple it instead of directly
attach. Why is that?

I figured out the background noise... I had a TV, 3 lights, and a fan
motor going within about 15 ft of me. The background noise dropped off
a lot once I cut them off.

Dave

Hi Dave,

Well, you can couple the signal from the antenna into the radio in
different ways, and though multiple ways may work, one may be more
practical than another, and may be more efficient with available parts.
The feedpoint of an antenna--the place you couple signals in or
out--may look like a low impedance or a high impedance; it may have
quite a bit of reactance in it (what look like capacitance or
inductance). Getting the most signal out depends on matching to that
impedance. Fortunately, for most medium-wave signals (standard
broadcast), you can have inefficient coupling and still have a very
useable signal, because the signals have to be pretty strong to be
useable. They have to be larger than the atmospheric noise, which is
rather high at those frequencies.

A disadvantage of simply connecting an antenna wire to the "hot" side
of the LC at the receiver input is that a random wire will likely have
quite a bit of effective capacitance, causing the LC to be detuned from
what you expect. And if the wire is fairly long, it may represent a
relatively low resistance, which will make the tuning much less
sharp--it will lower the LC (tank circuit) Q--and you'll be listening
to more than one signal at a time with your simple receiver design.

It can be an advantage to make the coupling variable. You can do that
by changing the number of turns, or by moving the ferrite rod into or
out of those turns, with the inductive coupling. Or you can connect
the wire to the "hot" side of the LC tuning tank through a small
"trimmer" capacitor.

There's a lot more to the story, which you'll learn by studying,
experimenting, and keeping an open mind and high level of curiosity
about things. Joel in your other thread told you some about bandwidth
of LC tanks...you can determine the Q by measuring the 3dB bandwidth of
the tank. Then the Q is center frequency divided by bandwidth. If you
have a good coil and capacitor, and no circuit loading the LC, the Q
might be, say, 200, limited by losses in the coil and capacitor
themselves. If you add a resistor--either a part designed to be a
resistor or a circuit that has an effective resistance--it will lower
the Q and broaden the bandwidth. If the power lost in the resistor is
equal to the power lost in the coil and capacitor, you will have a Q
that's half as large--100 in this case. A receiver designer will pick
a Q that doesn't limit the bandwidth too much, but gets rid of
interfering signals. And if the receiver relies on two or more LC
tanks to track together, the Q will be picked a bit lower to allow
errors in tracking without hurting the performance.

Again...this is only scratching the surface. You should probably look
for some books like "Radio Amateur's Handbook" for a lot more
discussion of things like tuned circuits.

Cheers,
Tom