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#1
August 16th 05, 05:04 AM
 [email protected] Guest Posts: n/a

from scratch and having a problem with the three IF stages oscillating
at the IF frequency (455KHz). The IF coils are salvaged from other
radios and the ground connections are to a common bus wire that
connects all the IF coil shields. It works reasonably well around 5
volts, but breaks into oscillation (at the IF frequency) if the supply
voltage is increased 1/2 volt and the signal drops off significantly if
the supply voltage is reduced 1/2 volt. So, it only works within a very
small supply voltage range of around 5 to 5.5 volts. The IF stages are
decoupled from the supply with a small resistor in series and bypass
cap to ground which helps, but doesn't solve the problem. I'm wondering
what can be done to stop oscillations and increase gain?

How did they manage to avoid the oscillation problems in the old tube
radios that were hand wired without any PC board?

-Bill

#3
August 17th 05, 12:48 AM
 tim gorman Guest Posts: n/a

#4
August 17th 05, 10:14 AM
 Pete KE9OA Guest Posts: n/a

I don't know what you mean by "small resistor" in series with the V+ line.
Each amplifier stage should have a series resistor of 100 ohms and a bypass
capacitor of .01uF. Actually, you can bypass every other stage if you are
using common emitter or common source stages, but I would do it with every
amplifier stage.
As far as physical distance for each stage, you should allow 1/2 inch for
every 40dB of gain. You shouldn't have this type of problem. A ground plane
is always a good idea, but at the very least you should have quite a bit of
ground flood on the circuit board. Although it is possible if you have quite
a bit of copper foil tape around the board, you can build this circuit on
perfboard but you have to be very careful, as you have already discovered. I
don't know if they have perfboard that has copper on only one side, but this
is one solution.
You had a question about the old hand-wired tubed equipment. With a metal
chassis, you would keep the wiring down close to the chassis itself, which
would in effect provide that ground plane. A ground is very important to
provide a place for the return currents for all of the signals, whether
physical wires or copper traces are used.

Pete

wrote in message
ups.com...
from scratch and having a problem with the three IF stages oscillating
at the IF frequency (455KHz). The IF coils are salvaged from other
radios and the ground connections are to a common bus wire that
connects all the IF coil shields. It works reasonably well around 5
volts, but breaks into oscillation (at the IF frequency) if the supply
voltage is increased 1/2 volt and the signal drops off significantly if
the supply voltage is reduced 1/2 volt. So, it only works within a very
small supply voltage range of around 5 to 5.5 volts. The IF stages are
decoupled from the supply with a small resistor in series and bypass
cap to ground which helps, but doesn't solve the problem. I'm wondering
what can be done to stop oscillations and increase gain?

How did they manage to avoid the oscillation problems in the old tube
radios that were hand wired without any PC board?

-Bill

#5
August 18th 05, 03:13 AM
 [email protected] Guest Posts: n/a

I don't know what you mean by "small resistor" in series with the V+ line.
Each amplifier stage should have a series resistor of 100 ohms and a bypass
capacitor of .01uF.

I've tried several values, 220,330,510, ect. with 0.05uF bypass.
Doesn't make much difference.

As far as physical distance for each stage, you should allow 1/2 inch for
every 40dB of gain. You shouldn't have this type of problem. A ground plane
is always a good idea,

The IF cans are spaced 1 inch apart and I disconnected the 2nd IF stage
so I only have an oscillator, mixer and single IF amp stage. Same
problem, it always oscillates when the voltage is raised to obtain
reasonable gain.

Although it is possible if you have quite a bit of copper foil tape around the
board, you can build this circuit on perfboard but you have to be very careful,

I took a look inside a AM/FM clock radio I have and checked the
ground connections. There are 3 RF transformers, oscillator, mixer and
single IF stage, and all of the metal shields of the transformers are
isolated. There are no physical connections between the shields of the
3 transformers.

Obviously, I am missing something. Grounding all the transformer
housings on a massive ground plane does not seem to be the answer.

-Bill

#6
August 18th 05, 12:36 PM
 W3JDR Guest Posts: n/a

Bill,

Transistor amplifiers with inputs and outputs tuned to the same frequency
turn out to be oscillators, especially if the the tuned frequency is very
low compared to the transistor's Ft. If you're using a modern transistor
with an Ft of say, 800 Mhz, and you're tuning it to say, 500KHz, then you
could have available gain of over 60 dB in one stage. This is a sure-bet
oscillator waiting to happen.

You can lower the gain by 'loading' the input and output tuned circuits with
parallel resistors (try 1K for starters and work up from there if it stops
the oscillation. Just remember that this technique, in addition to lowering

You can also try a small resistor (less than a couple hundred ohms) in
series with the collector lead of the transistor, or in series with the base
lead. This technique will lower the stage gain and actually increase the
impedance seen by the tuned circuits, improving selectivity.

Keep in mind that raising the selectivity too much can cause tuned circuit
losses.

Joe
W3JDR

wrote in message
oups.com...
I don't know what you mean by "small resistor" in series with the V+
line.
Each amplifier stage should have a series resistor of 100 ohms and a
bypass
capacitor of .01uF.

I've tried several values, 220,330,510, ect. with 0.05uF bypass.
Doesn't make much difference.

As far as physical distance for each stage, you should allow 1/2 inch for
every 40dB of gain. You shouldn't have this type of problem. A ground
plane
is always a good idea,

The IF cans are spaced 1 inch apart and I disconnected the 2nd IF stage
so I only have an oscillator, mixer and single IF amp stage. Same
problem, it always oscillates when the voltage is raised to obtain
reasonable gain.

Although it is possible if you have quite a bit of copper foil tape
around the
board, you can build this circuit on perfboard but you have to be very
careful,

I took a look inside a AM/FM clock radio I have and checked the
ground connections. There are 3 RF transformers, oscillator, mixer and
single IF stage, and all of the metal shields of the transformers are
isolated. There are no physical connections between the shields of the
3 transformers.

Obviously, I am missing something. Grounding all the transformer
housings on a massive ground plane does not seem to be the answer.

-Bill

#8
August 18th 05, 02:10 PM
 Pete KE9OA Guest Posts: n/a

A ground plane should make the system more stable. The radios that I have
been designing over the years have gains in the I.F. systems that range from
90 to 120dB, depending on the topology. You are definitely getting signal
feedback through your power supply rails. 100 to 120 ohms is about as high
as you want to go as far as decoupling resistor values. .05uF is too high of
a value for a decoupling cap..............01uF is what you should use for
local decoupling.
You should also have at least one bulk decoupling cap that has a value in
the range of 10 to 47uF. The larger decoupling cap cancels out the
inductance of your power supply wiring while the smaller local decoupling
caps cancel out the inductance of the bulk cap.

Pete

wrote in message
oups.com...
I don't know what you mean by "small resistor" in series with the V+
line.
Each amplifier stage should have a series resistor of 100 ohms and a
bypass
capacitor of .01uF.

I've tried several values, 220,330,510, ect. with 0.05uF bypass.
Doesn't make much difference.

As far as physical distance for each stage, you should allow 1/2 inch for
every 40dB of gain. You shouldn't have this type of problem. A ground
plane
is always a good idea,

The IF cans are spaced 1 inch apart and I disconnected the 2nd IF stage
so I only have an oscillator, mixer and single IF amp stage. Same
problem, it always oscillates when the voltage is raised to obtain
reasonable gain.

Although it is possible if you have quite a bit of copper foil tape
around the
board, you can build this circuit on perfboard but you have to be very
careful,

I took a look inside a AM/FM clock radio I have and checked the
ground connections. There are 3 RF transformers, oscillator, mixer and
single IF stage, and all of the metal shields of the transformers are
isolated. There are no physical connections between the shields of the
3 transformers.

Obviously, I am missing something. Grounding all the transformer
housings on a massive ground plane does not seem to be the answer.

-Bill

#9
August 19th 05, 03:30 AM
 Ken Scharf Guest Posts: n/a

#10
August 19th 05, 04:37 AM
 [email protected] Guest Posts: n/a

Well, I don't have an exact dB gain figure, but I hooked up the mixer
stage by itself and I get about 300 to 400 mV p-p with a generator
connected to the antenna coil using 3 turns of wire around the coil and
a 50 ohm resistor in series. The generator is set to 400mV p-p and I
get about the same amount out which is fairly stable from 9 volt supply
to 6 volts.

The first stage seems to be working right, no oscillations,
and stable output as the supply changes, and reasonable gain.

The transistor is a 2N4123 with a Ft of 250 MHz and
hfe of 50-200. It is biased using a 1k emitter resistor
at 1 volt, or about 1 mA.

I assume this stage works ok and will investigate
number 2.

-Bill

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