Andrea Baldoni wrote:



wrote:

: If we can get back to the original claim of "increased
: noise with AGC applied" we might be able to help Andrea
: some. We don't know what Andrea has for a main receiver
: and interjecting some "badness" remarks by the ARRL about
: a certain IC isn't going to help clarify Andrea's problem.

Sorry for the short answer. The receiver is a Yaesu FR-101 with his
internal 2m converter. Actually, I'm doing some recalibration but I didn't
found at now nothing really out. I just noted that, mostly noticeable in
FM mode (by noise increasing) but present in all modes looking s-meter,
the response of the IF was not a gaussian with a flat top. In the center
there was a little decreasing area. If you tune it perfectly, you had a
sligtly less signal than little up or down in freq.


................ snip ...........................


Ciao,
AB

Are you sure you are seeing an AGC problem? What you describe above, with
slightly less signal in the center, is typical of a *filter* with dip in
the middle of the passband. This is *not* a result of increased signal
strength causing more AGC and thus more noise when the receiver is tuned to
exact center on a signal. It is the result of a filter design choice (lower
cost) or the result of the filter tuning (tuned for better shape at the
edges which causes more dip in the middle).

Without knowing more about the receiver I can't make any guesses as to what
is in play here but I question if this is an AGC artifact.

tim ab0wr

tim gorman wrote:

: Are you sure you are seeing an AGC problem? What you describe above, with
: slightly less signal in the center, is typical of a *filter* with dip in

I have the filter dip; it was not cured with realigning, but realigning was
anyway helpful to gain a dB or two.
I also see that disabling AGC cause less noise in FM while listening to 2m
converted to HF by the internal converter of the Yaesu FR-101. I didn't check
if enabling or disabling AGC cause any change in the filter dip, anyway I'll
check and report soon.

: Without knowing more about the receiver I can't make any guesses as to what
: is in play here but I question if this is an AGC artifact.

It uses a MC1496G as mixer and two CA3053 as IF. Plus some DG FETs, in first
RF amplifier, after the mixer... What information do you need?

Ciao,
AB

.... Andrea Baldoni, 2002: messaggio non protetto da copyright.

Andrea Baldoni wrote:



tim gorman wrote:

: Are you sure you are seeing an AGC problem? What you describe above,
: with slightly less signal in the center, is typical of a *filter* with
: dip in

I have the filter dip; it was not cured with realigning, but realigning
was anyway helpful to gain a dB or two.
I also see that disabling AGC cause less noise in FM while listening to 2m
converted to HF by the internal converter of the Yaesu FR-101. I didn't
check if enabling or disabling AGC cause any change in the filter dip,
anyway I'll check and report soon.

: Without knowing more about the receiver I can't make any guesses as to
: what is in play here but I question if this is an AGC artifact.

It uses a MC1496G as mixer and two CA3053 as IF. Plus some DG FETs, in
first RF amplifier, after the mixer... What information do you need?

Ciao,
AB

... Andrea Baldoni, 2002: messaggio non protetto da copyright.

A block diagram would be helpful. Do you know if the AGC is being derived
from the audio chain or from a sampling circuit in the IF chain?

Where are you turning off the AGC? In the FR-101? It sounds like you have a
frequency converter feeding an HF receiver with an FM position. Is that
correct? Could you just as easily tune in 2-m SSB as well as 2-m FM?

I can't seem to grasp why turning off the AGC would result in *less* noise,
especially on FM.

The "noise factor" of the system is probably fixed by the converter, not the
receiver. It would be like hooking an antenna to an HF receiver. Usually
the noise in the receiver is set by the atmospheric noise the antenna picks
up, not by the noise factor of the receiver. (On the higher HF bands, 15m
and 10m, this may not always be the case) The same would apply for the 2m
converter. Unless it is designed very well its contribution to the noise at
the antenna of the receiver would probably mask the noise factor of the
receiver itself.

If you could kill the power to the converter you could probably test for
this by just killing the power and seeing what happens to the noise out of
the receiver speaker. If it goes down, then the noise factor of the
receiver is irrelevant. If it doesn't change then the converter is
contributing less noise than the receiver itself.

If turning off the AGC causes less noise output then my first guess would be
to look at what "turning off the AGC" is actually doing. Is it actually
breaking the AGC loop so the AGC inputs to the CA3053 amps are left
floating? Or does turning off the AGC actually mean putting a fixed bias on
the CA3053's? Either case could potentially cause the gain of the CA3053's
to actually go down with the AGC turned off and that might be what is going
on.

Can you actually monitor the AGC loop to see what happens to the AGC voltage
when the AGC is turned off?

tim ab0wr

tim gorman wrote:

: A block diagram would be helpful. Do you know if the AGC is being derived
: from the audio chain or from a sampling circuit in the IF chain?

The AGC is derived sampling the RF level at the last IF. In the same point,
(using another diode, capacitor, etc.) there is the AM detector. The same RF
signal go to a diode balanced modulator along with BFO signal for CW and SSB
and to the input of FM IF amplifier (TA7061AP).

: Where are you turning off the AGC? In the FR-101? It sounds like you have a
: frequency converter feeding an HF receiver with an FM position. Is that
: correct? Could you just as easily tune in 2-m SSB as well as 2-m FM?

I have a FR-101 with the onboard 2m converter. Yes, I could tune 2m SSB but
it's difficult to find SSB on 2m here usually so I didn't any test. The FR-101
is CW/SSB/AM/Narrow AM in segments of the HF range, plus you could buy (as a
option) a onboard 2m converter, a onboard 6m converter and a onboard FM
detector. I have all boards installed, so I eventually could use FM on
HF as well as 2m or 6m.

: If you could kill the power to the converter you could probably test for
: this by just killing the power and seeing what happens to the noise out of
: the receiver speaker. If it goes down, then the noise factor of the
: receiver is irrelevant. If it doesn't change then the converter is
: contributing less noise than the receiver itself.

Turning on the 2m converter don't seem to change noise level, while instead
turning on the 6m converter seems to double the noise. Maybe it needs
realigning, I never use it so I don't know if it's working well.

: If turning off the AGC causes less noise output then my first guess would be
: to look at what "turning off the AGC" is actually doing. Is it actually

The AGC line is derived from a fixed voltage using a 9V zener, then the RF
GAIN pot permit to divide this voltage from 100% to ground and feed it (trough
a resistor) to the first RF amplifier of the HF receiver (a DG FET) as well as
the first RF amplifier of the 2m converter, and the same for 6m converter.
It is also fed to the last but one CA3053. Other amplifiers are fixed gain
I suppose.
Everything in the receiver needs to reduce gain, lower this voltage by more
or less shorting it to ground.
For instance, the standby button shorts it to ground, silencing the receiver
completely. The RF level at the last IF instead reduce it by means of common
emitter transistor: the AGC voltage from zener at the collector and the
rectified and filtered IF at the base.
When you disable AGC, you disconnect the collector of this transistor, thus
the signal is let alone to the level adjusted with RF gain pot (normally at
maximum, so it is 9V).

Ciao,
AB

.... Andrea Baldoni, 2002: messaggio non protetto da copyright.

Andrea Baldoni wrote:

.....................................


The AGC line is derived from a fixed voltage using a 9V zener, then the RF
GAIN pot permit to divide this voltage from 100% to ground and feed it
(trough a resistor) to the first RF amplifier of the HF receiver (a DG
FET) as well as the first RF amplifier of the 2m converter, and the same
for 6m converter. It is also fed to the last but one CA3053. Other
amplifiers are fixed gain I suppose.
Everything in the receiver needs to reduce gain, lower this voltage by
more or less shorting it to ground.
For instance, the standby button shorts it to ground, silencing the
receiver completely. The RF level at the last IF instead reduce it by
means of common emitter transistor: the AGC voltage from zener at the
collector and the rectified and filtered IF at the base.
When you disable AGC, you disconnect the collector of this transistor,
thus the signal is let alone to the level adjusted with RF gain pot
(normally at maximum, so it is 9V).

Ciao,
AB

Ok, have you checked the Dual Gate FET to insure that the bias supplied by
the RF gain control puts the device at maximum gain when the AGC is off?

Dual Gate FET's have transducer gain curves that are peaked curves.
Depending upon the Gate 2 voltage, the transducer gain can actually go down
as the Gate1 to Source voltage goes up.

I would still be interested in knowing *exactly* what the AGC voltage on the
gate of the DGFET is for 1) AGC on, no signal, RF gain wide open, and 2)
AGC off, no signal, RF gain wide open. It would also be interesting to know
what the Gate 1 bias voltage is for each state as well.

I'll bet you'll find an interesting interaction between the bias voltages
and the actual stage gain as the controls are manipulated.

tim ab0wr

On Sat, 26 Aug 2006 21:30:47 -0500, tim gorman
wrote:

Andrea Baldoni wrote:

....................................


The AGC line is derived from a fixed voltage using a 9V zener, then the RF
GAIN pot permit to divide this voltage from 100% to ground and feed it
(trough a resistor) to the first RF amplifier of the HF receiver (a DG
FET) as well as the first RF amplifier of the 2m converter, and the same
for 6m converter. It is also fed to the last but one CA3053. Other
amplifiers are fixed gain I suppose.
Everything in the receiver needs to reduce gain, lower this voltage by
more or less shorting it to ground.
For instance, the standby button shorts it to ground, silencing the
receiver completely. The RF level at the last IF instead reduce it by
means of common emitter transistor: the AGC voltage from zener at the
collector and the rectified and filtered IF at the base.
When you disable AGC, you disconnect the collector of this transistor,
thus the signal is let alone to the level adjusted with RF gain pot
(normally at maximum, so it is 9V).

Ciao,
AB

Ok, have you checked the Dual Gate FET to insure that the bias supplied by
the RF gain control puts the device at maximum gain when the AGC is off?

Dual Gate FET's have transducer gain curves that are peaked curves.
Depending upon the Gate 2 voltage, the transducer gain can actually go down
as the Gate1 to Source voltage goes up.

I would still be interested in knowing *exactly* what the AGC voltage on the
gate of the DGFET is for 1) AGC on, no signal, RF gain wide open, and 2)
AGC off, no signal, RF gain wide open. It would also be interesting to know
what the Gate 1 bias voltage is for each state as well.

Tim,

Your hitting the specifics of what I refered to earlier. I'll repeat
it for emphasis.

What I have seen in some cases is where the no
signal resting point for gain control bias voltage is not correct and
the gain can go up a bit before going down. Often seen on oder
recievers where the large part of the radio is discrete devices
and the various setpoints have drifted from age or componenet
changes.

I have seen this on older radios where parts have been replaced or
the original parts used were at opposing ends of the allowable
tolerence. Occasionally a part like a zener doide can drift form
heating. The end result is the full gain voltage can be off or full
gain for a single stage can be off (high or low from optimum).
In one case it was a mechanical switch (agc/manual) causing
difficulty (leakage path). Other suspect components seen in
Japanese built radios are those commonly used ceramic disk
caps for bypasses, they can and do go leaky(high resistance),
or short and I've even seen microphonic. I have a reciever I
repaired where the DGfet developed a substrate to gate2 short
which casued all manner of unusual problems.

The worst case by far was one that the agc bias point had drifted
a bit high. When on manual agc the RX was hot. When agc was
enabled the RX sensitvity would drop noticeably. The problem
was the higher agc bias point had the IF and RF running harder and
producing more noise and when agc was turned on it would see the
noise and pull the agc voltage. That sounds ok save for the front
end was more agc sensitivve than the overbiased IF and the front
end would loose gain faster (it was 2 jfets cascode) rendering the
reciever less sensitive. The fix was repairing the internal voltage
regulator that fed 9V to most of the circuits (it was running at 11v
due to open zener).

Just a few examples of what can occur. I havent even gone into the
golden screwdriver problems when pots are tweeked for "more".


I'll bet you'll find an interesting interaction between the bias voltages
and the actual stage gain as the controls are manipulated.

I'd be inclined to agree.

Allison