Maker Pro
Maker Pro

Noise problem in existing circuit.

J

Jamie

Jan 1, 1970
0
Please look at circuit in question.

Side Band detector



1u
|| audio out
+---------+-------+---+||+---+
| 8.4 + | ||
| --- |
| --- 0.022|
+ | |
7.8Mhz RX sig .87 |/ GND |
inject here +----+----------+| .-.
| | |> | |
.-. .-. 0.226 | |
| | | | |7.8mhz carrier '-'
10k| | | |100k .-.inject here |
'-' '-' | | 9.4V
| | | |
GND '-'
9.47 |
+-----+
.-. |
| | ---
680 | | ---1u
'-' |
+-----+
|
===
GND

THe above circuit is also used in the detection section of the AM
receiver and that works fine there because the 7.8Mhz carrier is not
active at the emitter and there is no noise.

Problem, everything in this circuit seems to be fine with the manual
etc.. The problem is, while in a SSB mode and very low 8.7mhs RX signal,
the injected 8.7Mhz carrier at the emitter is generating a level of
white noise, which can he heard in the audio section when advancing
gain and reducing RF gain to nothing, it seems to be excessive. This
noise would remind you of listening to FM with no signal, rush noise.

Using the Tek 485, I see the carrier (BFO if you want), at 2.1VPP
with a .5us sweep. It matches exactly what should be there, according to
the manual, however, It appears that this carrier is phase shifting some
how? The 485 is displaying it in a widen fat line view, which tells me
it is being Phase/Freq modulated some how with possibly some foreign
noise ?
I am on vacation so I don't have access to the lecroy I would normal
use to measure this. I do have a Regal I could connect up but I have a
hard time getting away from my Tek485 :)

Looking at the circuit above, the audio out, my scope at 50mv input
with the 10:1 probe hardly shows any noise but it is there because the
next audio stages amplify it nicely. I do have active fet probes with a
preamp if that is needed. I also have other RF tools if I have to get
out the big guns.

THis noise is reminding me of what you'd see at the base of a BJT in
R.F. surroundings or just lots johnson noise, but I can't seem to find
noise at the base, but a little barely showing at the emitter.

Could there be a problem with the 7.8mhz inject carrier at the emitter
, producing this noise, due to some other issue else where? Like the
7.8mhz shifting being detected at this stage. Do you think I would be
able to detect this as actual audible noise with this type of config?

I have not yet sub the signal with the use of the HP gen, that's next.

As far as I know, the 485 should show a stable view of that at a .5us
sweep. It has never failed me before.

I do have a other tools to work with here if needed. It's beginning to
look like a modification to the circuit may be needed.

Jamie
 
J

Joerg

Jan 1, 1970
0
Jamie said:
Please look at circuit in question.

Side Band detector



1u
|| audio out
+---------+-------+---+||+---+
| 8.4 + | ||
| --- |
| --- 0.022|
+ | |
7.8Mhz RX sig .87 |/ GND |
inject here +----+----------+| .-.
| | |> | |
.-. .-. 0.226 | |
| | | | |7.8mhz carrier '-'
10k| | | |100k .-.inject here |
'-' '-' | | 9.4V
| | | |
GND '-'
9.47 |
+-----+
.-. |
| | ---
680 | | ---1u
'-' |
+-----+
|
===
GND

THe above circuit is also used in the detection section of the AM
receiver and that works fine there because the 7.8Mhz carrier is not
active at the emitter and there is no noise.

Problem, everything in this circuit seems to be fine with the manual
etc.. The problem is, while in a SSB mode and very low 8.7mhs RX signal,
the injected 8.7Mhz carrier at the emitter is generating a level of
white noise, which can he heard in the audio section when advancing
gain and reducing RF gain to nothing, it seems to be excessive. This
noise would remind you of listening to FM with no signal, rush noise.

Using the Tek 485, I see the carrier (BFO if you want), at 2.1VPP with
a .5us sweep. It matches exactly what should be there, according to the
manual, however, It appears that this carrier is phase shifting some
how? The 485 is displaying it in a widen fat line view, which tells me
it is being Phase/Freq modulated some how with possibly some foreign
noise ?


Two things come to mind:

Could it be that you are feeding too much amplitude in? What does
"inject here" mean at the emitter? Current feed? How much? 2.1Vpp
(assuming into 50ohms) sound like a ton of bricks to me.

Listen to the carrier with something else. Do you have a communications
receiver available at the place of vacation? Maybe somewhere behind the
shelf with the tequila and the margarita glasses :)

I am on vacation so I don't have access to the lecroy I would normal
use to measure this. I do have a Regal I could connect up but I have a
hard time getting away from my Tek485 :)

Don't spend too much time with electronics or ham radio during the
vacation or you wife might not be happy :)

[...]
 
J

Joerg

Jan 1, 1970
0
Jamie said:
Joerg said:
Jamie wrote:
[...]
I am on vacation so I don't have access to the lecroy I would normal
use to measure this. I do have a Regal I could connect up but I have a
hard time getting away from my Tek485 :)


Don't spend too much time with electronics or ham radio during the
vacation or you wife might not be happy :)

[...]
My wife has been reporting to work every day this week so that's a
bust.. My father did come down from the intelligent state (Maine) to
visit ;)

Ah, a vacation in Front-porchia :)

Those can be the best, finally you get around to doing all those things
you've always wanted to tackle.
 
J

Jamie

Jan 1, 1970
0
Joerg said:
Jamie said:
Please look at circuit in question.

Side Band detector



1u
|| audio out
+---------+-------+---+||+---+
| 8.4 + | ||
| --- |
| --- 0.022|
+ | |
7.8Mhz RX sig .87 |/ GND |
inject here +----+----------+| .-.
| | |> | |
.-. .-. 0.226 | |
| | | | |7.8mhz carrier '-'
10k| | | |100k .-.inject here |
'-' '-' | | 9.4V
| | | |
GND '-'
9.47 |
+-----+
.-. |
| | ---
680 | | ---1u
'-' |
+-----+
|
===
GND

THe above circuit is also used in the detection section of the AM
receiver and that works fine there because the 7.8Mhz carrier is not
active at the emitter and there is no noise.

Problem, everything in this circuit seems to be fine with the manual
etc.. The problem is, while in a SSB mode and very low 8.7mhs RX signal,
the injected 8.7Mhz carrier at the emitter is generating a level of
white noise, which can he heard in the audio section when advancing
gain and reducing RF gain to nothing, it seems to be excessive. This
noise would remind you of listening to FM with no signal, rush noise.

Using the Tek 485, I see the carrier (BFO if you want), at 2.1VPP with
a .5us sweep. It matches exactly what should be there, according to the
manual, however, It appears that this carrier is phase shifting some
how? The 485 is displaying it in a widen fat line view, which tells me
it is being Phase/Freq modulated some how with possibly some foreign
noise ?



Two things come to mind:

Could it be that you are feeding too much amplitude in? What does
"inject here" mean at the emitter? Current feed? How much? 2.1Vpp
(assuming into 50ohms) sound like a ton of bricks to me.

Listen to the carrier with something else. Do you have a communications
receiver available at the place of vacation? Maybe somewhere behind the
shelf with the tequila and the margarita glasses :)


I am on vacation so I don't have access to the lecroy I would normal
use to measure this. I do have a Regal I could connect up but I have a
hard time getting away from my Tek485 :)


Don't spend too much time with electronics or ham radio during the
vacation or you wife might not be happy :)

[...]
My wife has been reporting to work every day this week so that's a
bust.. My father did come down from the intelligent state (Maine) to
visit ;)

Jamie
 
T

Tauno Voipio

Jan 1, 1970
0
For SSB detection use a real quadrature demodulator (do you guys call that 'gilbert cell?),
and make sure the BFO is a perfect sinewave, symmetrical too, just one frequency.

A Gilbert cell is a four-quadrant analog multiplier. It makes a good
doubly-balanced modulator or demodulator.

For quadrature demodulation, you have to split the IF into two branches,
and feed the BFO to the branches in 90 degree phase difference. The
resulting audio channels (usually called I and Q) have to be combined
with a wideband 90 degree phase difference network (Hilbert
transformer).
 
T

Tim Williams

Jan 1, 1970
0
Jan Panteltje said:
why do sidebands of the BFO cause noise?
Say you have a pure BFO fb and a station fx,

Now you listen to
fb + fx and fb - fx

If you have a BFO that is fb 2fb 3fb 4fb etc...

What? Those aren't sidebands. Those are harmonics. You can't even get
basic terminology right!

Noise sidebands could be represented as a variation in amplitude and phase
or frequency over time (i.e., the sidebands need not be symmetrical, but
they will be distributed normally for small increments of any of the three
quantities). This could be written as fb + epsilon, for an epsilon which
is a normally distributed free variable with FWHA of sigma. Then, sigma
is the bandwidth of your BFO.

Simply, if you reduce the mean frequency of the BFO by mixing it with a
nearby frequency, and selecting the lowest product, sigma remains the same
magnitude, but it's a larger percentage of the total. Mixed down to 0Hz
(baseband), the carrier disappears completely and the noise bandwidth is
simply band-limited white noise. Typically, the distribution is peaked on
the carrier, so the baseband version is peaked at 0Hz, i.e., it's at least
pink noise, but more likely brown noise -- which has a "rushing" sort of
sound.
then you listen to

fb + fx, fb - fx, 2fb + fx, 2fb - fx, 3fb + fx, 3fb - fx, 4fb + fx,
4fb - fx

Depending on filters present.. anyways the noises add up sum of squares
I would think.

False, only one of those frequencies are present after selecting a
particular sum or difference with a suitable filter. Typically in SSB, fb
= fx, so the difference frequency lands in the base band, and a lowpass
over the audio range excludes other products.

If you had written, fb +/- delta fn, then it would be obvious that the
difference
(fb +/- delta fn) - fx
when fb ~= fx is
+/- delta fn
in other words, a variation in carrier frequency produces a variable tone,
rather than a zero beat. If that variable tone follows some distribution,
like a normal distribution, then the noise will have a random sound, and a
shape corresponding to some color of noise, like brown noise.

Tim
 
J

Jamie

Jan 1, 1970
0
Jan said:
PS
why do sidebands of the BFO cause noise?
Say you have a pure BFO fb and a station fx,

Now you listen to
fb + fx and fb - fx

If you have a BFO that is fb 2fb 3fb 4fb etc...
then you listen to

fb + fx, fb - fx, 2fb + fx, 2fb - fx, 3fb + fx, 3fb - fx, 4fb + fx, 4fb - fx

Depending on filters present.. anyways the noises add up sum of squares I would think.

OK, more
BEEP BEEP BEEP


-. ,-
output xform )|(
+ )|(
+-----------------------+-' '-
+---------+
7.8Mhz Filter + |
| |
.--------. IF amp + +
| | || |/ \| TX switch
| |-||+--------+-| |+------+------+-----+
| | || | |> <| | + +
'--------' .-. | + | --- .-.
| | + | + --- | |
| | | | .-. | | |
'-' | | | | === '-'
| +---------+ | | GND |
+-----------------+ | | '-' ===
--- .-. | GND
0.039uf --- | |
+ | | 330 xmitt signal level negator
| '-'
+----|
===
GND

The 0.039uf film poly ws the problem. Using the S/A I found what
looked like noise at the emitters. If I biased the TX switch a few uAs,
I could get it to stop however, the design is not intended to work that
way. So, did a in circuit test on the cap and didn't get anything form
it. It's hard to get at so I broke it off from the board and then solder
a new one on the bottom side.

The noise has diminish to a sencible level now and it has also
increased the gain in the IF. So I guess we're golden now :)

Jamie
 
T

Tim Williams

Jan 1, 1970
0
Jamie said:
| |
.--------. IF amp + +
| | || |/ \| TX switch
| |-||+--------+-| |+------+------+-----+
| | || | |> <| | + +
'--------' .-. | + | --- .-.
| | + | + --- | |
| | | | .-. | | |
'-' | | | | === '-'
| +---------+ | | GND |
^^^^^
Syntax warning: low base impedance.

Caps on bases make good oscillators. Suggest a few ohms (22-100ish) in
series with TX switch base.

Tim
 
T

Tim Williams

Jan 1, 1970
0
Jan Panteltje said:
Yes harmonios idiot!
Try learning to read : waveform distortion = harmonics.
Fourrier asshole
PLONK

OP didn't mention distortion, neither did you. You mentioned sidebands,
which you still got wrong.

Like a previous argument: sidebands are like how a waveform's envelope and
phase change over time, harmonics define how "lumpy" the waveform looks
within a cycle. Harmonics are farther spaced in frequency, which means
they affect things at small time scales, and sidebands vice-versa.
Fourier knows this. And please learn to spell...

Tim
 
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