Noise problem in existing circuit.

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

Reply to
Jamie
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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 :-)

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

[...]
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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Could be the transistor is oscillating. Try adding a base resistor,

100 ohms maybe.
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John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Reply to
John Larkin

[...]

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.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

Reply to
Jamie

On a sunny day (Fri, 07 Sep 2012 14:20:57 -0400) it happened Jamie wrote in :

Ok, I will have a go. In this kind of mixer, the oscilator signal injected at the emittor gets loaded by a variable impedance (up and down current, and changing capacitance).

This causes wave form distortion of that BFO signal at that point, or 'sidebands', those attriubute to noise. 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. Buffer it if need be.

Replacing this circuit with a dual gate MOSFET should decrease any variable loading on the BFO somewhat. Diode ring detector (passive mixer), etc etc etc etc diff amp wth BFO in current source.. BEEP

beep beep beep

halso

Reply to
Jan Panteltje

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

Reply to
Jan Panteltje

'gilbert cell?),

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).

--

Tauno Voipio
Reply to
Tauno Voipio

On a sunny day (Sat, 08 Sep 2012 16:23:48 +0300) it happened Tauno Voipio wrote in :

'gilbert cell?),

frequency.

Yes, I once made a ssb exciter that way, with 2 double triodes... The carrier canceling was drifting all the time due to tube changing (heating up etc). Then I went for a diode ring modulator and a crystal filter (XF-9a and XF-9b were good 9 MHz filters at that time).

Audio 90 degrees phase shift was not that exact either even with the RSGB handbook component values for that filter.

Much later I did the quadrature modulation thing with 4053 switches for PAL color chroma generation.

The demodulator can be done exactly the same way. And that should also work for SSB detection.

As to the noise, and this is a very old secret from the time of analog tape recorders, the studio recorders has a waveform symmetry adjustment for the tape bias, set for minimum noise. Very important to have a clean waveform for the BFO or whatever you are mixing with.

Dilbert cell?

Reply to
Jan Panteltje

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.

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

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Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

would think.

-. ,- output xform )|( + )|( +-----------------------+-' '- +---------+ 7.8Mhz Filter + | | | .--------. IF amp + + | | || |/ \| TX switch | |-||+--------+-| |+------+------+-----+ | | || | |>

Reply to
Jamie

"Jamie" wrote in message news:cYK2s.1941$ snipped-for-privacy@newsfe18.iad...

^^^^^ Syntax warning: low base impedance.

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

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

On a sunny day (Sat, 8 Sep 2012 11:16:56 -0500) it happened "Tim Williams" wrote in :

Yes harmonios idiot! Try learning to read : waveform distortion = harmonics. Fourrier asshole PLONK

Reply to
Jan Panteltje

On a sunny day (Sat, 08 Sep 2012 13:21:46 -0400) it happened Jamie wrote in :

would think.

Yea, but replacing it all with a more advanced ciruit would reduce the noise evern further :-)

Reply to
Jan Panteltje

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

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

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