Unstable Gunn oscillator

R

Rene Tschaggelar

Guest
With a 2Ghz varactor tuneable Q band (34-36GHz) GUNN
from the junk pile we encountered extremely high
phase noise. Instead of being as stable as 100kHz
with phasenoise down 70dB at 200kHz as measured
with a previous one, this one jumps around 2MHz
and the phase noise in no more than 50dB down farther
out. The power supply is stable to below a few 10uV
and the varactor supply is also stable to below 100uV.
Replacing them didn't change anything. The temperature
is also stabilized.
Is it thinkable that an unfortunate mounting of the
subparts creates unstable modes ? It could also have
been dropped. Or that it would become more stable
with more or less drive current ?

{ It slowly becomes apparent that the junk pile has
no ISO 9000 quality control. }

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
In article <42b15d65$0$1157$5402220f@news.sunrise.ch>, none@none.net
says...
With a 2Ghz varactor tuneable Q band (34-36GHz) GUNN
from the junk pile we encountered extremely high
phase noise. Instead of being as stable as 100kHz
with phasenoise down 70dB at 200kHz as measured
with a previous one, this one jumps around 2MHz
and the phase noise in no more than 50dB down farther
out. The power supply is stable to below a few 10uV
and the varactor supply is also stable to below 100uV.
Replacing them didn't change anything. The temperature
is also stabilized.
Is it thinkable that an unfortunate mounting of the
subparts creates unstable modes ? It could also have
been dropped. Or that it would become more stable
with more or less drive current ?

{ It slowly becomes apparent that the junk pile has
no ISO 9000 quality control. }

Rene
Click to expand...
Both of those figures sound fairly bad at first blush.

For fun, I tried phase-locking a high-quality (relatively speaking) 10
GHz Gunn transmitter awhile back, using a relatively-narrow PLL to get
an idea of the unit's broadband noise profile. I saw about -107 dBc/Hz
at a 100-kHz offset (see graphs at
http://www.qsl.net/ke5fx/gunnpll.htm). At 200 kHz it was perhaps 6-7 dB
worse, around -100 dBc/Hz.

I'd expect these readings to degrade by about 6 dB/octave as you go up
in frequency, so I'm thinking the 200 kHz reading on your 35 GHz part
should be around -90 dBc/Hz rather than -70.

Do you have a large capacitor across the Gunn diode's terminals? Large
electrolytics can do funny things there, due to the diode's negative-
resistance properties. Easy to make an audio oscillator. I'd say try
playing with different bypassing schemes, and maybe take the diode out
and clean its contacts with ProGold or something like that.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------
 
John Miles wrote:

Both of those figures sound fairly bad at first blush.

For fun, I tried phase-locking a high-quality (relatively speaking) 10
GHz Gunn transmitter awhile back, using a relatively-narrow PLL to get
an idea of the unit's broadband noise profile. I saw about -107 dBc/Hz
at a 100-kHz offset (see graphs at
http://www.qsl.net/ke5fx/gunnpll.htm). At 200 kHz it was perhaps 6-7 dB
worse, around -100 dBc/Hz.

I'd expect these readings to degrade by about 6 dB/octave as you go up
in frequency, so I'm thinking the 200 kHz reading on your 35 GHz part
should be around -90 dBc/Hz rather than -70.

Do you have a large capacitor across the Gunn diode's terminals? Large
electrolytics can do funny things there, due to the diode's negative-
resistance properties. Easy to make an audio oscillator. I'd say try
playing with different bypassing schemes, and maybe take the diode out
and clean its contacts with ProGold or something like that.
Click to expand...
Thanks for the reference to your page. I wasn't aware the
Gunns were that good in the X band, At 35GHz they are a
bit noisier :
http://www.ibrtses.com/projects/qband_afc.html
With the newer controller and Gunn :
http://www.ibrtses.com/products/gunnps.html
There are only about 220uF parallel to the Gunn, but back in the
powersupply. The negative resistance applies to the microwave only,
not to the DC I guess. I'm feeding 7.1V at 750mA or so and get 60mW
out of it. This particular Gunn doesn't have a mechanical adjustment
screw, just a varactor. and yet it does 2GHz between 5 and 22V.
Yes, that scales down to 130kHz/mV.
I wouldn't want to open it, the waveguide is a WR28 with just 4 by
10mm crossection

Rene
 
In article <42b1d5ac$0$1154$5402220f@news.sunrise.ch>, none@none.net
says...
John Miles wrote:
Do you have a large capacitor across the Gunn diode's terminals? Large
electrolytics can do funny things there, due to the diode's negative-
resistance properties. Easy to make an audio oscillator. I'd say try
playing with different bypassing schemes, and maybe take the diode out
and clean its contacts with ProGold or something like that.

Thanks for the reference to your page.
Click to expand...
Looks like you figured out that I left an 'l' off the end :) -- sorry.
It should have been http://www.qsl.net/ke5fx/gunnpll.html .

There are only about 220uF parallel to the Gunn, but back in the
powersupply. The negative resistance applies to the microwave only,
not to the DC I guess.
Click to expand...
Like Steve pointed out, that's not correct. Try running your diode from
a variable supply with an ammeter, and watch what happens to the current
as you raise the voltage. Be careful -- you can exceed the diode's
current spec by running it under its rated voltage!

I'm feeding 7.1V at 750mA or so and get 60mW
out of it. This particular Gunn doesn't have a mechanical adjustment
screw, just a varactor. and yet it does 2GHz between 5 and 22V.
Yes, that scales down to 130kHz/mV.
Click to expand...
Yep... add a frequency discriminator and you have a neat parametric amp!

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------
 
Steve Kavanagh wrote:

Rene:

The negative resistance applies at other frequencies. Think about
it...under normal operating conditions the DC current drops as the DC
voltage is increased. So John Miles' suggestion of a bypass cap right
at the Gunn might help - it is common practice to use one.
Click to expand...
I wasn't aware of this. Possibly the reson we blew one.
Took us 2000$ and 12 weeks.

If you are comparing the phase noise of a 2 GHz tuning range oscillator
with the 100 MHz tuning range unit you describe on your website, part
of the difference might just be due to degradation of cavity Q by the
more tightly coupled varactor. You could also be getting external RF
signals coupled onto the more sensitive varactor.
Click to expand...
Yes, the Q is somewhat lower but that doesn't mean
there must be competing modes as in a laser. The
current setup involves only two isolators, an
attenuator, a PLL locking counter and a spectrum
analyzer.

I'll try the suggestions, thanks.

Rene
 
Steve Kavanagh wrote:

Rene:

The negative resistance applies at other frequencies. Think about
it...under normal operating conditions the DC current drops as the DC
voltage is increased. So John Miles' suggestion of a bypass cap right
at the Gunn might help - it is common practice to use one.
Click to expand...
Meanwhile I reinserted the previously installed 1.2
Ohm series resistor, to no avail. I also tried a low
ESR 10000uF cap, to no avail.

Does anyone has any hints as to what happens with
the output power when the voltage increases and
the current decreases ? There should be an optimum
efficiency somewhere. Not necessarily where the
noise is lowest.I guess I have to do some measurements.

Rene
 
Steve Kavanagh wrote:
Yes, typically there is a maximum output bias setting, usually near the
rated voltage. This is normally a substantially higher voltage than
the minimum required for negative resistance. It is rather hard to
find experimentally as the optimum matching varies with supply voltage
(partly because the frequency changes, too).
Click to expand...
Oh, the matching changes too. Could this mean that
off the optimum point, the Q is getting smaller ?
The specification said as powersupply range 6 to 8V,
and for this particular one suggested 7.5V, while
I for some reason did choose 7.1V.
The matching is somewhat difficult to measure as we have
two isolators in series after the Gunn.

Being confident that we do have the better measurement
equipment than the junkyard :
Could the optimum power supply voltage be found
with a network analyzer ?

Rene
 
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