[Nico Coesel]

Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL chip to
serve as a simple RF generator. The ADF4350 can output a signal
between 140MHz to 4.4GHz. It seems to work but the output shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpg showing 2
traces). I can't test beyond 2.6GHz The board is a 4 layer FR4
PCB.

So far I tried to change the decoupling capacitors (0402), add extra
wires to the power supply pins, change the output termination, use
different cables (SMA) but this changes nothing at all.

Is there an easy way to pin this problem down without spending another
fortune on equipment? I guess there is a parallel resonator somewhere.
To make matters worse I'm not quite sure my SA is 100%. I'm thinking
about simulating the board using Sonnet or putting together a simple
power level meter.

Any other ideas?

 

[Jamie]

Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL chip to
serve as a simple RF generator. The ADF4350 can output a signal
between 140MHz to 4.4GHz. It seems to work but the output shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpg showing 2
traces). I can't test beyond 2.6GHz The board is a 4 layer FR4
PCB.

My service monitor gives me a couple of issues on some frequencies, to
verify this, I placed a current meter inline with the circuit to see if
there was a noticeable change at the same place as on the spectrum
sweep. Of course, the change most likely is minimum but enough that it
should tell you if you actually do have a resonator on board somewhere
loading down your signal.

So far I tried to change the decoupling capacitors (0402), add extra
wires to the power supply pins, change the output termination, use
different cables (SMA) but this changes nothing at all.

Is there an easy way to pin this problem down without spending another
fortune on equipment? I guess there is a parallel resonator somewhere.
To make matters worse I'm not quite sure my SA is 100%. I'm thinking
about simulating the board using Sonnet or putting together a simple
power level meter.

Any other ideas?

Jamie

 

[mike]

Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL chip to
serve as a simple RF generator. The ADF4350 can output a signal
between 140MHz to 4.4GHz. It seems to work but the output shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpg showing 2

traces). I can't test beyond 2.6GHz The board is a 4 layer FR4
PCB.

So far I tried to change the decoupling capacitors (0402), add extra
wires to the power supply pins, change the output termination, use
different cables (SMA) but this changes nothing at all.

Is there an easy way to pin this problem down without spending another
fortune on equipment? I guess there is a parallel resonator somewhere.
To make matters worse I'm not quite sure my SA is 100%. I'm thinking
about simulating the board using Sonnet or putting together a simple
power level meter.

Any other ideas?

20 years ago, I had a computer motherboard that had resonances in
the 400MHz range with harmonics going up as high as I could measure.
Turned out to be the ground and power planes resonating.
I tried adding/deleting caps. I could change the Q a little,
but couldn't change the frequency much. The impedance was so low
that adding discrete parts didn't do much.

I got curious and TDR'd and frequency swept some other production
boards. They had the same problem.

I decided I was trying to fix something that wasn't broke.

You don't have that luxury. You probably don't need a power plane.

I'd try a TDR on the output looking back...or a directional coupler
and sweeper. Another thing to try is to sweep the power and ground
planes (powered off)
and look at the output connector to see if anything is coupling in.
Or stuff the resonant frequency in the output and look for signal
on the board. But that's gonna be a difficult probing problem.

 

[Nico Coesel]

Got a pic of the PC board?

This is the HF part:
http://img96.imageshack.us/img96/1460/img0506bc.jpg

The next layers are ground and power. I already tried to make the 'S'
shorter with a wire but no difference at all.

 

[Nico Coesel]

Is that 'S' shape even 50 ohm trace?

It should be

I use femm 4.2 to calculate trace widths [very accurate], or an old
DOS tool called DIMSTRIP.exe that Engineers claimed better than 1%
accuracy to 1GHz. If you want a copy of the 50k program let me know.

Given a box lid 1 inch above surface of PCB, FR-4 dielectric 4.2 and
loss tangent of 0.06, 1 mil copper metallization layer [balance
between 0.7 and 1.4 mil], dielectric thickness of 20mils; Dimstrip
calculates the S curve trace width should be 38.9 mils to get Zo=3D50
ohms.

These are the specs from the board house concerning the layer
distances:
http://www.eurocircuits.com/index.php/technology-guidelines/pcb-multilayer-build-ups

The dielectric is 14 mils. The S trace is 26 mils wide.

What did the PCB Fab house suggest for the trace width? They have
their own programs.

I've used an online trace impedance calculator.

A bit of the reason for the whole exercise is to be able to take some
measurements to see whether there are serious mismatches in circuits.
I'm planning on ordering a directional coupler for that purpose.

 

[Joerg]

Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL chip to
serve as a simple RF generator. The ADF4350 can output a signal
between 140MHz to 4.4GHz. It seems to work but the output shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpg showing 2
traces). I can't test beyond 2.6GHz The board is a 4 layer FR4
PCB.

So far I tried to change the decoupling capacitors (0402), add extra
wires to the power supply pins, change the output termination, use
different cables (SMA) but this changes nothing at all.

Is there an easy way to pin this problem down without spending another
fortune on equipment? I guess there is a parallel resonator somewhere.
To make matters worse I'm not quite sure my SA is 100%. I'm thinking
about simulating the board using Sonnet or putting together a simple
power level meter.

Any other ideas?

How did you measure that? IOW how did you keep the spectrum analyzer and
the ADF4350 in sync?

Did you make sure to turn all WLAN gear in the area off? Because that
operates right around 2.5GHz. Not sure who uses 2.3GHz over there but
that's a miniscule dip.

 

[Nico Coesel]

How did you measure that? IOW how did you keep the spectrum analyzer and
the ADF4350 in sync?

The ADF4350 outputs the signal for much longer than the sweep time of
the spectrum analyzer. I've wrote some software for the controller
which lets the ADF4350 step through a range of frequencies. I have
tried various hold and sweep times but all with the same end result.

Did you make sure to turn all WLAN gear in the area off? Because that
operates right around 2.5GHz. Not sure who uses 2.3GHz over there but
that's a miniscule dip.

True but except for the PCB the cabling is shielded.

 

[Joerg]

The ADF4350 outputs the signal for much longer than the sweep time of
the spectrum analyzer. I've wrote some software for the controller
which lets the ADF4350 step through a range of frequencies. I have
tried various hold and sweep times but all with the same end result.

Good, that should exclude aliasing.

True but except for the PCB the cabling is shielded.

Then I'd put the PCB into a metal can. Danish butter cookie cans are
good for that but some volunteers have to eat the contents first.

The other way to find out is probe before the filter at the ADF4350. If
it's clean there then the filter is the culprit. Although it looks like
there isn't too much filtering going on. One of the grounds in the
middle of the filter looks a bit iffy but hard to see.

 

[Nico Coesel]

A short trace like that won't have much effect at low frequencies like
2.5 GHz. The board layout looks fine to me.

I assume that the SMA goes to a 50 ohm coax into a 50 ohm spectrum
analyzer. Did you try different lengths of coax to your SA? If the SA

I tried a different cable and a different SMA to N converter. No
change.

were a bad 50 ohm match, you could get reflections. Quick check: stick
a 6 dB attenuator on one end of the cable.

I used a 20dB attenuator but no change.

What value of resistors (inductors?) did you use for the output
pullups on the ADF4350?

I use two 100 Ohm 0402 resistor in parallel. The downside of the
ADF4350 is that it doesn't output a sine wave. AD cleverly covered
that up by specifying different filters for specific frequency ranges.

I wonder if it's falling out of lock in the dip region. I think you
need some other instrument to cross-check the SA. A diode detector
would be a good start, or a sampling oscilloscope.

I tried a diode detector much like this with a BAT54 diode:
http://www.minikits.com.au/images/probe.jpg

That doesn't work very well to say the least. Maybe I should dig up
some PIN diodes.

That ADF4350 looks nice. I may have a use for it, if I ever get around
to designing my pulse generator. But the digital interface is awesome:
I've seen entire microprocessors that were simpler.

Wait until you get to the math to calculate the frequency The rest
of my PCB is a USB gadget BTW.

 

[Nico Coesel]

a bit more work to wire up, but maybe something like this:
http://www.analog.com/en/rfif-components/detectors/ad8314/products/product.html

That actually looks very interesting and doable. I've ordered some
samples!

 

[Nico Coesel]

That did bother me, on the data sheet. The output stage is apparently
an npn current-steering diffamp, and I'd expect it to look like a
square wave current source with finite edge rates.

Well the output is a square wave (with loads of harmonics).

No, PINs are very slow. A low-c, low-barrier schottky, designed to be
a detector, would be best.

I'll order a couple of those then

Or just buy an SMA detector from Mini-circuits.

Do you have access to a sampling (or fast digital) oscilloscope? It
wouldn't even need to be triggered, although that would be
instructive, to see the actual waveforms.

The fastest digital scope I have goes to 500 MHz so thats a no-go. I
think I'll build an RF detector around an ADI RF detector chip. That
would turn the entire gadget into a much more versatile device.

Yeah, that does look formidable. They should furnish a nice
parameterized C source routine, so their customers don't have to do it
hundreds of times.

If you reduce the function to (out * divider) /refin=... the math
becomes quite easy. A few lines of code does the job if you can live
with a few kHz resolution.

 

[Nico Coesel]

On Apr 11, 2:51=3DA0am, [email protected] (Nico Coesel) wrote:

Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL chip to
serve as a simple RF generator. The ADF4350 can output a signal
between 140MHz to 4.4GHz. It seems to work but the output shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpgshowing2
traces). I can't test beyond 2.6GHz The board is a 4 layer FR4
PCB.

So far I tried to change the decoupling capacitors (0402), add extra
wires to the power supply pins, change the output termination, use
different cables (SMA) but this changes nothing at all.

Is there an easy way to pin this problem down without spending anoth= er
fortune on equipment? I guess there is a parallel resonator somewher= e.
To make matters worse I'm not quite sure my SA is 100%. I'm thinking
about simulating the board using Sonnet or putting together a simple
power level meter.

Any other ideas?

Got a pic of the PC board?

This is the HF part:http://img96.imageshack.us/img96/1460/img0506bc.jp=

g

The next layers are ground and power. I already tried to make the 'S'
shorter with a wire but no difference at all.

Is that 'S' shape even 50 ohm trace?

It should be

I use femm 4.2 to calculate trace widths [very accurate], or an old
DOS tool called DIMSTRIP.exe that Engineers claimed better than 1%
accuracy to 1GHz. If you want a copy of the 50k program let me know.

Given a box lid 1 inch above surface of PCB, FR-4 dielectric 4.2 and
loss tangent of 0.06, 1 mil copper metallization layer [balance
between 0.7 and 1.4 mil], dielectric thickness of 20mils; Dimstrip
calculates the S curve trace width should be 38.9 mils to get Zo=3D3D50
ohms.

These are the specs from the board house concerning the layer
distances:http://www.eurocircuits.com/index.php/technology-guidelines/pcb= -multi...

The dielectric is 14 mils. The S trace is 26 mils wide.

What did the PCB Fab house suggest for the trace width? They have
their own programs.

I've used an online trace impedance calculator.

A bit of the reason for the whole exercise is to be able to take some
measurements to see whether there are serious mismatches in circuits.
I'm planning on ordering a directional coupler for that purpose.

I get around 51 ohms, so with thicker metallization from the tinning
it seems right. But 14 mils is an odd 42 mil thick board? Please tell

Its a 62 mil board.

me you DIDN'T go 14, 32, 14 ?? You'd be far better off with a 32 mil
board and go 10, 10, 10 than do that. If you need to stay at 62, add
two layers and interlace power and GND with 8 mils each, be a lot
better off.

How much improvement would that get me? The 'problem' is that when it
comes to these kind of low volume tinker projects there is not much
choice in board stackup. I have to go with what a pooling service
offers. I could choose a 6 layer stackup for the next version but it
is serious overkill for the rest of the board which could easely be on
a 2 layer board

14 mil thick and 26 mils wide moves a LOT of field up into the air!
changes the 4.2 to 3.2 effective. Not sure it is good to have that
floating around above your circuitry.

What do you mean by 'serious mismatches in circuits'?

A slightly different subject: AFAIK there are at least two ways to
determine mismatch. TDR or by using a directional coupler and see what
is reflected back. I'd like to get a setup with a directional coupler.

=3D =3D =3D
not even going to ask: from URL
<http://www.edaboard.com/thread194703.html>
You have to program it with every frequency in your chosen sweep. i.e.
you have to program every step manually.

You can alter the output power via one of the control registers and
this might help flatten the output power but my guess is that your
output stage from the 4350 will contribute a lot of the unequal power
delivery.

From memory you have to reprogram registers 0 and 4 each time you
update for a new frequency.

You could run it into a limiter amplifier and then compensate for the
rolloff in the limiter with a simple lumped network to get a very flat
output power across the full range.

Note: You will get fairly poor spectral purity from the ADF4350
because of the way it operates via internal dividers. i.e. you will
get fairly high harmonic content from it. If you use a limiter to
level the output then the purity will get worse but it might still
meet your requirements.

Thats another problem I'm going to think about later on. I'm not quite
happy with the amount of harmonics produced by the 4350.

Your sweep speed will be limited by the speed of your SPI interface to
the ADF4350 and the number of programmed step points you choose..
=3D =3D =3D

The SPI interface can go up to tens of MHz. Thats not really a problem

I may be wrong, but it you programmed output+ and output- to be the
same, you have a MUCH lower transmission line impedance right at the
terminals of the IC. More like 43 ohm differential 57 ohm com mode
yield around 31 ohm per line! pretty bad mismatch.for about 100 mils

Why is that? The outputs are open collectors. More or less current
sinks with infinite (tm) impedance. They outputs are terminated into
50 Ohms from the power supply.

Anyway, I have removed one of the output traces with a dremel-like
tool to make sure but the problem persists.

 

[Martin Riddle]

On Apr 11, 2:51=A0am, [email protected] (Nico Coesel) wrote:
On Tue, 10 Apr 2012 22:54:19 GMT, [email protected] (Nico
Coesel)
wrote:

Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL
chip to
serve as a simple RF generator. The ADF4350 can output a
signal
between 140MHz to 4.4GHz. It seems to work but the output
shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpgshowing2
traces). I can't test beyond 2.6GHz The board is a 4 layer
FR4
PCB.

So far I tried to change the decoupling capacitors (0402), add
extra
wires to the power supply pins, change the output termination,
use
different cables (SMA) but this changes nothing at all.

Is there an easy way to pin this problem down without spending
another
fortune on equipment? I guess there is a parallel resonator
somewhere.
To make matters worse I'm not quite sure my SA is 100%. I'm
thinking
about simulating the board using Sonnet or putting together a
simple
power level meter.

Any other ideas?

Got a pic of the PC board?

This is the HF
part:http://img96.imageshack.us/img96/1460/img0506bc.jpg

The next layers are ground and power. I already tried to make
the 'S'
shorter with a wire but no difference at all.

----

Is that 'S' shape even 50 ohm trace?

It should be

I use femm 4.2 to calculate trace widths [very accurate], or an
old
DOS tool called DIMSTRIP.exe that Engineers claimed better than 1%
accuracy to 1GHz. If you want a copy of the 50k program let me
know.

Given a box lid 1 inch above surface of PCB, FR-4 dielectric 4.2
and
loss tangent of 0.06, 1 mil copper metallization layer [balance
between 0.7 and 1.4 mil], dielectric thickness of 20mils; Dimstrip
calculates the S curve trace width should be 38.9 mils to get
Zo=3D50
ohms.

These are the specs from the board house concerning the layer
distances:
http://www.eurocircuits.com/index.php/technology-guidelines/pcb-multi...

The dielectric is 14 mils. The S trace is 26 mils wide.

Appcad puts that at 49 ohms. The trace is basically perfect, and is
short enough that it wouldn't matter much anyhow.

--

John Larkin Highland Technology
Incwww.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Not very perfect right at the IC and the run to the SMA is into the
wavelength size, which is significant length.

wavelength INSIDE dielectric is approx 300e6/2.4GHz/.0254/sqrt(4.2)/
0.0254 = .2.4 inches

that means the 1/4 wavelength where all the 'magic' rotations occur,
is around 0.6 inches!

There's no way his transmission line structure can explain the huge
dip he's seeing. It's very nicely matched in the DC-3 GHz sort of
range. I do boards that look just about like that, on FR4, and go a
lot faster.

I think its external interference. He should do a site scan out of
curiosity. Or like suggested, put the board in a metal can.

Those 6 control lines on the left look lonely, for kicks I'd place a
ferrite bead or core across them and see what happens.

Cheers

 

[Joerg]

[...]

14 mil thick and 26 mils wide moves a LOT of field up into the air!
changes the 4.2 to 3.2 effective. Not sure it is good to have that
floating around above your circuitry.

What do you mean by 'serious mismatches in circuits'?

A slightly different subject: AFAIK there are at least two ways to
determine mismatch. TDR or by using a directional coupler and see what
is reflected back. I'd like to get a setup with a directional coupler.

Measure the output without the filter. You can do that without building
or buying anything. Then if you have determined that it is really the
filter you can add 10-20% or so to a cap here and there and see which
one moves the resonance. If your analyzer has a track gen you can also
plot the filter passband. There are so many ways to peel the onion here.

If you really have to have a directional coupler, Mini-Cicuits carries
those. Not sure how good, always meant to buy one but so far did all
such jobs without.

Thats another problem I'm going to think about later on. I'm not quite
happy with the amount of harmonics produced by the 4350.

If you need a wide frequency range to be convered by the chip consider a
tracking filter.

The SPI interface can go up to tens of MHz. Thats not really a problem



Why is that? The outputs are open collectors. More or less current
sinks with infinite (tm) impedance. They outputs are terminated into
50 Ohms from the power supply.

Anyway, I have removed one of the output traces with a dremel-like
tool to make sure but the problem persists.

On really short stretches (a centimeter or so) such mismatches don't
affect things much.

 

[Joerg]

On Apr 11, 9:07 am, John Larkin
On Apr 11, 2:51=A0am, [email protected] (Nico Coesel) wrote:
On Tue, 10 Apr 2012 22:54:19 GMT, [email protected] (Nico
Coesel)
wrote:
Every now and then I spend some time tinkering with HF stuff.
I've put a board together with an ADF4350 fractional-N PLL
chip to
serve as a simple RF generator. The ADF4350 can output a
signal
between 140MHz to 4.4GHz. It seems to work but the output
shows a
large dip at 2.487 GHz and small one at 2.287 GHz (see
http://img201.imageshack.us/img201/4512/img0498ry.jpgshowing2
traces). I can't test beyond 2.6GHz The board is a 4 layer
FR4
PCB.
So far I tried to change the decoupling capacitors (0402), add
extra
wires to the power supply pins, change the output termination,
use
different cables (SMA) but this changes nothing at all.
Is there an easy way to pin this problem down without spending
another
fortune on equipment? I guess there is a parallel resonator
somewhere.
To make matters worse I'm not quite sure my SA is 100%. I'm
thinking
about simulating the board using Sonnet or putting together a
simple
power level meter.
Any other ideas?
Got a pic of the PC board?
This is the HF
part:http://img96.imageshack.us/img96/1460/img0506bc.jpg
The next layers are ground and power. I already tried to make
the 'S'
shorter with a wire but no difference at all.
----
Is that 'S' shape even 50 ohm trace?
It should be
I use femm 4.2 to calculate trace widths [very accurate], or an
old
DOS tool called DIMSTRIP.exe that Engineers claimed better than 1%
accuracy to 1GHz. If you want a copy of the 50k program let me
know.
Given a box lid 1 inch above surface of PCB, FR-4 dielectric 4.2
and
loss tangent of 0.06, 1 mil copper metallization layer [balance
between 0.7 and 1.4 mil], dielectric thickness of 20mils; Dimstrip
calculates the S curve trace width should be 38.9 mils to get
Zo=3D50
ohms.
These are the specs from the board house concerning the layer
distances:
http://www.eurocircuits.com/index.php/technology-guidelines/pcb-multi...
The dielectric is 14 mils. The S trace is 26 mils wide.
Appcad puts that at 49 ohms. The trace is basically perfect, and is
short enough that it wouldn't matter much anyhow.

--

John Larkin Highland Technology
Incwww.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Not very perfect right at the IC and the run to the SMA is into the
wavelength size, which is significant length.

wavelength INSIDE dielectric is approx 300e6/2.4GHz/.0254/sqrt(4.2)/
0.0254 = .2.4 inches

that means the 1/4 wavelength where all the 'magic' rotations occur,
is around 0.6 inches!

There's no way his transmission line structure can explain the huge
dip he's seeing. It's very nicely matched in the DC-3 GHz sort of
range. I do boards that look just about like that, on FR4, and go a
lot faster.

I think its external interference. He should do a site scan out of
curiosity. Or like suggested, put the board in a metal can.

Yup. Hand waving can show this already, literally. Cup the hands over
the board and if the dip changes, bingo.

Those 6 control lines on the left look lonely, for kicks I'd place a
ferrite bead or core across them and see what happens.

The usual ferrite material is no good up there anymore. Forming 1-2
loops around a chop stick would likely yield better results. And it
ain't gonna cost nothing if you pull them chop sticks back out

 

[Joerg]

Use your fingers!

when you touch the part that is resonant, it will shift frequency
lower and de Q.

Someday you can attain RF wizard status like me.

I can fix things simply by touching them.

Now if the whole table starts to hover when you do that I'd say it's not
an RF effect

 

[Nico Coesel]

[...]

14 mil thick and 26 mils wide moves a LOT of field up into the air!
changes the 4.2 to 3.2 effective. Not sure it is good to have that
floating around above your circuitry.

What do you mean by 'serious mismatches in circuits'?

A slightly different subject: AFAIK there are at least two ways to
determine mismatch. TDR or by using a directional coupler and see what
is reflected back. I'd like to get a setup with a directional coupler.

The above is a slightly different topic. Once I get this setup going
I'd like to use it for testing some odd ways to connect ethernet. RJ45
is not always the most practical connector.

Measure the output without the filter. You can do that without building
or buying anything. Then if you have determined that it is really the
filter you can add 10-20% or so to a cap here and there and see which

Actually there is no filter at the output

I ordered some RF detector schottky diodes. If all goes well they'll
arive tomorrow. I really need to figure out whether the problem is in
the SA or in my circuit.

If you really have to have a directional coupler, Mini-Cicuits carries
those. Not sure how good, always meant to buy one but so far did all
such jobs without.

I've use a directional coupler from Mini-Circuits before for antenna
matching. But that was only 430MHz. Fortunately I can buy direct from
their UK office.

If you need a wide frequency range to be convered by the chip consider a
tracking filter.

That sounds like a good plan. But I'll need to read more about that
first.

On really short stretches (a centimeter or so) such mismatches don't
affect things much.

As I noticed There is some change (a few dB) in the output level
though.

 

[Joerg]

The above is a slightly different topic. Once I get this setup going
I'd like to use it for testing some odd ways to connect ethernet. RJ45
is not always the most practical connector.


Actually there is no filter at the output

I ordered some RF detector schottky diodes. If all goes well they'll
arive tomorrow. I really need to figure out whether the problem is in
the SA or in my circuit.

The spectrum analyzer can easily be tested for that. Hang a preamplifier
onto it with nothing connected to it's input. This will raise the noise
floor. Most spectrum analyzers are sensitive enough by themselves
though, you can test that by shorting the input and seeing if the noise
floor changes. A dip in the noise floor at 2.45GHz should be clearly
visible and if there is none the analyzer can hardly be at fault here. I
still think you might have been tripped up by your or someone else's
WLAN. Or by some other device that transmits around 2.45GHz (there are a
lot).

I've use a directional coupler from Mini-Circuits before for antenna
matching. But that was only 430MHz. Fortunately I can buy direct from
their UK office.

That's nice. In Germany we always had to go through a distributor in the
90's. But they were quite competent. Might still be that way.

That sounds like a good plan. But I'll need to read more about that
first.

It's essentially just a filter with varicaps in there. I always liked
those from the good old Philips Gloeilampenfabrieken ... ahm ... NXP

Muxed individual filters work as well but not so great if this is to be
continually sweeped.

As I noticed There is some change (a few dB) in the output level
though.

A few dB? That's a bit much, almost sounds like you have a resonance in
some part.

 

[Fred Bartoli]

Joerg a écrit :

Now if the whole table starts to hover when you do that I'd say it's not
an RF effect

Of course it is. It's the Race Freak effect...

 

[Nico Coesel]

Nico Coesel wrote:
[...]


14 mil thick and 26 mils wide moves a LOT of field up into the air!
changes the 4.2 to 3.2 effective. Not sure it is good to have that
floating around above your circuitry.

What do you mean by 'serious mismatches in circuits'?
A slightly different subject: AFAIK there are at least two ways to
determine mismatch. TDR or by using a directional coupler and see what
is reflected back. I'd like to get a setup with a directional coupler.

The above is a slightly different topic. Once I get this setup going
I'd like to use it for testing some odd ways to connect ethernet. RJ45
is not always the most practical connector.

Measure the output without the filter. You can do that without building
or buying anything. Then if you have determined that it is really the
filter you can add 10-20% or so to a cap here and there and see which

Actually there is no filter at the output

I ordered some RF detector schottky diodes. If all goes well they'll
arive tomorrow. I really need to figure out whether the problem is in
the SA or in my circuit.

The spectrum analyzer can easily be tested for that. Hang a preamplifier
onto it with nothing connected to it's input. This will raise the noise
floor. Most spectrum analyzers are sensitive enough by themselves
though, you can test that by shorting the input and seeing if the noise
floor changes. A dip in the noise floor at 2.45GHz should be clearly

I tried that but the noise floor stays flat at -70dBm.

Anyway, I've tried the simple RF detector (
http://1.bp.blogspot.com/_vfmOyxDCr...AGpc/IZAMFN1e-_I/s1600/RF_Probe_Schematic.jpg
with a HSMS-286B detector diode).
The output isn't flat but there is no extreme dip either. When I
change the output power by 6dB the detector output also changes
accordingly. The dip is about -20dB which should result in the
detector output going near 0.

So far I'm leaning towards the SA being bad. I'll wait for the RF
detector chips to arrive and put one on a PCB to (hopefully) get a
better measurement than the simple detector provides and reach a final
verdict.

It's essentially just a filter with varicaps in there. I always liked
those from the good old Philips Gloeilampenfabrieken ... ahm ... NXP

Any references? Google seems useless on this subject