Also, you don't say what the input caoacitance is. In my modeling, with an
assumption of about 4pf for chip and strays, the voltage gain is about 9
(19db), the resonance is a little below 400 mhz and you'd be about 10 db
down at 434 mhz. I agree with Tim...tune it.

Joe
W3JDR


"Tim Wescott" wrote in message
...
David wrote:

I am having trouble matching to the input of a SA605 at 434 MHz.

If I apply a sig gen directly to pin 1 via a 1n cap I get around - 95Bm
sensitivity. I then add my match and the sensitivity drops more than
10dB. I am expecting 10dB increase in sensitivity when input is matched
correctly.

First I tried matching to 800R as per input impedance listed in the
Philips APP note. I then noticed most applications matched to 1k5.

My current circuit is C-Tap (33pF to ground and series 5p6 to inductor).
18.5nH inductor (Q = 100) across input pins 1 and 2.
10n decoupling cap from pin 2 to ground.

Any help much appreciated.

Regards

David

You don't mention any adjustable components -- you _are_ tuning this
thing, right? Your circuit will have a loaded Q of around 5, so you
should see a pretty broad peak, but you will have to tune out all your
stray capacitance.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Yes, I tune the series capacitance of the tap(3-10p trimmer), the 5.6pF
is the calculated value.
The input capacitance of the IC is assumed to be around 2.6pF and I
allowed a couple of pF stray circuit capacitance.

As the match is so broad due to low Q, I would assume the tuning is not
critical and that any matching closer to the device input impedance
would show a significant improvement in sensitivity.

Does anyone know what the impedance of the SA605 is at 434 MHz?

Philips APP note AN1994 shows a table that states 1785 Ohms // 2.5 pF at
250 MHz and dropping to 588 Ohms // 2.75pF at 500 MHz. If I interpolate
the values between these points and assume the change is almost linear
then I would expect the impedance to be around 800R // 2p5 or 2.6.

I do not have a network analyser to determine the impedance.

Thanks

Regards

David


W3JDR wrote:
Also, you don't say what the input caoacitance is. In my modeling, with an
assumption of about 4pf for chip and strays, the voltage gain is about 9
(19db), the resonance is a little below 400 mhz and you'd be about 10 db
down at 434 mhz. I agree with Tim...tune it.

Joe
W3JDR


"Tim Wescott" wrote in message
...
David wrote:

I am having trouble matching to the input of a SA605 at 434 MHz.

If I apply a sig gen directly to pin 1 via a 1n cap I get around - 95Bm
sensitivity. I then add my match and the sensitivity drops more than
10dB. I am expecting 10dB increase in sensitivity when input is matched
correctly.

First I tried matching to 800R as per input impedance listed in the
Philips APP note. I then noticed most applications matched to 1k5.

My current circuit is C-Tap (33pF to ground and series 5p6 to inductor).
18.5nH inductor (Q = 100) across input pins 1 and 2.
10n decoupling cap from pin 2 to ground.

Any help much appreciated.

Regards

David
You don't mention any adjustable components -- you _are_ tuning this
thing, right? Your circuit will have a loaded Q of around 5, so you
should see a pretty broad peak, but you will have to tune out all your
stray capacitance.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Your 10 nanofarad capacitor as a bypass on the low RF input is ridiculous.
It has to have a self-resonance down in the low MHz., if not the kHz.

Use a bypass something on the order of 50 pf if a leaded capacitor, and
something like 200 pf if a chip.

Jim




"David" wrote in message
...
Yes, I tune the series capacitance of the tap(3-10p trimmer), the 5.6pF is
the calculated value.
The input capacitance of the IC is assumed to be around 2.6pF and I
allowed a couple of pF stray circuit capacitance.

As the match is so broad due to low Q, I would assume the tuning is not
critical and that any matching closer to the device input impedance would
show a significant improvement in sensitivity.

Does anyone know what the impedance of the SA605 is at 434 MHz?

Philips APP note AN1994 shows a table that states 1785 Ohms // 2.5 pF at
250 MHz and dropping to 588 Ohms // 2.75pF at 500 MHz. If I interpolate
the values between these points and assume the change is almost linear
then I would expect the impedance to be around 800R // 2p5 or 2.6.

I do not have a network analyser to determine the impedance.

Thanks

Regards

David


W3JDR wrote:
Also, you don't say what the input caoacitance is. In my modeling, with
an
assumption of about 4pf for chip and strays, the voltage gain is about 9
(19db), the resonance is a little below 400 mhz and you'd be about 10 db
down at 434 mhz. I agree with Tim...tune it.

Joe
W3JDR


"Tim Wescott" wrote in message
...
David wrote:

I am having trouble matching to the input of a SA605 at 434 MHz.

If I apply a sig gen directly to pin 1 via a 1n cap I get around - 95Bm
sensitivity. I then add my match and the sensitivity drops more than
10dB. I am expecting 10dB increase in sensitivity when input is matched
correctly.

First I tried matching to 800R as per input impedance listed in the
Philips APP note. I then noticed most applications matched to 1k5.

My current circuit is C-Tap (33pF to ground and series 5p6 to
inductor).
18.5nH inductor (Q = 100) across input pins 1 and 2.
10n decoupling cap from pin 2 to ground.

Any help much appreciated.

Regards

David
You don't mention any adjustable components -- you _are_ tuning this
thing, right? Your circuit will have a loaded Q of around 5, so you
should see a pretty broad peak, but you will have to tune out all your
stray capacitance.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Jim,

I replaced the ridiculously large 10n with 200pF as suggested. Not a
gnat's wisker difference in the response. Should this have some
noticeable difference ?

The Philips app note AN1993 shows a 152 MHz RF receiver using even
larger and more ridiculous 100nF values for the RF decoupling throughout
the circuit. Why would the IC manufacturer use these values ?

Regards

David

RST Engineering wrote:
Your 10 nanofarad capacitor as a bypass on the low RF input is ridiculous.
It has to have a self-resonance down in the low MHz., if not the kHz.

Use a bypass something on the order of 50 pf if a leaded capacitor, and
something like 200 pf if a chip.

Jim




"David" wrote in message
...
Yes, I tune the series capacitance of the tap(3-10p trimmer), the 5.6pF is
the calculated value.
The input capacitance of the IC is assumed to be around 2.6pF and I
allowed a couple of pF stray circuit capacitance.

As the match is so broad due to low Q, I would assume the tuning is not
critical and that any matching closer to the device input impedance would
show a significant improvement in sensitivity.

Does anyone know what the impedance of the SA605 is at 434 MHz?

Philips APP note AN1994 shows a table that states 1785 Ohms // 2.5 pF at
250 MHz and dropping to 588 Ohms // 2.75pF at 500 MHz. If I interpolate
the values between these points and assume the change is almost linear
then I would expect the impedance to be around 800R // 2p5 or 2.6.

I do not have a network analyser to determine the impedance.

Thanks

Regards

David


W3JDR wrote:
Also, you don't say what the input caoacitance is. In my modeling, with
an
assumption of about 4pf for chip and strays, the voltage gain is about 9
(19db), the resonance is a little below 400 mhz and you'd be about 10 db
down at 434 mhz. I agree with Tim...tune it.

Joe
W3JDR


"Tim Wescott" wrote in message
...
David wrote:

I am having trouble matching to the input of a SA605 at 434 MHz.

If I apply a sig gen directly to pin 1 via a 1n cap I get around - 95Bm
sensitivity. I then add my match and the sensitivity drops more than
10dB. I am expecting 10dB increase in sensitivity when input is matched
correctly.

First I tried matching to 800R as per input impedance listed in the
Philips APP note. I then noticed most applications matched to 1k5.

My current circuit is C-Tap (33pF to ground and series 5p6 to
inductor).
18.5nH inductor (Q = 100) across input pins 1 and 2.
10n decoupling cap from pin 2 to ground.

Any help much appreciated.

Regards

David
You don't mention any adjustable components -- you _are_ tuning this
thing, right? Your circuit will have a loaded Q of around 5, so you
should see a pretty broad peak, but you will have to tune out all your
stray capacitance.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

"David" wrote in message
...
Jim,

I replaced the ridiculously large 10n with 200pF as suggested. Not a
gnat's wisker difference in the response. Should this have some noticeable
difference ?

Only if the low side RF input grounding makes a bit of difference in the
performance. It seems as though this device is rather numb to doing
anything with the other balanced input.




The Philips app note AN1993 shows a 152 MHz RF receiver using even larger
and more ridiculous 100nF values for the RF decoupling throughout the
circuit. Why would the IC manufacturer use these values ?

Several reasons come to mind. The aforementioned numb unused input. They
found some interesting motorboating (low frequency) proclivities and started
throwing capacitors at it until they found the right value. The
applications engineer was two weeks out of school on his first real RF
circuit.

Jim

"RST Engineering" wrote in message
...
Several reasons come to mind. The aforementioned numb unused input.
They
found some interesting motorboating (low frequency) proclivities and
started throwing capacitors at it until they found the right value. The
applications engineer was two weeks out of school on his first real RF
circuit.

Something I've found myself -- and several considerably experienced RF
designers have verified -- is that RF coupling and bypass capacitors are
often nowhere near as critical as you might think they are. On a real
circuit board, by the time you start getting close to the SRF of even a
100nF capacitor, there's usually enough parasitic coupling that what the
capacitor is doing just doesn't matter that much. I've used 100nF coupling
caps in paths all the way up to 3GHz, and while you *can* see a difference
in S21 compared to using, e.g., 10pF, it's really, really small... like,
under 1/4dB, and thus the overall response is usually swamped by active
devices, loads, etc.

Thus I'm not at all surprised the OP saw absolutely no different when he
reduced the 10nF caps to 200pF.

That being said, I wouldn't throw a larger-than-necessary cap into an RF
path without good reason. My main point is that the problem of providing
wideband coupling is often nowhere near as difficult as it first appears.
On the other hand, this implies the well-known corollary that isolation is
often a lot *harder* to achieve than we'd like.

---Joel