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

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