Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]()
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 |
#2
![]() |
|||
|
|||
![]()
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 |
#3
![]() |
|||
|
|||
![]()
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 |
#4
![]() |
|||
|
|||
![]()
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 |
#5
![]() |
|||
|
|||
![]()
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 |
#6
![]() |
|||
|
|||
![]()
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 |
#7
![]() |
|||
|
|||
![]()
David,
http://www.geocities.com/markaren10/misc/SA605.gif You had this in mind ?? -Mark. 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 |
#8
![]() |
|||
|
|||
![]()
Mark,
Yes, that's the circuit I am having trouble with. Regards David MarkAren wrote: David, http://www.geocities.com/markaren10/misc/SA605.gif You had this in mind ?? -Mark. 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 |
#9
![]() |
|||
|
|||
![]()
David,
What type of inductor are you using ? SMT can be sensitive with proximity issues. Are you 100% sure that capacitor values are as you hoped - SMT can be a pain to verify. Note, I did drop the 33pF to 15pF to improve return loss. The 6.5pF cap is the most sensitive of all parts in the cct. Any chance of a photo of the input network and signal coupling ? My current bet is an inadvertently loaded wrong value component. Been there, done that etc. Regards, Mark =========== David wrote: Mark, Yes, that's the circuit I am having trouble with. Regards David MarkAren wrote: David, http://www.geocities.com/markaren10/misc/SA605.gif You had this in mind ?? Mark. 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 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
New program. Input Z of a loaded line | Antenna | |||
Input stage for VHF frequency counter in an FPGA? | Homebrew | |||
What's this inductor doin'? | Homebrew | |||
Antenna matching 433MHz | Antenna | |||
Reflected power ? new thread, new beginning, kinda ? | Antenna |