How to improve SNR in Antenna Cirsuit Design

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I am looking into an existing GPS mini-size patch antenna design (1575MHz),
which includes an LC low-pass filter, an amplifier with RFI suppression
(BFP640), and a pi attenuator about 3 dB. The SNR values are generally 5dB
lower than normal. So I am wondering if I remove the pi attenuator, will the
SNR be able to get improved?

I also found the antenna ground was not big enough (about 26mm* 26mm), and I
suspect that it was the reason that caused bad SNR values. Any idea to
increase the SNR values without significantly increasing the Antenna ground?
Can I modify the LC low-pass filter and amplifier to improve the SNR as
well?

Thanks.

Johnson



Re: How to improve SNR in Antenna Cirsuit Design
On Thu, 10 Mar 2005 11:44:30 -0700, "Johnson Liuis"

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An LC filter made of lumped components at 1.5 GHz ????

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Is the attenuator between the antenna and preamplier or after the
preamplifier ?

If the attenuator is in front of the amplifier, removing it would
improve the SNR by 3 dB. The only sensible reason for putting a
resistive attenuator in front of the preamplifier would be that the
amplifier was unstable, when connected to a highly reactive antenna.
Inserting a resistive attenuator would create a nearly 50 ohm
resistive source impedance and the amplifier would be stable. However,
this would be a very bad way to tame the amplifier.

Paul


Re: How to improve SNR in Antenna Cirsuit Design
Thanks for your quick reply, Paul,

I just made a mistake. The LC network is not a low-pass filter, actually it
is a bandpass/bandstop resonant filter around 1575MHz. I just calculated the
resonant frequency and to my surprise, I found the calculated frequency is
about 1.9GHz instead of 1.575GHz, so it seemed that the previous designer
took the parasitic capacitor and inductor into account. Could you please let
me know if it is reasonable to leave about 0.3GHz room for parasitic
capacitor and inductor?

BTW, the RF signal is first feed into the LC bandpass filter, then to a
Low-noise Amplifier, then to the Pi Attenuator.

If the attenuator is behind the LNA, can I still get some nenefits in
improving SNR?

You mentioned that it is a very bad way to tame the amplifier by adding Pi
attenuator, do you have any better way?

Thanks.

Johnson

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(1575MHz),
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Re: How to improve SNR in Antenna Cirsuit Design
On Thu, 10 Mar 2005 13:36:20 -0700, "Johnson Liuis"


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I would not use discrete inductors and capacitors at such frequencies
due to the low Q and spurious resonances, but rather make a bandpass
filter of a 1/4 wavelength resonator grounded at the other end. This
could be made of a stripline or microstrip construction. While this
has spurious responses at 3f, 5f etc., the transistor gain has dropped
quite a lot at these frequencies anyway.

There are a quite a lot circuits published in amateur radio amateur
literature for the 1.3 GHz (23 cm) and 2.4 GHz (13 cm) amateur bands,
which should be easily adaptable to 1.5 GHz.

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Only if the cable to the actual receiver is long and lossy or the
actual receiver noise figure is _very_ bad.

The preamplifier gain (including any post-amplifier attenuators) must
be large enough, in order to "mask" the cable or receiver noise. With
current semiconductors, you can get quite a lot of forward gain at
these frequencies, so I do not think this is an issue.

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It is a bad way to put an attenuator in front of a low noise
preamplifier, since the attenuation is directly added to the noise
figure and thus dropping the SNR by the amount of attenuation.

If an amplifier has a lot of leakage from output to input (the s12
parameter, reverse transducer gain), the undesired wave travels from
amplifier output to input and continues towards the antenna. The
antenna is well matched, this wave radiates into space through the
antenna. However, if there is a mismatch, the signal is reflected back
and enters again the amplifier input port, it is again amplified and
possibly causing oscillations, if the phase relations are favourable.

When you put an attenuator between the mismatch and the amplifier the
s12 wave is once attenuated going through the attenuator, it is then
reflected from the mismatch, attenuated again in the attenuator before
entering the amplifier input port and amplified. Due to the increased
attenuation of this unwanted wave, the ability to sustain oscillation
is greatly reduced.

To avoid the need for this attenuator, look at the overall input
matching and also look at s-parameters (s11, s21, s12 and s22) to find
a more suitable active device. It should be noted that if some input
filtering is used, there are certainly going to be mismatches at some
frequencies (unless some diplexer system is used), the s-parameters
should be studied at all frequencies at which the active device has
forward gain and not just on the desired reception frequency.

Now thinking about the attenuator between the preamplifier and the
actual receiver may have something to do with a bad mismatch between
the amplifier output and receiver input, which might also cause a
strong reflection at the receiver input, which would travel through
the preamplifier reverse gain to the input, reflected at some input
mismatch and entering the preamplifier input, causing oscillation. The
preamplifier output attenuator would also attenuate the wave reflected
at receiver input and thus reducing the risk of oscillation. Provided
that the preamplifier gain is large enough to mask the receiver input
noise, this attenuator between preamplifier and receiver should not
harm the noise total figure.

However, since you experienced lower SNR than expected, either the
preamplifier gain is too low (below 10-15 dB) or the preamplifier is
very noisy in the first place.
 
Paul


Re: How to improve SNR in Antenna Cirsuit Design
Thank you very much, Paul,

You mentioned that there are some circuits published in amateur radio
amateur
literature for the 1.3 GHz (23 cm) and 2.4 GHz (13 cm) amateur bands. Could
you please recommend a few websites or literatures to me?

Johnson



Re: How to improve SNR in Antenna Cirsuit Design
On Sun, 13 Mar 2005 21:43:16 -0700, "Johnson Liuis"


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Designing circuits for frequencies in the upper UHF range and above
requires in practice that you think about the various components in a
circuits as sections of transmission lines. For instance "The ARRL
UHF/Microwave Experimenter's Manual" will give some easy to read
theoretical background to this. It was published in 1990, so there is
not much point of copying the designs, since better components are
available. The book might still be available from www.arrl.org

For web searches, I would suggest including search expressions like
"23 cm", "1.3 GHz" or "1296 MHz" should help you find some modern
designs.

Have you looked for application notes from various GPS chip set
manufacturers. Even if you are not using the same chip-set,
application notes about antennas and preamplifiers should still be
usable. It should be noted that the line widths and lengths as well as
the PCB material as PCB thickness should be the same if copied, unless
you know how to scale them.

Take a look at www.maxim-ic.com they might also have some application
notes.

Paul
 

Re: How to improve SNR in Antenna Cirsuit Design
Thanks  Paul,.
You mentioned instead of using a discrete inductors and capacitors at such
frequencies
due to the low Q and spurious resonances, I'd better make a bandpass
filter of a 1/4 wavelength resonator grounded at the other end. This
could be made of a stripline or microstrip construction.Could you please l
et me know what is the Pros and Cons of this 1/4 wavelength resonator
compoared to adding a ceremic filter directly to the RF circuit before LNA?
Thanks in advance.
Johnson



Re: How to improve SNR in Antenna Cirsuit Design
BTW, 1/4 of the GPS wavelength is about 5cm. Does it mean that my antenna
circuit board has to be more than 5 cm in length to fit the 1/4 wavelength
resonator?
I am just an amateur about antenna design, sorry if my question is too
"naive".
Johnson



Re: How to improve SNR in Antenna Cirsuit Design

Sorry I made a mistake. The LC network is not a low-pass filter, actually it
is a bandpass/bandstop resonant filter around 1575MHz. I just calculated the
resonant frequency and to my surprise, I found the calculated frequency is
about 1.9GHz instead of 1.575GHz, so it seemed that the previous designer
took the parasitic capacitor and inductor into account. Could anybody please
let me know if it is reasonable to leave about 0.3GHz room for parasitic
capacitor and inductor?

BTW, the RF signal is first feed into the LC bandpass filter, then to a
Low-noise Amplifier, then to the Pi Attenuator.

Any idea?

John





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