Elementary RF questions

What're the importance of the following:
- Antenna matching
- Ground sheilding for PCBs
-Local Oscillator frequency is greater than RF frequency and higher in amplitude?

Thanks in advance,
Ahmad,

1) Antenna matching: depend if the antenna is trasmitting or receiving. If the antenna is in trasmission and the antenna it is not matched, there is signal that return to the transmitter (PA usually) and you can break it. If you are receiving, the return signal can create some hole in the received signal.
2) The ground shield is very important in the PCB, specially if you work at RF. It is the reference point for the signal and shield the RF tracks of the other signals. It is very important to have a very good ground shield.
3) If yo mean the LO due to the PLL it is often so, because usually the PLL works at a frequency x2 or x4 of the RF signal. It is preferred to work so because it is more simple to integrate the VCO inductor and it is simple to have the quadrature signals (I and Q) from the division.

By

Well, I konw that shielding is important, but i want to know the theoritical reason for it..

Hi ahmad,

antenna matching is done to achieve maximum signal reception/transmission to the receiver or transmitter (the antenna has an impedace to be calculated and be matched to the output impedance of a transmitter or the input impedace of the receiver) (ahem.. please correct me if Im wrong or misunderstanding you)

"Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system." -a friend

What're the importance of the following:
- Antenna matching

In order to maximize the received energy signal from the air to the IC you need to use match antenna to the impedance of the ckt.
you can derivate the when you have source and load with the same impedance the power transfer is maximized.

- Ground sheilding for PCBs
you want radiated signals from the air not to couple into your circuit. basically, you ground sheild your circuit and thus protecting it from any interferers surrounding your ckt.

-Local Oscillator frequency is greater than RF frequency and higher in amplitude?

LO which is bigger then the RF signal requires slighly more complicated LO and higher phase noise due to the high frequency.
It is much easier to implement a low frequency oscillator then high (mostly because you are not affected from parasitics as much as high LO).

Regards.
Jimmy

Jim cage

what you say about the LO frequency vs. RF frequency is not always true. If you design an application with discrete components it is true, but if you works in an integrate chip it is better to work with LO > RF frequency. This why the VCO inductor is lower at higher frequency. In thiscase when you divide you obtain an improvement in the VCO phase noise of 6 dB dividing by two. This improvement is 3 dB in the flat zone of the PLL.

By

As far as the LO is concerned...
This is usually due to the fact so as to convert the RF signal into intermediate frequency (IF) signal...the IF stage is optimized for signal processing...

For LO in a PLL i agree with others...

If the IF frequency is low, there's no need lo frequency being higher than rf frequency.

Hi Ahmad,

In most cases I generally agree with what the others say, but there
are still two things about the LO that need to be said. But ....
because asking questions is a lot easier than giving answers, I'm
gonna ask you two questions:

1. Suppose you build two identical receivers, the first with the LO lower
than the wanted signal and the second with the LO higher than the
wanted signal. Which of the two receivers is gonna have the largest
tuning range ?

2. Where do you think the mirror frequencies will be with respect to the
wanted signal and what are their distances from the wanted signal ?

Regards,

on1aag.

on1aag,
I understood what you say about tuning range and mirror frequencies
of course tunining range is higher incase of higher LO frequency than RF and mirror frequency will be much away of the band i want to work in.

But i was asking about LO amplitude is higher than RF (much higher than it)
And i knew the answer,

Anyway,
Thanks for help
Regards,
Ahmad,

BTW, we frequently are talking about "antenna matching" and "antenna length should be 1/4 of the wave length". My question is that are these two requirement unified? Or they are actually two requirements instead?