Designing a wideband receiver ?

[yolande_yj]

Can I design a system that can do the following job?

It monitors a signal which

1. has variable power (I am not sure the power level at this moment)
2. has frequency from 1GHz to 6GHz (The freq is specified by the user)

The signal is down convered to IF (and further converted to digital signal by ADC).

Actually this is part of a VNA's job, but I want a simple solution. There must be many issues/difficulties. Can anyone suggest me a possible solution? Thanks.

 

[throwaway18]

>Can I design a system that can do the following job?

In principle, yes it can be done. Whether you can design one depends on your knowledge and skill.

More information is needed such as the bandwidth of the signal being received.

 

[yolande_yj]

Hi throwaway18, yes, I know it can be done. In fact, network analyzer, spectrum analyzer are everywhere. I am wondering if there is simple solution for my particular requirement.

The signal is narrow band, or one tone signal. But it can be changed by the user from 1GHz to 6GHz. In the very begining and for the demo purpose, I want to use just a signal gen to generate the LO signal. I don't mind to use expensive component such as SAW filter for image rejection.

Let's say if I use broadband LNA followed by a mixer to downconver the signal to IF. The conversion gain varies in different frequency (do you have any idea how much the ConvGain changes?). A complete flat gain is impossible I think. So how to "calibrate" this? Do I need a PGA followed by the mixer?

For image rejection, since the signal's frequency changes, so does the image freq. So I think it is better to filter the image after the mixer. Do you think I should use off chip SAW filter or on chip Complex Filter/PPF?

Should I use single conversion or double conversion?

If I use descrete component, I think I have problem in matching between stages. Should I use several channels? If yes, how to implement the switch to divert the signal?

 

[throwaway18]

I assume we are talking about a radio receiver that will get a signal from an antenna.

I havn't built much equipment for >440MHz so I'm not too hot on the specifics of microwave stuff.

>The signal is narrow band, or one tone signal.
A single rf carrier or an audio tone modulated onto a carrier?

Let's say if I use broadband LNA followed by a mixer to downconver the signal to IF.

This will inevitably result in a receiver with an image reception problem at either
tuned frequency + 2*IF or tuned frequency - 2*IF depending on whether you put the
LO at F+IF or f-IF. This might be acceptable, the receiver will work when there is
not much energy at the image frequency. I imagine you would be most likly to have
a problem when the image frequency is in the 2.4GHz microwave oven band or the 1800/1900MHz
mobile phone bands.

The only options to avoid image reception are either to filter the image frequency before
the mixer or to use a first IF which is higher than the signal frequency.

If you used an IF frequency of say 500MHz you could have a set of 1GHz wide front end
filters that you select depending on where you are tuned. Electronically switching between
filters with enough isolation would be a problem, perhaps you could do some research in
that area, minicircuit's make some **broken link removed**but I can't find a price off hand so they are probably not cheap, alternativly a big
mechanical switch designed for rf use (constant characteristic impedance) or coaxial relays.
Perhaps you could just plug in whichever filter you need.

How about a first IF at around 7GHz, second IF at say 500MHz ?

>So I think it is better to filter the image after the mixer.
You can't. The problem is that the wanted signal and the signals at the image frequency
are shifted to the same frequency after the mixer.

>The conversion gain varies in different frequency

Either use AGC or in an FM system you may get away with running the amps into clipping when the signal is strong.

>Should I use single conversion or double conversion?

Depends on the performance you require.
You will genereally be able to be get better sensativity and selectivity with a double conversion receiver.

 

[springf2000]

the most difficult is to design the LO for your receiver! in fact wideband LO is not easily to do, if you want the phase noise of the LO is good and then the tuning bandwidth is good, then it is not a easy job!but you can use seperate bandwidth coverge and then use the SWITCH to select your desired LO!

 

[yolande_yj]

Guys, thanks a lot.

Can I use a single mixer or two mixers if double conversion is used to cover the whole frequency range (1-6GHz)?

Can I use discrete components to setup the system? I think matching between stages is a big issue if discrete components are used.

To throwaway18,

A single rf carrier or an audio tone modulated onto a carrier?

Single rf carrier.

I imagine you would be most likly to have a problem when the image frequency is in the 2.4GHz microwave oven band or the 1800/1900MHz mobile phone bands.

Yes, I will use metal to shield the entire system. The image problem is it probably add 3dB noise figure to my system.

You can't. The problem is that the wanted signal and the signals at the image frequency are shifted to the same frequency after the mixer.

Yes, I can. IQ mixer + complex/ Polyphase filter can do this job. But I think the trade-offs are: the balence of the IQ mixers, the linearity of the complex filter or the loss of the PPF.

How about a first IF at around 7GHz, second IF at say 500MHz ?

My frequency range is from 1GHz to 6GHz

To springf2000:

use the SWITCH to select your desired LO

What kind of switch can do this job?