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kmoravec

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Plano, TX

151 MHZ Filter Design Questions
Mar 14^th, 2008, 10:28am

I am designing a digital radio transciever for the 151-154 MURS band. I am not a RF designer, but since we could not find someone locally they told me to do it since I have a General Class Ham License. :)

I have the general design:

For transmitting, I am starting with a Analog Devices ADF7021, amplifying the transmission with a AD8354 RF Gain Block and then going into a ADG919 RF Switch to a LCL pi matching circuit, to an antenna. (I have to be < 2W transmit power)

For Receiving, Starting at the antenna the signal goes through the LCL pi matching circuit, to the ADG919 RF Switch, to the ADF7021.

First question is on the transmitter side. What is standard practice for a low pass filter? Should I put a low pass filter between the ADF7021 and the AD8354 gain block, or between the AD8354 gain block and the ADG919 or in both places? What order of filter?

Second question. On the receive side the datasheet has an "Optional Band Pass Filter". What determines ahead of time, whether I need it or not? I am going to probably put it in to be safe, but what order of band pass do I need? I do not expect a lot of RF signals near by as this will be used mostly in rural areas. What is good practice?

Thanks,
Kip

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didac

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Re: 151 MHZ Filter Design Questions
Reply #1 - Mar 14^th, 2008, 1:51pm

Hi,
Maybe I'm not the most suitable forum member since I'm more in integrated design than system level but here I go:
I haven't worked with MURS but making a google search I found that it's FCC regulated(http://www.provide.net/~prsg/murs_faq.htm) so your filter requirements should be derived from the spectral mask dictated in the rules http://www.provide.net/~prsg/mursrule.htm#95.635, from this requirements you could trace a asymptotic filter and derive the order.

You could place a filter between the IC and the gain block and another between the gain block and the switch, it depends if the IC generates spurs that could be useful to filter prior to postamplificate, and then you could filter again to be mask compliant.

I've checked the datasheet of both the IC and the block gain(quick reading) you specify and maybe I'm missing something but accordint to ADF7021 the output power of the internal PA it's 13dBm and the CP1dB of the gain block it's something like 2dBm at your band so I think that it's not a suitable block for this application.

For the receiver filter it applies the same, the order it's determined bby the worst case of unwanted emission. You should check the rules and see how much power will be outside your band then determine the filter order.

Hope it helps,

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kmoravec

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Plano, TX

Re: 151 MHZ Filter Design Questions
Reply #2 - Mar 14^th, 2008, 3:58pm

My bad. I wrote down the wrong part number. Your email just about gave me a heart attack. ;D

The Gain block is ADL5542 which has a gain of about 20 db. The ADF7021 has varing output, but the maximum is 13 dbm. So together we should be able to have 33 dbm output.

I started looking at the maximum output of the ADL5542 in dbm, and I do not recognize the number. It needs to be more than 33 dbm. I may still be in trouble.

Is there an RF amplifier that will do what I need, or do I have to build it out of discrete parts? Do you have a favorite that you would recommend to start me in the correct direction?

Thanks for the feedback.
Kip

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loose-electron

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Re: 151 MHZ Filter Design Questions
Reply #3 - Mar 14^th, 2008, 7:18pm

Hm....

A lot to ask of a radio ham.... (I am WA1QDI/6, but not active in many years)

You need to keep track of certain things:

On the receiver at all stages:
Amplitude of signals
Noise referenced back to the antenna
Linearity (IP2 and IP3)
Spectral masking/filtering profile and resulting amplitudes of all OOB (out of band) interfereing signals.

On the transmitter at all stages:
Amplitude of signals
Spectral spreading due to noise in the TX path
Linearity, spectral spurs
Spectral masking/filtering profile and resulting amplitudes of all OOB (out of band) spectral content that makes it to the antenna.

Typically the above can be done on an Excel spreadsheet, or using a simulator of the system (Matlab, or ADS)

The good news - Analog Devices seems to have produced a chip set for this application, so get in touch with their applications engineers and get the configuration from them to use the chip set. Usually there is apretty extensive set of application notes for a chip set.

Then it is a bit easier to get it done.

hope that helps,
vy 73,
Jerry

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Jerry Twomey
www.effectiveelectrons.com
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Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical

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