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FSK Receiver - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

 Post subject: FSK Receiver
Posted: Wed Jun 08, 2011 5:06 pm 

Joined: Wed Jun 01, 2011 9:36 pm
Posts: 2
I am trying to design an FSK receiver with a high data rate. (1 Mb/s) The operating frequency is 978 MHz. I have found a transceiver IC that will handle that rate but the frequency range is 900-928 MHz.

I have a reference design for that IC. It goes as:

Antenna -> SAW Filter -> LNA -> IC

Can I add a mixer to the chain to downconvert to, say, 900 MHz?


Antenna -> SAW -> LNA -> MIXER -> IC

The mixer would have to be capable of producing a high frequency IF. I could use a 78 MHz LO to produce 900 MHz and 1056 MHz and hope the IC's bandpass blocks the high side. Or perhaps I would have to put another SAW filter after the mixer?

I appeal to the experts.


 Post subject: Re: FSK Receiver
Posted: Thu Jun 23, 2011 2:02 pm 
Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany

It should be stated in the IC data sheet what is the out of band rejection around the carrier. If it is not mentioned, then it is better to use an additional filter.

Take into consideration the losses of the mixer (Conversion loss) in your chain. In addition, a for the LO source a driver amp. is most probably required.

Best regards,

- IR

 Post subject: Re: FSK Receiver
Posted: Wed Jul 06, 2011 7:48 am 

Joined: Wed Aug 16, 2006 8:20 am
Posts: 23
Location: India
It should not be difficult to find transceivers at 978 MHz.. 978 MHz is generally used in Airborne communications.. I am not sure about the data rate...
Since the IC you have has input range of 900-928 MHz it should have good rejection for the 1056 MHz signal.. also if you use a down conversion mixer, the 1056MHz at the mixer outpurt would be highly attenuated. Check the level of this signal at the mixer output.. you may save some money by using a simple LPF instead of an expensive SAW filter if the 1056MHz signal is small

Ashish Bondia,
Design Engineer- RF

 Post subject: Re: FSK Receiver
Posted: Wed Jul 06, 2011 12:57 pm 

Joined: Wed Jun 01, 2011 9:36 pm
Posts: 2
Yes, it is hard to find a transceiver that supports that data rate. RFMD makes one.

I have not heard of using a simple LP filter at these frequencies, it has been a hassle finding SAW filters, basically we have to spend $$$.

Right now the design will use a mixer to down convert to an IF that will feed an IF SAW filter and then a Quadrature demodulator.

This, of course, may change.



 Post subject: Re: FSK Receiver
Posted: Tue Sep 06, 2011 6:39 am 

Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
I would take something like a CC1101 or ADF7023, run it at 433 MHz, and down/upconvert your band to there. TI makes a chip with a LNA and transmit amp built into the same chip with a cheap switch to share the antenna that you could use as a front end.

Analog devices has some PLLs with onboard VCOs that could be the LO, or you could use another one of those transceiver chips just as a cw LO.

The only tricky thing is the input bandpass filter, as it is not a common band. maybe make a simple lumped element BPF since it is not a hard reject band for image suppression.

Maguffin Microwave Consulting

Posted  11/12/2012

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