frequency downconverter

[siyom]

I need to down convert a frequency of around 88 - 108MHz to about 40kHz.
I thought about using an AD831 mixer,but at an IF frequency of 10MHz I'll only go down a few MHz not to 40khz so I will need multiple mixers,and the AD831 is very expensive,what other mixers can i use that are less expensive?

Or are there mixers that perform dual operations?

 

[cowboybob]

That range? All frequencies between 88 to 108 MHz to a single frequency of 40kHz?

I'm curious. What for?

<EDIT> Right off the top of my head, just modulate the range 88 to 108MHZ of a sig gen's output with a constant 40kHz and demodulate that signal.

 

[siyom]

the 88 - 108 is generally just FM spectrum,I need to convert it to about 40-50kHz for digital signal processing purposes.
Will I need more than 2 mixers in this case?

 

[cowboybob]

Just off the top of my head, have you considered feeding the AD832, for example, a 98MHz unmodulated carrier, mixing it with a 98.040MHz signal and running the resultant signal through a very tight 40kHz band pass filter (BPF)?

(An example only)

The BPF might conceivably introduce unacceptable noise.

 

[crutschow]

You would need a tuneable source of 88MHz to 108MHz to mix with the signal to get the 40kHz output (signal source being 40kHz above or below the signal being tuned) and a high-order 40kHz bandpass filter after the mixer. But note that the bandwidth of a standard FM signal is much greater than 40kHz (about 150kHz) so you cannot make an FM radio that way, if that is your intention.

 

[dr pepper]

The fm stereo sub carrier is at 38kc, so if you were to use this on broadcast stations you'll have a strong 2 kc content.

 

[crutschow]

The fm stereo sub carrier is at 38kc, so if you were to use this on broadcast stations you'll have a strong 2 kc content.

That's for the spectrum of AM modulation. For FM the spectrum of the 38Khz is determined by the modulation index of the FM transmitter and would likely appear much higher than 38KHz for commercial FM modulation indexes. See this for more info.

 

[dr pepper]

Yes, I think I got that.

 

[ronsimpson]

The fm stereo sub carrier is at 38kc, so if you were to use this on broadcast stations you'll have a strong 2 kc content.

The carrier is transmitted at 19khz. There should not be anything at 38khz. Filter to get 20hzto 15khz and you will get a good mono signal.

 

[dr pepper]

I dont mean to argue, and this isnt my best subject, but isnt the 19kc signal a pilot tone that the receiver uses to tell if the incomming signal is stereo?

 

[EinarA]

Yes, 19khz is the pilot tone. But the stereo signal is modulated so that there is no carrier at 38Khz, so the 19Khz is used to regenerate the carrier for demodulation.
To be a little clearer: the R+L signal is combined with a R-L signal that has been suppressed carrier amplitude modulated at 38Khz and these signals then frequency modulate the transmitted signal. The receiver can't demodulate the R-L signal without a reference frequency so, for reasons I have forgotten, it is sent as a 19Khz pilot tone.

 

[crutschow]

The stereo R-L subcarrier is 38kHz but that carrier signal can't pass through the receiver audio bandpass so the reference carrier is transmitted at 19kHz. This signal is then doubled to 38kHz in the receiver to provide for synchronous demodulation of the stereo R-L signal.

 

[EinarA]

The R-L is SCAMed so occupies a 23 KHz to 53 KHz band and there is essentially no 38 KHz present. The signal from the demodulater has this amount of bandwidth ( and more). It is then sent to the stereo decoder which locks a 38 K Osc to the 19 K pilot tone ( and does the rest of recreating separate R and L channels.)
Reading my audio handbook I find that it is actually L-R, the demodulater bandwidth is around 1 MHz so as not to upset the phase of the signals, and the decoder Osc is 76 KHz and is divided down twice.

 

[siyom]

Ok so basically I will either need to either find a way around my DSP's dynamic range?or just use multiple Mixers...I thought the 98MHz to 98.04MHz was a good idea but the 150KHz FM spectrum will be a problem.

 

[cowboybob]

Are you using a "normal" FM receiver (mono and/or stereo) for your DSP project? The 150kHz spectrum is just a value (range) within which the content is placed. No content, no problem.

Or just that frequency range from a simple, non-modulated generator?

 

[siyom]

Yes its a normal FM reciever,nothing too complicated..I've just realised that I need to do a lot of reading with regards to this

 

[JimB]

To try and clarify your thinking, have a look at the attached picture taken from the screen of a spectrum analyser which was looking at an off air FM radio signal.

The analyser was set to 3KHz Resolution Bandwidth and the horizontal sweep is set to 20KHz per division.
The vertical scale is 10dB per division.
The picture is a composite of several sweeps across the screen of the analogue storage scope display.

You will see that the signal occupies far more than 40KHz.

JimB

 

[davenn]

indeed

Isnt it ~ 120 - 150 kHz for FM broadcast ?

Dave