how to convert 100Hz signal to intermediate frequency

[yyan71]

Hi there,

I am designing a device which is planning to transmit a ECG signal from a portable device to remote base station.

Now ECG front end circuit has been down and I can obtain the normal ECG signal under 150Hz.

Now I am not very sure how to convert the low frequency from under 150Hz to intermediate frequency (like 100MHz to 850MHz). The reason I need IF is that I will use a LO and Mixer to up-converting the signal to RF later.

I have two ideas:

1. Find a AM/FM modulator to modulate the signal and convert the signal frequency to IF (like 100MHz to 850MHz). Now I am using AD630, but this one can only give me a fair output at 10KHz (at most 50kHz).

2. Use a frequency multiplier to convert the low frequency to IF (like 100MHz to 850MHz). I haven't try this method yet.

My questions are:

1. Is there any modulator in market can help me to get an AM/FM modulated output at IF (like 100MHz to 850MHz).

2. Whether the second idea mentioned above is feasible? If it's feasible, is there any frequency multiplier in market can help me to get an output at IF (like 100MHz to 850MHz) with the input of 10kHz to 50kHz.

Welcome all the suggestions!!

Thank you very much for your advices.

 

[vfone]

You need a mixer that supports near DC on RF input as the one below.

http://www.datasheetcatalog.org/datasheet/motorola/MC13143D.pdf

The topology will be: Low Pass Filter, Mixer, LO (LO=IF-150Hz; IF can be anywhere in the mixer range), IF filter, IF Amplifier, etc.

 

[yyan71]

You need a mixer that supports near DC on RF input as the one below.

http://www.datasheetcatalog.org/datasheet/motorola/MC13143D.pdf

The topology will be: Low Pass Filter, Mixer, LO (LO=IF-150Hz; IF can be anywhere in the mixer range), IF filter, IF Amplifier, etc.

Thanks vfone for your suggestion. I have read the datasheet of MC13143, but I found that MC13143 has a differential IF outputs and need a z transformer. Because of lack of experience, I don't know whether balanced Mixer is better than the simple Y-mixer or not.

By the way, MC13143 is no longer manufactured. I find another component LT5560 which seems can be competent. Please see the datasheet below:

http://www.farnell.com/datasheets/72197.pdf
or http://cds.linear.com/docs/Datasheet/5560f.pdf

But this one is a double balanced mixer, I'm not quite sure about its performance and whether the transformers would bring some unpredictable troubles for the circuit.

My questions are:

1. Whether LT5560 could be used for my design, even if it is a double balanced mixer.

2. Based on LT5560's datasheet, I can not find any information about its performance under frequency of 10kHz. So, I'm not sure whether this component is like what is labeled on its title '0.01MHz to 4GHz Low Power Active Mixer' . Please give me some advice about the 0.01MHz.

Thanks a lot!!!!!!!!!!!!!!!!!!!

Best regards

Yang

 

[FvM]

It seems to me, that the question title is misleading. You are intending to transmit the analog ECG signal, so you actually need a
common FM transmitter, as it could be used e.g. for an audio signal.

 

[vfone]

Single or Double Balanced doesn't matter too much for this application. DBM is better anyway.
You can try to use LT5560 and see if it works with your low input frequency. Should work, but with less conversion gain.

 

[yyan71]

It seems to me, that the question title is misleading. You are intending to transmit the analog ECG signal, so you actually need a
common FM transmitter, as it could be used e.g. for an audio signal.

Yes, like you say. I am intending to transmit the analog ECG signal. I have a AM modulator in hand and I can get a 10kHz modulated AM signal. The problem is how to find a proper mixer to take the 10kHz as the input of the mixer. As I know most of the commercial mixers are designed for intermediate frequency.

 

[biff44]

There are plenty of noise sources at around 150 Hz, so sending it will be non-trivial. I would recommend either digitizing it, and sending it with some modern form of digital modulation.

If you want to send it with analog direct modulation, I would use FM with a large deviation. AM will pick up a lot of low frequency AM noise (atmospheric, etc). PM modulation would require a very low phase noise source on the transmit or receive ends. FM does not require such a low phase noise source for accurate transmission.

Are you sure they are 150 Hz signals? I thought ecg signals were more impulsive in nature, and you would then have to transmit the harmonics as well to get the right waveshape.

Rich

 

[yyan71]

There are plenty of noise sources at around 150 Hz, so sending it will be non-trivial. I would recommend either digitizing it, and sending it with some modern form of digital modulation.

If you want to send it with analog direct modulation, I would use FM with a large deviation. AM will pick up a lot of low frequency AM noise (atmospheric, etc). PM modulation would require a very low phase noise source on the transmit or receive ends. FM does not require such a low phase noise source for accurate transmission.

Are you sure they are 150 Hz signals? I thought ecg signals were more impulsive in nature, and you would then have to transmit the harmonics as well to get the right waveshape.

Rich

Thanks Rich, your suggestion about AM modulation is helpful and I will consider FM at the same time.

About the ECG signal, yes, you are right, actually its frequency range is from nearly 0 to 150Hz. For my project, I don't need to get the pretty ECG waveshape, just need to count the heart beats. Try to use analog transmission is because I just want to make the on body wireless device simple and small. I'm not sure if analog transmission will be feasible or not for my application.

Anyway, I am a fresh man in RF and I really appreciate your advices.

Thanks again.

Added after 2 minutes:

It seems to me, that the question title is misleading. You are intending to transmit the analog ECG signal, so you actually need a
common FM transmitter, as it could be used e.g. for an audio signal.

Thanks FvM,I will consider FM transmitter...maybe will borther you later...

cheers

 

[biff44]

check out nordic semiconductor, they have some low power fm transceiver chips that would work well.