FM Radio Module Breakout Board - NS73M

Wireless & IoT
1

I have buy a FM Radio Module Breakout Board - NS73M

All is ok , but i have a problem

The pll is lock but i have some spurius near the local frequency.

Why happen this;

2

I have three units and they all seem to be clean. I don’t have access to a spectrum analyzer, so I can’t be sure though. One of my radios in the kitchen appears to get swamped by the strong RF signal from the board. Perhaps that is what you are seeing as well. Try putting some distance between the transmitter and the radio and see if that helps.

3

riden:
I have three units and they all seem to be clean. I don’t have access to a spectrum analyzer, so I can’t be sure though. One of my radios in the kitchen appears to get swamped by the strong RF signal from the board. Perhaps that is what you are seeing as well. Try putting some distance between the transmitter and the radio and see if that helps.

I test with spectrum analyzer.

If i lock the pll to 100MHZ there is another signal to 100,6Mhz , 25dB below.

This happen at all frequency.

4

I went back and checked a link that I seen before at http://cba.sakura.ne.jp/sub04/jisaku36.htm. Take a look at some of the photos near the bottom of the page. It seems to be showing similar results. The local oscillator is supposed to operating at a 304KHz offset, so maybe that’s what you are seeing. You may require some time of very sharp bandpass (or notch) filter to clean the signal up.

5

riden:
I went back and checked a link that I seen before at http://cba.sakura.ne.jp/sub04/jisaku36.htm. Take a look at some of the photos near the bottom of the page. It seems to be showing similar results. The local oscillator is supposed to operating at a 304KHz offset, so maybe that’s what you are seeing. You may require some time of very sharp bandpass (or notch) filter to clean the signal up.

Yes , i have the similar result as the pictures at this site.

Yours don’t have the similar result;

6

savnik:
Yes , i have the similar result as the pictures at this site.

Yours don’t have the similar result;

I probably do have the same situation; I just never noticed it probably because its 25 db down and the transmitter isn’t next to the radio. I have a look tonight.

7

Is the spurious radiation coming out the antenna or from the local oscillator ? Would shielding the PCB eliminate the off frequency radiation ?

Gus

8

riden:

savnik:
Yes , i have the similar result as the pictures at this site.

Yours don’t have the similar result;

I probably do have the same situation; I just never noticed it probably because its 25 db down and the transmitter isn’t next to the radio. I have a look tonight.

Do you have look;
9

Sorry about not responding sooner. I can pick up the extra signal if I place the radio near the receiver and I look for it. It appears that it is the local oscillator, but I can’t determine if the signal is present at the antenna output or if it is radiating from the board. For my purposes (transmitting the output of my iPod to my car radio), it isn’t an issue for me.

If the spurious outputs are a problem for your project, you could try a shielded case as puppetboy suggested. If the signal is present at the output pin, you’ll have to add a tuned circuit to eliminate the unwanted output.

10

To Riden Thanks for the info about adding a tuned circuit

Best

Gus

11

The spurious radiation coming out of the antenna.

I test with a spectrum analyzer.

The tuned circuit is a problem for whom don’t have a spectrum analyzer to tune.

12

savnik:
The spurious radiation coming out of the antenna.

I test with a spectrum analyzer. The tuned circuit is a problem for whom don’t have a spectrum analyzer to tune.

At first I was a bit surprised by that, but after thinking about it, it makes sense that the chip manufacturer doesn’t want to have to incorporate a variable tuned circuit to filter out the local oscillator that tracks the output frequency. I would imagine that this is more of an issue if one were using these chips as a basis of an exciter feeding an amplifier instead of a standalone short range FM transmitter. I would also agree getting the tuned circuit tweaked without a spectrum analyzer can be tricky over the entire FM band. If the transmitter was on a fixed frequency (or a few frequencies), you might be able to use a crude field strength meter for tuning.

13

savnik:
The spurious radiation coming out of the antenna.

I test with a spectrum analyzer.

The tuned circuit is a problem for whom don’t have a spectrum analyzer to tune.

You don’t need a specturm analyser, a simple FM radio

will do.

Knowing that the spur is at 600kHz offset, tune the

Tx to a know frequency (say 100MHz) and the FM

radio to 100.6MHz. Insert a trap to reduce the spur,

and tune it for least signal when the detector (FM radio)

is on the spur’s frequency.

A trap that tracks the FM TX would be hard to do.

Adding it inside the IC package would be more so.

A better FM exciter plan would be to place the LO

outside the Tx band and trap it with a bandpass

filter before the TX output.

The 600KHz offset is probably the output from

the IC modulator, and would require a double conversion

chain to push it out of band.

That IC’s flawed RF performance is probably acceptable

for the stated use in many FM radio mics and iPod to

FM radio adapters.