Synthesized Transmitter for Radio Control

[hadihf]

Hi everybody,

I have built fm synthesized rf deck by GAA at mstar2k.com. Works great so far. Just had a chance to look at SA plot recently and noted that Bandwidth is a bit out at -55dB. Tried all sort ot tweaks with no improvement and things focus on the loop filter values. Can somebody suggest an improvement? The modulation is +1.5KHz. The spectrum w/o modulation is very clean. Phase detector frequency is 10Khz. Spur at 10KHz interval is also visible. Any help will be highly appriciated.

Best Regards
hadihf

 

[BigBoss]

It's very hard to see 1.5kHz sidebands with this low cost Spectrum Analyzer.And also, it's hard to say something about Loop Filter components without knowing VCO characteristics such as Kvco.
In additional to this, Charge Pump current specification is also unknown so therefore we can't say anything about Loop Filter.

 

[hadihf]

Actually i wantd to learn how can i reduce modulation peaks. In this design reference is mudulated instead of VCO.

 

[BigBoss]

Actually i wantd to learn how can i reduce modulation peaks. In this design reference is mudulated instead of VCO.

It probably uses Frequency Modulation and in this modulation scheme the sidebands are visible by frequency of modulation steps.It's normal...

 

[hadihf]

Yes FM. I think Fcomp can be reduces to narow bandwidth. At present it is 10KHz. 2.5KHz might work. Loop filter has to be redesigned.

 

[BigBoss]

Yes FM. I think Fcomp can be reduces to narow bandwidth. At present it is 10KHz. 2.5KHz might work. Loop filter has to be redesigned.

In this case you will use NBFM and all you should do is to decrease modulation signal amplitude.It will be NBFM..

 

[hadihf]

For RC a minimum of plus or minus 1.5KHz deviation is required. I can reduce the modulatin depth but some RC recievers will become unresponsive.

Regards