PLL RF synthesizer - question with covering the UHF band

[santerror]

PLL RF synthesizer

Hello all, i'm trying to make a synthesizer with this basics specs: freq. range: aprox. 300 MHz to 900 MHz (UHF Band), step size (6MHz). For now, i need help just to achieve this specs, im not worried about output level, phase noise, etc.. I'd like to now if to cover the UHF band it's necessary to use many vco in parallel, or if exist some vco to cover whole band with large Kvco, or some pll and vco integrated, etc... The application is a mobile tuner digital TV.
Tnks in advance

 

[vfone]

Re: PLL RF synthesizer

Hard to get only one VCO covering two octaves in UHF band.
But why you need this wide frequency range, when I think the range for mobile TV in UHF is 470MHz to 770MHz.

 

[biff44]

Re: PLL RF synthesizer

I think the trick is to use a high side tunable LO to upconvert to a 1st IF frequency around 1.2 GHz, and then a 2nd fixed LO to downconvert to the low 2nd IF.

 

[santerror]

Re: PLL RF synthesizer

ok vfone u r rigth, 300 to 900 was an approximate example, the right frequency for uhf digital tv in brazil is from 473 to 803.
biff44, the IF i need is 500khz, can you please explain again, with more detail, your suggestion?

 

[biff44]

Re: PLL RF synthesizer

to downconvert 473 - 804 MHz to a 0.5 Mhz IF, you can use an LO that tunes from 472.5 to 803.5 MHz, and do it in one downconversion step. You would need a 52% bandwidth vco to do that.

If instead you upconverted to 1.2 Ghz first, that LO would be 1673 to 2004 MHz. That is an 18% bandwidth. Of course, since the IF is at 1200 MHz, you would then mix it with a fixed 1999.5 MHz LO to generate the 2nd IF at 0.5 Mhz.

BTW, why is the output frequency so low? 0.5 MHz?

 

[santerror]

Re: PLL RF synthesizer

its a Brazilian standard for mobile digital tv tuner..

 

[biff44]

Re: PLL RF synthesizer

doesn't seem to be enough bandwidth for a tv signal! I guess with enough compression....

 

[santerror]

Re: PLL RF synthesizer

the signal before ADC need to be in base band.

Added after 5 minutes:

u are right, this application its to digital tv on cell phones and not need high definition

 

[biff44]

Re: PLL RF synthesizer

I guess the reference designs use chips that have an image reject mixer application--two mixers fed with a 0/90 degree phase shift in the LO, such as:
https://www.analog.com/en/audiovideo-products/mobile-tv-tuners/ADMTV202/products/product.html

In these, you run the LO at 4 X the desired LO frquency (I think), and use a divide by 4 to get the 0/90 degree phase shift. So you still need a big LO bandwidth, but since the LO frequency is now microwave, instead of UHF, you can use tricks like switching in a couple of different fixed capacitor values to the resonator, in parallel with the varactor capacitance. In this way you can get the tuning range you desire.

Interesting stuff.

 

[santerror]

Re: PLL RF synthesizer

im searching for a frequency divider with quadrature output, but i dont found it.
can you show me some link for this component

 

[mmic1978]

LC VCO just use switching cap array

1. band with switching

2. tuning with vtune

the problem is the variation of Kvco

 

[ledum]

https://www.analog.com/en/rfif-components/pll-synthesizersvcos/ADF4350/products/product.html - synthesizer and VCO from 137.5 MHz to 4400 MHz
and https://www.analog.com/en/rfif-components/modulatorsdemodulators/adl5387/products/product.html - down converter to I and Q IF, local oscillator must be 2xF - but there is no problem with ADF4350 IC

 

[BigBoss]

If you use hyperabrupt varactor diodes, you can obtain a VCO that covers UHF band entirely.Hyperabrupt varactors can have more than 6-10 capacitance ratios between 0.8V-33V varactor voltages.But PN performance can not be very good..
Or, as mentioned above, using a VCO that will be functional between 600MHz-1800 MHz and then you divide it by 2 ( it improves PN by 6 dB)