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St7ven

Comtech 1.2Ghz Tun frequency

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Hi,

I'm looking for some help.

I want to make a 1.2ghz frequency scanner with an old 1.2Ghz receiver. In fact, I replace the current receiver with the comtech module. And, rather than throughing away the replaced module, I have planned to use it.

I have two question in mind about how works this type of module :

- The Tun voltage is 9V for 1500Mhz. All frequency I'm want to scan go up to 1500Mhz. Is that a good choice to apply 5V (1250MHz) instead ? 

- From a post, I read that 

ADDR = tuner module I2C address
DIV1 & DIV2 = PLL divider
CTRL = PLL control byte

Band switch byte is not send and therefore not used.
From the control byte the PLL is set to:
– 50uA charge pump current
– normal operation mode & charge pump on
– reference divider 1/64 resulting in 62.5 KHz reference frequency

The divider is set to 12476. With the following math it’s possible to calculate the reception frequency:
fosc = 2 x fr x divider = 2 x 62.5 KHz x 12476 = 1559.5 MHz

To high? No, you must subtract the 1. IF with 480 MHz, so 1559.5 MHz – 480 MHz = 1079.5 MHz

Where does he find this formula? and the 62.5KHz?

 

 

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- The Tun voltage is 9V for 1500Mhz. All frequency I'm want to scan go up to 1500Mhz. Is that a good choice to apply 5V (1250MHz) instead ? 

You will need 9V for TUN and 5.0V for the logic power (+5 pin).

 

Where does he find this formula?

Details are in the SP5055 data sheet. http://radiomods.free.fr/f6csx/comtech/SP5055.PDF

 

and the 62.5KHz?

2 x 62.5KHz = 125KHz. This is the channel step size (frequency resolution).

 

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Thank you for your answers and the link oif the datasheet !

 

=> " The programmed frequency can be calculated by multiplying the programmed division ratio by 16 times the comparison frequency Fcomp. [...]  Note that the comparison frequency is 7·8125kHz when a 4MHz reference is used. " => 7·8125 * 16 = 125kHz

 

What about the input impedance, which is given for 75Ohms in RMRC web site? Is it right to say my 50Ohms antenna will no match correctly with this receiver and I should by a 75Ohms antenna ? Rare Antenna Impedance is given by the retailer, so I don't have a clue where I should get one....

 

PS /

 

I got my answer :

 

" Comtech tuner modules were developed for the set top box satellite ** TV ** industry about 20 years ago. (2) TV coax is 75 ohms.  Antenna impedance varies depending on the design. For example, a traditional halfwave dipole (in a theoretical perfect world) is 73 ohms. Regardless, there's no reason to panic if the antenna is 50 ohms and the vRx is 75; It works well enough that it is unlikely that you will notice any difference during your FPV flight."

 

Edited by St7ven

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