[clug] Microwave/Wireless - DTV Overload

Neil Pickford neilp at goldweb.com.au
Fri Feb 4 11:37:03 GMT 2005

Sorry I meant 2.4 GHz for WiFi not 1.2 GHz - typo

However the Intermediate Frequency (IF) could still be in the 1.2 GHz 
range, however I suspect it is somewhere more like 1600 MHz.

Time to get a bit technical to explain this.

Generally this type of receiver mixes a high side synthesized Local 
Oscillator (LO) {the tuning frequency} with the incoming Radio Frequency 
(RF) signal {the wanted TV signal} to get a difference beat at the first 
IF ready to filter out adjacent interfering signals (other unwanted TV 

Unfortunately because all mixing is both sum AND difference there is 
another frequency that will be moved to the IF as well. we call this the 
  Image Frequency.  Your WiFi problems may be Image Frequency related.

Because the WiFi Transmit signal will be very strong in the vicinity of 
your computer with the DTV card, any up front Image rejection filters 
are likely to not be much use.
How good is the RF shielding around your DTV tuner card?

Some examples of mixing with simple numbers.
eg 1800MHz LO - 200MHz RFin = 1600MHz 1stIF with RfImageFreq=2400MHz
or 1800MHz LO + 600MHz RFin = 2400MHz 1stIF with RfImageFreq=4200MHz
or 1800MHz LO - 600MHz RFin = 1200MHz 1stIF with RfImageFreq=2400MHz

Where ever you see a number near 2400MHz for the IF or Image Frequency 
you will have a potential ElectroMagnetic Interference (EMI) problem.

I suspect the numbers in the first example are around the values that 
are causing the problem as most of the DTV transmissions in Canberra are 
around 180-230 MHz. (Except SBS)

Your LO would probably tune between around 1.7 GHz through to 2.3 GHz

Hope this makes the observed problem a little less obscure.

Neil Pickford

Brad Hards wrote:
> On Fri, 4 Feb 2005 09:08 am, Neil Pickford wrote:
>>Although Digital TV runs at lower frequencies (174-230 MHz VHF & 520-800
>>MHz UHF) than WiFi/Microwave (1.2 GHz) it is quite possible that the
>>first Intermediate Frequency used by the tuner card is in the 1200 MHz
> Huh? If the highest RF is 800MHz, how do you get 1200MHz IF? 
> Brad

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