# [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
stations).

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
VK1KNP

> 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
>>area.
>
> Huh? If the highest RF is 800MHz, how do you get 1200MHz IF?
>