Re: Open challange target

On Thu, 1 Dec 2005 13:04:34 +0100, "Meindert Sprang"
<mhsprang@xxxxxxxxxxxxxxxxxxx> wrote:

>"Peter" <moocowmoo@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
>news:dmmk4s$ptq$1$830fa795@xxxxxxxxxxxxxxxxxxx
>>
>> Detect the frequency of the local oscillator, which as somebody else has
>> mentioned has been used in the UK for decades. The real challenge would be
>> to use ultrasonic to do it.
>
>And what if you dont't know whether the tuner uses up or down conversion and
>it's MF?


While non-standard IF frequencies and local oscillators can be used to
fool e.g. radar detector detectors, it would be quite problematic to
implement on a TV receiver.

I have never seen a TV-tuner that would do upconversion. Upconversion
is mainly used in HF general conversion receiver with the first IF in
the 40-90 MHz range or in spectrum analyzers with the first IF at 2-4
GHz.

The TV IF is in the 30-40 MHz area for historical reasons and since
SAW filters are used these days, it would be quite hard to do some
special arrangements.

To cover the Band I VHF channels in the 47-68 MHz area, the local
oscillator (LO) would have to be above the received channel (Rx) to
keep the local oscillator tuning range reasonable. Running the LO
below the Rx frequency also creates the problem of LO harmonics
hitting the received channel. When the LO is above the Rx frequency,
the harmonics can never hit the received frequency.

Anyway, since the LO must be above RF on Band I, the IF spectrum is
inverted, moving the sound carrier from the top end of the channel to
the bottom of the IF passband. Sound traps etc must be set at this
frequency.

While it would technically be possible to put the LO below Rx on VHF
band III and UHF IV/V, this would also require moving the IF sound
trap to the opposite end of the IF-passband for these channels.

Thus, the LO is above Rx also on these channels to avoid this
inconvenience.

Paul

.

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