[opendtv] Re: 5th generation Test
- From: Ron Economos <k6mpg@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 17 Mar 2005 03:05:19 -0800
At my work location in Milpitas, CA 95035, turning
on the FM trap on the distribution amplifier makes a
world of difference with the Samsung T165. Antenna
is a small VHF/UHF log-yagi on the roof of a one story
building that's pointed in no particular direction (rotor
controller is in a lab that I don't have access to).
KQED-DT on RF channel 30 goes from massive
break-ups to perfect copy.
KBHK-DT on RF channel 45 goes from occasional
break-ups to perfect copy.
KNTV-DT on RF channel 12 (8.9 kW ERP) goes from
*not recoverable* to perfect copy.
Of course, the strength of FM stations in the Bay Area
is due to their location on mountain tops. A paper clip
on the input of a spectrum analyzer will show these FM
carriers way above the noise.
The point is, a little front-end filtering can go a long way.
To answer Tom's question directly, yes you can add
a bandpass filter to the antenna input of your receiver.
For HF receivers, these devices are called pre-selectors
and (at least in the olden days) were manually tuned.
I don't know of any out-board tunable bandpass filters
for television frequencies, but manual tuning is a non-
starter for consumers electronics. The correct solution
is a tracking (electronically tuned) bandpass filter in the
receiver.
Ron
Tom Barry wrote:
>Manfredi, Albert E wrote:
> >>How much extra are we talking about here?
> >
> >
> > Well, since everyone was marveling at the low cost of
> > the first gen Freeview boxes, and they were dual
> > conversion designs, the cost cannot be prohibitive.
> > However, it won't follow Moore's law, so the cost
> > will not erode over time quite so fast.
>
>Is this RF front end something that could be sold separately and stuck
>on the antenna input of existing receivers?
>
>- Tom
>
>
>
>>Tom Barry wrote:
>>
>>
>>
>>
>>>It sounds like "IM3 products" (whatever they are) and
>>>good RF frontends are supposed to be known technology.
>>>
>>>
>>IM3 is third order intermodulation distortion products.
>>Created in receivers when multiple incoming carriers
>>and the local oscillator beat together. Ideally, any
>>such IM products would fall way outside the IF passband
>>and be attenuated nicely. But with single conversion
>>tuners, they are not all outside the passband.
>>
>>It appears that the low cost of single conversion
>>tuners is enough of an incentive that manufacturers
>>seem to jump at the chance of going that route. As we
>>were informed a couple of weeks ago, even COFDM did
>>this. As Al Limbert and others explained, if your
>>RF amp up front is tuned to the channel you want and
>>can reduce its gain in the presence of strong signals,
>>the IM distortion can be greatly reduced, so single
>>conversion tuners can work okay.
>>
>>With co-located transmitters, the problem might not be
>>so acute. Because you will never see unwanted signals
>>that are louder than the desired signal. But if the
>>unwanted signal is louder than the wanted signal, then
>>this can be a problem. The numbers we saw showed that
>>a 3 dB increased level to the receiver, from unwanted
>>signals, can cause up to 9 dB increase in this in-band
>>IM3 distortion (IIRC). So, big deal. And a 3 dB
>>increase into a receiver is not so hard to imagine if
>>there are loud transmitters close by, even if your RF
>>amp is tuned. The shoulders of the RF amp's passband
>>won't be vertical. Dual conversion, i.e. where the
>>receiver goes through two IF conversions, takes care
>>of this nicely.
>>
>>
>>
>>
>>>But if that's the case then maybe we (collectively,
>>>not me) also know what it would cost to add them to a
>>>box with an LG5 chip in it.
>>>
>>>How much extra are we talking about here?
>>>
>>>
>>Well, since everyone was marveling at the low cost of
>>the first gen Freeview boxes, and they were dual
>>conversion designs, the cost cannot be prohibitive.
>>However, it won't follow Moore's law, so the cost
>>will not erode over time quite so fast.
>>
>>Bert
>>
>>
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