In article , Patrick Turner
wrote:
John Byrns wrote:
In article , Patrick Turner
wrote:
John Byrns wrote:
7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to
see why
this should be true.
Because for the same Q value, the pass band would be 4 times wider
Where is it written that the same loaded Q must be used for both filters?
If you can change the center frequency, why can't you change the loaded Q?
The lower the Q, the more IFTs required for a given amount of pass band and
attenuationout of band.
Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are. There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.
To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:
One One
455 kHz IFT 2.0 MHz IFT
Q = 15.167 Q = 66.667
Fc-60 kHz -24.30 dB -24.18 dB
Fc-50 kHz -21.22 dB -21.09 dB
Fc-40 kHz -17.56 dB -17.42 dB
Fc-30 kHz -13.22 dB -13.07 dB
Fc-20 kHz -8.72 dB -8.60 dB
Fc-15 kHz -7.09 dB -7.02 dB
Fc-10 kHz -6.27 dB -6.24 dB
Fc-05 kHz -6.04 dB -6.04 dB
Fc kHz -6.02 dB -6.02 dB
Fc+05 kHz -6.03 dB -6.03 dB
Fc+10 kHz -6.19 dB -6.22 dB
Fc+15 kHz -6.86 dB -6.96 dB
Fc+20 kHz -8.34 dB -8.50 dB
Fc+30 kHz -12.75 dB -12.95 dB
Fc+40 kHz -17.15 dB -17.32 dB
Fc+50 kHz -20.88 dB -21.01 dB
Fc+60 kHz -24.01 dB -24.12 dB
The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.
Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.
Its difficult to make a 455kHz typical old IFT produce a nice flat topped
20 kHz wide BW. Its either pointy nosed, undecoupled, or flat
topped, critical
coupled,
or over critical or rabbit eared.
I have tried all that.
So you have tried all that and rejected the "pointy nosed", "flat topped",
and "rabbit eared" response curves. I am left to wonder what sort of
response curve you were looking for? Why not settle for a nice "flat
topped" response curve and be done with it?
I didn't say I had rejected the flat topped critical coupled IF response.
Then what did you say? You said you had "tried all that" but now it
appears that you were telling a little fib and hadn't actually tried a 455
kHz IF designed to produce the desired response.
Regards,
John Byrns
Surf my web pages at,
http://users.rcn.com/jbyrns/