here is my opinion about the 10.7 mhz if. yes, i agree that 45 mhz is
better. it makes it easier to eliminate image frequencies, you only need one
low pass filter to do it. also, you do not have the problem of rejecting
signals that are at the if frequency. in this case, 10.7 mhz. if i had a
source for the 45 mhz crystal filters that i could purchase in small
quantities, i would have used them instead of the 10.7. since i could not
locate such a source, i chose to use the more readily available 10.7 mhz
filters for my if. there is no problem with receiveing signals at 10.7 mhz.
they will tune just like any other. the one problem that i was concerned
with was being able to reject signals at 10.7 mhz when the radio is tuned to
other frequencies. there are 3 things that help with this problem however.
they are:
1) a doubly balanced mixer, such as the tfm-2, has some built in rejection
between the rf port and the if port. even though the manufacturer's spec
sheets do not normally list this, i was able to measure the rejection of the
rf signal at the if mixer port and found it to be about 30 db. this is a
great help in overcoming the weakness of the 10.7 mhz if design. this is
also why you do not want to use another type of mixer such as the ne602.
2) here in the united states, the frequency of 10.7 mhz is not normally used
to transmit on because this is also the common if frequency of home fm stereo
receivers that almose every home has. if someone did transmit on that
frequency, they would likely interfere with many peoples home receivers. i
have listened in on 10.7 mhz many times and have never heard any strong
signals there. you may want to check and see if 10.7 mhz has any strong
signals where you live.
3) in anticipation of this possible problem, i have designed and built a 10.7
mhz notch filter that can be inserted in the antenna path to further
eliminate interference on 10.7 mhz. it can provide about 40 db of extra
attenuation at 10.7 and has a bandwidth of only about 1 mhz. although i have
this filter, i have never needed to use it because like i said before, there
are no strong signals on 10.7 mhz here.
so in summary, i would say that the 10.7 mhz if design is not as weak as it
might first appear. the main disadvantage that i see is the need for six
bands of image filters that could be replaced with only one if a 45 mhz if
was used. understand that i was trying to make a design that would be easier
for others to obtain the parts needed to build it. the greatest design in
the world does no one any good if they cannot get the parts. i am sorry that
i could not anticipate what parts would be available in other parts of the
world.
