If the LO was on the low side, with a 9 MHz IF the LO would be on 8.5
MHz. The LO signal to the mixer is quite a bit (20 dB) stronger than the
IF signal, so its harmonic is the most troublesome. Its better to mix
the LO to the high side, 18.5+9 = 27.5 MHz which won't come through the
18.9 MHz band pass filter nearly as much as 2 x 8.5 = 17 MHz. Other than
birdies, receive isn't much of a problem, transmit birdies are a
problem.
Getting 8.5 from a 5 MHz VFO would be a pain too. The 3.5 MHz crystal
would tend to come through that mixer at 7 MHz also. That would make the
injection dirty.
73, Jerry, K0CQ
--
FAQ on WWW: http://www.contesting.com/FAQ/tentec
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
|