On Fri, 28 Feb 2014 15:24:11 -0500, Michael Black wrote:

On Fri, 28 Feb 2014, Stuart Longland VK4MSL wrote:
Deciding on a suitable IF is the next challenge. The higher I make it,
the easier it becomes to make a filter that will reject images. I
considered a bog-standard 10.7MHz IF at first, but then considered what
would happen at 70cm where the band goes from 430-450MHz.

At one point, there was a wave of FM communication receivers that used
21.4MHz (twice 10.7MHz) IFs, so those should be reasonably available.
Of course, I've seen some odd filters in the HF range used in such
receivers too.

When early clunky cellphones were obsolete, I was able to buy some at
garage sales. This was the mid-nineties. They all had crystal filters
in the frequencies above 30MHz, some around 45MHz but also some around
70MHz if I remember. The good thing was they all converted a second
time to 455 or 450KHz, so there was a conversion crystal there too.
[…]
WIth all the recent portable shortwave receivers on the commercial
market,
there may be some now that are broken, offered up at a low cost, but
having an IF filter around 45MHz, and then a conversion to 455KHz or
450KHz, the latter filters not perfect for SSB but in some receivers
there is a narrower one than the AM filter.

I just had a look around, and found I can get an off-the-shelf crystal
filter for 45MHz, and a crystal for 44.545MHz. The filter has a 15kHz
bandwidth.

That sounds like a better combo. I was aware of IFs of 10.7MHz, wasn't
aware of 45MHz. 45MHz would work okay for what I'm doing.

The fun bit being now, working out how to best filter the image. Suppose
I wanted a signal on 146.400MHz… LO1 would need to be tuned to 101.400MHz
(it won't reach 191.4MHz). That'll give me an image at 56.4MHz.

I fiddled with doing filters to knock out such frequencies: the 45MHz IF
will be easier than 30MHz, but I still had a great deal of trouble
synthesizing a lumped impedance filter. Transmit is of particular
concern, since in addition, the filter has to handle high power. (If I
understand correctly, it goes *after* the PA to reject any harmonic
products from the amplifier?)

What rejection figures do people normally aim for when rejecting these
images?