On Tue, 20 Jun 2006 13:58:31 -0500, Ben Jackson wrote:

I'm still working on my HF APRS beacon idea (25MHz-14.85MHz = 10.151)
and I've done some experiments using a 74HC86 quad XOR with one gate for
each oscillator and one as a mixer. In spice simulations, the '86 as a
mixer produces lots of products resulting from the odd harmonics of the
inputs, some of which are close to or below the 10.151MHz I want. If I
run that output through a 2nd order Butterworth 30m bandpass filter I
get a clean result.

Since the filter loss depends on the ratio between the loaded-Q and
the unloaded-Q and in order to get a steep bandpass filter, the losses
are going to be large with ordinary LC components.

If you put the steep bandpass filter after the power amplifier, a
large amount of expensive RF fundamental power is lost in the filter.
However, if you put the steep filter before the power amplifier, the
loss of signal level is easily compensated for by an extra low level
amplification stage, with minimal extra cost.

Depending of the linearity of the power amplifier, some low pass
filtering may be needed after it, but now you have to worry only on
the harmonics of the 10 MHz signal, not the various mixing products.
If you use a push-pull power amplifier, the first significant harmonic
would be the 3rd, which is above 30 MHz and since this is a single
band design, it might even make sense to use an elliptic low pass
filter, with a resonant notch at the 3rd harmonic.

Paul OH3LWR