On Sun, 14 Mar 2004 23:35:56 GMT, Gary Morton
wrote:

I'm experimenting with various VCO configurations/designs for a general
coverage short wave receiver.

I assume that this is an up conversion design, with the (first) IF
well above the intended Rx band.

The output of the VCO will drive a MOSFET mixer.

I assume this is a single ended dual-gate MOSFET mixer (and not some
kind of MOSFET quad).

In dual gate MOSFET mixers (as well as in heptode mixers) the load
current is proportional to both the gate voltages, thus some
multiplication occurs between the various gate signals and the
trigonometric relation sin(A)*sin(B) = 1/2*(cos(A-B)-cos(A+B)) can be
used to estimate the amplitudes of the mixing products.

I am able to vary the gain of the oscillator. I get the best looking sine wave
at the point that oscillation just starts. The 2nd harmonic is around -40dbC.
The voltage as scoped at the tank is around 1V ptp. If the gain is increased
the 2nd harmonic raises to -30dbC and the tank voltage to 4V ptp. I think this
is about the right drive level for the MOSFET mixer (yet to be built).

The second and higher harmonics will no doubt produce mixing products
with the antenna spectrum and will cause a lot of unwanted responses
at 2LO+/-IF and 3LO+/-IF and so on, but this is mainly a problem, if
the IF is comparable to the received signal (e.g. 9 MHz or 10.7 MHz)
or the LO is below the received signal, in which case the second
harmonic could fall close to the received frequency. In all these
cases, it is important, that there is a sufficient selectivity in
front of the mixer, usually requiring at least one and preferably
several constantly tunable front end stages as in high quality old
receivers.

However, in up-converting systems, with say 40 MHz IF, the LO range is
40-70 MHz, the second harmonic is 80-140 MHz and the unwanted signals
that can produce the 40 MHz IF signal are at 40-180 MHz. Good
shielding and a steep fixed low pass filter at 30 MHz should take care
of these higher image responses.

While the unwanted mixing products may still occur, the mixing is
between the internally generated front end thermal noise and the 2LO,
the amplitudes are insignificant, especially when receiving the noisy
HF range.

If you are using the traditional general coverage up-conversion
system, I would not be too concerned about the amplitude of the second
harmonic.

One advantage of using some balanced, hard driven mixer, is that the
first strong spurious response is at 3LO, the spurious image responses
are at even higher frequencies, simplifying the input filtering.

Paul OH3LWR