EMRFD Message Archive 8433

Message Date From Subject
8433 2013-03-31 18:16:13 Dana Myers Simple local-oscillator for diode mixer - comments?
I've uploaded a file njf-vfo-3.png to the folder K6JQ.

Basic goal is a simple, VCO-able LO for a TUF-series mixer.

This is a simple J310-based Hartley VFO, using a 74AHC08 to square
and buffer the output for use with a TUF-series mixer. LTSPICE modeling
with a 'HC08 model was encouraging. I have some AHC logic on hand,
and decided to use an 'AHC08 with 3 gates paralleled for output
drive. Before adding the 'AHC08 I measured the drain current at
around 600uA @ 5V for the J310. The 'AHC08 uses one gate for
squaring to drive the others. Note that the TI AHC series has
approximately 200mV of hysteresis around 50% of Vcc, and modeling
shows the oscillator output is close to 5Vp-p though I am not
equipped to measure it in practice.

I've measured the output power with the exact circuit shown as
around +12.2dBm RMS (M3 freq/power meter), which is around +9.2
dBm after adjusting for square vs. sine. I suppose that might be
little hot for a TUF-series mixer, particularly as square-wave
drive so I'm likely to increase the attenuator in practice.

Output level is +/- .1dBm across a 1MHz range, since the output
is determined primarily by the 5V supply. The 'AHC08 also seems
to a pretty good job of isolating the oscillator from load

A pair of paralleled MV104s don't give me the desired tuning range,
but that's something else to work on.

Anything I'm missing here with respect to using the 'AHC logic
this way? Is it particularly noisy?

Dana K6JQ
8438 2013-03-31 20:34:03 William Carver Re: Simple local-oscillator for diode mixer - comments?
If you put a 50 ohm resistor from AHC output(s) to the mixer LO terminal
then the LO port of the mixer will be fairly well matched to absorb
signals originating from the other ports.

If you design or adjust the square wave for perfect 50/50 symmetry the
mixer third order intercept is enhanced. The LO port will be getting a
bit more signal than the rated sine-wave input, but that won't hurt the
mixer or performance.

If you can listen to the second harmonic of the VFO frequency FROM THE
SQUARER (use some resistors to the squarer, you don't want to ALSO be
hearing a VFO harmonic!) and adjust the bias for a sharp null, or deep
reduction, in that second harmonic signal.

One thing you should consider: the multiple gate packages inside a
multi-gate package SHARE one ground and one Vcc pin between them. Even
with zero lead length of ground-pin-to-groundplane, and zero lead length
SMD bypass on Vcc-to-groundplane, the section you doing the squaring
will see a "glitch" on its Vcc and ground when the other three sections
pump current to the mixer, and that glitch changes the input threshold
at that instant. Sometimes (often?) this produces an brief burst of a
few cycles of few-nS oscillation on the rising and falling edges of the
LO waveform. The mixer will see those.

I would suggest squaring the VFO with its own dedicated chip, no other
functions performed by any other gates in the package (don't use them).
This squarer chip would have its own +5V bypass capacitor, and I'd
decouple the 5V right at the chip with a -43 ferrite bead or maybe a 10
ohm resistor. If you were purchasing chips for this function, to save
wiring/space I'd suggest using a "Tiny Logic" single gate (NC7SZ04 for
example). I've used it effectively to square up a 100 MHz VFO.

Drive the mixer with a second chip. Your three (or four) AHC08 sections
in parallel would be fine. Using two chips uses some extra space and
maybe looks a little clumsy. But it does a better job!

Bill W7AAZ
8447 2013-04-01 18:49:58 Dana Myers Re: Simple local-oscillator for diode mixer - comments?