VCO Questions
 

I'm experimenting with various VCO configurations/designs for a general
coverage short wave receiver. The output of the VCO will drive a MOSFET mixer.

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).

Just what difference will the level of 2nd harmonic make? Does it matter
whether I have -40 or -30dbC? I'm not sure what effect it will have on the
performance of the final receiver, but my gut feeling is that the better the
input to the mixer the less unwanted components coming out from the mixer.
Then again this is not meant to be a high performance receiver.

I have seen designs which take the output from the source of the FET (for the
design under test). The waveform here varies from bad to attrocious - not the
kind of thing that should IMHO be fed into the mixer, although of course it
will still work.

The variable gain is necessary to obtain oscillation as the tuning capacitance
varies. Later I want to add an AGC circuit.

I have also experimented with buffering the tank, as this is the place with
the cleanest waveform. A fet source follower didn't seem to work as expected
and I now think that this type of circuit is only good for low level signals,
let's say 1V ptp if the pinch voltage is around -3 or -4V. Comments?

In a previous circuit I used an emitter follower using two 100k resistors to
bias the transistor base to Vdd/2. It seemed to work and certainly I
understand far better the voltage swing limitations of this circuit. The tank
will be loaded by 50k (depending upon beta and emitter load), however an
experiment with the tank loaded with a 12k resistor showed that this had
little detrimental effect on the circuit, although the gain control needed to
be increased. The general opinion that I have read appears to recommended
loading the tank as lightly as possible, although the advantages as not
clearly spelt out (at least to me!).

regards...

--Gary

On Sun, 14 Mar 2004 16:48:53 -0800, "Tim Wescott"
wrote:

I doubt that there's any real advantage to feeding a nice clean sine wave to
your MOSFET mixer -- the best distortion performance that you'll get out of
the mixer will be if you're driving it hard (i.e. turning it on and off).
Even with sine waves in you'd get lots of harmonics internally in the mixer.

By the way, why not use an off-the-shelf mixer like the NE602? I doubt that
you'd get much better performance out of a single MOSFET unless you're a
real RF circuits whiz, and the NE602 is quite easy to apply.

agreed, I'd concentrate on the loop filter section, if he's even using one.








Remove "HeadFromButt", before replying by email.

On Sun, 14 Mar 2004 16:48:53 -0800, "Tim Wescott"
wrote:

I doubt that there's any real advantage to feeding a nice clean sine wave to
your MOSFET mixer -- the best distortion performance that you'll get out of
the mixer will be if you're driving it hard (i.e. turning it on and off).
Even with sine waves in you'd get lots of harmonics internally in the mixer.

By the way, why not use an off-the-shelf mixer like the NE602? I doubt that
you'd get much better performance out of a single MOSFET unless you're a
real RF circuits whiz, and the NE602 is quite easy to apply.

agreed, I'd concentrate on the loop filter section, if he's even using one.








Remove "HeadFromButt", before replying by email.

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

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

Leon Heller wrote:
snip
You could put an LP filter on the buffered oscillator output.
snip

The VCO operates over a wide range, making the control of such a filter quite
tricky. There will be three ranges and this would add to the complexity.

--Gary

Leon Heller wrote:
snip
You could put an LP filter on the buffered oscillator output.
snip

The VCO operates over a wide range, making the control of such a filter quite
tricky. There will be three ranges and this would add to the complexity.

--Gary

Tim Wescott wrote:

snip

I doubt that there's any real advantage to feeding a nice clean sine wave to
your MOSFET mixer -- the best distortion performance that you'll get out of
the mixer will be if you're driving it hard (i.e. turning it on and off).
Even with sine waves in you'd get lots of harmonics internally in the mixer.

By the way, why not use an off-the-shelf mixer like the NE602? I doubt that
you'd get much better performance out of a single MOSFET unless you're a
real RF circuits whiz, and the NE602 is quite easy to apply.

snip

I built a short wave receiver many years ago. The design was from a Babani
Press book (highly recommended). It worked OK, but I always hated not knowing
the exact frequency. At the time I had no tools to even calibrate the dial.

The front end was a single transistor oscillator and mixer. I tried the
"improved" design which used a MOSFET, but I didn't have the right Denco coil
and the performance was degraded.

The intention is to add digital tuning to this old project.

--Gary

Tim Wescott wrote:

snip

I doubt that there's any real advantage to feeding a nice clean sine wave to
your MOSFET mixer -- the best distortion performance that you'll get out of
the mixer will be if you're driving it hard (i.e. turning it on and off).
Even with sine waves in you'd get lots of harmonics internally in the mixer.

By the way, why not use an off-the-shelf mixer like the NE602? I doubt that
you'd get much better performance out of a single MOSFET unless you're a
real RF circuits whiz, and the NE602 is quite easy to apply.

snip

I built a short wave receiver many years ago. The design was from a Babani
Press book (highly recommended). It worked OK, but I always hated not knowing
the exact frequency. At the time I had no tools to even calibrate the dial.

The front end was a single transistor oscillator and mixer. I tried the
"improved" design which used a MOSFET, but I didn't have the right Denco coil
and the performance was degraded.

The intention is to add digital tuning to this old project.

--Gary

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. The output of the VCO will drive a MOSFET mixer.

I am able to vary the gain of the oscillator. I get the best looking sine wave
at the point that oscillation just starts.

That is the nature of oscillators. As you then increase the gain, the amplitude
increases until some non-linearity prevents it - usually the available voltage
swing in a device. This non-linearity causes the harmonic content to appear and
rise to fairly significant levels, as you have observed.

Whether the harmonic levels you noted are of great significance depends on the
mixer attributes.

One technique often used is to ensure the limiting non-linearity occurs in a
separate gain stage following the oscillator. Gain control derived from that
stage is then fed back to the oscillator to keep it in that "just oscillating"
region where harmonic content is lowest. A separate take-off from the
oscillator through a second buffer stage is then used as the actual output.