Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm
and output impedances of approx. 50 Ohms.

There must be an explanation.

Thanks
Andreas

"andreas magun" wrote in message
...
Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm and
output impedances of approx. 50 Ohms.

Have you checked the RF power from your quadrature driver with resistive
loads? It can be very misleading if you measure it whilst connected to the
mixers.

Leon

Leon Heller wrote:
"andreas magun" wrote in message
Thanks for your reply,

I have checked the output power (voltage) from my quadrature driver
with a 50 Ohm load. The output voltage dropped by a factor 2,
indicating a good match. The same happened when I attached the
mixers (SBL-1) instead. I assumed that the LO-input resistance of an
SBL-1 at 7 MHz is close to 50 Ohm and purely resistive.

If there is not a plausible explanation for my results I will redo
the measurements, to exclude measuring errors.

Andreas


...

Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm and
output impedances of approx. 50 Ohms.


Have you checked the RF power from your quadrature driver with resistive
loads? It can be very misleading if you measure it whilst connected to the
mixers.

Leon

"andreas magun" wrote in message
...
Leon Heller wrote:
"andreas magun" wrote in message
Thanks for your reply,

I have checked the output power (voltage) from my quadrature driver with
a 50 Ohm load. The output voltage dropped by a factor 2,
indicating a good match. The same happened when I attached the
mixers (SBL-1) instead. I assumed that the LO-input resistance of an SBL-1
at 7 MHz is close to 50 Ohm and purely resistive.

Diode mixers make a real mess of the input LO signal, if you look at what
happens to it with a 'scope. They have to be fed from a 50 ohm resistive
source, but they are not really resistive themselves.

Leon

On Wed, 23 Feb 2005 04:28:27 -0000, "Leon Heller"
wrote:

Diode mixers make a real mess of the input LO signal, if you look at what
happens to it with a 'scope. They have to be fed from a 50 ohm resistive
source, but they are not really resistive themselves.

yes they do. I've also used square wave drive and found that you have
to be sure your really driving them with 7-10DBM. A 74F74 really
can't do that though it looks that way. I found I had to use a
parallel group of 74HCT244 buffers (minimum of three) to get good
drive.

Allison

wrote in message
...
On Wed, 23 Feb 2005 04:28:27 -0000, "Leon Heller"
wrote:

Diode mixers make a real mess of the input LO signal, if you look at what
happens to it with a 'scope. They have to be fed from a 50 ohm resistive
source, but they are not really resistive themselves.

yes they do. I've also used square wave drive and found that you have
to be sure your really driving them with 7-10DBM. A 74F74 really
can't do that though it looks that way. I found I had to use a
parallel group of 74HCT244 buffers (minimum of three) to get good
drive.

I suppose what the OP needs is the square wave equivalent of a +7 dBm sine
wave. I'm not sure what that is.

Leon

Did the audio go through a LPF for the phasing unit?
Remember that a square wave is made up of the sum of many harmonics -
most of these will have been removed and the amplituyde of the
fundamental may not be that large.

Richard


andreas magun wrote:
Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm
and output impedances of approx. 50 Ohms.

There must be an explanation.

Thanks
Andreas

Thanks for replying.

The problem, was that I had not enough LO-power from the 74F74 outputs.
I have buffered them with a 50 Ohm TTL Driver 74AS804 that could deliver
plenty of current into the LO-inputs. I have limited it to approx 10 mA
by a series resistor of 300 Ohm. At a voltage drop of approximately .550
mVpp across the LO-inputs (approx. 5 mW) the exciter works now as expected.

Andreas

Richard Hosking wrote:
Did the audio go through a LPF for the phasing unit?
Remember that a square wave is made up of the sum of many harmonics -
most of these will have been removed and the amplituyde of the
fundamental may not be that large.

Richard


andreas magun wrote:

Hi,

I have assembled a T2 phasing exciter (KK7B) and looked
at its output spectrum with a spectrum analyzer.

When pumping the exciter with quadrature sine wave signals it
works as expected (output power roughly 3 mW, sideband supression
better than 40 db).

However, when pumping with quadrature squarewave
signals of approximately the same power, the output power
drops by a factor of at least 20 db! I would have expected
much more power, similar to that obtained with sine wave pumping.

The quadrature phase signal was generated with a 74F74, the
quadrature signals being attenuated to give a power of approx. 10 dbm
and output impedances of approx. 50 Ohms.

There must be an explanation.

Thanks
Andreas

On Mon, 07 Mar 2005 17:47:56 +0100, andreas magun
wrote:

Thanks for replying.

The problem, was that I had not enough LO-power from the 74F74 outputs.
I have buffered them with a 50 Ohm TTL Driver 74AS804 that could deliver
plenty of current into the LO-inputs. I have limited it to approx 10 mA
by a series resistor of 300 Ohm. At a voltage drop of approximately .550
mVpp across the LO-inputs (approx. 5 mW) the exciter works now as expected.

That is good to hear. I've had excellent results with T2 using
squarewave drive.

Allison