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

"Leon Heller" wrote in message
...
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.

To answer my own post: I think it should be 1V p-p for the equivalent square
wave, which is the same power as a 0.5V rms sine wave into 50 ohms (+7 dBm).

Leon

To answer my own post: I think it should be 1V p-p for the equivalent square
wave, which is the same power as a 0.5V rms sine wave into 50 ohms (+7 dBm).

Leon



1 Vpp square wave into 50 Ohms would be +13 dbm. What is the maximum
power a SBL-1 mixer will survive?

Andreas

"andreas magun" wrote in message
...

To answer my own post: I think it should be 1V p-p for the equivalent
square wave, which is the same power as a 0.5V rms sine wave into 50 ohms
(+7 dBm).

Leon

1 Vpp square wave into 50 Ohms would be +13 dbm.

I don't think it is, the average value (same as RMS) will be 0.5V, which is
+7 dBm.

What is the maximum
power a SBL-1 mixer will survive?

I think it will take +13 dBm, performance will suffer if it is overdriven.

73, Leon




--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller

Keepthis in mind:
+7dbm (the recommended RMS input for a level 7 mixer) is about 1.5Vpp sine.
This holds up to reason, as a DBM has two diode-drops that the LO must
overcome in order to switch the diodes properly. Therefore, if you use a
square wave, you also need to supply about 1.5Vpp, or the diodes won't
switch.

There are two common misconceptions many people hold about these devices.
One is that they are inherently 50 ohm devices. In reality you will only see
50 ohms on a given port if the other two ports are also terminated in 50
ohms. You could run the device just as well at 75 ohms if you keep it
balanced properly. Also, many people think that the LO input is power
dependent. This only partially true. It's really voltage dependent,
requiring sufficient drive voltage to turn on the diodes and pump an amount
of current through them that is large compared to the expected input RF
signal.

There is no magic in these things. They're just diode DPDT switches that
reverse the polarity of the RF signal at a rate determined by the LO
frequency. This is essentially the same as multiplying the RF signal by +/-1
at the LO rate. Simple math shows that this multiplication process yields
the sum and difference frequency products. The more symmetrical the LO
switching waveform, the more perfect the multiplication. This is why square
wave LO drive is preferred.

Joe
W3JDR




"andreas magun" wrote in message
...

To answer my own post: I think it should be 1V p-p for the equivalent
square wave, which is the same power as a 0.5V rms sine wave into 50 ohms
(+7 dBm).

Leon

1 Vpp square wave into 50 Ohms would be +13 dbm. What is the maximum power
a SBL-1 mixer will survive?

Andreas